Condensation of a vapor-noncondensable gas mixture within a horizontal air-cooled tube

The equations governing the coupled heat and mass transport mechanism are derived for the condensation of a pure vapor—non condensable gas mixture within a horizontal finned tube cooled by air in cross flow. Assuming that temperatures of both the gaseous and liquid phase vary linearly in a short length tube element, where the problem is posed, a couple of equations are obtained for the exit temperatures of the streams as a function of the inlet and the interphase ones. These equations can be handled iteratively as a sub-routine of a simulation program already implemented by Urbicain and Paloschi (1). The heat transfer mechanism is governed by an overall coefficient U defined between the main gaseous stream and the cooling air, calculated from the individual resistances which operate on two different sections of the condensing film. The procedure has been succesively tested on a water vapor-carbon dioxide finned air cooled condenser and represents a generalization of the step by step program mentioned above.     

Investigation on heat transfer characteristics of aircraft icing including runback water

The heat transfer characteristics of aircraft icing process were investigated based on the theories of liquid–solid phase change and film flow. The heat transfer model which couples runback water flow with liquid–solid phase change was established and the influence of airflow parameters on the characteristics of icing growth was analyzed. The results indicate that the runback water on the icing surface will accelerate the liquid–solid phase change and the icing process. The shear stress caused by the airflow is the key factor to the runback water flow. The higher the airflow velocity, the greater the shear stress and stronger the runback water flow. Under the condition of runback water flow, the velocity and temperature of the airflow are the main causes effecting on the icing accretion. The higher the airflow velocity or the lower the temperature is, the greater the icing rate will be. The liquid water content (LWC) and the collection efficiency have weak effect on the icing rate comparatively.     

Optimal distribution of cooling during gas compression

This paper describes the optimization of the distribution of heat transfer (cooling) during the process of gas compression. The coolant is a stream of cold liquid. There is a fundamental tradeoff between the savings in compressor power, which are due to distributed cooling, and the pumping power required to circulate the coolant. The tradeoff is revealed on the basis of a combined model of multi-stage gas compression, resistance to fluid flow, and area-constrained counter- or co-current heat exchange between the gaseous stream and the liquid stream. The results are illustrated for the compressor of an actual ammonia refrigeration plant, for which the distributed-cooling design is highly recommended because the compressor discharge temperature in such units is high. It is shown that there is an optimal coolant (water) flow rate such that the total power requirement is minimized. The optimized distribution of gas compression and cooling is robust with respect to the selection of the water flow rate.     

A droplet size dependent multiphase mixture model for two phase flow in PEMFCs

A droplet size dependent multiphase mixture model is developed in this paper, and the droplet size in the gas channel can be considered as a parameter in this multiphase mixture model, which includes the effect of gas diffusion layer (GDL) properties and the gas drag function and cannot be considered in the commonly used multiphase mixture model in the references. The three-dimensional two phase and non-isothermal simulation of the PEMFCs with a straight flow field is performed. The effect of droplet size on the liquid remove, the effect of liquid water on the heat transfer and the effect of gas flow pattern on the heat and mass transfer are mainly investigated. The simulation results show that the large droplet is hard to be dragged by the gas, so it produces large water saturation. The results of the heat transfer show that the liquid water hinders the heat transfer in the GDL and catalyst layer, so it produces the large relative high temperature area, and there are large temperature difference and water saturation in the PEMFCs operated with coflow pattern compared with counter flow pattern.     

Numerical Study on Phase Change of Water Flowing Across Two Heated Circular Cylinders in Tandem Arrangement

This paper focuses on fluid flow and heat transfer analysis over two heated cylinders arranged in tandem. The flow of water over heated cylinders faces a phenomenon of phase change from liquid (water) to vapor phase (steam). The mechanism of this phase change is studied through a numerical simulation supplemented with verification and validation of the code. The problem is simulated when flows from two cylinders in a tandem arrangement become interacting and noninteracting. The Eulerian model is used during simulation to comprehend the multiphase phenomena. The effect of spacing between these two cylinders and the Reynolds number effect are studied in this paper. The volume fractions of both the water and vapor phases and the heat transfer coefficients of both the cylinders have been computed and presented as findings of the problem.     

