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ISSN 1000-0585
CN 11-1848/P
Started in 1982
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  • Table of Content
      , Volume 58 Issue 5 Previous Issue    Next Issue
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    Optimization of an intermediate heat exchanger for a natural circulation molten salt pebble-bed reactor
    XUE Chunhui, DONG Yujie
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 445-449.   DOI: 10.16511/j.cnki.qhdxxb.2018.22.021
    Abstract   PDF (2640KB) ( 472 )
    The nuclear hot spring design concept for molten salt pebble-bed reactors features full power natural circulation with the intermediate heat exchanger (IHX) providing most of the pressure drop in the primary loop outside the core. The IHX flow resistance efficiency is improved here using numerical simulations of the flow and heat transfer characteristics in the flow channel of the IHX. Response surfaces are given for the influences of the structural parameters on the flow resistance coefficient, the Colburn factor and the comprehensive evaluation factor. Optimal fin dimensions are given for small pressure drops but high heat transfer efficiencies based on a multi-objective genetic optimization scheme. An IHX is then designed using the optimized fin parameters through the Aspen software with a 30% decrease of the overall volume of the IHX.
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    Mass estimation model for Brayton cycle space nuclear power systems
    YOU Ersheng, SHE Ding, SHI Lei
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 450-455.   DOI: 10.16511/j.cnki.qhdxxb.2018.25.007
    Abstract   PDF (1338KB) ( 384 )
    Due to their high power and long lifetimes, nuclear power systems are being considered for long space missions in the future. Unlike current terrestrial nuclear facilities, the system mass and size are very important for space applications and define the system performance. A mass estimation model for closed Brayton cycles is studied in this paper. The MATLAB code SPRBC was used to calculate the total mass and the specific mass of a Brayton cycle space power system, including the heaviest components of the nuclear reactor, the shielding, the Brayton rotating unit, the regenerator and the heat radiator. The specific mass is nearly 30 kg/kWe for a system with hundreds of kilowatts electric power and less than 10 kg/kWe for megawatt systems. A 1.76 MW system had a specific mass of 6.14 kg/kWe and a radiator area of about 665 m2. The total mass was nearly 10.8 t, with the nuclear reactor be 22.5% of the mass, the shielding being 22% and the Brayton rotating unit being 26.4%.
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    Experimental study of the behavior of soil-gravel mixtures
    WANG Teng, HU Daisong, ZHANG Ga
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 456-460.   DOI: 10.16511/j.cnki.qhdxxb.2018.22.018
    Abstract   PDF (1366KB) ( 355 )
    The strength and deformation characteristics of the soil-gravel mixtures must be studied for disaster prevention and to improve safety. Serialized plain strain tests are conducted on various soil-gravel mixtures with aluminum bars used to simulate the gravel. The tests show that the gravel content and gravel shape significantly affect the strength and deformation of the soil-gravel mixtures. The peak strength and dilatancy characteristics increase as the gravel content increases and the gravel becomes more psephitic. Measurements of the gravel movement show that the gravel impedes the formation and development of the shear band by providing a rolling resistance which increases the peak strength.
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    Microseismic characteristic analysis in deep TBM construction tunnels
    TANG Zhili, LIU Xiaoli, LI Chaoyi, QIN Pengxiang, XU Qianjun
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 461-468.   DOI: 10.16511/j.cnki.qhdxxb.2018.26.017
    Abstract   PDF (5278KB) ( 390 )
    The increasing depth of underground construction tunnels has led to instabilities of the surrounding rock caused by the excavation unloading is the rock structures. A real-time microseismic monitoring system was built to monitor and evaluate the stability of the rock during excavation foe a tunnel in Tibet, China. The system measured the temporal and spatial evolution of the microseismic activities during tunnel boring machine (TBM) excavation. The results were used to relate the b-values of the microseismic events and the stability risks of the surrounding rock. The results show that the number of microseismic events and the energy released increase with increasing TBM activity. A large number of microseismic events are concentrated in space and time before collapse of the rock mass and the gathering area coincides with the position of the rock mass collapse. The b-value is an important index to evaluate the surrounding rock stability. The results verify the feasibility of microseismic monitoring to evaluate the stability of the surrounding rock and a reference for microseismic monitoring in similar projects.
