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 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.     

Flow maldistribution and thermal performance deterioration in a cross-flow air to air heat exchanger with plate-fin cores

The inlet and outlet duct geometry in an air to air compact heat exchanger is always irregular. A skewed Z-type arrangement is popular between the impinging flow and the core. Such duct placements usually lead to a non-uniform flow distribution on core surface. In this research, the flow maldistribution and thermal performance deterioration in cross-flow air to air heat exchangers are investigated. The inlet duct, the core and the outlet duct are combined together to calculate the flow distribution on core inlet face. First, a CFD code is used to calculate the flow distribution, by treating the plate-fin core as a porous media. Then a heat transfer model between the two air flows in the plate-fin channels is set up. Using the flow distribution data predicted, the heat exchange effectiveness and the thermal performance deterioration factor are calculated with finite difference scheme. Experiments are performed to validate the flow distribution and heat transfer model. The results indicate that when the channel pitch is below 2.0 mm, the flow distribution is quite homogeneous and the thermal deterioration due to flow maldistribution can be neglected. However, when the channel pitch is larger than 2 mm, the maldistribution is quite large and a 10–20% thermal deterioration factor could be found. The study proves that the inlet duct, the outlet duct, and the core should be coupled together to clarify flow maldistribution problems.     

Analysis of evaporating mist flow for enhanced convective heat transfer

Enhancement of forced convective heat transport through the use of evaporating mist flow is investigated analytically and by numerical simulation. A two-phase mist, consisting of finely dispersed water droplets in an airstream, is introduced at the inlet of a longitudinally-finned heat sink. The latent heat absorbed by the evaporating droplets significantly reduces the sensible heating of the air inside the heat sink which translates into higher heat-dissipation capacities. The flow and heat transfer characteristics of mist flows are studied through a detailed numerical analysis of the mass, momentum and energy transport equations for the mist droplets and the airstream, which are treated as two separate phases. The coupling between the two phases is modeled through interaction terms in the transport equations. The effects of inlet mist droplet size and concentration on the thermal performance of the heat sink are analyzed parametrically. The results provide insight into the complex transport processes associated with mist flows. The simulations indicate that significantly higher heat transfer coefficients are obtained with mist flows as compared to air flows, highlighting the potential for the use of mist flows for enhanced thermal management applications.     

Two-phase flow and transport in the air cathode of proton exchange membrane fuel cells

Two-phase flow and transport of reactants and products in the air cathode of proton exchange membrane (PEM) fuel cells is studied analytically and numerically. Single- and two-phase regimes of water distribution and transport are classified by a threshold current density corresponding to first appearance of liquid water at the membrane/cathode interface. When the cell operates above the threshold current density, liquid water appears and a two-phase zone forms within the porous cathode. A two-phase, multicomponent mixture model in conjunction with a finite-volume-based computational fluid dynamics (CFD) technique is applied to simulate the cathode operation in this regime. The model is able to handle the situation where a single-phase region co-exists with a two-phase zone in the air cathode. For the first time, the polarization curve as well as water and oxygen concentration distributions encompassing both single- and two-phase regimes of the air cathode are presented. Capillary action is found to be the dominant mechanism for water transport inside the two-phase zone of the hydrophilic structure. The liquid water saturation within the cathode is predicted to reach 6.3% at 1.4 A cm⁻² for dry inlet air.     

The effect of confinement on the flow and turbulent heat transfer in a mist impinging jet

Numerical study of the effect of confinement on a flow structure and heat transfer in an impinging mist jets with low mass fraction of droplets (M_L1 ? 1%) were presented. The turbulent mist jet is issued from a pipe and strikes into the center of the flat heated plate. Mathematical model is based on the steady-state RANS equations for the two-phase flow in Euler/Euler approach. Predictions were performed for the distances between the nozzle and the target plate x/(2R) = 0.5–10 and the initial droplets size (d₁ = 5–100 μm) at the varied Reynolds number based on the nozzle diameter, Re = (1.3–8) × 10⁴. Addition of droplets causes significant increase of heat transfer intensity in the vicinity of the jet stagnation point compared with the one-phase air impinging jet. The presence of the confinement upper surface decreases the wall friction and heat transfer rate, but the change of friction and heat transfer coefficients in the stagnation point is insignificant. The effect of confinement on the heat transfer is observed only in very small nozzle-to-plate distances (H/(2R) < 0.5) both in single-phase and mist impinging jets.     

