竖直管内降膜流动气液两相运动数值模拟

对较高流速下强迫竖直降膜进行了模拟.建立了描述竖直管内强迫降膜的物理模型和数学模型,采用RNG k-ε模型描述管内气体和液体的复杂湍流流动过程,采用VOF方法对气液相界面进行追踪.通过观察流动过程中的液膜前端速度矢量变化、整个流场中的压力分布变化过程,对降膜前端的"托举"现象进行了分析,对竖直管内液膜形成过程中的两相复杂运动进行了分析研究,指出了避免"托举"现象出现的条件.     

Interaction between turbulence and thermal radiation in combined heat transfer channel flows

The aim of the present work is to examine the effects of interaction between turbulence and thermal radiation on the fully developed turbulent channel flow with variable properties in the presence of combined mixed convection‐radiation heat transfer. The vertical and horizontal channels under study are formed by differentially heated flat parallel plates. Large eddy simulation and the low Mach number approach are used to solve the governing equations. Also, the radiative transfer equation is solved using the method. The results are achieved by developing a solver in an open‐source computational fluid dynamics toolbox. The main focus is to find out whether neglecting turbulence‐radiation interaction (TRI) is a valid assumption for such flows under consideration. The present results show that, in both configurations, the maximum values of emission TRI and incident TRI are 2% and 3%, respectively. These results are consistent with the previous findings suggesting that in the nonreactive flows, these two terms are negligible.     

Measurement and modeling of liquid film thickness evolution in stratified two-phase microchannel flows

Polymer electrolyte membrane (PEM) fuel cells incorporating microchannels (D < 500 μm) can benefit from improved fuel delivery and convective cooling. However, this requires a better understanding of two-phase microchannel transport phenomena, particularly liquid–gas interactions and liquid clogging in cathode air-delivery channels. This paper develops optical fluorescence imaging of water films in hydrophilic channels with varying air velocity and water injection rate. Micromachined silicon test structures with optical access and distributed water injection simulate the cathode channels of a PEM fuel cell. Film thickness data vary strongly with air velocity and are consistent with stratified flow modeling. This work facilitates the study of regime transitions in two-phase microchannel flows and the effects of flow regimes on heat and mass transfer and axial pressure gradients.     

Numerical studies of heterogeneous reaction in stagnation flows using one-dimensional and two-dimensional Cartesian models

Flow over a thin strip is one of the major experimental configurations in the study of catalytic combustion. 1D laminar stagnation-flow models are generally employed to investigate the catalytic combustion characteristics in this experimental configuration. In this study, a 2D Cartesian model is developed to examine the conditions under which 1D models are applicable. The results show that the 1D model is only appropriate in the experimental configurations with a length-to-width ratio less than 1 to 3 (depending on the flow conditions). The 1D model failed because of the neglecting of transverse diffusive mass transfer (at Re < 30) and of the oversimplified convective mass transfer (at Re > 60). The calculations of the 1D and 2D models are also compared against the OH measurements performed in a typical stagnation-flow reactor with a length-to-width ratio of 16. The results show that the computations of the 2D model are in better agreement with the experimental data.     

等离子发生器燃烧流场的数值模拟

采用漩涡破碎(EBU)燃烧模型、κ-ε双方程湍流模型及SIMPLEC算法对等离子发生器内部的燃烧流场进行了数值模拟,得到温度场、压力场以及湍流脉动动能、湍流平均动能耗散率等参数分布图。     

Numerical Computations of Confined Swirl Flows in Augmentor of Jet Engines and Comparisons with Experiments

This paper describes the design of four types of swirlers which produce different velocity profiles.The dataof confined turbulent swirling flowfields are obtained from experiments and numerical calculations.The actualflow profiles at the swirler exit basically agree with the desired profiles.Two kinds of turbulence models,k-ε model and DDC model,are used to predict the flowfields producedby swirl augmentors.The results show that the DDC model,based on interaction between dissipation anddispersion,can overcome the defect of k-ε model and predict the swirling flowfields more accurately than k-εmodel.     

