15 March 2020, Volume 60 Issue 3

    

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SPECIAL SECTION: AEROSPACE AND ENGINEERING MECHANICS
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  Review of research on Solar System dust dynamics

  LIU Xiaodong

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 189-197. https://doi.org/10.16511/j.cnki.qhdxxb.2020.26.001

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  Solar System dust dynamics is a relatively new research topic which is important for understanding the origin and evolution of the Solar System as well as for space missions. This paper presents a review of recent studies on Solar System dust dynamics. The dust mass production model based on experiments by previous research for the impact-ejected process is introduced, and the initial mass distribution inferred from the in-situ detections and experiments is shown.This paper also reviews research on the modelling of perturbation forces and commonly used analytical methods. Modelling and experimental studies are presented for both dust charging and sputtering effects which affect the particle dynamics. Finally, the typical formation/migration mechanisms of particle populations as well as their significance on the dust distributions are summarized.
  
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  Modeling and control of a micro aero-engine thrust vector system

  LI Yingjie, WU Linfeng, LI Chunwen

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 198-205. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.042

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  A micro aero-engine model was used to study the control of the thrust vector system of a sub-scaled flight demonstrator. A mechanism model was combined with experimental data to model the thrust vector system. Aerodynamic declination and thrust loss coefficients were introduced to modify the mechanism model. The control laws were based on an improved generalized minimum variance method. Simulations show that the new method improves the model sensitivity to the control parameters compared with the traditional generalized minimum variance method. A control scheme was developed for the thrust vector system with simulations showing that the predictions are acceptable for the control of sub-scaled fighters.
  
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  Numerical simulations of ignition by soak-back heat through the injection panel in a premixed C₂H₄/N₂O thruster

  WANG Weilong, ZHANG Huiqiang

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 206-211. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.033

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  The premixed C₂H₄/N₂O propulsion system has significant potential for attitude and orbit control thrusters since the propellant is not toxic and the systems are efficient and simple. However, methods are needed to prevent flashback. This study used a numerical model to analyze ignition in a thruster chamber with a single injector unit with a premixed C₂H₄/N₂O propellant. The model included the plenum chamber, injection panel, combustion chamber and nozzle. The numerical model included a detailed chemical reaction mechanism for the ignition in the combustor and plenum chamber by coupling the reacting flow in the entire model and the heat transfer in the injection panel. The model predicts the spatio-temporal evolution of the unsteady ignition process in the thruster chamber and the steady characteristics are consistent with the results of chemical equilibrium calculations. Reducing the hole to the panel area ratio significantly increased the soak-back heat from the combustion chamber to the plenum chamber through the injection panel which eventually led to ignition of the premixed C₂H₄/N₂O propellant in the plenum chamber. This is a new flashback mode induced by structural heat transfer through the injection panel. This process has a critical hole to panel area ratio for this propellant with a conventional chamber head. Thus, a new constraint criterion is given for the injection panel design of a premixed C₂H₄/N₂O propulsion system.
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  Development and verification of a fuel slinger atomization test bench

  GAO Xiang, WANG Fang, XIAO Yangchun, WU Zhaoyang, JIN Jie

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 212-218. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.034

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  Fuel slingers are commonly used fuel injection devices for small and medium-sized aeroengines. This study developed a fuel slinger atomization test bench to provide reference data to improve fuel slinger designs and combustion chamber performance tests. The test bench included a water supply system, rotating system, ventilation system, test system, control system, and data acquisition system. Analyses of the key operating parameters such as the water supply pressure, water supply flow rate and vibrations of the rotating system shows that the test bench gives reliable, accurate experimental data. A phase Doppler particle analyzer (PDPA) was used to measure the particle size statistics and the variation of the Sauter mean diameter (SMD) for various rotational speeds. The experimental results show that the particle size in the fuel slinger has a normal distribution and the SMD decreases with increasing fuel slinger speed with an SMD of 41 μm above 12 000 r/min. The experimental results are consistent with the conclusions of previous papers, further indicating the reliability of the test bench.
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  Atmospheric drag on satellites flying in lower low-earth orbit

  JIN Xuhong, HUANG Fei, CHENG Xiaoli, WANG Qiang, WANG Bing

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 219-226. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.030

