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ISSN 1000-0585
CN 11-1848/P
Started in 1982
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  • Table of Content
      , Volume 64 Issue 1 Previous Issue    Next Issue
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    BIG DATA
    Two-stage fusion multiview graph clustering based on the attention mechanism
    ZHAO Xingwang, HOU Zhedong, YAO Kaixuan, LIANG Jiye
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 1-12.   DOI: 10.16511/j.cnki.qhdxxb.2024.21.001
    Abstract   HTML   PDF (8474KB) ( 433 )
    [Objective] Multiview graph clustering aims to investigate the inherent cluster structures in multiview graph data and has received quite extensive research attention over recent years. However, there are differences in the final quality of different views, but existing methods treat all views equally during the fusion process without assigning the corresponding weights based on the received quality of the view. This may result in the loss of complementary information from multiple views and go on to ultimately affect the clustering quality. Additionally, the topological structure and attribute information of nodes in multiview graph data differ significantly in terms of content and form, making it somewhat challenging to integrate these two types of information effectively. To solve these problems, this paper proposes two-stage fusion multiview graph clustering based on an attention mechanism.[Methods] The algorithm can be divided into three stages:feature filtering based on graph filtering, feature fusion based on the attention mechanism, and topological fusion based on the attention mechanism. In the first stage, graph filters are applied to combine the attribute information with the topological structure of each view. In this process, a smoother embedding representation is achieved by filtering out high-frequency noise. In the second stage, the smooth representations of individual views are fused using attention mechanisms to obtain the consensus smooth representation, which incorporates information from all views. Additionally, a consensus Laplacian matrix is obtained by combining multiple views' Laplacian matrices using learnable weights. To obtain the final embedded representation, the consensus Laplacian matrix and consensus smooth representation are inputted into an encoder. Subsequently, the similarity matrix for the final embedded representation is computed. Training samples are selected from the similarity matrix, and the embedded representation and learnable weights of the Laplacian matrix are optimized iteratively to obtain a somewhat more compressed embedded representation. Finally, performing spectral clustering on the embedding representation yields the clustering results. The performance of the algorithm is evaluated using widely-used clustering evaluation metrics, including accuracy, normalized mutual information, an adjusted Rand index, and an F1-score, on three datasets:Association for Computing Machinery (ACM), Digital Bibliography & Library Project (DBLP), and Internet Movie Database (IMDB).[Results] 1) The experimental results show that the proposed algorithm is more effective in handling multiview graph data, particularly for the ACM and DBLP datasets, compared to extant methods. However, it may not perform as well as LMEGC and MCGC on the IMDB dataset. 2) Through the exploration of view quality using the proposed methods, the algorithm can learn weights specific to each view based on quality. 3) Compared to the best-performing single view on each dataset (ACM, DBLP, and IMDB), the proposed algorithm achieves an average performance improvement of 2.4%, 2.9%, and 2.1%, respectively, after fusing all views. 4) Exploring the effect of the number of graph filter layers and the ratio of positive to negative node pairs on the performance of the algorithm, it was found that the best performance was achieved with somewhat small graph filter layers. The optimal ratio for positive and negative node pairs was around 0.01 and 0.5.[Conclusions] The algorithm combines attribute information with topological information through graph filtering to obtain smoother representations that are more suitable for clustering. The attention mechanisms can learn weights from both the topological and attribute information perspectives based on view quality. In this way, the representation could get the information from each view while avoiding the influence of poor-quality views. The proposed method in this paper achieves the expected results, greatly enhancing the clustering performance of the algorithm.
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    Robust verification of graph contrastive learning based on node feature adversarial attacks
    XING Yujie, WANG Xiao, SHI Chuan, HUANG Hai, CUI Peng
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 13-24.   DOI: 10.16511/j.cnki.qhdxxb.2024.21.002
    Abstract   HTML   PDF (4793KB) ( 210 )
    [Objective] Many recent studies have indicated that graph neural networks exhibit a lack of robustness when facing adversarial attacks involving perturbations in both graph structures and node features, and the subsequent predictions of these networks may become unreliable under such circumstances. This issue affects graph contrastive learning methods similarly. However, the existing evaluation of robustness methods is often entangled with attack algorithms, data labels, and downstream tasks, which are best avoided, especially within the self-supervised setup of graph contrastive learning. Therefore, this paper introduces a robustness verification algorithm for graph contrastive learning to assess the robustness of graph convolutional networks against node feature adversarial attacks.[Methods] To begin with, considering the nature of positive and negative pairs found in graph contrastive learning models, this paper defines the robustness verification problem of graph contrastive learning as a similarity comparison between adversarial samples and the target node along with its negative samples. This problem is then expressed as a dynamic programming problem, which avoids dependency on attack algorithms, data labels, and downstream tasks. To address this dynamic programming problem, a series of novel and effective methods are proposed in this paper. For the binary attributes commonly used in graph data, corresponding perturbation spaces are therefore constructed here. Considering the challenge posed by a large negative sample space in graph contrastive learning, a negative sample sampling strategy is designed to improve the efficiency of problem-solving. In cases where binary discrete attributes and nonlinear activation functions render the dynamic programming problem difficult to address, this paper employs relaxation techniques and uses dual problem optimization methods to further improve the solution's efficiency.[Results] To assess the effectiveness of the proposed graph contrastive learning robustness verification algorithm, we conducted experiments using the classic GRACE model on the Cora and CiteSeer datasets. Employing the robustness verification algorithm introduced for graph contrastive learning, we evaluated its robustness. As the perturbation intensity increased, the proportion of nodes that were verified as robust decreased rapidly, and the proportion of nodes that were verified as non-robust increased significantly. Simultaneously, the proportion of unverifiable nodes remained at a lower level. These observations show the effectiveness of the proposed framework for verifying the robustness of graph contrastive learning. Additionally, the experiments revealed that the robustness of the contrastive learning model ARIEL, designed against specific attack algorithms, lacks generalizability and exhibits poor verifiable robustness performance, suggesting its vulnerability to other attack algorithms. Besides, ablation experiments identified the adversarial attack components of ARIEL as the main reason for its diminished verifiable robustness. Lastly, parameter experiments demonstrated the reasonability of the proposed negative sample sampling strategy. The results showed that sampling 20 negative samples is sufficient to achieve a favorable performance of our robustness verification algorithm with high efficiency.[Conclusions] Through the analysis of our methods and experimental results, the graph contrastive learning robustness verification algorithm proposed in this study not only eliminates dependency on attack algorithms, data labels, and downstream tasks but also presents a more comprehensive measurement compared to traditional robustness metrics. It can verify robustness in multiple directions, thereby boosting the development of comprehensively robust graph contrastive learning algorithms.
