Numerical Study of Interaction of Propagated Shock with Boundary Layer Behind Contact Surface in Ducts

The interactions of the shock with the boundary layer of the cold gas behind the contact in many different conditions, i.e. three kinds of test gases and three kinds of sound speed ratios across the contact, were explored by numerical study. The trajectories of the transmitted shock in cold gas flow and the development of shock bifurcation in the process of interaction with boundary layer are illustrated by many kinds of figures (e.g. the time-distance diagrams of the acoustic impedance contours on the axis, the pressure and density contours and the static pressure distributions on the axis).     

Numerical and experimental study on contact face and shock wave motion in the receiving tube of gas wave refrigerator

The contact face and shock wave motion in an open ends receiving tube of gas waverefrigerator are investigated numerically and experimentally.The results show that,velocity of the contact face rises rapidly as gas is injected into the receiving tube,anddrops sharply after a steady propagation.However,velocity of the shock wave in thetube is almost linear.With increasing of inlet pressure,velocity of the shock waveand steady velocity of contact face also increase.In addition, time and distance ofcontact face propagation in the receiving tube become longer.     

Numerical study of shock tube flows with homogeneous and heterogeneous condensations in rarefaction wave

<正>Shock tubes are devices in which the state of a gas is changed suddenly from one uniform state to another by the passage of shock and expansion waves.In the theory of ideal shock tube flow,it is customarily assumed that the unsteady expansion and shock waves generated by diaphragm rupture are a perfectly centered plane wave.However, such waves are generally not centered,or may not even by plane in practice.In the present research,the time-dependent behavior of homogeneous and heterogeneous condensation of moist air in the shock tube is investigated by using a computational fluid dynamics work.Further,the numerical and experimental studies were carried out in order to investigate the effect of the diaphragm rupture process on the flow characteristics of expansion and shock waves generated near the diaphragm.     

Numerical study of three recirculation zones in the unilateral sudden expansion flow

Numerical simulations of the two-dimensional laminar flow over a backward-facing step channel using two commercially-available Computational Fluid Dynamics (CFD) code are reported. The subject is to analyze the three recirculation regions of the flow in a unilateral sudden expansion. Results are presented for laminar air flow for Reynolds number lower than 2500. The mathematical model equations (mass conservation and momentum) were solved with finite element (FEM) and finite volume (FVM) methods and a segregated approach. To get the grid independent, intensive refinement studies were carried out. Results obtained are compared to experimental data presenting a good agreement. The numerical results of this work show a non-linear increase of the reattachment length as also shown by the experimental results extracted from literature.     

Numerical study of flow regimes in microchannel with dynamic contact angle

Liquid water flow behavior in the microchannel is of crucial importance to the water management in proton exchange membrane fuel cells (PEMFCs). In this study, the liquid water flow regimes in a single straight microchannel are numerically investigated using the volume of fluid (VOF) method with the dynamic contact angle (DCA). Various air and liquid water inlet flow velocities are considered in the simulation to study their effects on the gas-liquid behaviors and flow structure. It was found that the liquid water injection rate is the dominant factor for the formation of different flow regimes: the increase of water inlet velocity will lead to the transition from squeezing flow to partial-jetting flow and jetting flow. Meanwhile, the air inlet velocity can also significantly affect the flow patterns: the higher inertial air flow will facilitate the detachment of liquid water under the squeezing flow, and greatly accelerate the transition process from liquid water blob to the film under the jetting flow.     

A study of external surface pressure effects on the properties for lithium-ion pouch cells

Lithium-ion pouch cells are widely used in electric vehicles because of their high energy density than other structures. There is an unavoidable external surface pressure between the cells in the process of packing and driving of electric vehicles. The influence of external surface pressure on the main properties of the lithium-ion pouch cell has been studied, which is of great significance to packing batteries and reusing retired cells. In this study, a testing device applied for the measurement of constant external surface pressures of lithium-ion pouch cells was first proposed and the different pressure stress-strain distribution on the external surface of cells under semirigid material pads were analyzed by simulation. The effects of pressure on the capacity, internal resistance, and open circuit voltage of fresh and aged LiNi_xCo_yMn_zO₂ (NCM) lithium-ion pouch cells are analyzed through experiments under 1 Mpa external surface pressure. The results show that the internal resistance of fresh cells tends to decrease. The average percentage of the maximum reduction of internal resistance is 13.28%. The experiments also demonstrate that the capacity of aging cells increased by 2.3%. Irreversibility of capacity improvement indicates applying appropriate external surface pressure can improve the secondary utilization efficiency of aging cells.     