Heat transfer enhancement in a small pipe by spinodal decomposition of a low viscosity,liquid–liquid,strongly non-regular mixture

We report experimental evidence of a 20–40 % enhancement of the effective heat transfer coefficient for laminar flow of a partially miscible binary liquid–liquid mixture in a small diameter horizontal tube that obtains when phase separation occurs in the tube. A mixture of acetone–hexadecane is quenched into the two-phase region so as to induce spinodal decomposition. The heat transfer rate is enhanced by self-induced convective effects sustained by the free energy liberated during phase separation. The experimental heat transfer coefficients obtained when separation occurs are compared to the corresponding values predicted for flow of a hypothetic mixture with identical properties but undergoing separation. For such comparison, the energy balance equation must carefully take into account both the sensible heat and the excess enthalpy difference between the inlet and the outlet streams because our liquid–liquid binary mixture is a very asymmetric system with large excess enthalpies. The non-ideal mixture thermodynamic properties needed for the energy balance are obtained by an empirical procedure from the experimental data available in the literature for our mixture. The experimental setup and calculation procedure is tested by experiments performed using single-phase water flow and single-phase mixture flow (above the critical point). The convective heat transfer augmentation that results in the presence of liquid–liquid phase separation may be exploited in the cooling or heating of small scale systems where turbulent convection cannot be achieved.     

一种适用于缸盖水腔沸腾传热计算的模型

基于VC 6.0开发了一种单相流沸腾传热模型,通过引入空泡份额的概念将沸腾发生时的流场看作一个气液均匀混合的单相流,从数学上对该模型进行了描述并介绍了模型的数值实现方法。通过与实验结果的对比,表明模型适用于缸盖冷却水腔内沸腾传热计算。实验和计算结果还表明,压力对沸腾传热的影响较为明显。最后以226B型发动机水腔为工程应用对象,计算出了水腔内的空泡份额分布和水腔内的流度分布情况。     

相变微胶囊悬浮液强制对流换热实验研究

提出了一种可以同时作为储能介质与传热流体的新型相变微胶囊悬浮液(MPCS),设计和搭建试验台,分别在层流和湍流条件下在等热流密度的光滑圆管中对MPCS进行了强制对流换热实验,研究了悬浮液浓度、流量、泵送功率和加热速率对其流动及传热特性的影响。结果表明:对于质量分数为5%的MPCS,当微胶囊中相变材料分别处于固体、固体-液体和液体状态时,对应的努塞尔数平均增大了23.9%、20.5%和9.1%;与纯水相比,MPCS作为在热力系统应用的传热流体可以实现强化传热,但是需要在传热实验中控制好相变过程才能使MPCS的传热性能优于水。     

Numerical simulation of three-dimensional gas/liquid two-phase flow in a proton exchange membrane fuel cell

Investigation into the formation and transport of liquid water in proton exchange membrane fuel cells (PEMFCs) is the key to fuel cell water management. A three-dimensional gas/liquid two-phase flow and heat transfer model is developed based on the multiphase mixture theory. The reactant gas flow, diffusion, and chemical reaction as well as the liquid water transport and phase change process are modeled. Numerical simulations on liquid water distribution and its effects on the performance of a PEMFC are conducted. Results show that liquid water distributes mostly in the cathode, and predicted cell performance decreases quickly at high current density due to the obstruction of liquid water to oxygen diffusion. The simulation results agree well with experimental data. Translated from J Tsinghua Univ (Sci & Tech), 2006, 46(2): 252–256 [译自: 清华大学学报]     

A theoretical study of evaporative heat transfer in high‐velocity two‐phase flow of air–water in a small vertical tube