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    VEHICLE AND TRAFFIC ENGINEERING
    Dynamic analysis of flexible coupling for an electric wheel with a suspended drive motor
    LUO Rongkang, YU Zhihao, WU Peibao, HOU Zhichao
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 25-32.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.038
    Abstract   HTML   PDF (5739KB) ( 193 )
    [Objective] Compared with the commercially available centralized motor drive systems, in-wheel drive motors are installed inside the wheels and provide high transmission efficiency, enhanced control flexibility, and ease of modular vehicle design. In mainstream configurations of in-wheel drive motors, the power and transmission devices are rigidly connected to the chassis, thereby increasing the unsprung mass of the vehicle and the rotational inertia of the wheels. This increase in unsprung mass and rotational inertia results in higher vertical acceleration of the vehicle body and increased dynamic loads on the wheels. Moreover, due to the lack of buffering effect from the centralized driving vehicle's half shaft, the rotational vibration of the wheels and the longitudinal vibration of the vehicle are more prominent. Electric wheels with suspended drive motors can not only extend motor life but also alleviate comfort problems as a result of excessive unsprung mass. Designing flexible coupling within limited wheel space in an electric wheel is a critical challenge.[Methods] In this paper, a flexible coupling was proposed for an electric wheel with a suspended drive motor. The coupling was a planar parallel mechanism composed of several connecting arms, each of which consisted of two parallelogram mechanisms linked in series. As only the rotational joints existed and the stiffness of the input and output disks were high, the analysis assumed that the connecting rod only experienced axial deformation and the input and output disks were treated as rigid bodies. Under these assumptions, kinematic and dynamic models were established for the coupling utilizing the geometric relationships between the rods with the principles of mechanical equilibrium. Coupling eccentricity and the static and dynamic loads of the components were then derived. To demonstrate, a set of parameters were selected for the coupling, and numerical calculations were performed for the eccentricity, static load, and dynamic load of the coupling.[Results] The results show that the coupling can transmit power with constant velocity and allows the transmission system to jump in vertical directions during power transmission. The total eccentric stroke of this coupling reaches 58.2 mm, which is sufficient for an in-wheel suspended drive motor. Application of 400 N·m torque and 1 000 r/min to the input shaft leads to a maximum static load on the connecting rod of approximately 1 300 N and a maximum dynamic load on the output disk of approximately 35 N. Under the same conditions, the dynamic load on the proposed coupling is only 1/10 of that on the Oldham coupling.[Conclusions] The coupling proposed in this paper can have a large eccentric stroke while occupying little axial space and imposing negligible dynamic load. This coupling can be helpful in developing electric wheels with suspended drive motors. The flexibility of the coupling allows the motor and transmission components to move independently from the wheel, which reduces the unsprung mass and solves the problem of vibration and noise. This coupling has significant advantages over previous solutions in terms of size, dynamic load, and mechanical efficiency. This work provides a reference for designing in-wheel suspended drive motors.
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    Calculation method of hypoid gear meshing efficiency of drive axles with considering system deformation
    WANG Qin, HE Di, GUI Liangjin, HU Zhiyu, PENG Jin, FAN Zijie
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 33-43.   DOI: 10.16511/j.cnki.qhdxxb.2023.21.020
    Abstract   HTML   PDF (10533KB) ( 102 )
    [Objective] The face-hobbing hypoid gear is a crucial component of drive axles owing to its continuous indexing processing capabilities. However, in practical engineering applications, gear pairs rarely operate under ideal conditions. Instead, they frequently experience heavy-load and high-speed conditions. These conditions result in substantial system deformation and a complex meshing state of the tooth surface. Under heavy-load conditions, considerable skewing of the load on the tooth surface can occur, greatly affecting the performance of the gear pair and drive axles. Currently, the system deformation factor under actual working conditions is not sufficiently considered, making it challenging to address the aforementioned issue. Consequently, this paper proposes the analysis of the tooth surface load distribution by employing a semi-analytic loaded tooth contact analysis method to accurately predict the tooth surface load distribution. Based on a load distribution analysis, a highly accurate calculation method for gear meshing efficiency is proposed.[Methods] This paper proposes a calculation method for gear meshing efficiency under mixed-lubrication conditions for hypoid gears in drive axles operating under complex working conditions. First, a multi-support shaft system modeling method is employed to analyze the drive axles system. This method can calculate the forces acting on various components, such as gears and bearings, as well as the gear misalignment caused by system deformation under various load conditions. Second, by simulating the spatial motion process of the actual gear machining machine, the coordinates of the tool cutting point are transformed to the coordinate system of the gear blank via coordinate transformation. This process results in the correspondence of the tooth profile with the actual machining parameters. The time-varying friction coefficient distribution of the tooth surface under different working conditions is derived by combining the point contact mixed-lubrication friction coefficient model of the tooth surface with its relative motion relationship. Then, taking into account the tooth surface deformation equilibrium equation, tooth surface torque equilibrium equation, and tooth surface contact pressure equilibrium equation, a gear frictional loaded tooth contact analysis method is established. This method accurately calculates the tooth surface load distribution and mesh efficiency of gears under different working conditions through an iterative solution. Finally, the calculation results of the tooth surface load distribution under various working conditions are compared with the experimental results obtained from a loading experiment conducted on the entire drive axles. The meshing efficiency of the gear pair under various working conditions is determined by conducting a system no-load efficiency experiment and loading efficiency experiment and comparing the results with those obtained by calculations.[Results] In the gear no-load experiment, the contact patterns of the gear tooth surface were compared under forward and reverse working conditions. The experimental results were found to be in good agreement with theoretical calculations, verifying the accuracy of the tooth surface calculation method and no-load tooth contact analysis. Subsequently, a loading experiment was conducted on the drive axles system, indicating that the system deformation had a considerable impact on the load distribution of the hypoid gear tooth surface. For instance, in the experiment drive axles, under heavy-load conditions, system deformation caused the tooth surface load on the driving side of the gear pair to shift toward the outside, while the tooth surface loaded on the driven side shifts toward the inside. The meshing efficiency experiment results revealed that system deformation considerably impacted the gear meshing efficiency under heavy-load and high-speed working conditions. In addition, the vehicle speed had a considerable impact on the meshing efficiency, with an increase in speed from 10 to 80 km/h, resulting in a 1% improvement in meshing efficiency.[Conclusions] By comprehensively considering gear meshing misalignment caused by system deformation and the mixed-lubrication state of the tooth surface, the tooth surface load distribution and meshing efficiency can be accurately calculated under loaded conditions. Therefore, the proposed process enhances the accuracy of calculating the drive axles system efficiency while providing a solid foundation for gear optimization research.