Boundary-velocity gradient and premixed flame blowoff in U-bend tubes with secondary flow

The effect of a non-uniform boundary-velocity gradient along the rim of a circular nozzle burner on flame stabilization, including partial liftoff and blowoff, has been investigated experimentally by using U-bend tubes as nozzles for both laminar methane/air and propane/air premixed flames. Secondary flow, induced by the imbalance between pressure force and centrifugal force inside the U-bend tube, generated non-uniform and non-axi-symmetric flow. The intensity of the secondary flow was controlled by varying the flow rate and the radius of curvature of the U-bend tubes. Unique features of flames were visualized with direct photography and planar laser-induced fluorescence for OH-radicals. As the flow-rate increases, the flame lifted off partially from the nozzle rim, and the nozzle attached region decreased with increasing flow rate. Finally, blowoff occurred. Stability of flames was mapped as functions of equivalence ratio, nozzle-exit velocity, and the radius of curvature. The flames in the U-bend tubes had larger velocities at blowoff compared to the case with a straight-tube burner. Flow-field measurement using a laser Doppler velocimeter showed that local boundary-velocity gradients at the critical conditions of partial liftoff and blowoff in the U-bend tubes agreed well with those in the straight-tube burner. Also, such conditions can be described with overall flow characteristics in U-bend tubes, represented by the Dean number.     

Uncertainty-quantification analysis of the effects of residual impurities on hydrogen–oxygen ignition in shock tubes

This study addresses the influences of residual radical impurities on the computation and experimental determination of ignition times in H₂/O₂ mixtures. Particular emphasis is made on the often-times encountered problem of the presence of H-atoms in the initial composition of H₂/O₂ mixtures in shock tubes. Two methods are proposed for quantifying experimentally H-residual impurities in shock tubes, namely, an a priori method that consists of detecting OH traces upon shocking unfueled mixtures, and a posteriori method in which the amount of impurities is inferred by comparing fueled experimental autoignition data with calculations. A stochastic Arrhenius model that describes the amount of H-radical impurities in shock tubes is proposed on the basis of experimental measurements as a function of the test temperature. It is suggested that this statistical model yields a probability density function for the residual concentration of hydrogen radicals in standard shock tubes. Theoretical quantifications of the uncertainties induced by the impurities on autoignition times are provided by using the 5-step short chemistry of Del Álamo et al. [1]. The analysis shows that the relative effects of H-impurities on delay times above crossover become more important as the dilution increases and as the temperature and pressure decrease. Below crossover, the effects of H-impurities on the ignition delay vanish rapidly, and are negligible compared to the departures produced by the non-ideal pressure rise that is seen in some shock-tube experiments at such low temperatures. The influences of kinetic uncertainties on the ignition time are typically negligible compared to the effects of the uncertainties induced by H-impurities when the short mechanism is used, except for air at high temperatures for which kinetic uncertainties dominate. Furthermore, calculations performed with the short mechanism show that correlations between the uncertainties in the rates of branching and termination steps have only some small influences on the ignition-time variabilities near crossover, where a global sensitivity analysis shows an increasing importance of the recombining kinetics. Computational quantifications of uncertainties are carried out by using numerical simulations of homogeneous ignition subject to Monte-Carlo sampling of the concentration of impurities. For the conditions analyzed, these computations show that the variabilities produced in ignition delays by the uncertainties in H-impurities are comparable to the experimental data scatter and to the effects of typical uncertainties of the test temperature when the Stanford chemical mechanism [2] is used. The calculations also unveil that the utilization of two other different chemical mechanisms, namely San Diego [3] and GRI v3.0 [4], yields variations in the ignition delays which are within the range of the uncertainties induced by the H-impurities. Finally, the effects of residual impurities in kinetic-isolation experiments and in supersonic-combustion ramjets are briefly discussed.     