A theoretical study was performed to investigate the evaporative heat transfer of high‐velocity two‐phase flow of air–water in a small vertical tube under both heating conditions of constant wall temperature and constant heat flux. A simplified two‐phase flow boundary layer model was used to evaluate the evaporative heat transfer characteristics of the annular two‐phase flow. The analytical results show that the gravitational force, the gas–liquid surface tension force, and the inertial force are much smaller than the frictional force and hence can be neglected for a small tube. The evaporative heat transfer characteristics of the small tube with constant wall temperature are quite close to those of the small tube with constant heat flux. The mechanism of the heat transfer enhancement is the forced convective evaporation on the surface of the thin liquid film. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 430–444, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10110     

Numerical study of liquid film cooling in a rocket combustion chamber

A numerical study is reported to investigate the liquid film cooling in a rocket combustion chamber. Mass, momentum and heat transfer characteristics through the interface are considered in detail. A marching procedure is employed for solution of the respective governing equations for the liquid film and gas stream together. The standard turbulence k–ε model is used to simulate the turbulence gas flow and a modified van Driest model is adopted to simulate the turbulent liquid film flow. Radiation of gas stream is also considered and simulated with the flux model. Downstream of the liquid film the gaseous film cooling is numerically studied simultaneously. Results are presented for a mixed gases–water system under different condition. Various effects on the liquid film length are examined in detail. There is a good agreement between the numerical prediction and experimental result on the liquid film length.     

FINITE ELEMENT ANALYSIS OF COUPLED HEAT AND MOISTURE TRANSFER IN CONCRETE SUBJECTED TO FIRE

A system of coupled transient differential equations governing heat, mass transfer, and pore pressure built up in porous media (concrete), subjected to intensive heating, is derived. Water vapor and liquid water are considered separately in the mass transfer formulation. The primary unknowns are temperature, water vapor content, and pore pressure of the gaseous mixture. A finite element formulation and corresponding flowchart of computations of all required data are presented. The numerical example solved represents a cross section of a concrete column exposed to fire. The domain and time distributions of temperature, pore pressure, water vapor, and liquid water content are presented. Computed pore pressure is higher than those usually reported by other analytical studies. The influence of some initial parameters (permeability, initial water content, and porosity) on maximum pore pressure is investigated.     

Numerical investigation of the optimal Nafion ionomer content in cathode catalyst layer: An agglomerate two-phase flow modelling

A two dimensional, across the channel, isothermal, two-phase flow model for a proton exchange membrane fuel cell is presented. Reactant transport in porous media, water phase transfer and water transport through the membrane are included. The catalyst layer is modelled as a spherical agglomerate structure. Liquid water occupies the secondary pores of the cathode catalyst layer to form a liquid water coating surrounding the agglomerate. The thickness is calculated by coupling the two-phase flow model with the agglomerate model. Ionomer swelling is associated with the non-uniform distribution of water in the ionomer determined from several processes occurring simultaneously, namely (1) water phase transfer between the vapour, dissolved and liquid water; (2) membrane/ionomer water content depending on the water vapour pressure; (3) a water film covering the catalyst agglomerate; (4) water transport through the membrane via electro-osmotic drag, back diffusion and hydraulic permeation. The model optimises the initial dry ionomer content in the cathode catalyst layer. The simulation results indicate that, to achieve the best fuel cell performance, the initial dry ionomer volume fraction should be controlled around 10%, corresponding to 0.3 mg cm⁻². By considering the effect of ionomer swelling on the reduction in CCL porosity and the increase in oxygen mass transport resistance, the accuracy of the model prediction is improved, especially at higher current densities.     