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    Estimation algorithm of driver's gaze zone based on lightweight spatial feature encoding network
    ZHANG Mingfang, LI Guilin, WU Chuna, WANG Li, TONG Lianghao
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 44-54.   DOI: 10.16511/j.cnki.qhdxxb.2023.26.045
    Abstract   HTML   PDF (8115KB) ( 111 )
    [Objective] The real-time monitoring of a driver's gaze region is essential for human-machine shared driving vehicles to understand and predict the driver's intentions. Because of the limited computational resources and storage capacity of in-vehicle platforms, existing gaze region estimation algorithms often hardly balance accuracy and real-time performance and ignore temporal information.[Methods] Therefore, this paper proposes a lightweight spatial feature encoding network (LSFENet) for driver gaze region estimation. First, the image sequence of the driver's upper body is captured by an RGB camera. Image preprocessing steps, including face alignment and glasses removal, are performed to obtain left- and right-eye images and facial keypoint coordinates to handle challenges such as cluttered backgrounds and facial occlusions in the captured images. Face alignment is conducted using the multi-task cascaded convolutional network algorithm, and the glasses are removed using the cycle-consistent adversarial network algorithm. Second, we build the LSFENet feature extraction network based on the GCSbottleneck module to improve the MobileNetV2 architecture, since the inverted residual structure in the MobileNetV2 network requires a significant amount of memory and floating-point operations and ignores the redundancy and the correlation among the feature maps. We embed a ghost module to improve memory consumption and integrate the channel and spatial attention modules to extract the cross-channel and spatial information from the feature map. Next, the Kronecker product is used to fuse eye features with facial keypoint features to reduce the impact of the information complexity imbalance. Then, the fused features from the images at continuous frames are input into a recurrent neural network to estimate the gaze zone of the image sequence. Finally, the proposed network is evaluated using the public driver gaze in the wild (DGW) dataset and a self-collected dataset. The evaluation metrics include the number of parameters, the floating-point operations per second (FLOPs), the frames per second (FPS), and the F1 score.[Results] The experimental results showed the following:(1) The gaze region estimation accuracy of the proposed algorithm was 97.08%, which was approximately 7% higher than that of the original MobileNetV2. Additionally, both the number of parameters and FLOPs were reduced by 22.5%, and the FPS was improved by 36.43%. The proposed network had a frame rate of approximately 103 FPS and satisfied the computational efficiency and accuracy requirements under in-vehicle environments. (2) The estimation accuracies of the gaze regions 1, 2, 3, 4, and 9 were over 85% for the proposed algorithm. The macro-average and micro-average precisions of the DGW dataset reached 74.32% and 76.01%, respectively. (3) The proposed algorithm provided high classification accuracy for fine-grained eye images with small intra-class differences. (4) The visualization results of the class activation mapping demonstrated that the proposed algorithm had strong adaptability to various lighting conditions and glass occlusion situations.[Conclusions] The research results are of great significance for the recognition of a driver's visual distraction states.
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    Merging decision behavior model based on multivariate adaptive regression splines
    LI Gen, ZHAI Wei, HUANG Haibo, REN Jiaolong, Wang Dengzhong, WU Lan
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 55-62.   DOI: 10.16511/j.cnki.qhdxxb.2023.21.019
    Abstract   HTML   PDF (2764KB) ( 206 )
    [Objective] Weaving areas are bottlenecks of freeways, and lane-changing behavior is one of the main reasons for the capacity decline and traffic congestion in weaving areas. Frequent merging behaviors may lead to traffic flow disturbance upstream from the weaving area, affect the normal running of surrounding vehicles, and in severe cases may even lead to multi-vehicle accidents. An in-depth understanding of merging decision behavior in the weaving area is essential to reduce the vehicle collision risk and improve the traffic safety level. A newly developed nonparametric regression model-multiple adaptive regression splines (MARS)-is adopted to model the gap selection decision during merging to study the merging behavior in the freeway weaving area.[Methods] This study investigates complex interactions between merging and surrounding vehicles during merging. Trajectory data are extracted from the US-101 dataset provided by the dataset of next generation simulation program, and the symmetric exponential moving average filter method is used to smooth the data. Merging vehicles are influenced by surrounding vehicles in the auxiliary and adjacent main lanes. Thus, explanatory variables such as speeds, speed differences, gaps, and locations are calculated. Longitudinal and lateral collision risk indicators and time-to-collision are also considered to study the influence of collision risk on merging behaviors. Finally, 925 observations are obtained and randomly divided into two subdatasets to train and test the model. The MARS model is compared with four state-of-the-art machine learning techniques:classification and regression tree, gradient boosting decision tree (GBDT), random forest, and logistic regression models.[Results] The speed difference between the merging vehicle and vehicles in the adjacent main lane played the most important role in gap selection. Interactions of influencing variables were observed. In particular, the best interaction level was 4 in the final model. The comparison showed that GBDT and MARS had the lowest rates of prediction error at 0.138 and 0.141, respectively.However, MARS could provide explicit expression functions that reflect the interaction between the influencing variables, which was beneficial to engineering applications.[Conclusions] By using the optimal variable transformation and potential variable interaction in the regression modeling scheme, MARS could easily handle complex nonlinear relationships in merging behaviors. This model could accurately predict the gap selection behavior and provide explicit expression functions, thus simplifying its understanding and application to driver assistance systems and autonomous driving systems.
