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ISSN 1000-0585
CN 11-1848/P
Started in 1982
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  • Table of Content
      , Volume 63 Issue 10 Previous Issue    Next Issue
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    PUBLIC SAFETY
    Experimental study on the characteristics of fire spread on porous sand bed infiltrated by high flash point liquid fuel
    CHEN Changkun, SHI Lang, BAO Yipeng, ZHANG Yulun
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1493-1501.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.043
    Abstract   HTML   PDF (16569KB) ( 224 )
    [Objective] Previous studies on propagation-type combustion have yielded fruitful results. However, relatively few studies have investigated the characteristics of fire spread behavior of nonvolatile high flash point liquid fuel in porous media. [Methods] A series of combustion tests were performed on typical high flash point liquid fuels (kerosene, 0# diesel) infiltrating quartz sand beds of various particle sizes. The average particle sizes of quartz sand under the designed experimental conditions were 0.428, 0.715, 1.200, 2.180, 3.675, and 4.500 mm. The fire spread was recorded using a high-definition video camera at 25 frames per second, with the camera positioned 100 cm away from the experimental tank. The mass loss rate was measured using an electronic balance with a 0.1 g accuracy, and the fire spread rate was determined from the flame front position data using MATLAB. A series of K-type armored thermocouples with a diameter of 0.5 mm were arranged on the side wall of the experimental tank to measure the temperature distribution in the quartz sand bed. [Results] The experimental results reveal that the heat transfer rate of a fine-grained sand bed (d=0.428 mm) is faster than that of a coarse-grained sand bed (d=2.180 mm) at the same depth. This difference can be attributed to the effect of capillary action on the mass transfer efficiency of the sand layer, which in turn affects the heat transfer rate during the combustion reaction process. Furthermore, a preheating zone of a certain length exists near the flame front, which is primarily dominated by heat conduction and has minimal effect on convection and radiation heat transfer. In the process of fire spreading on the quartz sand bed with liquid fuel, the mass loss rate increases over time. However, the growth rate of mass loss rate varies under different particle size conditions. In the quasi-stable combustion stage, the growth rate of mass loss rate initially decreases and then increases with increasing particle size. As the particle size increases, the average fire spreading rate of kerosene and 0# diesel on the surface of the quartz sand bed first decreases and then increases. Because of the relatively high flash point and viscosity of 0# diesel, its average fire spreading rate on the surface of the quartz sand bed is lower than that of kerosene. [Conclusions] This study analyzes the characteristics of fire spread on the surface of a typical porous sand bed infiltrated with high flash point liquid fuel and reveals the mechanism of the effect of particle size on the characteristics of fire spread to a certain extent. The research results provide references for the security protection and decontamination treatment of the fire spread problem when liquid fuel leaks into porous media.
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    Compartment fire behavior with two opposite openings under crosswind
    DUAN Junrui, HE Mingming, HU Haowei, JI Jie
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1502-1511.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.040
    Abstract   HTML   PDF (9957KB) ( 134 )
    [Objective] External wind almost always exists around high-rise buildings. Due to the superimposed or competitive effect of wind pressure, buoyancy, and thermal expansion, the fire behaviors in high-rise building enclosures with two openings exhibit more complex dynamic evolution characteristics than those without external wind effects. In situations with such effects, new models are required to systematically analyze the fire behavior evolution mechanism. [Methods] In this study, we established a more practical compartment fire scenario and performed numerical analyses using the computational fluid dynamics (CFD) code, fire dynamics simulator (FDS), to study the evolution of the fire behavior of a compartment with two opposite openings. A total of 48 simulations were performed, wherein different wind speeds and heat release rates were considered. The fire source was a propane burner in the center of the compartment. The simulation duration was set at 350 s. The use of numerical simulations opened up the potential for a direct evaluation of a wide range of variables (e.g., the mass rate of inflow and outflow through openings and temperature) and even more complex quantities (e.g., the heat release rate within predefined volumes such as windward side space, leeward side space, and space inside the compartment). To obtain accurate simulations, sensitivity analysis was performed. The impact of crosswind speed on the behavior of fire spill plume on the windward and leeward sides, the temperature inside the compartment, and the flow pattern across the two openings were analyzed. Combined with the energy conservation equation, the average temperature rise model inside the compartment at over-ventilated conditions under the crosswind was established based on dimensional analysis. A critical wind speed (vc) was determined for the conversion of gas from bidirectional to unidirectional flow according to the pressure differences among the windward side, the compartment, and the leeward side. [Results] This study finds that a uniform mesh size of 2 cm is sufficient to achieve convergence. The results indicate that (1) the fire spill plume on the windward side at under-ventilated conditions gradually disappears as the wind speed (v) increases. Afterward, the fire spill plume on the leeward side also disappears gradually. At the same time, the heat release rate inside the compartment gradually increases. The critical criterion for the occurrence of the fire spill plume is that the global equivalence ratio Φ equals 0.645. When Φ≤0.645, combustion occurs only within the compartment. When Φ>0.645, the fire spill plume appears outside the compartment. (2) As the wind speed increases, the average temperature within the compartment constantly decreases at under-ventilated conditions and constantly increases at over-ventilated conditions. When v<vc, there is a bidirectional flow through the openings on the windward or leeward side; when vvc, the flow through the openings on both sides is unidirectional. The accuracy of the critical wind speed model is further verified based on an analysis of the mass flow rate through the openings on both sides. [Conclusions] The outcomes and findings of this study will help improve the existing theories of enclosure fire dynamics and provide theoretical and technical support for the fire protection of high-rise buildings.
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    Experimental analyses and modeling of pool fires with different ullage heights
    ZHU Huacheng, HU Zhenqi, ZHAO Zhishan, CHEN Cheng, ZHAO Jinlong, YANG Rui
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1512-1519.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.041
    Abstract   HTML   PDF (4352KB) ( 122 )
    [Objective] In recent years, floating-roof tanks have been widely used for liquid fuel storage in China. However, these tanks suffer from a considerable fire risk owing to the accumulation of highly flammable fuel vapors when the liquid fuel level is low. The risk of pool fires, confined by ullage heights, seriously threatens fuel storage and transportation. Furthermore, the flame characteristics, including burning rates and flame heights, are notably affected by ullage heights. However, the research on pool fires with different ullage heights is few. Hence, this study aims to experimentally study the pool fires for different ullage heights and analyze the key parameters (burning rate and flame height). Additionally, a correlation based on dimensionless analysis is proposed to predict the down-reaching flame height. [Methods] This study investigates the effect of ullage heights on pool fires. A series of pool fire experiments were conducted using a transparent quartz glass tray and heptane fuel. Different ullage heights (ranging from 3 cm to 50 cm, measured as the vertical distance from the tank top to the liquid fuel surface) were considered. The burning rate was measured using a Sartorius balance and video cameras were installed to record the burning process and flame heights. Flame heights were calculated by converting the flame videos into binary images. Subsequently, the key parameters were analyzed. [Results] The experimental results reveal the following: (1) The ullage height considerably affects the burning process, particularly at the initial and steady stages. At the initial stage, the burning rate sharply increases for cases with a low ullage height, while a burning rate decrease trend is observed after the rapid increase for cases with a large ullage height. This is mainly caused by the continuous uplift of the flame base, moving away from the fuel surface. (2) At the steady stage, the burning rate first decreases and then increases, followed by a final decrease with the increase of the ullage height. The increase in the distance between the flame base and the fuel surface results in this burning rate decrease trend. The burning rate increases because the flame base enters the tray. (3) Based on the flame shape in cases with a large ullage height in the steady stage, total flame height can be divided into the upper flame height outside the tray and the down-reaching flame height inside the tray. The experimental data demonstrates that the upper flame height decreases as the ullage height increases, whereas the down-reaching flame height shows an opposite trend. (4) A correlation is developed using the experimental data and dimensionless analysis to calculate down-reaching flame height with different ullage heights, in which the characteristic tray diameter is revised by considering the influence of ullage heights and air entrainment. [Conclusions] The findings of this study will contribute to the understanding of the burning behaviors of fires at different ullage heights, with practical implications in providing guidance for quantitative risk assessment in tank fires.