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  The test particle Monte Carlo method for flow in the free-molecular flow regime was integrated with various state-of-the-art atmospheric models in a general, three-dimensional code to calculate the atmospheric drag for user-defined spacecraft geometries of arbitrary complexity. Then, the code was applied to the geodetic GOCE satellite to evaluate the effects of flight altitude, orbit latitude and orbit longitude on the atmospheric drag to assess the sensitivity of the satellite drag on the atmospheric model. The results show that increasing the satellite height significantly reduces the atmospheric drag while increasing the drag coefficient. The results also illustrate the sensitivity of the satellite drag prediction to the atmospheric model. The orbit latitude and longitude affect the drag coefficients and the satellite drag indirectly by changing the atmospheric temperature and the molecular mass since the satellite drag is determined by the drag coefficient and the atmospheric density. Both the satellite drag and the drag coefficients are nonlinear functions of the orbit latitude and longitude. For the conditions considered here, two newer atmospheric models, JB2008 and DTM-2013, predict similar satellite drag forces with three older models, USSA-1976, Jacchia-1977 and NRLMSISE-00, yielding comparable but somewhat larger drag predictions.
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  Positions and stability of equilibrium points of minor celestial bodies with mascon

  JIANG Yu

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 227-232. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.027

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  This paper investigates the influence of a variety of mascon properties on the positions, topological conditions, and stability of the equilibria of minor bodies with application to asteroid 2867 Steins. Four types of mascon models are considered, the single-layer similar mascon, single-layer spherical mascon, multi-layer similar mascon, and multi-layer spherical mascon. The results show that the mascon may change the stability and topological conditions of the equilibrium points. For asteroid 2867 Steins, the topological condition and stability of external equilibrium point E4 differ between the mascon and the non-mascon cases. If the asteroid has no mascon, E4 is unstable and the topological condition is Case 5. If the asteroid has a mascon with the assumed density, E4 is stable and the topological condition becomes Case 1. The mascon case has larger distances between the external equilibrium point and the center-of-mass than the non-mascon case and smaller distances between the inner equilibrium point and the center-of-mass. If the position, density, and volume of two mascons are essentially the same, the shape has little effect on the equilibrium point position and no influence on the topological condition and stability of the equilibrium point. This study provides further understanding of the influence of the internal asteroid structure on the gravitational field around asteroids and provides a reference for gravitational field reconstruction, orbit design, and reliability analyses of asteroid exploration missions.
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  Effect of feeding mode and flow distribution on SiO₂ synthesis from D4

  ZHANG Han, HUANG Zeyu, ZHANG Guojun, OUYANG Baohua, WU Xuemin, ZHENG Lili

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 233-238. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.050

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  Burner design and flow rate control strongly impact combustion stability, SiO₂ generation, soot quality, and key flue gas components. This paper presents a numerical study of SiO₂ soot synthesis by the CVD method with octamethylcyclotetra-siloxane (also called ‘OMCTS’ or ‘D4’) as the precursor. The study investigates the effects of various feeding modes and D4 and oxygen mass flow rates on the SiO₂ generation efficiency and the distribution of key components on the surface. The SiO₂ generation rate and deposition uniformity can be significantly improved by using the multi-point feeding mode which increases the SiO₂ generation rate and reduces the OH concentration even with similar D4 inlet velocities. Proper control of the oxygen flow rate relative to the D4 mass flow rate improves the SiO₂ generation rate, but also increases the OH concentration. The multi-point feeding mode with the same D4 mass flow rate can reduce the D4 inlet velocity with then reduces the deposition efficiency some. Multi-point feeding with the proper increased D4 flow rate and flame temperature control improves the SiO₂ generation rate and deposition efficiency.
NUCLEAR ENERGY AND NEW ENERGY
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  Flow characteristics of free swirling jet with special structures

  HU Yu, GONG Yingli, SUN Xinyu, HUANG Xingliang, QI Haiying

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 239-247. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.001

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  Vortex thermal-intensification was studied by measuring the cold flow characteristics of a tornado-like vortex. The experiments measured the effect of the inlet channel area of a vortex generator on the initial vacuum and velocity distribution with strong swirl. The results show that the air mass flow rate strongly influences the initial vacuum with the vacuum increasing with the mass flow rate. All the velocity components of the tornado-like vortex can be described by semi-empirical vortex models with the velocity distribution varying significantly as the swirl intensity decreases. A strong swirl intensity is found crucial to the formation and maintenance of the hollow-core structure. The occurrence of radial entrainment indicates a significant decrease in the swirl intensity. These observations extend the understanding of the tornado-like vortex related to vortex thermal-intensification and provide important reference for exploring these mechanisms in the future.
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  Instantaneous measurements of granular blast furnace slag thermal parameters

  ZHANG Yanguo, ZHANG Ying, YANG Xiaoxiao

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 248-253. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.048