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    POWER AND ENERGY
    Experimental study on the evaporation characteristics of aviation kerosene with multidroplets
    WANG Fang, HAN Qiwei, LI Dianwang, JIN Jie
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 63-74.   DOI: 10.16511/j.cnki.qhdxxb.2023.26.048
    Abstract   HTML   PDF (8394KB) ( 86 )
    [Objective] The fuel within the combustion chamber of an aircraft engine is fragmented and atomized into a group of droplets, including dense small droplets, through devices such as fuel injectors or fuel throwers. These droplets evaporate in the high-temperature convective environment of the combustion chamber and subsequently mix with high-pressure air for combustion. Previous studies have focused on single-droplet evaporation characteristics rather than actual droplet group characteristics, thereby neglecting the interaction between droplets. To address this issue, multidroplets evaporation experiments are conducted in this study. This article provides experimental data to support the numerical simulation theory of multidroplet evaporation and accordingly supplements and corrects the droplet evaporation model.[Methods] In the multidroplet evaporation experiments, the evaporation processes of single, double, and triple jet fuel droplets suspend by quartz fiber under static conditions at high temperatures of 400, 500, and 600℃ are captured using a high-speed camera. Thereafter, the images are processed at a specific frame rate. A self-programming software is used to isolate the main body of the droplet, and the images are imported into MATLAB data processing software to obtain the droplet evaporation characteristic curve. Linear fitting is performed on the stable section of the evaporation characteristic curve to determine the evaporation rate of the droplet. The range of droplet spacing select in the multidroplet evaporation experiments is 1-4 times the droplet diameter.[Results] The results showed that at the same temperature, with increasing droplet spacing, the average evaporation rate of multiple droplets increased while the rate of change in evaporation rate decreased. The evaporation rate of multiple droplets tended to approach that of a single droplet and followed an approximate exponential trend. When the distance between droplets exceeded the critical distance, the mutual influence between droplets became weak enough to be ignored, and double droplets could be regarded as two independent single droplets. At this point, there was no difference between the evaporation rate of multiple and single droplets. However, when two droplets were sufficiently close, the mutual influence between them reached its maximum, and the average evaporation rate of the two droplets dropped to 80%-90% of that of a single droplet. As the temperature increased, the mutual influence between droplets became stronger, and accordingly, the critical droplet distance increased.[Conclusions] These experimental results are consistent with the theoretical model derivation. When the droplet spacing is less than the critical droplet spacing, the evaporation rate increases monotonously with increasing droplet spacing, and the rate of change gradually decreases to zero. When the distance between droplets exceed the critical distance, the evaporation rate of multiple droplets remain constant and is approximately equal to that of a single droplet. During the investigation of the empirical formula for multidroplet evaporation, the goodness of fit of the selected exponential form ranges between 0.899 and 0.983. This paper obtains experimental data on double droplets and triple droplets evaporation and obtains empirical formulas for droplet spacing and temperature of double droplets. The experimental data are consistent with theoretical analysis conclusions, providing experimental data support and theoretical innovation for multidroplets evaporation models.
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    Integrated design of ship propellers considering hydrodynamics, cavitation, and low noise
    YANG Qiongfang, HUANG Xiuchang, LI Ye
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 75-89.   DOI: 10.16511/j.cnki.qhdxxb.2023.26.043
    Abstract   HTML   PDF (13271KB) ( 142 )
    [Objective] Designing ship propellers is a comprehensive engineering task that synergistically considers hydrodynamics, cavitation, vibration, and noise performance. To address the limitations of current design guidance cases and existing open design software, an integrated software called OpenProp+ was developed. This application is designed to facilitate advancements in marine propeller design by incorporating theoretical formulas for noncavitation noise, empirical formulas for cavitation noise estimation, and the new Burrill diagram into the open-source software, OpenProp+.[Methods] The process of blade geometry design begins with determining the number of blades based on design requirements and empirical knowledge. The diameter is determined by the maximum power density limit and the optimal speed that meets the main engine's speed constraint, while the rotational speed is established according to the relative optimal efficiency. The three essential parameters for 3D blade section optimization along the radial direction are accomplished, including determining chord length distribution, applying a highly skewed angle, and suitably increasing the maximum thickness of the blade section from 0.70R to 0.95R. Notably, the chord length distribution should differ between five-blade and seven-blade propellers. The optimal skew value for the critical blade lies between 50.0% and 70.0%, with an initial recommended value of 60.0%. Appropriately increasing the tip thickness and its rake enhances anti-cavitation performance. Following design and optimization, performance prediction involves utilizing theoretical formulas of the propeller's free sound field by National Advisory Committee for Aeronoutics (NACA) to predict the source level of the sound pressure spectrum under noncavitation conditions and to illustrate its longitudinal acoustic direction diagram at discrete line spectrums. Factors such as ship speed, propeller rotating speed, diameter, blade numbers, thrust, and torque contributions to sound pressure are integrated into these formulas. The new Burrill spectrum can subsequently be employed to ascertain the presence of cavitation, estimate its range if it does exist, and qualitatively measure its noise performance under specific operating conditions. Finally, the Brown empirical formula estimates the propeller cavitation noise spectrum, while the Fraser empirical formula and International Council for the Exploration of the Sea (ICES) standard are used to quantitatively evaluate noise performance levels.[Results] The effectiveness of the integrated design software, OpenProp+, was validated through the design and performance prediction of a low-noise five-blade propeller, which yielded positive feedback. Within the full operating range, the open water performance curve of the designed blade almost coincided with the measured values of the original blade. Even on the off-design operating condition farthest from the designed advance ratio, the deviation between the thrust coefficient and torque coefficient compared to their measured value was only 4.65%. Considering the true ship wake flow distribution, the design point efficiency decreased by about 4% and the anti-cavitation margin decreased by about 12%. This indicated that the design program could effectively design the blades and reasonably predict their hydrodynamic and cavitation performance.[Conclusions] OpenProp+ not only reliably predicts the open-water performance of existing propellers but also designs new propellers and accurately forecasts their open-water performance. It can determine the presence of cavitation, quantify its range if present, predict the noncavitation noise source level, and estimate the cavitation noise spectrum source level. Thus, OpenProp+ and the complete design chart incorporated in the software can directly aid in the engineering application of ship propeller design.
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    Impact of inlet distortion on the flow characteristics of a heavy-duty gas turbine cylinder pressure
    SHI Yunjiao, ZHAO Ningbo, ZHENG Hongtao
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 90-98.   DOI: 10.16511/j.cnki.qhdxxb.2023.26.037
    Abstract   HTML   PDF (12798KB) ( 62 )
    [Objective] During the operation of a heavy-duty gas turbine, the flow field at the compressor outlet is impacted by the blade wakes and boundary layers. Consequently, the flow field entering the combustor becomes nonuniform, which can negatively impact the performance of the combustion chamber. Unfortunately, the existing design of combustion chambers often ignores the influence of this nonuniform inlet air, which is inconsistent with the actual operating conditions of a gas turbine, May cause poor uniformity of combustion chamber outlet temperature distribution or an increase in pressure loss. Therefore, we must consider the cylinder pressure as a crucial rectification component within the combustion chamber of a heavy-duty gas turbine. We can use the cylinder pressure to effectively address these issues and enhance the overall performance through performance analysis conducted under distortion conditions.[Methods] In this study, we conducted numerical simulations to investigate the flow characteristics of the cylinder pressure under various inlet distortions as well as the parameter response patterns associated with different distortion modes. The realizable k-ε turbulence model was employed, and a SIMPLE pressure-velocity coupling algorithm was applied. Second-order convergence precision was implemented to ensure accurate calculation of all physical quantities. We compared the calculated results with experimental data to validate the reliability of our numerical simulation method. The agreement between the two confirmed the credibility of our approach. We explored radial distortions at different levels of distortion and positions, as well as circumferential distortions under varying degrees of distortion. Our findings indicated that the appropriate range for inlet distortion degrees was between 0.18 and 0.47.[Results] The inlet distortion of the gas turbine combustion induced a more intricate vortex system within the cylinder pressure and altered the airflow separation position in the inlet section. Both the position and shape of the high-speed region with speeds greater than 80 m/s, were affected upon modifying the inlet distortion mode. Circumferential distortion resulted in the expansion of the high-speed area near the gas turbine combustion chamber flame tube, with a distortion degree approximately twice that of the uniform incoming flow and radial distortion. The rectification effect diminished when the inlet was circumferentially distorted, leading to increased pressure loss. The airflow separation position advanced with the upward movement of the peak position of radial distortion velocity, enlarging the high-speed area beneath the combustion chamber. This caused an increase in the total pressure loss, and the peak distortion value progressed along the path. Additionally, the high-speed area on the upper side of the cylinder pressure outlet diminished, nearly disappearing in the presence of high speed on the inner side under inlet conditions. Within the same distortion mode, augmenting the degree of distortion exerted a minimal impact on the vortex structure and flow field distribution characteristics. However, augmenting the degree of distortion exerted a minimal impact and elevated the maximum airflow velocity within the combustion diffuser, resulting in an increased total pressure loss and outlet distortion.[Conclusions] Both the inlet distortion mode and its degree considerably impact the velocity distribution within the combustion diffuser as well as the structure of the vortex structure. In particular, when circumferential distortion occurs at the inlet, the rectification effect of the diffuser deteriorates. Consequently, we must prioritize the design process of the combustion chamber with regard to the structure's capability to withstand circumferential distortion.