Boiling heat transfer and critical heat flux of ethanol–water mixtures flowing through a diverging microchannel with artificial cavities

This paper presents an experimental study on the convective boiling heat transfer and the critical heat flux (CHF) of ethanol–water mixtures in a diverging microchannel with artificial cavities. The results show that the boiling heat transfer and the CHF are significantly influenced by the molar fraction (x_m) as well as the mass flux. For the single-phase convection region except for the region near the onset of nucleate boiling with temperature overshoot, the single-phase heat transfer coefficient is independent of the wall superheat and increases with a decrease in the molar fraction. After boiling incipience, the two-phase heat transfer coefficient is much higher than that of single-phase convection. The two-phase heat transfer coefficient shows a maximum in the region of bubbly-elongated slug flow and deceases with a further increase in the wall superheat until approaching a condition of CHF, indicating that the heat transfer is mainly dominated by convective boiling. A flow-pattern-based empirical correlation for the two-phase heat transfer coefficient of the flow boiling of ethanol–water mixtures is developed. The overall mean absolute error of the proposed correlation is 15.5%, and more than 82.5% of the experimental data were predicted within a ±25% error band. The CHF increases from x_m = 0–0.1, and then decreases rapidly from x_m = 0.1–1 at a given mass flux of 175 kg/m² s. The maximum CHF is reached at x_m = 0.1 due to the Marangoni effect, indicating that small additions of ethanol into water could significantly increase the CHF. On the other hand, the CHF increases with increasing the mass flux at a given molar fraction of 0.1. Moreover, the experimental CHF results are compared with existing CHF correlations of flow boiling of the mixtures in a microchannel.     

Experimental study of saturated flow boiling heat transfer in an array of staggered micro-pin-fins

This study concerns water saturated flow boiling heat transfer in an array of staggered square micro-pin-fins having a 200 × 200 μm² cross-section by a 670 μm height. Three inlet temperatures of 90, 60, and 30 °C, six mass velocities for each inlet temperature, ranging from 183 to 420 kg/m² s, and outlet pressures between 1.03 and 1.08 bar were tested. Heat fluxes ranged from 23.7 to 248.5 W/cm². Heat transfer coefficient was fairly constant at high quality, insensitive to both quality and mass velocity. Heat transfer was enhanced by inlet subcooling at low quality. Possible heat transfer mechanism was discussed.     

A direct formate microfluidic fuel cell with cotton thread-based electrodes

A direct formate microfluidic fuel cell with cotton thread-based electrodes is proposed. The palladium catalyst is directly coated on cotton threads by repeated dipping method to prepare electrodes, which integrates the flow channel and electrode together and provides exposed active sites for enhancing the mass transfer on the anode and cathode. The aqueous anolyte and catholyte transport through cotton threads by capillary force with aid of gravity, eliminating the use of any external pump and facilitating the integration and miniaturization of the whole system. In the experiment, a three-flow channel structure is employed. The fuel is sodium formate and the oxidant is hydrogen peroxide. 1 M Na₂SO₄ solution is introduced into the middle channel formed by cotton threads with no catalyst to alleviate the reactant crossover. Performance is evaluated under various catalyst loadings, fuel concentrations and differences in height between the inlet and outlet. Results show that the fuel cell produces an open circuit voltage (OCV) of 1.41 V. The maximum current density of 74.56 mA cm⁻² and the peak power density of 24.75 mW cm⁻² are yielded when the palladium loading is 1 mg cm⁻¹ and the difference in height between the inlet and outlet is 7 cm, using 4 M HCOONa as fuel. Furthermore, the performance of the fuel cell increases first and then decreases with increasing the palladium loading. The same variation is observed with increasing the fuel concentration. However, the performance gradually increases with increasing the difference in height from 3 cm to 7 cm. The proposed microfluidic fuel cell with cotton thread-based electrodes shows enormous potential as a micro power source for portable devices.     

Diagnostic tool to detect liquid water removal in the cathode channels of proton exchange membrane fuel cells

A transparent PEMFC with a single straight channel was designed to study liquid water transport in the cathode channel. The pressure-drop between the inlet and outlet of the channel was measured and used as a diagnostic signal to monitor liquid water accumulation and removal. This method was non-destructive for the fuel cell, and is capable of monitoring the water droplet buildup and removal in the channel on-line directly, and giving real-time liquid water buildup information. The proper velocity for liquid water removal can be determined according to the pressure-drop curve, which was very helpful to design a flow field and to optimize fuel cell operation. Under the study conditions, and to ensure liquid water discharge, the gas velocity should not lower than 2, 3 and 5 m s⁻¹ for 600, 1000 and 1200 mA cm⁻², respectively. The results were further verified by visualization in a transparent PEMFC.     