Modeling of hydrodynamics in microchannel reactor for Fischer–Tropsch synthesis

Hydrodynamics of liquid and gaseous products in microchannel reactor for Fischer–Tropsch synthesis is considered. It is supposed, that liquid and gaseous products of the synthesis move downward in annular flow regime. A microchannel with irregular internal walls is investigated in cylindrical symmetry. In proposed numerical technique the peculiarities of coating the microchannel walls by cobalt-based catalytic particles are taken into account. System of equations of two-phase hydrodynamics is based on the generalized equations for mass flowrate and momentum of liquid film and gaseous phase. Stable numerical algorithm for calculation the thermodynamic equilibrium in gas–liquid mixtures of synthesis products is proposed. Calculation results illustrate thermophysical properties of liquid and gaseous products. In the hydrodynamics model variations along a microchannel mass fractions and thermophysical properties of liquid and gaseous products were taking into account. Principal hydrodynamical difference between a smooth microchannel and a microchannel with random roughness is explained. Hydrodynamical parameters and gradient of pressure are investigated as functions of pressure, temperature, averaged diameter of a microchannel and chain growth probability.     

ZnO对不同流态下相变微胶囊悬浮液对流换热特性影响研究*

相变微胶囊悬浮液（MPCS）可作为热交换介质和储热流体,但其导热率较低导致其应用受到一定的限制。以水为基液使用相变微胶囊（MPCM）制备MPCS,加入氧化锌（ZnO）颗粒以提高MPCS导热率。使用旋转流变仪、差式热量扫描仪、导热仪分别测定了MPCS的黏度、相变潜热和导热系数等物理性质。设计并搭建了试验台,在内径6 mm的圆管中,使用水、MPCS以及ZnO@MPCS在层流和湍流下进行强制对流换热实验,通过对比其换热情况分析ZnO对MPCS换热特性的影响。结果表明：加入ZnO的MPCS具有良好的储热性和导热性,1%ZnO@5%MPCS导热系数较5%MPCS提高了17.9%。层流条件下MPCS的平均局部换热系数低于水,1%ZnO@5%MPCS平均局部换热系数比水高6.5%;湍流时,1%ZnO@5%MPCS在相同质量流量和功率下的平均局部换热系数相较于水提高了15.7%。     

Modeling two-phase flow in PEM fuel cell channels

This paper is concerned with the simultaneous flow of liquid water and gaseous reactants in mini-channels of a proton exchange membrane (PEM) fuel cell. Envisaging the mini-channels as structured and ordered porous media, we develop a continuum model of two-phase channel flow based on two-phase Darcy's law and the M² formalism, which allow estimate of the parameters key to fuel cell operation such as overall pressure drop and liquid saturation profiles along the axial flow direction. Analytical solutions of liquid water saturation and species concentrations along the channel are derived to explore the dependences of these physical variables vital to cell performance on operating parameters such as flow stoichiometric ratio and relative humility. The two-phase channel model is further implemented for three-dimensional numerical simulations of two-phase, multi-component transport in a single fuel-cell channel. Three issues critical to optimizing channel design and mitigating channel flooding in PEM fuel cells are fully discussed: liquid water buildup towards the fuel cell outlet, saturation spike in the vicinity of flow cross-sectional heterogeneity, and two-phase pressure drop. Both the two-phase model and analytical solutions presented in this paper may be applicable to more general two-phase flow phenomena through mini- and micro-channels.     

Flow investigation of phase change material (PCM) slurry as a heat transfer fluid in a closed loop system

Aqueous phase change material (PCM) particles are dispersed in an organic phase to constitute a slurry for using as a cold heat transfer medium for district cooling in refrigeration and air conditioning industry. The PCM contains 90% of water stabilized by a three‐dimensional network of polymer. The flow behaviour of the slurry is investigated in a small‐scale loop circuit with transparent pipes to allow observation of flow patterns. Data show that pressure drop increases with velocity and decreases with temperature, which can be explained by heterogeneities in flow for temperature higher than 0°C and for Reynolds number (based on the properties of the liquid phase) lower than 7000. A homogeneous particle field is observed for Reynolds number up to 7000, which guarantees a safe operation of the system without the occurrence of clogging in ducts. For this range of flow, the flow rate and the pump consumption for the PCM slurry decrease notably for the same heat transportation quantity compared with chilled water. Copyright © 2008 John Wiley & Sons, Ltd.     