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    Combustion properties of glass fiber/phenolic resin at low ambient pressures
    ZHANG Xiaoyu, JIA Xuhong, DAI Shangpei, TANG Jing, MA Junhao
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1520-1528.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.032
    Abstract   HTML   PDF (4265KB) ( 118 )
    [Objective] Accidental fires seriously threaten the safe operation of aircraft. Air transportation environments typically have low ambient pressures that can significantly influence the occurrence and spread of fire. The wallboards in civil aircraft are generally made of composite materials. The Federal Aviation Administration of the United States and the Civil Aviation Administration of China require that the fire resistance characteristics of these materials be experimentally verified. This study investigated a sandwich structure panel (panel A) and a laminated panel (panel B) of an Airbus aircraft to understand the influence of ambient pressure on aircraft fires and to enable the earliest possible detection, management, and prevention of aircraft fires at the low ambient pressures typically encountered in such situations. Panel A was composed of upper and lower resin base panels, with an aramid honeycomb core and adhesive middle layer, whereas panel B was a resin-based glass fiber-reinforced laminate. [Methods] The effects of ambient pressure on the thermal insulation, ignition time, mass loss, and smoke characteristics of the panels A and B were studied using self-built, low-pressure, oxygen-enriched combustors in Kangding, Sichuan Province (61 kPa) and Guanghan, Sichuan Province (96 kPa), respectively. The thermal insulation characteristics of the panels were studied by measuring the temperature on the back surface of the panel after heating the front surface for 60 s with a heating rod. The effect of pressure on the convective heat loss was studied using the ideal gas relation. The mass loss during the fire was recorded by an electronic balance, and the smoke generation was recorded in real time by a smoke analyzer. [Results] The temperature of the back surface of panel A was 692.3 ℃ at atmospheric pressure and 512.4 ℃ at low pressure with a decrease of about 26.0%. The temperature of the back surface of panel B at normal and low pressures was 810.5 ℃ and 820.9 ℃, respectively. Furthermore, the temperature variation as a function of time was almost the same under either pressure condition for panel B, indicating that changes in the ambient pressure in the range studied had almost no impact on the insulation of panel B. The heating rate of panel B was higher than that of panel A, demonstrating the superior thermal insulation performance of panel A. Regarding the effect of pressure on the convective heat loss, the measured ignition times were in good agreement with the analytical model. The ignition time for panel A was reduced from 24.16 s to 20.34 s, i.e., reduced by 16%. Pressure variations had less influence on the ignition time for panel B. Variations in the pressure affected the rate of combustion; the mass loss for panel A decreased from 8.7% to 4.9%, and the peak mass loss rate decreased from 68.7×10-3 g·s-1 to 22.8×10-3 g·s-1, whereas the mass loss for panel B decreased from 5.8% to 4.8% and the peak mass loss rate decreased from 35.0×10-3 g·s-1 to 12.5×10-3 g·s-1. The time of the maximum O2 consumption and the time of the CO and CO2 production peaks of either kind of panels were almost the same under different pressure environments, whereas the maximum O2 consumption and CO and CO2 production peaks in the low-pressure environment were higher than those at atmospheric pressure. [Conclusions] This preliminary study on the effect of pressure on the combustion characteristics of aircraft panels finds that pressure has a significant impact on the occurrence and spread of aircraft fires. This study can provide theoretical support for cabin fire prevention and fire rescue under different pressure environments.
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    Temperature distribution for fire in a forced ventilation subway car
    LIU Zhiyuan, LI Xing, ZHOU Xun
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1529-1536.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.042
    Abstract   HTML   PDF (10890KB) ( 89 )
    [Objective] Urban rail transit in China is developing rapidly due to the significant increase in subway lines and passenger flow. Although the subway system provides convenient transportation, the occurrence of subway car fires poses a potential risk to passengers. Subway cars are narrow and enclosed spaces, making them susceptible to significant property damage and casualties in the event of a fire. The existing research on subway car fires is primarily focused on natural ventilation conditions. Notably, subway cars are equipped with air supply systems to provide fresh air during normal operation, which continues to be operational during the initial stage of a fire. This air supply may considerably impact fire development and temperature distribution within the car during the fire. A clear understanding of the spreading of smoke and temperature distribution during subway car fires under air supply conditions can effectively determine the best position for fire detectors within the car. [Methods] This study establishes a comprehensive simulation model of a subway car. The car model comprises three main components: the passenger area, internal seating, and the top air supply system. The car includes four groups of air supply systems on the top. The length, width, and height of the car are 21.82, 2.92, and 1.86 m, respectively. After constructing the car model, we partition the grids and define the boundary conditions based on two ventilation scenarios: with and without a forced air supply. Further, two typical fire source locations—the center of the car and the area below the air supply outlet—and five different fire source powers are investigated. Analysis of this study primarily focuses on the airflow velocity distribution within the car under ventilation, and the temperature distribution within the car under the influence of air supply and different fire source positions. [Results] The results indicate that airflow vortexes are formed within the compartment when air is supplied, and the vortex position is directly related to the seating arrangement. The airflow velocity distribution within the compartment is significantly affected by the position of the air supply outlet. Under the experimental conditions, the presence or absence of ventilation within the compartment does not have a significant impact on the maximum temperature rise of the car ceiling. This may be attributed to the high temperature near the fire source, which results in significant thermal buoyancy. Hence, the air supply at the outlet does not affect the fire plume significantly. A stable smoke stratification may still occur during the car air supply, although the height of the smoke layer is considerably higher compared to natural ventilation. A strong correlation between the ceiling flue gas temperature and the position of the air supply outlet exists in the presence of the air supply. Each air supply outlet creates a low-temperature area, and the ceiling flue gas temperature at both ends of the compartment is significantly lower compared to natural ventilation. Furthermore, regardless of the presence or absence of an air supply, the smoke temperature on the compartment's ceiling decreases exponentially. [Conclusions] The findings of this research can serve as a reference for establishing subway car fire prevention strategies.