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  An instantaneous measurement method was developed to measure the surface temperature and surface emissivity of blast furnace slag for investigating dry granulation and waste heat recovery from blast furnace slag. The system was used to measure temperature-gray scale and emissivity-temperature relationships from granular blast furnace slag images captured by a high-speed camera based on the surface gray scale level. The surface temperature and the surface emissivity of the granular blast furnace slag could then be obtained by combining the measured surface gray scale with these relationships for the granular blast furnace slag. The results show that the surface emissivity decreases with decreasing slag surface temperature. The slag surface emissivity is about 0.89 at 1 402℃.
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  Polarization of the membrane electrode assembly in a proton exchange membrane fuel cell

  ZHAO Yang, WANG Shubo, LI Weiwei, XIE Xiaofeng

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 254-262. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.019

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  The performance of a proton exchange membrane fuel cell depends on the membrane electrode assembly (MEA) polarization. The characteristics of a home-made MEA with various polarizations were measured to calculate the internal losses, activation polarization, ohmic polarization, and mass transfer polarization. The results show that the activation polarization has a large effect, especially at low temperatures, while the ohmic polarization has little effect with a maximum difference of about 0.09 V for a current density of 1550 mA/cm². The mass transfer polarization effect increases rapidly as the current density increases. Moreover, 40%-44% of the oxygen transport resistance comes from the molecular diffusion resistance in the gas diffusion layer with 43%-50% of the resistance originating from the oxygen dissolution and diffusion through the ionomer layer.
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  Numerical study of the flow and heat transfer of supercritical CO₂ flowing in various vertical serpentine tubes

  HUANG Teng, LI Xuefang, CHRISTOPHER D M, BA Qingxin, CHENG Lin

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 263-270. https://doi.org/10.16511/j.cnki.qhdxxb.2019.21.031

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  As an environmentally friendly natural refrigerant, CO₂ has been increasingly used as the working fluid in heat pumps. The flow and heat transfer of supercritical CO₂ flowing in various serpentine tubes were modeled here to investigate the influence of the tube geometry on the heat transfer and to investigate the heat transfer enhancement mechanisms. Twelve full-size three-dimensional geometries were generated with different inner diameters and bend diameters to investigate the effects of the tube inner diameter and bend diameter on the flow and heat transfer of supercritical CO₂ for a given flow flux. The results show that the heat transfer coefficient decreases as both the bend diameter and the inner diameter decrease. Thus, the outer wall temperature increases more rapidly and is higher with larger inner diameters and tube bend diameters. Finally, the effects of flow direction on the heat transfer were also studied to show that upward flow results in a higher heat transfer coefficient than downward flow for inner diameters larger than 1 mm. The present results are useful for enhanced heat exchanger designs to improve system efficiencies.
CIVIL ENGINEERING
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  Resource-constrained project scheduling problem considering productivity and construction methods

  WANG Hengwei, LIN Jiarui, ZHANG Jianping

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 271-277. https://doi.org/10.16511/j.cnki.qhdxxb.2019.21.035

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  The multimode resource-constrained project scheduling problem (MRCPSP) is an essential mathematical model for construction schedule optimization. However, such models cannot easily simultaneously represent multiple relationships between activity duration, cost, and resource requirements. This paper presents an MRCPSP model that includes multiple relationships that is solved using constraint programming (CP). The model represents the relationships between activity duration, cost, and resource requirements by introducing a productivity function and the total resource requirements for combinations of activities. The model can simulate the construction productivity changes and the influence of construction methods on the results. The results then have a more explicit engineering meaning to improve actual projects than the traditional MRCPSP.
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  Bench-scale experimental study of the deflagration overpressure of a flammable liquid vapor in a tunnel

  CHEN Changkun, XU Tong, SHI Congling, ZHAO Xiaolong, ZHANG Yulun

  Journal of Tsinghua University(Science and Technology). 2020, 60(3): 278-284. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.032

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  The deflagration overpressure and flame propagation of ethanol vapor were investigated in a bench-scale tunnel. The characteristics of the deflagration flame and the overpressure were measured at liquid temperatures of 30℃, 40℃, 50℃, 60℃, 70℃ and 78℃ (boiling point). The results show that the explosion limit of vapor is affected by the initial liquid temperature. At low temperatures, the explosion limit range is quite small and the required detonation energy is very large. The overpressure of the ignited vapor had an obvious double-peak shape along the longitudinal direction of the tunnel. Meanwhile, when the overpressure reached peak at the first time, the pressure measuring points P1 above the fuel pins were lower than the overpressure of P2 and P3 away from the center. In addition, with increasing ethanol vapor concentration, the maximum detonation overpressure first increased and then decreased in the tunnel.