轮轨非对称接触及形面损伤问题分析

随着列车运行速度的提高,车轮形面损伤问题也日益严重。车轮形面损伤分为踏面剥离、车轮失圆和轮缘磨耗等类型。车轮形面损伤后会导致车辆的不平稳运行,使车辆一方面产生低频、中频和高频的振动,影响车辆的运行平稳性和乘坐舒适性,另一方面会引起车辆内部的噪声,车轮损伤严重时甚至会直接导致车辆脱轨事故的发生。车轮损伤问题是由轮轨接触非对称现象所引起的,轮轨非对称接触分为对称踏面与非对称钢轨的接触、非对称踏面与对称钢轨的接触以及非对称踏面与非对称钢轨的接触三种类型。当车轮形面或钢轨损伤后,就会导致轮轨非对称接触的发生,而轮轨非对称接触又会加剧车轮形面的损伤。为了提高车辆的动力学性能和运行稳定性,必须避免轮轨强非对称接触的发生。     

A Study of the Impulse Wave Discharged from the Exits of Two Parallel Tubes

The twin impulse wave leads to very complicated flow fields, such as Mach stem, spherical waves, and vortex ring. The twin impulse wave discharged from the exits of the two tubes placed in parallel is investigated to understand the detailed flow physics associated with the twin impulse wave, compared with those in a single impulse wave. In the current study, the merging phenomena and propagation characteristics of the impulse waves are investigated using a shock tube experiment and by numerical computations. The Harten-Yee's total variation diminishing (TVD) scheme is used to solve the unsteady two-dimensional compressible Euler equations. The Mach number Ms of incident shock wave is changed below 1.5 and the distance between two-parallel tubes, L/d, is changed from 1.2 to 4.0. In the shock tube experiment, the twin impulse waves are visualized by a Schlieren optical system for the purpose of validation of computational work. The results obtained show that on the symmetric axis between two-parallel tube     

Shock/expansion fan interaction with real gas effects for Earth and Mars atmospheric conditions

Numerical simulations are performed to investigate the real gas effects on shock/expansion fan interaction. Initial perfect gas simulations at low enthalpy capture the flow structures efficiently and outcomes are found to have excellent agreement with the analytical calculations. Furthermore, the simulations with the real gas solver for different enthalpies showed that the variation in enthalpy significantly changes the flow structures. It is observed that an increase in enthalpy leads to a decrease and increase in the postshock and postexpansion fan Mach numbers, respectively. Another important observation is the decrement in the peak pressure ratio with an increment in the enthalpy. These effects are noted to be more pronounced for Mars's environment due to the higher dependency of specific heat on temperature.     

液滴撞击不同固体表面的数值模拟研究

采用CLSVOF(coupled level set and volume of fluid)方法,对低We情况下液滴撞击不同固体表面的过程进行了数值模拟研究。分析了液滴撞击平板表面的动态过程,构建了液滴衰减振荡的数学模型,探究了不同的表面润湿性、撞击速度及表面微尺度结构对液滴动态特性的影响。结果表明:液滴撞击固体表面的过程包含铺展、回缩、振荡等多个现象,其最大铺展因子及振荡周期随着表面接触角的增大而减小,随着撞击速度的增大而增大;撞击表面的微尺度结构会对液滴的动态特性产生影响,微尺度结构会对液滴的铺展及回缩运动产生阻碍作用,导致液滴的振荡特性减弱;液滴在矩形沟槽表面达到最大铺展因子的用时最短,在三角形沟槽表面的最大铺展因子最小。     

太阳能电池背表面钝化的研究

利用PC1D模拟不同少子寿命的电池效率与背表面复合速率的关系,采用氮化硅和及其与二氧化硅薄膜的叠加层作为背面钝化膜,通过丝网印刷的方法形成条形局域背接触和局域背面点接触,条形接触的面积为背表面的25%,背面点接触孔径为250μm,间距2mm。经过RTP处理之后,两种不同的接触方式存在相同的问题,串联电阻大,并联电阻小,而利用腐蚀浆料的方法形成背面点接触,在电性能参数有少许改善。结果表明,在正常的烧结状态下,常规铝浆很难完全穿透氮化硅薄膜及其叠加层背面钝化层。而利用腐蚀浆料的方法形成背面点接触,在电性能参数有少许改善。     