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    Analysis of emergency rescue characteristics and evaluation of rescue capability for accidents associated with urban gas pipeline networks
    LI Cong, LU Yifei, CHEN Chen, XU Zixuan, YANG Rui
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1537-1547.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.026
    Abstract   HTML   PDF (4138KB) ( 122 )
    [Objective] Leakage accidents in urban gas pipeline networks occur from time to time, and most of them are accompanied by secondary disasters, such as explosions, fires, and building collapses, which seriously threaten the safety of people's lives and property. Previous research on gas accident rescue capability primarily focuses on gas enterprises or indoor gas emergencies, and research on accidents associated with gas pipeline networks is lacking. Some studies have limitations, such as broad evaluation indicators, vague content, and limited scope of assessment objects, which cause difficulties in applying the evaluation system in practice. This study aims to identify the weaknesses in the emergency rescue process for accidents associated with urban gas pipeline networks, effectively assess the emergency rescue capabilities for such accidents, and help improve the emergency rescue efficiency and gas safety guarantee level. [Methods] Herein, the emergency rescue characteristics for accidents associated with urban gas pipeline networks were analyzed and summarized, and the rescue capabilities for these accidents were evaluated. First, based on an in-depth analysis of emergency plans and accident cases associated with gas pipeline networks, the emergency rescue elements of accidents were extracted and sorted. Furthermore, the emergency rescue process was constructed. By summarizing the limitations in emergency rescue, an indicator system comprehensively reflecting emergency rescue capabilities was established based on four aspects, namely humans, pipelines, materials, and management. The system included 4 first-level indicators, 12 second-level indicators, and 27 third-level indicators. Second, the subjective-objective combination weighting method of the analytic hierarchy process (AHP) and the criteria importance through intercriteria correlation (CRITIC) method were used to calculate the weight of each indicator to reduce the possibility of excessive subjectivity caused by expert scoring to a certain extent. Combining the weight of each indicator can help identify and focus on the indicators with a high degree of importance. Finally, an emergency rescue capability evaluation model was established using the fuzzy comprehensive evaluation approach to realize the quantitative evaluation of the emergency rescue capabilities for accidents associated with gas pipeline networks in specific regions. The model was applied to Zhangwan District, Shiyan City, Hubei Province. [Results] The results show that indicators such as the “supply-demand ratio of rescue personnel”, “effectiveness of information transmission”, and “formulation and revision of emergency plans” account for a relatively large weight compared to other indicators. The indicators of “cooperation and coordination ability of rescuers”, “equipment performance”, and “emergency drill effect” are the weak links in the emergency rescue process of accidents associated with the gas pipeline networks in the region. Therefore, the departmental interaction needs to be strengthened, the construction of the rescue coordination mechanism needs to be improved, and joint prevention and control and coordinated rescue capabilities need to be enhanced. Furthermore, to standardize emergency drill training, the safety production investment guarantee for local gas companies should be increased, and to improve the design and planning of training content, online and offline integrated learning is necessary. [Conclusions] The feasibility and applicability of the evaluation system were verified through the application case. This evaluation system for the emergency rescue capability of accidents associated with urban gas pipeline networks offers a theoretical basis and feasible approach for establishing, improving, and evaluating emergency measures for accidents associated with urban gas pipeline networks.
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    Assessment of the heat transfer characteristics and cooling performance of firefighter cooling vests using thermal manikins
    CAO Kai, LI Yayun, FU Ming, GUO Xian, LIU Xiaoyong, SONG Yuhan
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1548-1557.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.044
    Abstract   HTML   PDF (9226KB) ( 125 )
    [Objective] Firefighting suits worn by firefighters in high-temperature radiation environments, coupled with moderate physical exertion, can lead to body heat accumulation and increased microenvironment temperature inside the suits, which triggers adverse physiological reactions due to severe heat stress, resulting in decreased work efficiency, heat cramps, and other harms. Hence, cooling vests are widely used by firefighters to effectively alleviate heat stress in firefighting scenarios. However, most contemporary cooling vests encounter issues such as severe cold stress, short cooling duration, and poor comfort. Therefore, it is necessary to conduct research on the cooling performance of these vests. [Methods] The cooling effectiveness of three types of firefighter cooling vests was assessed using a thermal manikin system in an environmental chamber under the following conditions: temperatures of 35 ℃, 40 ℃, and 45 ℃ and directed thermal radiation of 1.5 and 2.5 kW/m2, with low, moderate, and high levels of physical exertion. D1922L temperature sensors were deployed on the thermal manikin system to measure the temperatures of the outer, insulation, comfort, and inner layers of the cooling vest as well as the temperatures of the inner and outer layers of the firefighting suit. [Results] The experiments reveal that the cooling vests effectively reduce the microenvironmental temperature inside the firefighting suit, with a temperature difference of 6.6 ℃ between the inner and outer layers of the firefighting suit after 200 min of exposure. The cooling effect is most pronounced in the abdominal area with direct radiation. Among the three types of cooling vests assessed, Cooling Vest 1 exhibits the best performance, achieving a cooling power of 4.097 W and a cooling duration exceeding 2 h. Meanwhile, Cooling Vest 3 has a relatively poor cooling performance, with a cooling power of 0.753 W and a cooling duration of 90 min. As the physical exertion levels increase, the cooling effect of the vests is less pronounced. The linear fitting slopes of the temperature variation curves for the manikin's abdominal areas at 65, 110, and 165 W/m2 are 0.106 8, 0.273 4, and 0.508 6, respectively. Furthermore, the cooling performance of the vests diminishes with increasing environmental temperatures. At an environmental temperature of 45 ℃, the temperature gradients in the manikin's chest, shoulders, abdomen, and back areas increase by 50.0%, 40.6%, 60.0%, and 50.0%, respectively. Moreover, the directed thermal radiation has a significant impact on the cooling performance of the vests. The temperature of the manikin's chest area reaches 35.69 ℃ within only 7 min with a directed thermal radiation of 2.5 kW/m2. [Conclusions] These research findings are anticipated to serve as the preliminary data to enable the determination of rational work durations and intensity. In real fire rescue scenarios, the distribution of phase change materials for heat storage over the chest area and the duration of high-intensity work should be carefully considered. Additionally, these findings provide technical support for the development, testing, and evaluation of personal thermal protective equipment.