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    Optimal pole width ratio for high force density linear switched reluctance motors
    LÜ Jiangwei, ZHOU Kai
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 469-476.   DOI: 10.16511/j.cnki.qhdxxb.2018.22.017
    Abstract   PDF (3471KB) ( 294 )
    The double salient structure significantly reduces the forces in a linear switched reluctance motor (LSRM). The structure is then optimized for high force densities. The pole width is the key parameter in the double salient structure with this paper focusing on the optimal pole width ratio for a high force density LSRM using qualitative and quantitative analyses and experimental measurements. The qualitative analysis based on a linear flux linkage model shows a maximum average force for various translator and stator pole widths. Then, a finite element analysis (FEA) based parametric modeling method is used to validate the results and to calculate the optimal pole width range. Force measurements then agree well with the FEA results. They both show that the optimal stator pole width ratio is 0.4 to 0.45 and the optimal translator pole width ratio is 0.45 to 0.5 for a high force density in the LSRM. The optimal width ratio does not change with the air gap or slot depth.
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    Parametric adaptability of TIG arc-induced non-contact MIG arc ignition
    TANG Yingying, ZHU Zhiming, YANG Zhongyu, FU Pingpo
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 477-481.   DOI: 10.16511/j.cnki.qhdxxb.2018.25.025
    Abstract   PDF (4668KB) ( 343 )
    A series of arc ignition experiments using tungsten inert gas-metal inert gas (TIG-MIG) hybrid welding was conducted for various MIG welding parameters with simultaneous measurements of the arc voltage, the welding current and images of the TIG-MIG hybrid welding arc. These experiments studied the characteristics of the TIG arc-induced non-contact MIG arc ignition and its adaptability to the various MIG welding parameters. The results show that the initial TIG arc not only quickly enables the MIG welding to achieve non-contact arc ignition but also enables the MIG welding to enter a stable droplet-free transfer mode with a relatively low arc voltage and welding current. The weld appearance is more uniform than the conventional MIG welding with no spatter. The initial wire feed speed, welding torch inclination angle, shield gas flow rate and welding wire end diameter for the MIG welding could be varied over relatively wide ranges with non-contact arc ignition of the MIG welding when ignited by the TIG arc. If the initial wire feed speed is properly matched to the welding wire end diameter, no spatter is produced during the non-contact MIG arc ignition process even with large residual droplet on the end of the welding wire.
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    Natural formation of a nano/micro spacecraft cluster for multi-model distributed remote sensing
    FANG Yuankun, MENG Ziyang, YOU Zheng
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 482-488.   DOI: 10.16511/j.cnki.qhdxxb.2018.25.021
    Abstract   PDF (1421KB) ( 330 )
    Distributed remote sensing has attracted much attention for space exploration. The sensing ability is based on the system properties of the spacecraft cluster composed by multiple nano/micro spacecraft. In addition, the multiple nano/micro spacecraft formation can be rearranged for different applications. However, since the micro/nano spacecraft have little fuel, methods are urgently needed to maintain the stability of the distributed remote sensing nano/micro spacecraft formation for a relatively long period in a natural formation.The natural nano/micro spacecraft cluster formation is analyzed have for a multiple distributed remote sensing task scenario that optimizes the fuel usage and calculational efficiency. A direct parameter method was designed for the typical tasks of in-plane flying with a tree communication topology and leader-following topology modes. The orbit parameters of the followers are obtained directly by this method. STK simulations show this approach is accurate, robust, and useful for emergency remote sensing tasks.
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    Estimating the elasticities of gasoline demand for urban households in China
    CAO Jing, HU Wenhao
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 489-493.   DOI: 10.16511/j.cnki.qhdxxb.2018.22.026
    Abstract   PDF (858KB) ( 318 )
    To encourage low-carbon lifestyles and reduce vehicle air pollution, the Chinese government needs to implement effective policy tools to regulate gasoline consumption, such as gasoline taxes. This paper uses the Probit and Translog models to estimate gasoline price and income elasticities using urban household consumption data for 1992-2009. The results show that the household characteristics significant impact the gasoline consumption of urban households in China. The micro-data elasticities results are similar to those in western countries. The gasoline demand price elasticity is-0.445 and the sophisticated income elasticity is 1.307. Thus, a fuel tax can effectively regulate driving behavior as well as gasoline consumption. In addition, these results show that a gasoline tax is also progressive in China that reduces gasoline consumption and pollutant emissions without negative effects on household welfare.
