振荡作用下气液两相流的流动特性

为研究高强化活塞冷却油腔内气液两相流的振荡流动结构对其强化传热效果的影响,对矩形空腔内气液两相流的振荡流动过程进行了实验研究。结果表明:气液两相流在起振阶段和充分振荡阶段的振荡流动规律有着明显的区别,起振阶段两相流以"爬壁"运动为主,而充分振荡阶段两相流则以"绕壁"运动为主;振荡频率主要影响两相流中气泡的状态和湍流强度,而填充率则主要影响两相流的流动形态和运动规律,在起振阶段振荡频率和填充率对起振阶段时长和最大气泡尺度的变化都有显著影响,而在充分振荡阶段,振荡频率对于气泡尺度的影响要明显高于填充率。     

Two-Phase Flow Modeling in Microchannels and Minichannels

In this article, three different methods for two-phase flow modeling in microchannels and minichannels are presented. They are effective property models for homogeneous two-phase flows, an asymptotic modeling approach for separated two-phase flow, and bounds on two-phase frictional pressure gradient. In the first method, new definitions for two-phase viscosity are proposed using a one-dimensional transport analogy between thermal conductivity of porous media and viscosity in two-phase flow. These new definitions can be used to compute the two-phase frictional pressure gradient using the homogeneous modeling approach. In the second method, a simple semitheoretical method for calculating two-phase frictional pressure gradient using asymptotic analysis is presented. Two-phase frictional pressure gradient is expressed in terms of the asymptotic single-phase frictional pressure gradients for liquid and gas flowing alone. In the final method, simple rules are developed for obtaining rational bounds for two-phase frictional pressure gradient in minichannels and microchannels. In all cases, the proposed modeling approaches are validated using the published experimental data.     

Heat Transfer and Two-Phase Flow during Shell-side Condensation

This paper surveys the evolution of power condenser tube bundle arrangements and examines present-day designs. Condensation heat transfer during shell-side flow is reviewed, including the effects of vapor shear, condensate inundation, noncondensable gases, and enhancement techniques. The difficulties experienced in calculating vapor pressure drop through tube bundles are described, as well as recent attempts to obtain more reliable correlations. The modeling of these phenomena to predict shell-side condenser performance is reviewed, as well as the use of one- and two-dimensional computer codes. Appropriate topics for future research are identified.     

稠密气固两相流动过程模拟的改进模型与应用

提出了模拟稠密气固两相流动的改进模型。湍流流场采用改进的k-ε-εe模型，颗粒的聚合效应采用聚合力的当量直径折算模型计算。将颗粒团作为离散相，研究颗粒团的运动、碰撞、破碎与合并。应用上述模型数值模拟了循环流化床内的稠密气固两相流动。得到了床内气相速度、颗粒团分布、颗粒浓度分布及颗粒团大小分布等详细两相流场信息。计算结果合理，与前人实验结果相符。模拟结果详细揭示了循环流化床内稠密气固两相流动的基本特征。图8表2参8     

Two-Phase Flow and Boiling Heat Transfer in Small-Diameter Tubes

For the development of a high-performance heat exchanger using small channels or minichannels for air-conditioning systems, it is necessary to clarify the characteristics of vapor‐liquid two-phase flow and heat transfer of refrigerants in small-diameter tubes. In this keynote paper, the related research works that have already been performed by the author and coworkers are introduced. Based on the observations and experiments of R410A flowing in small-diameter circular and noncircular tubes with hydraulic diameter of about 1 mm, the characteristics of vapor‐liquid two-phase flow pattern and boiling heat transfer were clarified. In low quality or mass flux and low heat flux condition, in which the flow was mainly slug, the “liquid film conduction evaporation” heat transfer peculiar to small-diameter tubes prevailed and exhibited considerably good heat transfer compared to nucleate boiling and forced convection evaporation heat transfer. The effects of the tube cross-sectional shape and flow direction on the heat transfer primarily appeared in the region of the “liquid film conduction evaporation” heat transfer. A new heat transfer correlation considering all of three contributions has been developed for small circular tubes.     