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    Experimental investigations on flow-flame-acoustic dynamic characteristics of a central staged burner
    JIN Ming, LU Yudi, LI Yuansen, LIU Weijie, GE Bing, ZANG Shusheng
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 99-108.   DOI: 10.16511/j.cnki.qhdxxb.2023.21.017
    Abstract   HTML   PDF (14544KB) ( 79 )
    [Objective] To reveal the influence of interaction between pilot and main flames on flow and thermoacoustic instability characteristics, the flow and thermoacoustic dynamic characteristics of the pilot stage, main stage, and centrally staged flames in the combustion chamber of the central staged model combustor are studied in detail through experiments.[Methods] High-speed measurement methods such as high-frequency particle image velocimetry, high-speed camera, pressure sensors, and photomultiplier tube are used to study the flow-flame-acoustic dynamic characteristics of the central staged burner. During the experiments, the unsteady flow field, flame CH* signal distribution, global heat-release rate, and pressure pulsation characteristics are measured under different testing conditions.[Results] The results showed that the pilot swirling jet had a significant influence on the flow-flame-acoustic dynamic characteristics of the stratified burner. Furthermore, when the pilot stage used a swirling flame, the main recirculation zone was formed downstream of the nozzle outlet and the high-temperature burned gas was rolled back to improve the combustion stability of the swirl flame. The peak value of pressure was 683 Pa, and the main peak values of dynamic pressure and heat release were both visible at a frequency of 120 Hz. However, the dominant frequency of the flow field proper orthogonal decomposition (POD) time coefficient spectrum showed that the dominant frequencies of large-scale shedding vortex in the flow field were 68 and 109 Hz, indicating that the interaction between pilot and main flames caused the thermoacoustic instability frequency to be inconsistent with large-scale vortex shedding frequency. Moreover, the dominant frequency of pressure and heat release were slightly shifted when the pilot stage was operated with a swirling air jet, and it was reduced by 5 Hz and got to 115 Hz. However, the main recirculation zone disappeared, resulting in a strengthening of the thermoacoustic instability and an increase in the peak pressure to 2 947 Pa. The dominant frequency of the flow field POD time coefficient spectrum showed that the frequency of the shedding vortex generated by the swirling flow shear layer of the main stage was 115 Hz, which was well locked with the frequency of the thermoacoustic instability mode under this condition. The results, in this case, showed that acoustic-velocity-flame instabilities were mutually coupled under the single-swirling flame self-excited oscillation condition. However, for the flame interaction case, the flow instability frequency induced by flame and pressure oscillations changed significantly, and the coupling between acoustic-velocity-flame instabilities was also destroyed.[Conclusions] When the main stage is operated with swirling air, whether the pilot stage is operated with swirling air or swirling flame, the stable main recirculation zone can always be observed at the burner exit plane. However, when the main stage is operated with a swirling flame, the swirling air from the pilot stage destroys the main recirculation zone, which is detrimental to the main-stage swirling flame's combustion stability. The main recirculation zone reappears when the pilot stage is converted into a swirling flame. The increased heat load also causes the accelerated expansion of the gas, which induces a stronger vortex in the swirling flow shear layer. However, the interaction between the pilot flame and the main flame accelerates the dissipation of the vortex.
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    MECHANICAL ENGINEERING
    Clustering and selection method of microservices for intelligent manufacturing
    WANG Liping, SHI Huijie, WANG Dong
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 109-116.   DOI: 10.16511/j.cnki.qhdxxb.2023.21.023
    Abstract   HTML   PDF (2244KB) ( 234 )
    [Objective] Intelligent manufacturing requires the agile development of industrial software, and microservices architecture for industrial software application is an important research direction at present. To improve developmental efficiency of industrial software, functional components in the form of microservices must be published on the internet, and a microservices library needs to be built for management, enabling users to choose according to their needs. To meet the demands of the increasingly complex business needs of users, the microservices library must efficiently combine numerous contained microservices to expand achievable functions. Based on the binding mechanism between the registry and microservices library, the quality of the two links of retrieval and selection in the microservices combination significantly impacts user experience.[Methods] In the process of service retrieval, a service clustering algorithm based on Sentence BERT and SOM (SBERT-SOM-k) is proposed to efficiently find a group of microservices that meet specific functional requirements of the microservices library. This method converts microservices into a text vector through Sentence BERT, mapping them to the output layer nodes of SOM. The iterated weight vector represents these output layer nodes, which can effectively improve the disadvantage of the k-means algorithm that is sensitive to noise and outliers. Because of the absence of a special microservices set related to industrial software at present, the experiment uses the open web service test dataset OWLS-TC4 to compare SBERT-SOM-k with three clustering algorithms:SBERT-k, LDA-k, and TFIDF-k and uses three indicators of accuracy, recall, and F-measure to evaluate the performance of the algorithm. In the service selection phase, to further select the optimal combination of microservices, nonfunctional requirements, such as availability, reliability, and response time, are considered as quality of service (QoS) indicators, which transformed into a multiconstraint single-objective optimization problem based on QoS. Furthermore, an improved genetic algorithm is proposed, which is redesigned in terms of encoding, crossover, mutation, selection, and constraint punishment. The improved genetic algorithm is used to achieve stable solutions for multiconstraint single-objective optimization problems. The experiment used QWS2.0, a real web service dataset that exists on the internet, to compare the number of iterations and fitness values of the improved genetic algorithm (IGA), cross unimproved but constrained genetic algorithm (NICGA), and cross unconstrained improved genetic algorithm (NICCGA) for obtaining the optimal solution under different task node numbers.[Results] The results of service clustering experiments show that the average accuracy of different clustering algorithms is the same. However, SBERT-SOM-k still has significant advantages over SBERT-k, LDA-k, and TF-IDF-k, with the average recall rates increased by 29.56%, 19.36%, and 31.70%, and the corresponding average F-measure increased by 22.41%, 13.77%, and 25.33%, respectively. Additionally, IGA can improve the optimization speed and quality of the service composition, effectively preventing premature convergence.[Conclusions] The proposed clustering and selection method has achieved good results. It can rapidly select microservices from the microservices library, meeting the needs of users. Furthermore, integrating these microservices into an industrial software system registry based on the microservices architecture will enhance user satisfaction and help to realize the efficient use of microservices.