Droplet combustion in the presence of acoustic excitation

This experimental study focused on droplet combustion characteristics for various liquid fuels during exposure to external acoustical perturbations generated within an acoustic waveguide. The alternative liquid fuels include alcohols, aviation fuel (JP-8), and liquid synthetic fuel derived via the Fischer–Tropsch process. The study examined combustion during excitation conditions in which the droplet was situated in the vicinity of a pressure node (PN). In response to such acoustic excitation, the flame surrounding the droplet was observed to be deflected, on average, with an orientation depending on the droplet’s relative position with respect to the PN. Flame orientation was always found to be consistent with the sign of a theoretical bulk acoustic acceleration, analogous to a gravitational acceleration, acting on the burning system. Yet experimentally measured acoustic accelerations based on mean flame deflection differed quantitatively from that predicted by the theory. Phase-locked OH^∗ chemiluminescence imaging revealed temporal oscillations in flame standoff distance from the droplet as well as chemiluminescent intensity; these oscillations were especially pronounced when the droplets were situated close to the PN. Simultaneous imaging and pressure measurements enabled quantification of combustion-acoustic coupling via the Rayleigh index, and hence a more detailed understanding of dynamical phenomena associated with acoustically coupled condensed phase combustion processes.     

稀混合气低速过滤燃烧的数值模拟与理论分析

应用一维数值模拟方法,研究了多孔介质引起的热回流效应、多孔介质的导热系数、系统的热损失、混合气当量比对燃烧波波速和反应区最高温度的影响.分别将燃烧区简化为无限薄和极其狭窄的区域,从理论上得到了低速过滤燃烧波波速和反应区的最高温度两个关系式,构成了两者的封闭解.利用数值模拟和理论分析求得的燃烧波波速和反应区的最高燃烧温度,取得了与实验结果相同的趋势.     

温度及压力对柴油机燃烧室内混合气形成影响的数值模拟

用CFD分析软件对柴油机燃烧室内不同环境条件下柴油喷雾的混合气形成及撞壁混合过程进行了模拟计算，并与PLIF法取得的试验结果进行了对比，二者基本吻合．试验和模拟计算结果均表明，油束撞壁后主要沿燃烧室壁面向下移动，在燃烧室近壁区形成一层薄而浓、面积较大的混合气层，且随燃烧室内气体密度的增大，对应时刻及位置的速度、喷雾贯穿距、燃烧室壁面的燃油堆积量、近壁区浓混合气层的面积及混合气浓度减小．     

Experimental investigation of the two-phase theory in a fluidized-bed combustor

Though the two-phase theory of fluidization is well-accepted, no direct experimental measurements of the different gas concentrations predicted to occur in bubble and particulate phases could be found in the literature. For the first time, theoretical predictions of these different gas concentrations have been validated experimentally, using a combined oxygen/bubble probe. Based on the two-phase theory, a mathematical model was developed for the combustion of a batch of char particles in a fluidized-bed combustor. The experimental oxygen concentration in the particulate phase as a function of time was well predicted by the model. Slight discrepancies for the bubble phase values were eliminated when low-oxygen-concentration bubbles were excluded from the data, attributed to some char combustion occurring in bubbles being contrary to the model assumption. The temperature difference between char and bed particles (ΔT) was the only adjustable parameter in the model. A value of 20°C fitted the burnoff times measured by visual observation of the top of the bed, for both 5 and 10 g char batch masses. Model predictions of the oxygen concentrations were not sensitive to ΔT during the first half of burnoff, when mass transfer controlled the combustion rate, so the mass transfer processes were predicted correctly by the model effectively with no adjustable parameter. The ΔT value of 20°C was significantly lower than experimental measurements of maximum burning char particle temperatures, reported to be 70°C for the small-diameter bed particles used in this work. The discrepancy was attributed to two factors: (i) the decrease in char particle temperature towards the end of the burnoff, when kinetics significantly affected the combustion rate; and (ii) a lower char particle temperature in the particulate phase than in the bubble phase, with experimental char particle temperature measurements biased towards the higher bubble phase values. It was inferred: (i) that the maximum values of ΔT measured experimentally are too high for calculation of the char particle combustion rate during the kinetic-controlled latter stage of burnoff and (ii) that reported values of the heat transfer coefficient from burning char particles to the particulate phase deduced from these particle temperature measurements may have been underestimated.     