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    Multi-objective optimization method and case analysis for emergency drainage of pumped storage power station
    DAI Xin, CHEN Jushi, CHEN Tao, HUANG Hong, LI Zhipeng, YU Shuiping
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1558-1565.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.031
    Abstract   HTML   PDF (4027KB) ( 138 )
    [Objective] The severity of flood disasters in underground powerhouses is increasing with the expansion of pumped storage power stations, and the current emergency drainage system of pumped storage power stations cannot meet the large discharge and high-lift demands during periods of abnormal water inflow. This paper proposes a phased parallel emergency drainage method for pumped storage power stations to address the problem of ineffective and slow drainage during underground facility floods. Furthermore, a multi-objective optimization model for emergency drainage is established to solve critical parameters in the drainage plan. [Methods] This paper proposes a multi-objective optimization method for emergency drainage in pumped storage power stations. First, based on the engineering characteristics of the underground powerhouses of pumped storage power stations, the emergency drainage process is divided into three stages: early, middle, and late. Second, emergency drainage methods are developed for each stage. During the early stage of drainage, for a small amount of accumulated water in the area above the top floor of the generator, submersible pumps and drainage pipelines are installed in the entrance tunnel to discharge the water to the exit of the tunnel. The middle stage of drainage focuses on draining the accumulated water from the generator floor; multiple drainage vehicles are connected in series to drain the water to the surface. Late-stage drainage is aimed at the accumulated water below the generator floor. A small volume floating pump is used to extract water from each layer, which is then drained to the generator floor. Here, a water storage tank is set up to temporarily store the water extracted by the floating pump; the water is drained in parallel through the entrance tunnel and the maintenance and leakage drainage pipes. Finally, the proposed drainage method is mathematically modeled to minimize drainage time and operating costs as objectives. Furthermore, the non-dominated sorting genetic algorithm-Ⅱ (NSGA-Ⅱ) is employed to obtain the optimal solution for the drainage flow rate at each stage. [Results] The phased series-parallel comprehensive emergency drainage method proposed in this paper employs movable drainage equipment, enabling rapid deployment by onsite emergency rescue teams. In the case of a pumped storage power station, 34 drainage schemes are optimized, all of which meet the constraint conditions. Among them, the shortest emergency drainage time is about 52 h, and the minimum operating cost for drainage equipment is approximately 1.38 million RMB, providing a basis for selecting drainage schemes and procuring or customizing drainage equipment. [Conclusions] The optimal solution set of drainage flow rates corresponding to each drainage stage can be obtained by updating only the self-attribute data of the power station in the model input using the NSGA-Ⅱ-based multi-objective optimization model for emergency drainage of pumped storage power stations. This method reduces emergency drainage time and equipment operating costs while improving the emergency response level of flood accidents in pumped storage power stations.
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    Comprehensive evaluation method for dam safety considering fusion weight optimization and conflicting information sources
    WANG Lei, WANG Xiaoling, ZHANG Jun, YU Jia, WANG Jiajun
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1566-1575.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.039
    Abstract   HTML   PDF (7894KB) ( 119 )
    [Objective] Evidence theory, an effective multisource information fusion method, has been widely applied in the field of dam safety monitoring and health diagnosis. However, two limitations must be addressed: the accuracy of evaluation results of existing conflicting evidence-processing methods is low, and the existing fusion weight calculation methods may result in unreasonable fusion weights when the differences in the contribution of each monitoring type to the dam safety evaluation results are ignored. In this paper, we propose a comprehensive evaluation method for dam safety considering fusion weight optimization and conflicting information sources. The negative impact of conflicting evidence on the accuracy of information fusion is significantly eliminated by means of both decreasing conflicting information generation sources and preprocessing evidence bodies. Furthermore, reasonable fusion weights for each monitoring type can be obtained through subjective assignment. [Methods] In the data preprocessing stage, we propose the STL (seasonal-trend decomposition procedure based on loess)-iForest (isolation forest algorithm) method for temporal outlier detection. First, the original dam safety monitoring sequence data are decomposed into trend, seasonal, and residual terms using the STL temporal decomposition method, which eliminates the influence of data trend and periodicity on the total error rejection accuracy. Then, the iForest algorithm is used to remove outliers from the data of the residual term. Finally, the data are reconstructed using the Hermite interpolation method. Using the above methods, we suppress the generation of conflicting evidence at the source as much as possible. In the multisource information fusion stage, we calculate the evidence fusion weight of low-level information using the χ2 divergence and Shannon entropy, which can overcome the shortcomings of traditional distance metrics, such as the Euclidean distance, which have poor reliability of fusion results. For high-level information, we use the scaling method to determine the corresponding fusion weight to ensure that the weight reflects the difference in the contribution of different monitoring types to the dam safety evaluation results. [Results] The results with data preprocessing show that the STL temporal decomposition method separates the trend term and seasonal term of the data well and keeps the outliers in the residual terms, providing a good basis for outlier detection. Then, the iForest algorithm can detect all obvious outliers in the data, which shows that the proposed outlier detection method can ensure the high reliability of the multisource monitoring data by eliminating conflicting evidence from the sources. In the multisource information fusion method proposed in this paper, the discrete degree and uncertainty of the evidence are fully considered to ensure the accuracy of the evaluation results, especially in the case of conflicting evidence, which is consistent with the actual situation. Compared with evidence theory based on the Euclidean distance and the case of unexcluded data coarseness, the proposed method improves the probability corresponding to the basic normal state to 0.842, indicating that the method has better reliability. [Conclusions] The proposed method shows high-accuracy fusion results and high-reliability qualitative evaluation results. In addition, it can accurately quantify the differences in the contributions of each monitoring type to the comprehensive dam safety evaluation results. The proposed method has considerable potential in the diagnosis of dam safety, given its high accuracy and good robustness.
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    Interdependencies between city water and electricity supply networks based on attack tolerance
    SHI Zhengyi, HUANG Hong, ZHOU Shiwei
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1576-1583.   DOI: 10.16511/j.cnki.qhdxxb.2023.21.015
    Abstract   HTML   PDF (5133KB) ( 115 )
    [Objective] Historical events suggest that infrastructure interdependencies exhibit negative influence in an amplifying or cascading way, leading to remarkably outcomes on an urban, regional, or even national scale. Therefore, studies on infrastructure interdependencies play a vital role. The interdependency-caused vulnerability of water and electricity supply networks, as components of city lifeline, generally poses risks to people's life safety, daily life, and industry functioning. [Methods] A case study of water and electricity supply networks of a district was conducted to measure their interdependencies in the frame of attack tolerance, thus innovatively advancing from its usual use in studies on the World Wide Web and social networks. Infrastructure systems tend to possess certain spatial structures, especially network-like ones such as water and electricity supply networks, rendering topological methods effective and straightforward. An attack tolerance framework based on graph theory was applied in this research. Two graph models were established using the Python NetworkX library prior to checking the correctness of the modeling. The degree of each vertex was calculated, and its frequency density curve was drawn to match the network characteristics with those of an exponential network, such as a water or electricity supply network. Subsequently, the fragmentation of the networks was studied. When the vertices were removed to simulate a random external attack, the original network as a whole disintegrated into multiple disconnected small components known as vertex clusters. This process is called fragmentation. The quantitative characteristics of vertex clusters, together with the basic network index called network diameter, served as indices of attack tolerance. The curves of the indices above the proportion of removed vertices and the distribution scatters of cluster size at a certain removed proportion were drawn to measure and compare the attack tolerance between the two networks. In addition, the distribution scatters were fit to an exponential form including two parameters and the curve of the parameters to the removed proportion was drawn. Finally, this study introduced a measure of geographical interdependency between water and electricity networks based on a previous attack tolerance study. The two networks were placed in the same coordination system that had been gridded rectangularly with a certain fineness. The hypothesis was that the electricity of each water vertex was provided by the nearest electricity vertex; the former was removed when the latter malfunctions. Vertices in each rectangle of the grid were completely removed to simulate a blackout of a district, and a geographical interdependency index was defined for every grid reference in accordance with the fragmentation indices. This research visualized the spatial distribution of the interdependency index at a certain grid fineness, through which the critical sections could be identified. [Results] The attack tolerance of the water supply network was slightly remarkable, and the southeast region of the water supply network of this district was the most dependent on the electricity supply network. [Conclusions] This work introduced an interdependency analysis method based on the framework of attack tolerance of a topological network and provided guidelines for the protection of infrastructure systems, urban planning, contingency plans, and resilience enhancements.
