恒热流竖壁降膜发展段流动换热分析

采用边界相似解对恒热流竖壁下降液膜发展中的层流流动与换热特性进行了分析,获得了发展段长度,液膜厚度和无量纲换热系数的计算表达式。     

进口湍流对透平叶栅内部流动影响的数值研究

通过求解N S方程,分别采用Spalart Allmaras与k eps 湍流模型,对VKI低速轴流透平叶栅内部的湍流进行了数值分析。与实验结果比较,两种湍流模型计算结果与实验是一致的。叶栅流动损失的构成、分布以及计算准确性的主要因素是来流边界层状态。入口边界层是端部损失的主要来源。在高速流动条件下,叶栅损失主要是吸力面边界层径向串流及其尾迹。图14参8     

转捩模型在静叶栅三维粘性流数值模拟中的应用研究

边界层转捩流动的特征参数明显不同于层流或湍流，转捩流动的这些特征直接影响着流体的动力、传热特性。而通过有效地控制转捩起点、转捩区长度就可以有效改善流动效率。作者将修正的转捩模型应用到实际流场计算中，对计算结果进行了比较，从中得到了一些有意义的分析结果。     

横管降膜过程中液膜内流动和传热特性分析

横管降膜流动过程中,液膜速度和温度及其分布是影响传热传质的关键因素,由于实验研究方法的局限性,实验研究结果一般只是液膜内各参数的平均值,而液膜内部的速度和温度具体分布特性却很难得到。借助FLUENT软件,利用VOF模型研究了橫管外液膜速度和温度及其分布特性。通过建立三维数理模型,模拟研究了常温常压下,橫管外液膜无相变条件下横管液膜的传热过程,并从边界层的角度解释了液膜波动对传热过程的影响。     

边界层理论在陡坡坡面流速计算中的应用

坡面流是研究土壤侵蚀机理的关键,流速计算受多因素影响尤显复杂,为此提出了应用陡坡紊流边界层名义厚度方程求解坡面流流速的分布,探讨了经典边界层理论模型,修正了传统边界层的势流流速,推导出适用于描述坡面流流动特性的理论模型,考虑到下垫面及水温等的影响,构建了分类计算平均流速的计算模型,并验证了计算模型的可行性。     

零压力梯度平板边界层转捩的数值模拟

对平板边界层转捩试验T3A和T3B进行了数值模拟,计算值与实验值吻合的较好。通过对零压力梯度的平板转捩现象的研究得出以下的结论:转捩流动的复杂性对流场边界层的影响不容忽视;M-L转捩模型能比较准确地预测转捩的发生和发展过程,在高湍流度下使用全湍流模型来模拟转捩流动的误差较小。     

循环流化床锅炉燃烧室边界层的实验研究

循环流化床锅炉燃烧室的物料浓度影响传热和燃烧,在75t/h循环流化床锅炉上对燃烧室物料浓度分布的测量结果发现,循环流化床锅炉燃烧室的下降流边界层中在接近壁面处存在气体边界层,气体边界层的厚度与距离布风板高度有关,据此给出了下降流边界层厚度和气体边界层厚度的计算公式,并提出了循环流化床燃烧室双环模型。     

柴油机缸内壁面边界层的数值模拟

利用二维边界层微分方程数值模拟了缸内壁面速度和热边界层,获得了不同工况下的速度和热边界层厚度,将二维边界层微分方程的数值计算结果和在TY1100柴油机上的测量结果进行了对比分析,结果表明,二者的变化趋势是一致的,建立的数值模拟模型可以较好地预测缸内壁面速度和热边界层的变化规律．     

水平管外降膜蒸发微观传热特性

为了深入探究水平管降膜蒸发的微观传热特性,采用基于VOF法的计算流体模型对水平管外降膜蒸发进行数值模拟,通过求解控制方程得到液膜内的温度场和速度场。分析了不同入口边界温度和Re数下管外薄液膜内热边界层、无量纲温度和局部传热系数的微观传热特性变化规律,定量给出了热发展区与充分热发展区的边界位置。模拟结果表明:液膜入口温度越高,液膜热发展区覆盖的圆周角度越小;液膜内的热发展区覆盖的角度随Re数的增大而增加是平均传热系数随Re数增大的原因;管外圆周方向无量纲温度分布证明了液膜中的传热包含导热和对流传热;管外液膜内纯导热系数与局部传热系数的差值随倾斜角的增加而减少是由于对流效应沿管圆周方向减弱引起的。     