Water management studies in PEM fuel cells,part III: Dynamic breakthrough and intermittent drainage characteristics from GDLs with and without MPLs

The transport of liquid water and gaseous reactants through a gas diffusion layer (GDL) is one of the most important water management issues in a proton exchange membrane fuel cell (PEMFC). In this work, the liquid water breakthrough dynamics, characterized by the capillary pressure and water saturation, across GDLs with and without a microporous layer (MPL) are studied in an ex-situ setup which closely simulates a real fuel cell configuration and operating conditions. The results reveal that recurrent breakthroughs are observed for all of the GDL samples tested, indicating the presence of an intermittent water drainage mechanism in the GDL. This is accounted for by the breakdown and redevelopment of the continuous water paths during water drainage as demonstrated by Haines jumps. For GDL samples without MPL, a dynamic change of breakthrough locations is observed, which originates from the rearrangement of the water configuration in the GDL following the drainage. For GDL samples with MPL, no dynamic change of breakthrough location can be found and the water saturation is significantly lower than the samples without MPL. These results suggest that the MPL not only limits the number of water entry locations into the GDL (such that the water saturation is drastically reduced), but also stabilizes the water paths (or morphology). The effect of MPL on the two-phase flow dynamics in gas channels is also studied with multi-channel flow experiments. The most important result is that GDL without MPL promotes film flow and shifts the slug-to-film flow transition to lower air flow rates, compared with the case of GDL with MPL. This is closely related to the larger number of water breakthrough locations through GDL without MPL, which promotes the formation of water film.     

Numerical study of the hydrodynamics and heat transfer characteristics of liquid–liquid Taylor flow in microchannel

A numerical study of an oil–water Taylor flow is presented in this paper to explore its flow and heat transfer characteristics. Due to the large surface area to volume ratio in narrow channels, using slug flows, high heat and mass transfer rates could be achieved. Sound knowledge of the underlying physics of slug flow is required for the practical design of microfluidic devices. In this study, hydrodynamics and heat transfer characteristics of dispersed oil droplets flowing inside a vertically upward circular microchannel (D = 0.1 mm) with water being the carrier phase have been explored numerically. ANSYS Fluent was employed to capture the liquid–liquid interface using volume of fluid method. Two different boundary conditions were considered in the present study. First, an isothermal wall of 373 K and later a constant wall heat flux (420 kW/m²) were, respectively, prescribed over the wall of the microchannel. The numerical code was validated against the results available in the literature, and the significant results in the form of pressure drop and heat transfer rates have been discussed. A considerable increase in Nusselt number, up to 180% and 210%, was observed with the oil–water slug flow in contrast to the liquid‐only single‐phase flow inside the microchannel for isothermal and constant wall heat flux conditions, respectively.     

Numerical simulation of complex flow and heat transfer induced by localized laser heating on a urethane-coated substrate

A three-dimensional numerical simulation is conducted for complex flow and heat transfer that incorporate solid–liquid–vapor phase change and surface chemical reaction induced by localized laser heating on a urethane-coated stainless-steel substrate. The surface chemical reaction due to laser irradiation on the urethane-coated stainless-steel substrate, and heat and mass transfer due to melting/vaporization of the stainless steel are considered. The entire problem is solved within one computational domain that includes two solid regions and one gaseous region through a penalty method. One of the solid region is the paint that will decompose via chemical reaction to generate gaseous products and then mix with the air, and the other one is the stainless steel that melting and vaporization can occur due to extremely high temperature in the process. Moreover, the gas phase is considered as a multicomponent system that consists of O₂, N₂, CO₂, H₂O, NO₂, binder vapor, and stainless-steel vapor. In the present multiphysics simulation, the process of melting, vaporization and chemical reaction and the splash of the melted paint and stainless steel into the gas is observed.