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    Lidar positioning and path tracking of a single steering wheel automated guided vehicle forklift
    YAO Ming, DUAN Jinhao, SHAO Zhufeng, YUAN Shaolun, SU Yunzhou
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 117-129.   DOI: 10.16511/j.cnki.qhdxxb.2023.21.022
    Abstract   HTML   PDF (9753KB) ( 212 )
    [Objective] automated guided vehicle (AGV) forklift is an important material transportation equipment in the industrial field. Its positioning and path-tracking accuracy is an important basis for improving material transportation efficiency, factory automation, and intelligence. Thus, this paper uses a single steering wheel AGV forklift in an indoor structured environment of the pharmaceutical industry as an object, realizing the lidar positioning based on the reflector using the density-based spatial clustering of applications with noise (DBSCAN) and the fast iterative closest point (FICP) algorithms, and designing a proportional-integral (PI) controller to address the path-tracking problem of the AGV forklift.[Methods] First, the kinematics characteristics of the single steering wheel AGV forklift are analyzed, and its kinematics equations and state space equations are established. Subsequently, the DBSCAN and FICP algorithms were used to implement a reflector-based lidar positioning method for an accurate positioning problem. Moreover, a distance-based outlier elimination rule is proposed to address the problem of outliers interfering with the positioning process, which ensures the stability of the positioning results and the robustness of the algorithm. The Kalman filter algorithm is used to fuse the measurement data of the inertial measurement unit (IMU) and the angle sensor to improve the accuracy of the lidar positioning algorithm of the AGV forklift. This study establishes the position error and attitude error in the two core paths of straight lines and arcs based on the geometric relationship for the path-tracking problem. Following that, a PI controller is designed to realize the path tracking of the AGV forklift. Considering curvature discontinuity when the arc of equal curvature is connected with the straight-line path, the arc path based on the third-order Bézier curve was designed in this study. Furthermore, according to the limitation of the AGV forklift in the arc movement process, the parameters of the Bézier curve are analyzed and optimized to avoid the decrease of the path-tracking accuracy caused by the abrupt change of the path curvature.[Results] The experimental verification showed that the lidar positioning algorithm based on DBSCAN and FICP algorithms could achieve ±3 mm positioning accuracy. Stable AGV forklift positioning could be achieved when combined with the outlier elimination rules. Furthermore, the Kalman filter-based fusion of IMU and angle sensor data resulted in accurate AGV forklift positioning. The improved arc path based on the Bézier curve reduced the arc path tracking error by about 72% compared with the equal-curvature arc path. The AGV's position and attitude errors were controlled based on the PI controller, which could control the dynamic tracking accuracy to within 25 mm. Furthermore, the repeated positioning accuracy of the work site reached ±12 mm, meeting the expected design requirements.[Conclusions] This paper studies the lidar positioning and path-tracking technology of a single steering wheel AGV forklift in an indoor structured environment. An accurate and stable lidar positioning algorithm based on DBSCAN and FICP algorithms is realized by introducing outlier elimination rules and the Kalman filter. The AGV forklift's path tracking is realized using the PI controller, and the tracking accuracy of the arc path is improved using the Bézier curve. Finally, the positioning accuracy, path-tracking accuracy, and repeated positioning accuracy of the work site all met the expected design requirements.
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    ELECTRONIC ENGINEERING
    Distributed printed differential low-pass filters for data acquisition terminals
    LIU Jinquan, WANG Qian, XU Jian, HU Qiang, ZENG Qing, JU Qianyu
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 130-134.   DOI: 10.16511/j.cnki.qhdxxb.2023.21.018
    Abstract   HTML   PDF (3391KB) ( 149 )
    [Objective] In high-speed data acquisition terminals, differential lines are commonly employed to carry high-speed digital signals, enhancing anti-interference performance. However, mutual interference frequently occurs when high-speed acquisition terminals and wireless devices coexist. For instance, 5G base stations and Gbps high-speed signal acquisition terminals in current 5G high-speed acquisition systems interfere with each other. This issue is addressed by installing electromagnetic interference (EMI) filters on a printed circuit board (PCB); however, EMI filters have disadvantages, including fixed frequency points and high cost, which hinder their practicability. In this work, a design method for a low-cost, easy-to-implement distributed printed differential low-pass filter is proposed based on a step impedance filter that can be designed using simple synthesis tools and applied to multilayer PCBs of various radiofrequency systems.[Methods] Based on a typical LC low-pass filter, step impedance resonators were introduced as a solution to high-frequency parasitic parameters and harmonic suppression. The high- and low-impedance transmission lines serve as an inductor and capacitor, respectively, realizing the transformation from a single-end LC filter to a single-end microstrip filter. Due to the common-mode suppression requirements of differential circuits, symmetrical design and equivalent processing were performed for conversion into a microstrip differential low-pass filter, where the high-impedance line is realized by loosely coupled differential lines and the low-impedance line is realized by tightly coupled differential lines. The larger the impedance difference between high and low impedance in the design, the more it contributes to reducing the filter size, and the device size can be further reduced by adhering to this principle. In addition, considering the influence of the transmission line on the ground impedance, an independent non-grounded pair of differential lines was selected for the design. The PCB board used FR4 with a dielectric constant of 4.2 and a multilayer structure with thicknesses of 0.127 mm and 0.508 mm for the two dielectric layers. The final dimensions were 22 mm×16 mm.[Results] Simulation results showed that the distributed differential filter is a 0-3.5 GHz low-pass filter under ideal conditions. The passband and return loss values were less than 0.5 dB and greater than 20 dB, respectively. Two baluns were added to the circuit design used for differential circuit measurement. Test results revealed that the balun possesses considerable interference of approximately 600 MHz, but the curve without the balun's influence is highly consistent with the simulated curve. The passband and return loss values were <5 dB and <17 dB, respectively.[Conclusions] The findings of this work indicate that the method of using differential step impedance to realize a distributed printed filter based on a multilayer PCB board is feasible and can be effectively applied in high-speed data acquisition terminals as a solution to the mutual interference between high-speed acquisition and wireless interconnection.