水平单管外混合气体对流冷凝换热的理论研究

采用修正的膜模型与Nusselt凝结理论结合的方法,对含湿混合气体自上而下横掠水平管外时的对流冷凝换热机理进行研究,建立了液膜流动和传热模型,进行数值求解并分析了雷诺数、壁面温度及水蒸汽浓度等因素对混合气体冷凝换热的影响。计算结果表明:水管外壁液膜厚度分布很大程度上受气体边界层对液膜剪切力的影响。而局部努谢尔数不同于纯蒸气的的冷凝换热,它受气相热阻的影响很大,其分布状况类似于单相气体管外的对流换热。     

竖壁降膜波动特性数值模拟研究

建立了竖直壁面降膜流动的二维几何模型,运用VOF方法对雷诺数200～1 000的降膜的波动特性进行了数值模拟。研究了液膜的波动特征、液膜流动方向的速度变化以及液体雷诺数对液膜波动的影响规律。结果表明:根据液膜的形态可以将流动区域分为入口区、发展区和稳定区三部分。入口区的液膜相对比较平滑,发展区的液膜表现为频率较高的小幅波动,稳定区的液膜波动幅度增加而频率减小;入口区的最大流速大于初始流速,发展区的最大流速在初始流速上下波动,而稳定区流速小于初始流速;随着液体雷诺数的增加,液膜厚度增加而波幅降低。     

The numerical computation of the evaporative cooling of falling water film in turbulent mixed convection inside a vertical tube

An analysis has been developed for studying the evaporative cooling of liquid film falling inside a vertical insulated tube in turbulent gas stream is presented. Heat and mass transfer characteristics in air–water system are mainly considered. A low Reynolds number turbulence model of Launder and Sharma is used to simulate the turbulent gas stream and a modified Van Driest model suggested by Yih and Liu is adopted to simulate the turbulent liquid film. The model predictions are first compared with available experimental data for the purpose of validating the model. Parametric computations were performed to investigate the effects of Reynolds number, inlet liquid temperature and inlet liquid mass flow rate on the liquid film cooling mechanism. Results show that significant liquid cooling results for the system with a higher gas flow Reynolds number Re, a lower liquid flow rate Γ₀ or a higher inlet liquid temperature T_L0.     

斜槽管降膜吸收的数学模型及数值计算

提出一种新的计算低肋斜槽管管外单管降膜吸收时的数学模型,即把肋片形状为等边三角形、肋片高度<0 5mm的斜槽管管外液膜看成是由两部分———内层和外层组成的。通过理论分析,得出了内层液膜内的速度、温度和浓度分布的解析解,外层用数值计算的方法求解,内层和外层之间采用二阶藕合法,使内、外层之间能光滑过渡。应用上述数学模型,研究了这种传热管管外液膜中的速度、温度和浓度分布,讨论了肋片高度和肋片倾角对传热传质性能的影响。通过计算值与实验值的比较发现二者吻合较好。     

Lattice Boltzmann Simulation of Falling Film Flow under Low Reynolds Number

ABSTRACT

In this paper, a new two-dimensional simulation model was developed for the falling film flow under low Reynolds number (below 20). The phase-field multiphase lattice Boltzmann model was developed to simulate the flow pattern of the two-phase falling film with high density ratio. The approaches to treating the liquid-gas interface with high density ratio (up to 775), surface tension, gravity, inlet and outlet open boundary conditions as well as solid-liquid interface considering contact angle were developed firstly, and then implemented in the model. The dynamic characteristics of the film flow, including the development of the liquid-gas interface and the film thickness, were simulated under the Reynolds numbers between 1.0 and 20. The results show that the film is fully laminar under low Reynolds numbers. The falling film flow model developed in this study lays the foundation for the study of heat and mass transfer in the falling film based liquid desiccant dehumidifier.     