Two-Phase Flow Distribution to Tubes of Parallel Flow Air-Cooled Heat Exchangers

Abstract

This paper addresses two-phase flow distribution phenomena in multiple header–tube junctions used in heat exchangers. Because of phase separation, it is very difficult to obtain uniform two-phase flow distribution to the branch tubes. The flow distribution is strongly influenced by the header orientation (horizontal or vertical) and the number of branch tubes. Other factors that influence the flow distribution are the flow direction in the header (upflow or downflow), the header shape and tube end projection into the header, and the location and orientation of the inlet and exit connections. The source of maldistribution is the flow in the dividing headers. Work performed by the authors and others (including patents) are discussed. The possibilities for eliminating two-phase flow maldistribution are identified and discussed. This investigation shows that solutions, which provide uniform flow distribution, are very design-specific. Change of the geometry or operating parameters will require modification of the design.     

燃烧室内喷雾两相流场数值研究

采用贴体曲线网格、RNG k-e湍流模型和随机轨道法计算了火焰筒内的喷雾两相流动,构造了包括旋流器在内的主燃烧室计算网格,计算了5种旋流器下的主燃烧室的气流场和喷雾场,分析了不同旋流器对喷雾场和气流场的影响,以及喷雾对气流场的影响,并与实验结果进行了对比。结果显示旋流器叶片角度增大或者外内旋流器流量比的增加,都会导致液滴的喷雾锥角增大。喷雾使气流场的速度减小,旋流强度降低,随着旋流器叶片角度增加或者外内旋流器流量比的增加,喷雾场对气流场的影响增大。提出了主燃烧室内旋流器设计的改进方案。     

Adiabatic Two-Phase Pressure Drop of Refrigerants in Small Channels

The adiabatic pressure drop of two-phase refrigerant flow in small channels has been investigated. A rectangular channel with d _h = 148.0 μm has been tested with four refrigerants: R134a, R410A, propane (R290), and ammonia (R717). These data have been combined with data taken from five different channels, with d _h varying from 70 μm to 305 μm, of R134a. The measured pressure drops have been compared to many published separated-flow and homogeneous pressure drop models. A new correlation for C, the Chisholm parameter, has been developed based on the Reynolds number of the vapor phase (which contains the majority of the kinetic energy) and the dimensionless grouping ψ—a ratio of viscous to surface tension effects taken from the analysis of capillary flow performed by Sou and Griffith (1964). This allows the new correlation to account for the varying fluid properties (including surface tension) that are found in the different refrigerants. The new correlation takes flow regime into account by means of a Weber number based flow transition criteria, following the flow map of Akbar et al. (2003).     

Two-Phase Internal Flow: Toward a Theory of Everything

Two-phase flow and heat transfer is still an area of intense research and great uncertainty. Even severely restricting ourselves to just annular, internal two-phase flow does not significantly improve our chances of accurately predicting either pressure gradients or heat transfer coefficients for an arbitrary tube geometry or fluid. This article summarizes a series of investigations that aim to identify the fundamental governing physics of internal two-phase flow: a Theory of Everything. The techniques developed to do so have been varied, and novel approaches are presented here. At the macro scale, simultaneous visualization and measurement of pressure gradient have led to interesting observations about the relationship between flow regimes and these fundamental macroscale behaviors. Since the macroscale behaviors are governed, for the most part, by behaviors at the micro scale, a number of techniques have been developed to study this near-wall behavior in quantitative detail. In particular, dye-assisted planar laser induced fluorescence has provided the first accurate portrayals of the gas–liquid interface annular flow, and microscale multiphase particle image velocimetry has been used to obtain the velocity within the liquid film of annular flow from within micrometers of the wall to velocities within the waves 500 μm or more from the wall. Statistical analyses of these data point toward a general approach for modeling wall shear in two-phase concurrent internal flow.     

Numerical Investigation of Two-Phase Flow in Natural Gas Ejector

Increasing production and recovery from the mature oil and gas fields often requires a boosting system when the gas pressure is lower than that demanded by the transportation or process system. The supersonic ejector, considered to be a cost-effective way to boost the production of a low-pressure gas well, was introduced into the industrial field. However, the exploitation of natural gas often accompanies with water. The computational fluid dynamics (CFD) technique was employed to investigate the two-phase effect (water droplets) on the performance of natural gas ejector for the motive pressure ranging from 11.0 MPa to 13.0 MPa, induced pressure from 3.0 MPa to 5.0 MPa, and backpressure from 5.1 MPa to 5.6 MPa, while the injected water flow rate was less than 0.03 kg s^?1. The numerical results show that the entrainment ratio of the two-phase operation was higher than that of the single-phase operation with the variation of backpressure. Meanwhile, the entrainment ratio increased with the increase of injected water flow rate into the primary flow. When the water was injected into the secondary flow, the entrainment ratio decreased as the injected water flow rate increased, but the critical backpressure remained unchanged.     