Analysis of staggered tube bundle heat transfer to vertical foam flow

In general heat transfer intensity between solid surface and coolant (fluid) depends on three main parameters: heat transfer coefficient, size of heat exchange surface and temperature difference between surface and fluid. Sometimes the last two parameters (surface size and temperature difference) are strictly limited due to the process or technological requirements, and only increase of heat transfer coefficient is allowed. Simplest way offering sufficient increase in heat transfer rate (heat transfer coefficient as well) is to go from the laminar fluid flow regime to the turbulent one by increasing flow velocity. In many cases it helps despite such disadvantages like more complicated fluid supply system, rise of fluid flow mass rate and growth of energy usage for pumping. But in some cases, for example, in space application, in nuclear engineering, etc. there is not allowed to use high flow velocity of coolant – gas (due to vibration danger) or to apply high mass rate of coolant – liquid (due to limitation concerning weight or mass). One of the possible solutions of that problem could be the usage of two-phase flow as a coolant. An idea to use such two-phase coolant for heat removal from the solid surface is not new. Boiling liquid (water especially), gas flow with liquid droplets and other two-phase systems are widely used for heat and mass transfer purposes in various industries like food, chemical, oil, etc. An application of such two-phase coolants has lot advantages; high value of heat transfer coefficient is one of the most important. Unfortunately nothing is ideal on the Earth. Restrictions on vibration, on coolant weight (or mass rate); necessity to generate two-phase flow separately from the heat removal place; requirements on very low coolant velocities and other constraints do not allow using such type of two-phase coolant for purposes which were mentioned above (space application especially). As a possible way out can be usage of the statically stable foam flow produced from gas (air) and surfactant solutions in liquid (water). Our previous investigations [J. Gylys, Hydrodynamics and Heat Transfer under the Cellular Foam Systems, Technologija, Kaunas, 1998] showed the solid advantages of that type of two-phase coolant, including high values of heat transfer coefficient (up to 1000 W/m² K and more), low flow velocities (less than 1.0 m/s), small coolant density (less than 4 kg/m³), possibility to generate foam flow apart from the heat removal place, etc.This article is devoted to the experimental investigation of the staggered tube bundle heat transfer to the vertical upward and downward statically stable foam flow. The investigations were provided within the laminar regime of foam flow. The dependency of the tube bundle heat transfer on the foam flow velocity, flow direction and volumetric void fraction were analyzed. In addition to this, the influence of tube position in the bundle was investigated also. Investigation shows that the regularities of the tube bundle heat transfer to the vertical foam flow differ from the one-phase (gas or liquid) flow heat transfer peculiarities. It was showed that the heat transfer intensity of the staggered tube bundle to the foam flow is much higher (from 25 to 100 times) than that for the one-phase airflow under the same conditions (flow velocity). The results of the investigations were generalized using criterion equations, which can be applied for the calculation and design of the statically stable foam heat exchangers with the staggered tube bundles.     

Microchannel size effects on local flow boiling heat transfer to a dielectric fluid

Heat transfer with liquid–vapor phase change in microchannels can support very high heat fluxes for use in applications such as the thermal management of high-performance electronics. However, the effects of channel cross-sectional dimensions on the two-phase heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer of a dielectric fluid, Fluorinert FC-77, in microchannel heat sinks. Experiments are performed for mass fluxes ranging from 250 to 1600 kg/m² s. Seven different test pieces made from silicon and consisting of parallel microchannels with nominal widths ranging from 100 to 5850 μm, all with a nominal depth of 400 μm, are considered. An array of temperature sensors on the substrate allows for resolution of local temperatures and heat transfer coefficients. The results of this study show that for microchannels of width 400 μm and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size. Also, heat transfer coefficients and boiling curves are independent of mass flux in the nucleate boiling region for a fixed channel size, but are affected by mass flux as convective boiling dominates. A strong dependence of pressure drop on both channel size and mass flux is observed. The experimental results are compared to predictions from a number of existing correlations for both pool boiling and flow boiling heat transfer.     

Pressure Drop,Boiling Heat Transfer and Flow Patterns during Flow Boiling in a Single Microchannel

The boiling heat transfer and two-phase pressure drop of water in a microscale channel were experimentally investigated. The tested horizontal rectangular microchannel had a hydraulic diameter of 100 μ m and length of 40 mm. A series of microheaters provided heat energy to the working fluid, which made it possible to control and measure the local thermal conditions in the direction of the flow. Both the microchannel and microheaters were fabricated using a micro-electro-mechanical systems (MEMS) technique. Flow patterns were obtained from real-time flow visualizations made during the flow boiling experiments. Tests were performed for mass fluxes of 90, 169, and 267 kg/m²s and heat fluxes from 200 to 500 kW/m². The effects of the mass flux and vapor quality on the local flow boiling heat transfer coefficient and two-phase frictional pressure gradient were studied. The evaluated experimental data were compared with existing correlations. The experimental heat transfer coefficients were nearly independent of the mass flux and vapor quality. Most of the existing correlations did not provide reliable heat transfer coefficient predictions for different vapor quality values, nor could they predict the two-phase frictional pressure gradient except under some limited conditions.     