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    Military transport capacity evaluation of ports using entropy weight and TOPSIS
    ZHANG Jing, ZHANG Zhihui, LI Xiaodong, SUN Qi, QIAO Wei
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 494-499.   DOI: 10.16511/j.cnki.qhdxxb.2018.22.027
    Abstract   PDF (1139KB) ( 255 )
    There were few quantitative studies of port capacities for military transport with most focusing on the economic efficiency and civilian capacity although the ports often provide military sealift transport services. This study used a quantitative assessment method using entropy weight and the technique for order preference by similarity to an ideal solution (TOPSIS) to assess the military transport capacity. Indicators were defined to evaluate the port capacity from literature data and the Chinese military transport features. The methodology was then used to rank the military transport capacity of 27 coastal ports in China. The findings show that most Chinese ports pay little attention to military transportation. Only a few ports integrate military and civilian transport, such as Dalian which has the greatest military transport capacity.
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    API based sequence and statistical features in a combined malware detection architecture
    LU Xiaofeng, JIANG Fangshuo, ZHOU Xiao, CUI Baojiang, YI Shengwei, SHA Jing
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 500-508.   DOI: 10.16511/j.cnki.qhdxxb.2018.25.020
    Abstract   PDF (3026KB) ( 577 )
    This paper presents a combined machine learning framework for malware behavior analyses. One part of the framework analyzes the dependency relation in the API call sequence at the functional level to extract features to train and classify a random forest. The other part uses a recurrent neural network (RNN) to study the API sequence to identify malware with redundant information preprocessing using the RNN time series forecasting ability. Tests on a malware dataset show that both methods can effectively detect malwares. However, the combined framework is better with an AUC of 99.3%.
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    DNN-LSTM based VAD algorithm
    ZHANG Xueying, NIU Puhua, GAO Fan
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 509-515.   DOI: 10.16511/j.cnki.qhdxxb.2018.25.022
    Abstract   PDF (1357KB) ( 838 )
    Voice activity detection (VAD) algorithms based on deep neural networks (DNN) ignore the temporal correlation of the acoustic features between speech frames which significantly reduces the performance in noisy environments. This paper presents a hybrid deep neural network with long-short term memory (LSTM) for VAD analyses which utilizes dynamic information from the speech frames. A context information based cost function is used to train the DNN-LSTM network. The noisy speech corpus used here was based on TIDIGITS and Noisex-92. The results show that the DNN-LSTM based VAD algorithm has better recognition accuracy than DNN-based VAD algorithms in noisy environment which shows that this cost function is more suitable than the traditional cost function.
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    A robust time-delay estimation and dereverberation algorithm based on the coherence function
    FANG Yi, CHEN Youyuan, MOU Hongyu, FENG Haihong
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 516-522.   DOI: 10.16511/j.cnki.qhdxxb.2018.25.026
    Abstract   PDF (1357KB) ( 244 )
    The performance of traditional cross-correlation based time-delay estimation methods is sharply degraded in reverberation environments. Precedence effect models have been proposed with cross-correlation functions, but these models are quite parameter-sensitive and the front-end processes are very complex. This paper describes a method that first updates a function of the speech and noise based on the eigenvalues of the covariance matrix. Then, a coherence function of the speech is matched to the ideal coherence function for the time-delay estimate. Then, the estimated time delay and the noise coherence function are applied to the coherent-to-diffuse power ratio (CDR) estimator for reverberation suppression. Tests show that this scheme has higher localization accuracy than traditional methods and achieves higher PESQ (perceptual evaluation of speech quality) scores than other CDR estimators.
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    Process modeling and analysis of the parameters for oxidation of cyclohexane into cyclohexanone
    LI Xiuxi, CAO Liqi, WANG Xing
    Journal of Tsinghua University(Science and Technology). 2018, 58 (5): 523-528.   DOI: 10.16511/j.cnki.qhdxxb.2018.25.027
    Abstract   PDF (1091KB) ( 339 )
    Uncatalysed oxidation of cyclohexane into cyclohexanone is dangerous since the process will explode when the tail oxygen volume fraction reaches a critical limit. Thus, the tail oxygen volume fraction needs to be well characterized for various operating parameters for safe production. A mathematical model of the reaction is given here as a function of the mass transfer rate, hydraulics and cyclohexane volatilization. The reaction kinetics and material balance model are also given. Simulations are then in good agreement with actual production data. The results show that when the gas phase oxygen volume fraction is 28% and the feed gas flow rate is 6 750 Nm3·h-1, the oxygen tail gas volume fraction increases to 3%, which will cause an explosion. The reaction temperature, the gas phase oxygen volume fraction and the feed gas flow rate are the most sensitive parameters in the reaction process, with the model providing a theoretical basis for controlling safe production of cyclohexanone.
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