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    Analyzing the vulnerability of electrified transportation road networks
    WANG Hongping, HU Yanzhu, ZHANG Yufeng, WANG Song
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1584-1597.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.036
    Abstract   HTML   PDF (2151KB) ( 303 )
    [Objective] The rapid proliferation of electric vehicles (EVs) and the large-scale deployment of charging facilities have considerably increased the electrification of transportation road networks. However, road networks exhibit vulnerability to failure at several critical sections, which in turn may trigger a cascade of failures, ultimately leading to widespread road network disruptions. In the context of mixed electric and nonelectric vehicular flows, such adverse impacts may further spread and cascade due to EV-specific characteristics, such as limited EV range and required charging time. Protective measures for vulnerable road sections of electrified road networks against hazards could mitigate the risk of cascading failures and the further spread of disruptive events. Therefore, assessing the vulnerability of electrified transportation road networks and identifying critical road sections have become paramount. Given that the vulnerability of electrified transportation road networks has been scarcely explored in existing literature, this paper proposes a two-layered attacker-defender model to study the vulnerability of electrified transportation road networks. [Methods] The outer layer model aims to minimize system performance by targeting roads within the system for disruption, i.e., maximizing the total system travel time. The inner layer model serves as a defender, minimizing the total system travel time by dynamically and optimally distributing traffic flows containing both electric and nonelectric vehicles. The inner layer model is formulated based on an enhanced link transmission model, taking into consideration the critical characteristics of the electrified transportation road networks. This two-layered model can describe the temporal and spatial evolution of the mixed electric and nonelectric vehicular flows. Additionally, this paper provides a detailed solution method and theoretical analysis of this model. A mixed-integer quadratic programming problem is obtained by considering the dual of the inner problem and combining the inner problem with the outer problem. This problem is subsequently converted into a mixed-integer linear programming problem using the big M method. [Results] The proposed model is applied to a segment of the highway network in North Carolina, U.S. The experimental results reveal that (1) critical road sections as determined with and without EVs differ considerably. Therefore, it is necessary to incorporate EVs when analyzing the vulnerability of an electrified transportation road network. (2) The set of critical road sections varies depending on the level of attack resources. In particular, the set of critical road sections in the low attack resource level scenarios is not necessarily a subset of the critical road sections in the high attack resource level scenarios. (3) The experimental results confirm the existence of a critical point in the attack resource level. When this critical point is reached, the system performance displays a phase change phenomenon, marked by a notable decline. [Conclusions] The results verify that the proposed model can identify the set of critical road sections in the system and provide theoretical support to improve the vulnerability of the electrified transportation road networks.
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    A decision support model for crime investigation
    WANG Jia, WANG Weixi, WANG Litao, SHEN Shifei
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1598-1607.   DOI: 10.16511/j.cnki.qhdxxb.2022.25.029
    Abstract   HTML   PDF (3616KB) ( 188 )
    [Objective] Artificial intelligence and big data technologies have been used to solve many scientific problems, including crime analysis. The investigation of criminal cases has always been a critical and difficult point in the domain of crime analysis. The investigation stage of criminal cases primarily consists of evidence collection and evidence reasoning, and comprehensive and efficient collection and reasoning of evidence are critical to the rapid detection of cases. Simultaneously, the significance of the various pieces of evidence in the case varies. Evidence of high importance gathered during the investigation stage is critical for the accurate and efficient resolution of crime cases. However, existing research lacks the application of artificial intelligence methods to crime investigation decision support. [Methods] Aiming at the problem of crime investigation, this research proposes a decision support model based on the Bayesian network to help domain experts determine the direction of the investigation and reasoning the criminal facts. First, the Bayesian network is used to reason the hypothesis of criminal facts. Second, the weighted information entropy method is used to calculate the importance of criminal evidence. Four different types of weighted information entropy methods are employed to test the efficiency of the calculation method. The two methodologies are then combined to create the decision support model for crime investigation. Finally, the proposed model is applied to 420 crime cases to verify its accuracy, and the proposed model is also applied to a real case analysis to illustrate the application process of the model. [Results] The analysis of 420 crime cases reveals that calculations based on weighted information entropy are the best of all four methods. The top 3, 5, and 10 evidence pieces provided with the weighted information entropy method all have the highest coverage of importance, given any arbitrary evidence missing ratio. Meanwhile, when 50% of the evidence is missing, the output result's coverage of the top 3, 5, and 10 important pieces of evidence is greater than 50%, 65%, and 80%, respectively; when 90% of the evidence is missing, the coverage of the top 3, 5, and 10 evidence pieces is greater than 40%, 60% and 75%, respectively. These suggest that the model's detection recommendations are effective and can be used to assist in crime detection. Furthermore, the analysis of a real-world case also shows that the proposed model can generate effective investigation suggestions and provide decision support for evidence collection and reasoning during the investigation stage. [Conclusions] Finally, the proposed decision support model for crime investigation can analyze available case information and generate effective investigation suggestions and reasoning conclusions. However, it should be noted that the model developed in this study does not completely replace the role of professionals in the field of criminal investigations but rather provides analysis results to scientifically support the decisions of subsequent investigators in the initial stages of the investigation. Furthermore, this study focuses on the research of evidence collection and reasoning during the investigation stage of criminal cases but pays limited attention to the “evidence standard” involved in the process of evidence collection. Therefore, we can continue to investigate this aspect in the future to aid intelligence and standardization of evidence collection during the investigation stage.