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    Depth recognition thresholds of tactile perception for fine stripe texture of bar shapes
    ZHANG Shousheng, ZHUANG Tengfei, FANG Xingxing, ZHU Hua, TANG Wei
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 135-145.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.033
    Abstract   HTML   PDF (11894KB) ( 85 )
    [Objective] Although tactile perception plays a crucial role in human perception of the external world, human understanding of tactile perception remains limited due to the complexity of its mechanism and the multitude of perceptual units involved. The friction between surface textures and finger skin provides vibratory stimuli on the skin surface during tactile perception, thereby activating the somatosensory areas. It is necessary to evaluate the tactile perception of fine textures based on the friction behavior of skin and the related cortical activity in response to the texture stimuli.[Methods] Different fine stripe texture depths (5, 10, 15, 20, 25, and 30 μm) were designed and processed using laser engraving. The depth recognition threshold of tactile perception for fine texture was systematically investigated using subjective evaluation, surface friction and vibration, and the neurophysiological response of the brain. The effects of the texture stimulus intensity and neuronal excitability on tactile perception were verified by a single-channel neural mass model.[Results] An increase in the fine texture depth was associated with an increase in the subjective human texture sense, the degree of correct texture recognition, and the proportion of deformation friction. The average depth recognition threshold of tactile perception was found to be 11.60 μm. The load index, the maximum spectral amplitude of the vibration signal, the recurrence parameter entropy, the length of the longest vertical line segment, and the peak of P300 exhibited a substantial positive correlation with the fine texture depth. The latency of P300 showed a substantial negative correlation with the fine texture depth. When the texture depth exceeded the depth recognition threshold of tactile perception, the maximum spectral amplitude and nonlinear characteristic parameters of the touch vibration signal increased remarkably. The main frequency of the vibration signal also increased to be within the perceptual frequency range of the Pacinian corpuscle. As a result, the vibration signal system transformed from a homogenous state to a disrupted state. Furthermore, the intensity and the area of activation of the brain regions, the neuronal activity of the brain, the processing intensity, and the tactile recognition speed of the brain increased remarkably. Amplitude of the main frequency of the simulated electroencephalogram (EEG) signal increased with an increase in the mean value of the input signal. This trend was consistent with that of the real EEG signal, which indicated that the increase in the tactile intensity due to the increase in the texture depth was one of the reasons for the increase in amplitude of the main frequency of the tactile EEG signal. The main frequency of the simulated EEG signal decreased with an increase in the ratio of excitatory synaptic gain to inhibitory synaptic gain. This trend was consistent with that of the real EEG signal, which indicated that the increased excitability of the neuronal populations excited by the increase in texture depth was one of the reasons for the decrease in the main frequency of the tactile EEG signal.[Conclusions] The depth recognition thresholds of tactile perception for fine stripe textures, the finger touch tribological behavior, the frequency domain and nonlinear features of the touch vibration signals, and the time and frequency domain features of EEG signals undergo remarkable variations during touching and sensing. The single-channel neural mass model can effectively simulate real EEG signals.
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    ENGINEERING PHYSICS
    Cosmogenic radionuclides produced in high-purity germanium detectors during fabrication and transport
    ZHANG Qiuli, ZHANG Peng, JING Mingkun, ZENG Zhi, MA Hao
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 146-154.   DOI: 10.16511/j.cnki.qhdxxb.2023.22.025
    Abstract   HTML   PDF (5031KB) ( 99 )
    [Objective] Currently, experiments searching for rare event, such as the interaction of weakly interacting massive particles or the nuclear double beta decay, are important frontier topics in fundamental physics. Due to the considerably low probability of nuclear processes, experiments require ultralow background conditions, particularly for all materials in detection systems. Low-level high-purity germanium (HPGe) gamma-ray spectrometer with an extremely low threshold, high energy resolution, and ultralow radioactive background is critical in material selection for rare event experiments. However, cosmogenic radionuclides contaminate the germanium crystals and other materials of detectors during fabrication, storage, and transport. Effective removal of long-lived radionuclides inside germanium crystals, such as 60Co and 54Mn, within a short period is difficult, which can be considerably problematic for achieving the required sensitivity and seriously affect rare event searches. The direct, experimental information regarding the quantification of cosmogenic activation yields is scarce since analyzing cosmogenic radionuclides requires tracing the entire process from detector material preparation to the implementation of experiments. Most simulation methods were considered to quantify the activation yields of the long-lived radioisotopes that were cosmogenically induced on the ground in germanium crystals and cupreous detector components, such as YIELDX, ACTIVIA, TALYA, and GEANT4.[Methods] Herein, the cosmogenic activation of a newly customized low-level HPGe gamma-ray spectrometer was experimentally investigated. The detector was stored in underground plants except during assembly and manufacture. Immediately after the fabrication, the detector was transported by rail to the China Jinping underground laboratory (CJPL) for background measurement. The cosmogenic activation of materials in the underground can be considered negligible because the flux of cosmic nucleons in the CJPL was suppressed at depths of a few kilometers of water equivalent. Background measurement was performed with nitrogen flushing and multiple shields at different times. The cosmogenic radionuclides in the shielding materials can be ignored because they have been stored in the CJPL for more than six years. The characteristic gama peaks of the cosmogenic radionuclides 57Co, 58Co, 60Co, and 54Mn were observed in the energy spectra obtained during the experiment. Previous simulations indicated that both germanium crystals and copper bombarded by high-energy cosmic rays would produce these four radionuclides. The detection efficiencies of cosmogenic radionuclides were simulated using the GEANT4 Monte Carlo procedure. By fitting the curves that illustrated the changing characteristic peak counts over time, we obtained the specific activity of cosmogenic radionuclides in germanium crystals and copper.[Results] We assumed that cosmogenic radionuclides were produced during the entire process of fabrication and transport on the ground for approximately one month, and the specific activities of cosmogenic radionuclides inside the detector were calculated via spectral analysis, where the net area of a peak was determined under the assumption of a linear background. In the germanium crystals, the specific activities were obtained as 0.016 mBq/kg for 57Co, 0.046 mBq/kg for 58Co, and 0.012 mBq/kg for 54Mn, while in cupreous detector components, 0.452 mBq/kg for 57Co, 1.245 mBq/kg for 58Co, 0.382 mBq/kg for 54Mn, and 0.389 mBq/kg for 60Co, were obtained.[Conclusions] This research is essential for understanding and analyzing background spectra and creating background models for low-level radioactivity measurements. Moreover, the proposed method for estimating the specific activities of cosmogenic radionuclides provides a reference for assessing the cosmogenic background in rare event detection using HPGe detectors.