多流体模型在燃气轮机燃烧室三维反应流中的应用

对一种模型燃气轮机燃烧室中的三维反应流进行了数值模拟，模型燃烧室的燃料是CH4，燃烧类型是预混燃烧，在数值模拟过程中，采用了Spalding于1995年提出来的多流体模型来对燃烧室中的湍流预混燃烧进行了数值模拟，在数值模拟过程中考虑了辐射问题，采用了六通量辐射模型。通过数值模拟给出了速度，压力，湍流脉动动能，湍流动能耗散率，焓值，湍流粘度，温度，密度，燃烧产物质量分数，氧的质量分数，燃料/空气混合比，燃料质量分数，空间三个方向的辐射热通量以及各种流体的质量分数等变量的分布情况，此外，还采用传统的旋涡破碎模型对此燃烧室进行了数值模拟，并对两种方法的结果进行了分析比较，由分析可以看出多流体模型的结果接近于实际情况，对模型燃烧室进行三维反应流数值模拟的工作为今后对实际燃烧室反应流的数值模拟打下了一定的基础。     

Influence of convective heat transfer modeling on the estimation of thermal effects in cryogenic cavitating flows

The accuracy of numerical simulations for the prediction of cavitation in cryogenic fluids is of critical importance for the efficient design and performance of turbopumps in rocket propulsion systems. One of the main remaining challenges is efficiency in modeling of the physics, handling the multi-scale properties involved and developing robust numerical methodologies. Such flows involve thermodynamic phase transitions and cavitation bubbles that are on a smaller scale than the global flow structure. Cryogenic fluids are thermo-sensitive, and therefore, thermal effects and strong variations in fluid properties can alter the cavitation properties. The aim of this work is to address the challenge posed by thermal effects. The Rayleigh–Plesset equation is modified by the addition of a term for convective heat transfer at the interface between the liquid and the bubble coupled with a bubbly flow model to assess the prediction of thermal effects. We perform a parametric study by considering several values of and models for the convective heat transfer coefficient, h_b, and we compare the resulting temperature and pressure profiles with the experimental data. Finally, the results of a 2D simulation with a commercial CFD code are presented and compared with the previous results. We note the importance of the choice of h_b for the correct prediction of the temperature drop in the cavitating region, and we assess the most efficient models, underlining that the choice of h_b estimation model in a cryogenic cavitating flow is more important in the bubble growth phase than in the bubble collapse phase.     

用元素扩散方程和化学平衡热力计算的方法求解多组分反应流场

提出了一种新的方法来计算多组分化学反应流场。它首先引入了元素的分布方程，然后根据化学平衡的热力计算方法求解每一位置的组分及温度。这样，在计算低速流场时，就避免了处理化学反应源项引起的数值计算过程中的方程刚性问题，也减少了组分方程的数量。计算结果表明，这种方法的计算结果比EBU模型的计算结果更好的反应了物质热解离的情况。     

Numerical modeling of suppression of detonation waves in hydrogen-air mixture by system of inert particles clouds

The interaction of a detonation wave in a hydrogen-air mixture with a series of clouds of inert particles with diameters of 1, 10, and 100 μm is calculated. The concentration and geometric limits of detonation are determined. It was found that an increase in the particle diameter leads to an increase in the concentration limit and to decrease in the geometric limit of detonation. It has been found that an increase in the number of clouds in a series of clouds makes it possible to increase the length of the gaps between them for the successful failure of detonation. In addition, the longer the clouds of inert particles, the greater the maximum distance between them and the smaller their number is necessary to quench the detonation.     