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    MECHANICAL ENGINEERING
    Intelligent welding technology for large structures crawling robot
    FENG Xiaobing, WANG Jianjun, WANG Yongke, CHEN Suyun, LIU Aiping
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1608-1625.   DOI: 10.16511/j.cnki.qhdxxb.2022.26.057
    Abstract   HTML   PDF (14146KB) ( 234 )
    [Significance] Welding has reached a very important position in large structural workpieces. The welding of large structural members also has been covered many fields, including high-end manufacturing fields such as shipbuilding, oil and gas chemical industry, nuclear power engineering, energy and power, building steel structure, and rail transit. The problems of traditional manual welding are instability and low efficiency. Even semi-automatic welding such as gantry welding and rail robot welding, which has not yet to meet the requirements of more efficient and high-quality automation industry. The welding development trend of large structural parts is more intelligent and automatic welding. In this case, the welding robot that has no track and can crawl in all positions has become the representative of intelligent welding equipment, and gradually has promote and solve the automatic intelligent welding of large structural parts. [Progress] In this paper, the research status of intelligent robots for welding large structural parts at home and abroad was systematically introduced, and the welding robots were classified according to the welding robots with and without rails. The application scenarios, advantages and disadvantages of these two types of welding robots were compared. Through these comparisons, it was concluded that the welding robot without rail was more suitable for welding large structural parts because of its better flexibility, adaptability and convenience. There were many problems in the welding process of large structural parts, especially when multi-layer and multi pass thick plates were welded. At the same time, these problems caused low welding process accuracy and low welding quality. Problems included low machining accuracy, inaccurate assembly, metal thermal deformation, many weld passes, weld beads stacked together and many other reasons. In the welding of large structural parts, there were still many difficulties that need to be solved urgently. Automatic backing welding technology and multi-layer and multi-channel automatic routing technology were two very critical and difficult problems that hindered the progress of the welding industry. Until now, the backing welding of large structural parts has mainly depends on manual work, because backing welding was the first weld connecting two welding workpieces, which was very critical in the whole welding process. In the process of automatic backing welding, there were strict requirements on the assembly clearance, unfitness, blunt edge, curling and welding heat input. If these factors were not well controlled, a large number of welding defects such as missing welding and incomplete welding would occur in the process of backing welding. Multi layer and multi pass welding also depended on manual arrangement to cause unstable welding quality. Therefore, it was more and more important to automatically plan the number of welding passes and layers, arrange the welding sequence, determine the position coordinates of each weld pass and adjust the welding parameters. [Conclusions and Prospects] Based on this situation, this paper summarizes the exploration of related technologies from the aspects of automatic backing welding technology, seam tracking technology, multi-layer and multi-channel automatic lane arrangement technology, etc. Multi-modal deep learning and multi-sensor fusion technology are widely used in many fields, and have become a concern in military, industrial and high-tech development. Thus, these two technologies will be the key to developing welding robots and provide guidance for the intelligent welding of large structural parts. In the future, artificial intelligence technology will lead the welding robot to achieve better welding quality.
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    Braking noise characteristics and a simulation analysis method based on a multiscale braking test
    YANG Zhiyong, WANG Ziming, YE Shanshan, LI Zhiqiang, LI Weijing
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1626-1639.   DOI: 10.16511/j.cnki.qhdxxb.2023.26.001
    Abstract   HTML   PDF (32150KB) ( 106 )
    [Objective] Currently, iron or steel brake disks generally be selected to research braking noise. This research focuses mainly on the influence of the structural characteristics of the brake pads and brake disks on the braking noise, and few studies addresse how braking conditions or braking modes affect the noise characteristics. To clarify the causes of the abnormal noise in the braking process of different friction pairs, through a noise test of the multiscale bench braking test and the simulation method of complex eigenvalue analysis, this paper studys the braking noise characteristics and the generating mechanism of different friction pairs under different braking conditions and braking modes. [Methods] According to the scaled test scheme, a scaled test of three types of friction pairs is performed on the MM-1000/IV scaled test machine, and the braking noise characteristics of the friction pairs are compared and analyzed. For the selected friction pair of the SiCp/A356 composite brake disk and synthetic pad, a more realistic 1∶1 bench brake test is performed. On the basis of HyperMesh and Abaqus software, a finite element simulation model of the wheel-mounted disk foundation braking device with a wheel, disks, pads, pad holders, brake levers, etc. is constructed. [Results] Under the same working conditions, the sound pressure levels (SPL) of the two SiCp/A356 composite friction pairs were lower than those of FE-928W friction pair. The maximum value of maximum sound pressure level (Max SPL) was 96 dB for the FE-928W friction pair and 87 dB for the BJTU-928W friction pair. The braking noise type of the FE-928W friction pair was mainly a multifrequency squeal, and the braking squealing phenomenon occured in most working conditions. The noise types of the BJTU-928W friction pair and the KNORR-928W friction pair were mainly wide area noise and single frequency squeal. The friction surfaces of different friction pairs had different degrees of surface scratching, material transfer, and peeling, which led to differences in the braking noise. The main frequency of the squealing noise of the friction pair between the SiCp/A356 composite brake disk and the synthetic brake pad was concentrated near 2.20 kHz, and the Max SPL could reach 110 dB. Brake pads were the main source of braking noise. The greater the energy inputed, the more times of high-temperature continuous braking, the greater the squealing sound pressure level, and the more severe the squeal. In contrast, when braking at a lower energy input level, the disk state would be restored, and quiet braking could be achieved during subsequent braking. [Conclusions] Through the experiments and simulations, the simulation method of complex eigenvalue analysis can better predict the tendency of braking noise, and the braking noise characteristics and causes of different friction pairs under different braking conditions and braking modes are clarified. This research provides a scientific basis for noise reduction strategies such as friction matching, structural design optimization, and braking mode improvement of the friction pair between the SiCp/A356 composite brake disk and the synthetic brake pad.