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    NUCLEAR ENERGY AND NEW ENERGY
    Investigating the off-design performance of the secondary circuit system in a small modular reactor nuclear power plant
    WANG Xin, ZHAO Gang, QU Xinhe, WANG Jie, WANG Peng
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 155-163.   DOI: 10.16511/j.cnki.qhdxxb.2023.26.039
    Abstract   HTML   PDF (4665KB) ( 158 )
    [Objective] Small nuclear power plants, which find applications in various fields, are associated with such characteristics as compact size, low investment cost, and high flexibility. Analyzing the off-design performance of the secondary circuit in small modular reactor (SMR) power plants is crucial in terms of optimizing the operation and enhancing the overall performance of nuclear power units. Moreover, doing so serves as a valuable reference point for investigating prospective thermal utilization processes, including nuclear hydrogen production and urban heating.[Methods] This study examines the secondary circuit system of an SMR power plant, establishing a comprehensive and validated system model using the software EBSILON. The simulation is based on the principles of mass, momentum, and energy conservation, employing the Gauss-Seidel iteration method. The primary and secondary circuits of the power plant employ helium and water as coolants, respectively, with the main steam pressure and temperature set at 13.24 MPa and 566℃, respectively. The main steam pressure remains constant under off-design conditions. The system comprises two steam generators, a steam turbine, a condenser, a shaft seal heater, two high-pressure regenerative heaters, three low-pressure regenerative heaters, a deaerator, and pumps. The analysis focuses on exploring the effects of power level variations, turbine back pressure changes, and different regenerative heater removal methods on the system's performance. The power level ranges from 30% turbine heat acceptance (THA) to 100% THA, while the steam turbine back pressure varies from 2.5 kPa to 4.5 kPa. Four different heater removal methods are considered. Notably, the fourth method involves the normal operation of all heaters, serving as a control group for comparison purposes.[Results] The simulation results showed that:(1) Quantitative analysis revealed that power level changes considerably impacted various parameters, such as power generation efficiency, heat consumption rate, regenerative extraction coefficient, and high-/low-pressure cylinder output distribution. As the power level decreased from 100% THA to 30% THA, the power generation efficiency decreased by 5.427%, while the heat consumption rate increased by 1 210.487 kJ·(kW·h)-1. (2) Under the 100% THA operating conditions, reducing the turbine back pressure by 2.0 kPa boosted the power generation efficiency by 1.444%, decreased the heat consumption rate by 272.338 kJ·(kW·h)-1, and increased the power output by 7.120 MW. (3) Taking the main feedwater temperature as an example, partial or total removal of high-pressure heaters caused a decrease of more than 30℃ in the main feedwater temperature; however, the removal of only the low-pressure heater L1 slightly affected the main feedwater temperature. (4) Among the three variables, the power level had the most prominent impact on the thermal performance of the system.[Conclusions] The system's thermal performance is optimized under 100% THA conditions. It is conducive to improving the performance of the units by appropriately reducing the back pressure of the steam turbine. Additionally, cutting off the regenerative heaters can affect the system's thermal performance, but the impact varies with different cutting-off methods. Total removal of high-pressure heaters has a more prominent impact on the system's thermal performance than partial removal of high- or low-pressure heaters and causes a decrease in the feedwater temperature. This study can serve as a valuable reference for the operation and future expansion of SMR power plants.
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    PUBLIC SAFETY
    Effect of low-pressure and oxygen-enriched environment on combustion characteristics of typical fabrics
    JIA Xuhong, TANG Jing, MA Junhao, TIAN Wei, ZHANG Xiaoyu, DAI Shangpei, DING Sijie
    Journal of Tsinghua University(Science and Technology). 2024, 64 (1): 164-172.   DOI: 10.16511/j.cnki.qhdxxb.2023.26.040
    Abstract   HTML   PDF (10710KB) ( 88 )
    [Objective] Artificial oxygen enrichment devices are used in several situations to ensure the safety and health of workers and travelers in high-altitude regions, such as in high-altitude airport control command centers, VIP rooms, medical rooms, and luxury hotels. Indoor oxygen enrichment can meet the oxygen supplementation needs of people. However, the flammability of materials is affected in nonstandard atmospheric conditions such as low-pressure and oxygen-rich environments, resulting could cause additional fire hazards.[Methods] This study simulates the combustion of typical indoor fabrics in the Kangding Plateau (60.5 kPa) and Guanghan, Sichuan (95.8 kPa) inside a combustion chamber by adjusting the pressure and oxygen concentration. It explores changes in the core combustion parameters such as flame form, ignition time, mass loss rate, heat release rate, and total heat release amount of pure cotton and polyester at 60.5 kPa and various oxygen concentrations (21.0%, 27.0%, 33.0%, and 39.0%).[Results] Fabric combustion at low pressure involved the stages of thermal decomposition, ignition, intense burning, and flame decay until extinction. In a low-pressure environment with normal oxygen content, complete cotton combustion was achieved, resulting in the formation of residual char that was loose and easily pulverized. In contrast, polyester combustion exhibited an efficiency of only 11.1%, producing a considerable amount of black and brittle residual char. The rates of mass loss and heat release decreased during the combustion of cotton and polyester, resulting in lower flame heights. The ignition time of cotton decreased by 3.6%, while the ignition time of polyester decreased by 7.8%. The duration of combustion increased by 46.8% for cotton and 197.0% for polyester. Additionally, the burning time of melted polyester droplets increased by 296.0%. With an increase in the oxygen concentration, the ignition time of pure cotton and polyester decreased by 19.1% and 25.7%, respectively. The time of peak rates of mass loss and heat release for pure cotton and polyester were reduced by 78.1% and 52.1%, respectively. The flame height of both materials increased, and the peak mass loss rate and heat release rate significantly rised. The combustion efficiency of polyester was improved by 68.1%, and the total heat release was increased by 1.2 times. Additionally, the burning time of melted droplets was increased by 3.1 times. In contrast, the changes in these parameters were not considerable for cotton combustion. The decrease in the partial pressure of nitrogen in a low-pressure environment decreased the flame-retardant effect of the inert nitrogen gas. Thus, if the peak rate of heat release was taken as the criterion for a fire hazard, the combustion fire hazard of fabrics at a pressure of 60.5 kPa and oxygen concentration of 30.0% was equivalent to that of combustion under normal pressure and normal oxygen conditions. [Conclusions] This study analyzes the effects of the changes in oxygen concentration at low air pressure on the combustion characteristics and reveals the fire behavior characteristics of typical combustible materials such as cotton and polyester in low-pressure oxygen-rich environments. It provides a basis for the fire safety design of artificial oxygen enrichment environments in high-altitude regions.
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