Stability of conducting viscous film flowing down an inclined plane with linear temperature variation in the presence of a uniform normal electric field

Weakly nonlinear stability analysis of a thin liquid film falling down a heated inclined plane with linear temperature variation in the presence of a uniform normal electric field has been investigated within the finite amplitude regime. A generalized kinematic equation for the development of free surface is derived by using long wave expansion method. A normal mode approach and the method of multiple scales are used to investigate the linear and weakly nonlinear stability analysis of film flow, respectively. It is found that both Marangoni and electric Weber numbers have destabilizing effect on the film flow. The study reveals that both supercritical stability and subcritical instability are possible for this type of film flow. It is interesting to note that both the Marangoni and electric Weber numbers have qualitatively same influence on the stability characteristics but the effect of Marangoni number is much stronger compare to the electric Weber number. Scrutinizing the effect of Marangoni and electric Weber numbers on the amplitude and speed of waves it is found that, in the supercritical region amplitude and speed of the nonlinear waves increases with the increase in Marangoni and electric Weber numbers, while in the subcritical region the threshold amplitude decreases with the increase in Marangoni and electric Weber numbers. Finally, we obtain that spatially uniform solution is side-band stable in the supercritical region for our considered parameter range.     

CFD simulation of the two-phase flow for a falling film microreactor

Falling film microreactors, which provide very high specific interfacial area, have become a promising solution to the fast and strongly exothermic/endothermic gas–liquid reaction systems. A computational fluid dynamic simulation of the two-phase flow for a falling film microreactor is presented using the volume of fluid (VOF) model. The hydrodynamic characteristics, from both 2-D and 3-D simulations, including liquid film thickness, velocity, pressure and shear stress profiles, are analyzed. 2-D simulation is adopted for the study of the relationship of liquid flow rate and film thickness, as well as the effects of gas flow rate, surface tension, liquid viscosity and pressure difference on the liquid flow rate. 3-D simulation is necessary to provide the comprehensive flow profiles. Although the system is in the laminar flow regime, the liquid film features a wavy structure and the velocity profiles are complex.     

A study on flow and heat transfer characteristics for saturated falling-film evaporation of liquid oxygen in a vertical channel

In this study, a mathematical model for the laminar falling film is presented in order to simulate the evaporation heat transfer characteristics in falling liquid oxygen films. The model takes into account the effect of the interfacial shear. The values of the film thickness, the heat transfer coefficient as well as the interfacial shear are obtained under given conditions by solving the model with an iteration method. The influences of the inlet Reynolds number, channel length and the interfacial shear on the flow and heat transfer characteristics of the falling film evaporation are analyzed in detail. Effects of key factors on the circulation ratio of the inlet fluid mass flow rate to the generated vapor mass flow rate, an important design parameter for reboilers/condensers, are particularly analyzed. In addition, the variations of the average vapor velocity and interfacial film velocity are also discussed. The analysis results could provide theoretical guidance for the simulation and design of downflow reboilers/condensers applied in air separation units.     

沿垂直壁面气-液降膜流动传质过程的数值研究

在考虑气-液两相间质量源项和能量源项的条件下,基于VOF算法,建立了水和空气沿竖直平板壁面两相降膜流动传热传质的CFD模型。利用该模型研究了水和空气两相间的传质特性,分析了液膜波动、进气进液速度以及温度对传质的影响,计算结果表明:一定程度的液膜波动、进气速度和进液速度的提高、气-液之间的温差的增加,都能强化气-液之间的传质过程。     

Numerical investigation of effective parameters of falling film evaporation in a vertical-tube evaporator

Wastewater treatment is one of the most effective solutions to manage the problem of water scarcity. Falling film evaporators are excellent technology in wastewater treatment plants. These wastewater evaporators provide high heat transfer, short residence time in the heating zone, and high-purity distilled water. In the present study, the mechanism of turbulent falling film evaporation in a vertical tube has been investigated. A model has been developed for symmetrical two-dimensional pure and saline water flow in a vertical tube under constant wall heat flux. The numerical simulation has been carried out by a commercial computational fluid dynamics code. The evaporation of saturated liquid film is simulated utilizing a two-phase volume of fluid method and Tanasawa phase-change model. The main objective of this study is to evaluate the effects of water salinity, liquid Reynolds number, wall heat flux, and liquid film thickness on the two-phase heat transfer coefficient and vapor volume fraction. The numerical heat transfer coefficients are compared with the obtained results by Chen's empirical correlation. With a MAPE ≤ 11%, this study proves that the numerical method is highly effective at predicting the heat transfer coefficient. Moreover, the empirical coefficient of the Tanasawa model and the minimum thickness of the falling film are determined.     

自由表面摩擦和蒸发对过冷下降液膜传热的影响

从理论上对下降液膜在自由表面上存在反向剪切力和蒸发散热情况下的换热特性进行了分析,得到了膜厚、换热系数的无量纲关系式,讨论了剪切力、液膜雷诺数、壁面热流、蒸发率对流动和传热的影响。