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    Parameter tuning of the wafer stage compensation feedforward controller of the lithography machine
    LIU Tao, YANG Kaiming, ZHU Yu
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1640-1649.   DOI: 10.16511/j.cnki.qhdxxb.2023.26.002
    Abstract   HTML   PDF (7920KB) ( 177 )
    [Objective] The feedforward controller is crucial to achieving nano-level motion accuracy for the lithography wafer stage under high acceleration and deceleration conditions. Traditional 4-order feedforward is widely used to control precision motion systems because of its intuitive physical meaning and simple parameter tuning. However, its capacity to fit the inverse model is inadequate, and it is difficult to eliminate the repetitive error caused by the input trajectory. Therefore, a feedforward control architecture using the 4-order feedforward and an extra rational fraction compensator is proposed. [Methods] In this study,the input signal of the compensator is the higher-order derivative of the reference trajectory, and the numerator and denominator of the compensator use the delay unit as the basis function. Therefore, obtaining the unknown parameters of the basis function is crucial to the design. This paper proposes a data-driven iterative parameter tuning strategy for the compensation controller. The difficulty is that the tuning problem is a nonconvex optimization problem, making global parameter optimization challenging. This paper uses the relevant rules of system identification to address the issue at hand. The purpose of adding compensatory feedforward is to eliminate the residual error after using the 4-order feedforward, which is equivalent to achieving a zero-generalized error. Since the generalized error has a linear connection with the compensator parameters, the original nonconvex optimization problem is successfully transformed into a convex problem by minimizing the 2-norm of the generalized error. Through the above transformation, the global optimal point is obtained by the Gauss—Newton method, and the step size condition for ensuring iterative convergence is provided. In addition, the gradient and Hessian matrix of the objective function need to be incorporated into the parameter updating law, even though their exact values are difficult to obtain. This paper derives their unbiased estimates using two impulse response experiments and 2 trajectory tracking experiments. [Results] The proposed method was applied to the wafer stage of the lithography machine, and the experiment showed the following results: (1) Using the proposed method to tune three compensation controllers with different orders, their error 2-norm almost converged after five iterations. (2) After adding compensation feedforward, the acceleration and deceleration phase errors were reduced from ±35 nm to ±10 nm; the constant velocity phase error was almost equal to the positioning error, and its trajectory tracking effect was very close to that of iterative learning control (ILC) compensation. (3) Compared with the existing compensation controller parameter tuning method, the maximum moving average and moving standard deviation at velocity phase of the proposed method were smaller, and the lower the compensator order, the more obvious the advantage. (4) After changing trajectory, the proposed compensator could still achieve a better control effect than ILC compensation. [Conclusions] The above experiments verify the convergence performance of the proposed parameter tuning algorithm. It is shown that the proposed feedforward compensation architecture can effectively eliminate the residual repetition error of the 4-order feedforward; simultaneously, it can adapt to variable trajectories. In addition, compared to the current compensator tuning result, this method can achieve a superior trajectory tracking control effect while using a low-order compensation controller.
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    Special automatic spraying system for civil aircraft parts based on visual recognition
    JIANG Shuai, SONG Libin, CHEN Xiaoyong, ZHANG Peng, LIU Kecheng, CHANG Junhu
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1650-1657.   DOI: 10.16511/j.cnki.qhdxxb.2022.26.053
    Abstract   HTML   PDF (10547KB) ( 109 )
    [Objective] For a long time, manual operation is the only option for spraying aircraft parts. Uncertainty in a manual operation leads to problems including uneven paint film thickness, low spraying efficiency, and significant paint waste. More than 50 000 types of parts are available for various civil aircraft. With a wide variety, no fixed placement, and no clamping device, all these problems are far from being solved using existing robotic systems. To target the problems, an automatic spraying system for civil aircraft parts based on visual recognition is developed. [Methods] The system combines a two-dimensional (2D) camera and a three-dimensional (3D) camera to scan the spraying platform to realize the recognition and pose matching of parts. The 2D camera is used to obtain a 2D image of the parts on the spraying platform. Parts are preliminarily located and classified using the images, thereby reducing the interference of the environment on the point cloud. The 3D camera simply scans the partial space marked using the 2D camera, which effectively shortens the time compared to scanning the entire spraying platform space. The viewpoint feature histogram (VFH) descriptor is used to describe the point cloud features to recognize the type of parts. The k-dimensional tree (kd-tree) is used to establish the feature index. The search performance is much better than the global nearest neighbor search method. To solve the problem that the particle swarm optimization (PSO) algorithm is easy to fall into local optimal extremum, an improved PSO algorithm is proposed, where stagnation coefficient and the concept of decoupling are combined with the proposed algorithm. The spraying trajectory of the robot is planned, including the path of the spray gun and the trajectory of the robotic arm joints. The spray gun path planning means planning the path that the spray gun takes in the Cartesian space. Spraying paths are planned using the surface slicing method to realize the full coverage spraying of random poses and complex surface parts. Joint trajectory planning means planning the angle trajectory of each joint of the robot. The Bézier curve is used to plan the joint space trajectory to ensure the robot operated smoothly. [Results] The improved PSO algorithm performed better than the traditional PSO algorithm in convergence speed and accuracy. With an average accuracy of 2.11 mm, the automated point cloud registration for complex parts was completed in 240 s. Spray paths were simulated with Robot Studio, and simulations verified the method's effectiveness. [Conclusions] The spraying issue of numerous civil aircraft parts at arbitrary placements is resolved using the robot automatic spraying system based on visual recognition described in this study. Work efficiency and spraying quality of the system meet the production requirements. It is also found that the ambient light interferes with the work of segmenting parts in 2D images during the experiment. Manual adjustment of parameters cannot produce good results for complex image analysis tasks with high noise and shadow, so an adaptive parameter method should be proposed. The greedy algorithm is used for spraying path combinations, and the paths' global optimization method needs to be improved.
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    AEROSPACE ENGINEERING
    Preparation and process optimization of C/C composites based on the multi-channel flow field and thermal field control
    HUANG Xiaoqing, WANG Pengfei, ZHANG Song, ZHANG Hui
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1658-1671.   DOI: 10.16511/j.cnki.qhdxxb.2022.26.054
    Abstract   HTML   PDF (13159KB) ( 94 )
    [WTBZ] [Objective] Fiber-reinforced composite materials are widely used in aerospace due to high-temperature resistance and high specific strength. The popular chemical vapor infiltration (CVI) method slowly penetrates the porous preform through a gas source and decomposes the carbon source gas into pyrolytic carbon for deposition. This method offers the advantages of less fiber damage and designability of special-shaped components; however, the CVI preparation cycle is long, and the pore blockage on the fiber preform surface makes the densification of the porous preform uneven. To effectively improve the controllability and uniformity of the deposition of porous composite materials, a dual-process chemical vapor infiltration/deposition (CVI/CVD) system bases on a dual-temperature zone-dual-channel structure is proposed in this study, with strong designability and manufacturability and an improved CVI. [Methods] Bases on the influence of gas flow direction and temperature on the deposition process, CVD is introduced to combine the two parameters, thus improving the densification uniformity while shortening the fabrication cycle. Since the complex transport and reaction of carbon source and carrier gases is a multivariate multi-coupling process with a great impact on the deposition efficiency and uniformity, numerous modeling and experimental studies are required. In this paper, the process design and system optimization are conducted to generate carbon/carbon composites (C/C composite) bases on the integrated research idea of equipment design-build- theory-preparation-optimization. By establishing flow, heat transfer, and mass transfer reaction models, the effects of temperature, velocity, and concentration on the densification process are analyzed. The two-step deposition simulation of the porous composites is conducted via the dual-process CVI/CVD system. [Results] The results showed that reducing the deposition temperature could reduce the deposition rate, avoid saturated adsorption on the preform surface before complete deposition, and improve the deposition uniformity in the thickness direction. Increasing the temperature to increase the reaction rate could easily lead to surface deposition, causing the surface pores to close and block, thus hindering further deposition. The deposition position could be controlled by changing the spatial temperature gradient. For the same temperature gradient, the higher the control temperature, the larger the main deposition area; for a higher control temperature, the main deposition area decreased with the increasing temperature gradient. When the time for the reaction carbon source gas and the carrier gas to be transported to the deposition area was equal to that for the deposition reaction to consume the atmosphere source, the initial velocity matched the concentration, improving the densification efficiency. The two-step deposition simulation results showed that the porous preforms achieved a relatively uniform density in horizontal and vertical directions after the two-step CVI/CVD. [Conclusions] By combining the two processes, a dual-temperature zone-dual-channel CVI/CVD system is constructed. A mathematical and a physical model of densification are established, based on the basic theories of chemical reaction kinetics and heat and mass transfer to perform the variable parameter simulation experiment. The flow and thermal field control process optimization design is performed for the prepared C/C composite material, verifying the feasibility of the uniformity control and the process optimization of the C/C composite material deposition sample and providing significant guidance to conduct the future experiments.