A Coupled Pressure-Based Finite-Volume Solver for Incompressible Two-Phase Flow

This article reports on the formulation and testing of a coupled pressure-based algorithm for the solution of steady incompressible disperse two-phase-flow problems. The method is formulated within a Eulerian-Eulerian framework in the context of a collocated finite-volume scheme. An equation for pressure is derived from overall mass conservation following the segregated mass conservation–based algorithm (MCBA) approach and using an extended two-phase flow form of the Rhie-Chow interpolation technique. The newly developed pressure-based coupled solver differs from pressure-based segregated solvers in that it accounts implicitly for the pressure–velocity and the interphase drag couplings that are present in disperse multiphase flows to yield a system of coupled equations linking the velocity and pressure fields. The performance and accuracy of the coupled multiphase algorithm are assessed by solving eight one-dimensional two-phase flow problems spanning the spectrum from dilute bubbly to dense gas–solid flows. Each problem is solved over three grid systems with sizes of 10,000, 30,000, and 50,000 control volumes, respectively. Results are compared in terms of iterations and CPU time with similar ones generated using the segregated MCBA-SIMPLE algorithm. The newly developed coupled solver is shown to yield substantial decrease in the required number of iterations and CPU time, with the rate of solution acceleration varying between 1.3 and 4.6.     

排粉风机出口两相流动特性

通过利用9-26型高压离心风机,对排粉风机在风机叶轮不同转速、不同出口速度场条件下的风机出口两相流浓度进行了测量,测量发现除内侧附近区域外,沿着风机出口朝向外侧的方向固相浓度逐渐增高.测量结果给利用排粉风机出口的固相浓度分布特性来进行含粉气流的浓淡分离提供了参考.     

Exergy Efficiency of Two-Phase Flow in a Shell and Tube Condenser

This study deals with a comprehensive efficiency investigation of a TEMA “E” shell and tube condenser through exergy efficiency as a potential parameter for performance assessment. Exergy analysis of condensation of pure vapor in a mixture of non-condensing gas in a TEMA “E” shell and tube condenser is presented. This analysis is used to evaluate both local exergy efficiency of the system (along the condensation path) and for the entire condenser, i.e., overall exergy efficiency. The numerical results for an industrial condenser with a steam–air mixture and cooling water as working fluids indicate significant effects of temperature differences between the cooling water and the environment on exergy efficiency. Typical predicted cooling water and condensation temperature profiles are illustrated and compared with the corresponding local exergy efficiency profiles, which reveal a direct (inverse) influence of the coolant (condensation) temperature on the exergy efficiency. Further results provide verification of the newly developed exergy efficiency correlation with a set of experimental data.     

On Recent Advances in Modeling of Two-Phase Flow and Heat Transfer

General thermal design methods for two-phase heat exchangers are emerging that are based on local two-phase flow patterns and the flow structure of the two-phases. These methods promise to be much more accurate and reliable for predicting two-phase heat transfer coefficients and pressure drops than the older, statistically-derived empirical design methods that completely ignore flow regime effects or simply treat flows as stratified (gravity-controlled) or nonstratified (shear-controlled) flows, which greatly limits their accuracy, validity, and reliability and often results in prediction errors surpassing 100% within their supposed range of application. These new flow pattern and flow structure types of design methods are particularly suited for use in modern heat exchanger design software, which are typically incremental and hence require local methods that capture the real trends in experimental data. The status of these new developments is reviewed here for intube two-phase flow and heat transfer processes.     