Numerical modeling of liquid water motion in a polymer electrolyte fuel cell

A three dimensional transient model fully coupling the two phase flow, species transport, heat transport, and electrochemical processes is developed to investigate the liquid water formation and transport in a polymer electrolyte fuel cell (PEFC). This model is based on the multiphase mixture (M2) formulation with a complete treatment of two phase transport throughout the PEFC, including gas channels, enabling modeling the liquid water motion in the entire PEFC. This work particularly focuses on the liquid water accumulation and transport in gas channels. It is revealed that the liquid water accumulation in gas channels mainly relies on three mechanisms and in the anode and cathode may rely on different mechanisms. The transport of liquid water in the anode channel basically follows a condensation–evaporation mechanism, in sharp contrast to the hydrodynamic transport of liquid water in the cathode channel. Liquid water in the cathode channel can finally flow outside from the exit along with the exhaust gas. As the presence of liquid water in gas channels alters the flow regime involved, from the single phase homogeneous flow to two phase flow, the flow resistance is found to significantly increase.     

Numerical simulation of thermal convection of viscoelastic fluids using the grid-by-grid inversion method

Rayleigh–Bénard convection of viscoelastic fluids in a cavity is investigated using a newly developed grid-by-grid inversion method. In the grid-by-grid inversion method the hyperbolic constitutive equation is split such that the term for the convective transport of stress tensor is treated as a source. This renders the stress tensor a local function of the velocity gradient tensor as in the case of the Newtonian fluids and makes the algorithms for Newtonian fluids applicable to viscoelastic fluids. To corroborate the accuracy of the grid-by-grid inversion method, a linear stability analysis is performed to find the critical Rayleigh number and the domains of Hopf bifurcation and exchange of stabilities in the parameter space. The numerical results from the grid-by-grid inversion method are found to coincide with those of linear stability analysis exactly. Also considered is the standard benchmark problem of viscoelastic flow past a cylinder placed at the center between two plates to confirm the accuracy of the grid-by-grid inversion method.     

Fluid dynamic modeling of junctions in internal combustion engine inlet and exhaust systems

The modeling of inlet and exhaust systems of internal combustion engine is very important in order to evaluate the engine performance. This paper presents new pressure losses models which can be included in a one dimensional engine simulation code. In a first part, a CFD analysis is made in order to show the importance of the den- sity in the modeling approach. Then, the CFD code is used, as a numerical test bench, for the pressure losses models development. These coefficients depend on the geometrical characteristics of the junction and an experi- mental validation is made with the use of a shock tube test bench. All the models are then included in the engine simulation code of the laboratory. The numerical calculation of unsteady compressible flow, in each pipe of the inlet and exhaust systems, is made and the calculated engine torque is compared with experimental measurements.     

燃气镁合金熔化炉内气相燃烧过程的三维数值模拟

对燃气镁合金熔化炉内的三维湍流气相燃烧场特性进行了数值模拟,湍流模型采用了标准k-ε双方程模型,湍流燃烧采用有限速率/涡耗散模型,辐射换热P1模型,数值方法采用SIMPLE算法,实现了燃烧过程的数值再现,特别考察了烧嘴位置和入口流量速度因素对炉内温度场和流动场的影响,并为熔化炉的结构设计提供了一些合理化建议,为其实际运行提供了有益的参考价值。     

Mathematical modeling and the thermal performance of floating cum tilted-wick type solar still

In this paper, mathematical modeling for the thermal performance of floating cum tilted-wick type solar still has been presented. Explicit expressions for the temperatures of various components of the proposed system and its efficiency have been developed. The effect of mass flow rate due to capillary action of jute wick has been investigated for evaporative, convective, and radiative heat transfer from the evaporating surface to the condensing surface.It has been found that the mass flow rate of 2.5 x 10^?3 kg/s is optimum for effective distillation. Also, the effect of absorptivity of the wick surfaces (floating and tilted-wick) on the productivity of the solar still has been studied and found that α_w1 (absorptivity of the tilted-wick surface) and α_wf(absorptivity of the floating-wick surface) of 0.85 have given higher productivity.For enunciation of the analytical results, numerical calculations have been made using meteorological parameters for a typical winter and summer day in Coimbatore.The theoretical results are in good agreement with the experimental results.