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    Research and optimization of the mechanism of centrifugal compressor
    YAN Huihui, LI Haoyu, ZHOU Bohao, ZHANG Yuzhou, LAN Xudong
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1672-1685.   DOI: 10.16511/j.cnki.qhdxxb.2022.25.023
    Abstract   HTML   PDF (13597KB) ( 169 )
    [Objective] As the core component of an aeroengine, a compressor significantly affects the flow and power of the engine. Compared with the axial compressor, the centrifugal compressor is characterized by structural simplicity, manufacturing convenience, and high single-stage pressure ratio. Therefore, the compressor is highly suitable for turboshaft engines with low flow rates and low total pressure ratios. However, the piston engine plays a more important role in the market. Accelerating the research on centrifugal compressors used in small turboshaft engines is essential.[Methods] The design methods currently used in this project include experimentation, theoretical analysis, and numerical simulation. The numerical simulation method can eliminate the requirements of experimentation, overcome measurement difficulties, and eliminate the costs associated with the experiment process. Therefore, it is a relatively accurate and efficient method for flow and transfer analysis. In this paper, according to the theory of numerical simulation, the impeller and diffuser of the centrifugal compressor are designed under specified working conditions. A three-dimensional numerical simulation of the centrifugal compressor is conducted. The influence of typical parameters on the centrifugal compressors is studied, and the parameters of the preliminary design model are optimized to obtain the ideal model of the centrifugal compressor under the design conditions.[Results] The results of this study were obtained according to the static pressure distribution cloud map and the total pressure distribution cloud map of the meridional channel surface at the highest efficiency of the centrifugal compressor and design speed conditions. The efficiency of the optimized centrifugal compressor was 0.831; the corresponding pressure ratios was 8.771, which was 3.68% higher than that of the preliminary design; and the working margin was 18.44%, which was 4.79% higher than that of the preliminary design centrifugal compressor.[Conclusions] Through the numerical simulation results of an Eckardt impeller and comparison of the simulation with reference experimentation results, the reliability of the numerical simulation of a centrifugal compressor by FINE/Turbo is proved. The results demonstrate that the kinetic energy of the gas at the impeller outlet of the centrifugal compressor is basically transformed into pressure energy and that the supercharging effect is relatively good. The entropy increase mainly occurs at the tip clearance, where the leakage flow is relatively critical. The static pressure distribution of the B2B (blade to blade) section is compared with that of the meridional flow channel. The meridional section is a contraction channel along the flow direction caused by the large turning angle of the hub. Owing to the effect of centrifugal force, a low-speed zone is developed in the flow channel to form a separation zone and result in energy loss. The separation area can be reduced through the reduction of the inlet flow angle to improve the overall performance of the compressor. The research shows that properly reducing the inlet hub ratio and the inlet angle of the impeller blade root and reasonably selecting the blade tip clearance value and the relative width of the impeller outlet are beneficial to improving the efficiency and pressure ratio of the compressor.
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    ENVIRONMENTAL SCIENCE AND ENGINEERING
    Analysis of the standard requirements for achieving low-carbon transformation of coal power
    ZONG Jianfang, LI Pengcheng, DING Qing
    Journal of Tsinghua University(Science and Technology). 2023, 63 (10): 1686-1692.   DOI: 10.16511/j.cnki.qhdxxb.2023.25.007
    Abstract   HTML   PDF (1142KB) ( 230 )
    [Significance] The international and domestic coal power industries are working hard to achieve low-carbon coal power transformation. As the carbon dioxide emissions generated by coal-fired power generation account for >40% of the total emissions of the country, the low-carbon transformation of coal power will be the largest contributor to carbon reduction in the future. With the carbon peak and carbon neutral goal and the building of new power systems based on new energy sources, the low-carbon transformation of coal-fired power generation is the trend of the times. [Progress] Research on the low-carbon transformation of coal power is underway, and many policies on the low-carbon transformation of coal power have been issued at home and abroad. This paper provides a summary of the relevant research results for the low-carbon transformation technology path of coal power and an analysis of seven promising low-carbon transformation technological paths for coal power. The key tasks proposed by the main policy measures to promote the low-carbon transformation of coal power are highlighted. Based on a summary of the technical path and key tasks of policy measures for the low-carbon transformation of coal power, seven key standard requirements are proposed: relevant standards for controlling the energy efficiency level of new coal power projects, standards for energy conservation and consumption reduction of existing units, standards for flexible modification of coal power units, standards for the heating transformation of coal power units, standards for carbon capture utilization and storage (CCUS) of coal power, standards for biomass fuel and coal combined combustion power generation, and comprehensive energy service standards. The current situation of standardization work supporting the low-carbon transformation of coal power is summarized, and the applicability of relevant national, industrial, local, and international standards is evaluated and analyzed. From the analysis results, it is found that, in terms of national standards, there have been some standard achievements in such aspects as goal orientation, energy measurement, energy management, performance evaluation, and greenhouse gas emission accounting. In addition to the national standards, industrial standards, such as energy efficiency evaluation of coal-fired power plants, energy saving calculation, energy saving diagnosis, coal management, and greenhouse gas emission statistical indicators, provide complementary standards for the low-carbon transformation of coal power. The local standards for energy consumption quotas formulated by Shanghai, Chongqing, Shandong, and Hebei are also useful attempts to guide the low-carbon transformation and upgrade of coal power through standardization. Based on the evaluation results of the formulation and implementation of typical mandatory energy consumption quota standards and other typical coal power low-carbon transformation standards, this paper provides suggestions for improving the coal power low-carbon transformation standard system. [Conclusions and prospects] To better support the implementation of coal power low-carbon transformation policy and provide comprehensive and advanced standard system support, we should strengthen the overall planning of the standard system, deepen the research basis of the standards, and promote the coordinated interaction between coal power low-carbon technology innovation and standard upgrades. Moreover, we should encourage local governments to formulate advanced energy conservation and carbon reduction standards, and support the implementation of innovative low-carbon technologies and market-based mechanisms. In light of the advanced international standard, we should strive to achieve the international level of the low-carbon transformation standard for coal power. Further, we should effectively support the low-carbon development, transformation, and upgrade of coal power with a high-level and full-coverage standard system.
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