An Experimentally Validated Model for Two-Phase Sudden Contraction Pressure Drop in Microchannel Tube Headers

This paper presents the results of an experimental investigation of two-phase pressure loss of R134a in microchannel headers using various end-cut techniques. Novel experimental techniques and test sections were developed to enable the accurate determination of minor losses without obfuscating the problem with a lengthwise pressure gradient. This technique represents a departure from approaches used by other investigators that have extrapolated minor losses from air-water experiments and the combined effects of expansion, contraction, deceleration, and lengthwise pressure gradients. In this study, pressure losses were recorded over the entire range of qualities, from 100% vapor to 100% liquid. In addition, the tests were conducted for five different refrigerant mass fluxes between 185 kg/m²-s and 785 kg/m²-s using two different end-cut techniques. More than 790 data points were recorded to obtain a comprehensive understanding of the effects of mass flux and quality on minor pressure losses. High accuracy instrumentation such as coriolis mass flow meters, RTDs, pressure transducers, and real-time data analyses were used to ensure accuracy in the results. The results show that many of the commonly used correlations for estimating two-phase pressure losses significantly underpredict the pressure losses found in compact microchannel tube headers. Furthermore, the results show that the end-cut technique can substantially affect the pressure losses in microchannel headers. A new model for estimating the pressure loss in microchannel headers is presented, and a comparison of the end-cut techniques on the minor losses is reported.     

双流体颗粒-壁面碰撞模型在突扩两相湍流模拟中的应用

由于壁面粗糙度的双流体颗粒—壁面碰撞模型中，没有考虑颗粒—壁面碰撞前后颗粒速度相对于平均速度的差别，因此，修正了由此引起的颗粒雷诺应力的变化，并应用其对轴对称突扩流动进行了数值模拟，着重探讨了不同颗粒相边界条件的影响．结果表明，由于考虑了各方向雷诺应力之间的相互转化、雷诺应力从平均运动中得到能量以及壁面对颗粒运动的衰减作用等因素，包括摩擦系数、恢复系数和壁面粗糙度等影响因素的颗粒—壁面碰撞模型和由此得到的简化模型，给出的结果与实验值符合较好，而通常使用的零梯度颗粒壁面边界条件则给出失真的模拟结果。     

Comparison of Void Fraction Correlations for Different Flow Patterns in Upward Vertical Two-Phase Flow

A comparison of the performance of 52 void fraction correlations was made based on an unbiased experimental data set of 1208 data points. A comprehensive literature search was undertaken for the available void fraction correlations and experimental void fraction data for upward vertical two-phase flow. The performance of the correlations in correctly predicting the diverse data set was evaluated. Comparisons between the correlations were made and appropriate recommendations were drawn. The analysis showed that most of the correlations developed are very restricted in terms of handling a wide variety of data sets. Based on this analysis, void fraction correlations with the best predictive capability are highlighted.     

Fundamental Issues Related to Flow Boiling and Two-Phase Flow Patterns in Microchannels – Experimental Challenges and Opportunities

ABSTRACT

Flow boiling heat transfer in microchannels is used today in many diverse applications. The previous studies addressing the effect of channel size, heat flux, vapor quality, and mass flux on heat transfer during flow boiling are reviewed in the present paper. The relationship between flow characteristics and flow boiling heat transfer was studied experimentally for refrigerant R-C318 at moderate reduced pressures where the contribution of nucleate boiling is decisive. Flow boiling mechanisms were identified using an annular microchannel with transparent outer wall for successive visualization of boiling. The considerable suppression of nucleate boiling heat transfer was observed at transition to annular flow and explained by formation of a liquid flow with thin film and dry spots. A general equation for prediction of two-phase flow boiling heat transfer inside the circular, annular, and rectangular microchannels is proposed and verified using the experimental data. This equation accounts for the nucleate boiling suppression, forced convection, and thin film evaporative heat transfer in the form that allows to distinguish more clearly the contribution of each mechanism of heat transfer under the conditions, when it is predominant. A new approach for prediction of transition to the annular flow is proposed and verified, using the experimental data.     

On a Novel Dual-Grid Level-Set Method for Two-Phase Flow Simulation

In this work, a hypothesis is proposed for two-phase flow simulation: The number of grid points required for the grid-independent solution of the flow field is lesser than that required to capture the grid-independent interface. Based on this, a dual-grid level-set(LS) method (DGLSM) is proposed, where the LS and energy equations are solved on a uniform grid which is twice finer than the continuity and momentum equations. This novel grid arrangement is aimed at improving interface resolution with only a slight increase in computational time. Qualitative and quantitative results of the DGLSM are compared against analytical, experimental, and numerical results on three stringent test problems: dam break, Rayleigh-Taylor instability, and film boiling. The results show that the DGLSM is accurate and robust, even for surface tension-dominant flows. Moreover, the hypothesis is corroborated by showing that the DGLSM is nearly as accurate as the completely refined traditional LS method at substantially less computational expense.