Estimating the thermal contact conductance of metals by ultrasonic waves

Experiments and numerical simulations were performed to study heat transfer through a contact interface between two metallic solids in order to clarify the effects of roughness and waviness of the contact surface on the mean thermal contact conductance h_m. In the experiments, the characteristics of an ultrasonic transmission were also investigated so as to obtain a relation between h_m and the transmitted sound energy ratio, with and without attenuation, at the contact interface E_r. Ten pairs of brass specimens were tested in an atmospheric environment at room temperature. Each specimen was a cylindrical block 40 mm in diameter and 45 mm long. The contact surface was finished by polishing, grinding, milling, or turning. Mean nominal contact pressures p_m from 0.08 to 1.67 MPa were applied to the test columns. Our analysis takes account of the radial distribution of thermal contact conductance, using numerical solutions of the two-dimensional cylindrical heat conduction equation to simulate the heat transfer experiments. From the simulations, the effects of roughness and waviness on the temperature fields around the contact surfaces and on h_m were clarified. Finally, for flat rough surfaces, an empirical correlation expression between h_m and E_r is proposed. Using this expression, h_m can be predicted to within about ±50%. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(2): 130–141, 1998     

对影响铝管和铝管板胀接质量的研究

通过实验及在实际应用中影响铝管和铝管板胀接可靠性的分析，得出保证铝管和铝管板胀接质量的数据。     

Numerical study on the formation of Taylor bubbles in capillary tubes

Numerical simulations have been carried out for the transient formation of Taylor bubbles in a nozzle/tube co-flow arrangement by solving the unsteady, incompressible Navier–Stokes equations. A level set method was used to track the two-phase interface. The calculated bubble size, shape, liquid film thickness, bubble length, drift velocity, pressure drop and flow fields of Taylor flow agree well with the literature data. For a given nozzle/tube configuration, the formation of Taylor bubbles is found to be mainly dependent on the relative magnitude of gas and liquid superficial velocity. However, even under the same liquid and gas superficial velocities, the change of nozzle geometry alone can drastically change the size of Taylor bubbles and the pressure drop behavior inside a given capillary. This indicates that the widely used flow pattern map presented in terms of liquid and gas superficial velocities is not unique.     

Numerical study of bubble growth and boiling heat transfer on a microfinned surface

The bubble growth and boiling heat transfer on a microfinned surface are studied numerically by solving the conservation equations of mass, momentum and energy. The bubble shape is tracked by a sharp-interface level-set method, which is modified to include the effect of phase change and to treat the contact angle and microlayer heat flux on an immersed solid surface. The present computation demonstrates that the microfinned surface enhances boiling heat transfer significantly compared to a plain surface. The effects of fin spacing and height on the bubble growth and heat transfer are investigated to find the optimal conditions for boiling enhancement.     

Computational study of effects of contact resistance on a large‐scale vanadium redox flow battery stack

Computational models are developed to allow for a deeper understanding of design factors that affect the lifetime of a vanadium redox flow battery (VRFB) stack, particularly related with the contact‐resistance issue of end cells in a large‐scale stack. A simplified microcontact‐resistance model and a physics‐based macrocontact‐resistance model are constructed to investigate the effect of contact resistance on the performance and longevity of VRFB stacks. A microcontact‐resistance model predicts significant heat accumulation in the current‐collector plate that can result in irreversible damage of plastic materials and an electrical‐voltage loss if the contact resistance is not properly engineered in the stack design. Furthermore, the physics‐based macrocontact‐resistance model investigates abrupt voltage and current distortion in the bipolar plate that is in imperfect contact with the current collector; this results in the local corrosion of the bipolar plate. To ensure a long lifetime of VRFBs, a stack design with minimal contact resistance (less than 0.1 Ω cm²) is required. The structural design of the endplate as well as the selection of a high‐stiffness material is critical to mitigate the bending issue and reduce contact resistance.     

湍球塔内气流分布及除尘性能的试验研究

采用水、空气、填料球作为介质对湍球塔进行烟气除尘实验,重点研究了不同工况下塔内的流速分布及喷淋量、填料静止高度、入口粉尘浓度等因素对除尘效率的影响.实验结果表明填料球具有改善塔内流速分布的作用,添加填料球后,塔内气速稳定在1.5～2 m/s;添加表面活性剂能够显著提高除尘效率,与未加表面活性剂时相比,除尘效率增加超过J％.