Experimental study of R-134a evaporation heat transfer in a narrow annular duct

An experiment is carried out here to investigate the evaporation heat transfer and associated evaporating flow pattern for refrigerant R-134a flowing in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm. In the experiment, the effects of the duct gap, refrigerant vapor quality, mass flux and saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. For the duct gap of 2.0 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m² s, imposed heat flux q from 5 to 15 kW/m², vapor quality x_m from 0.05 to 0.95, and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the gap of 1.0 mm, G is varied from 500 to 700 kg/m² s with the other parameters varied in the same ranges as that for δ = 2.0 mm. The experimental data clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher G. Besides, the evaporation heat transfer coefficient also rises substantially at increasing q. Moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in T_sat, but the effects are less pronounced in the narrower duct at a low imposed heat flux and a high refrigerant mass flux. Furthermore, the evaporation heat transfer coefficient increases substantially with the refrigerant mass flux except at low vapor quality. We also note that reducing the duct gap causes a significant increase in h_r. In addition to the heat transfer data, photos of R-134a evaporating flow taken from the duct side show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. Finally, an empirical correlation for the present measured heat transfer coefficient for the R-134a evaporation in the narrow annular ducts is proposed.     

Experimental investigation of the forced convective boiling heat transfer of R-600a/oil/nanoparticle

In this paper an experimental study of convective boiling heat transfer of R-600a/oil/nanoparticle mixtures is investigated. The experimental setup was prepared with a smooth horizontal tube as a test section with the length and diameter of 9.5 and 103 mm, respectively, and pure R-600a was applied for evaluating the heat transfer enhancement. Six mixtures containing 1% weight fraction of R-600a/oil with different concentrations of CuO nanoparticles including 0.0, 0.5, 1.0, 1.5, 2.0 and 5.0% weight fraction of R-600a/oil/nanoparticle were used in our study.The mass velocity per cross area was considered at the range of 50–700 kg/m² s for low vapor quality (ϕ < 0.25). The results showed that the convective boiling heat transfer coefficient will be increased by increasing the mass fraction of nanoparticles up to 2%, while by increasing the mass fraction of nanoparticles up to 5% the heat transfer coefficient will be reduced.     

Experimental Study of Condensation in a Shell and Tube Heat Exchanger in the Presence of a Noncondensable Gas

In this paper, an experimental study of the condensation of water vapor from a binary mixture of air and low‐grade steam has been depicted. The study is based upon diffusion heat transfer in the presence of high concentration of noncondensable gas. To simplify the study, experimental analysis is supported by empirical solutions. The experimental setup is custom designed for testing a new shell and tube type heat exchanger supplied by the manufacturer. Air–vapor mixture at 80 °C (max) and 20.2% relative humidity enters the heat exchanger at a mass flow rate of 480 kg/h and condenses 27 kg/h vapor using cooling water at an inlet temperature of 7 °C to 10 °C and mass flow rate of 3500 kg/h. By using the experimental data of constant inlet air mass fraction, mixture gas velocity, and different volumetric flow rate of the cold fluid, the local heat transfer coefficients are obtained. The main objective of this work is to establish an approximate value for surface area and overall heat transfer coefficient of a horizontal shell and tube condenser used in process space. Under designed working conditions, the condenser is found to work efficiently with 90% vapor condensation by mass.     

Numerical prediction of airfoil characteristics in a transonic droplet-laden air flow

A transonic airfoil moving in an air–water droplet two-phase flow is investigated numerically to study the effect of droplet size and volume fraction. The droplets we consider are in the size 1–100 μm and the volume fraction is in 0.01%–10%. A compressible two-fluid model is solved by the WAF-HLL scheme developed earlier by the authors which includes drag force, heat transfer, phase change, and droplet fragmentation of the droplets. The numerical results show that the droplet breakup layer can be extended to a later distance as large as about 60% of a chord length at the trailing edge. Also the droplets have made the shock wave dissolved in compression waves and the airfoil performance seriously deteriorated.     

Water transport study in a high temperature proton exchange membrane fuel cell stack

A study of water transport in a high temperature phosphoric acid doped polybenzimidazole (PBI) membrane fuel cell stack is reported. Tests with different stoichiometries of dry cathode and different humidity levels of anode are performed. It is found that water transport across the membrane electrode assembly (MEA) is noteworthy and that water vapor partial pressure on the anode outlet is almost always higher than on the cathode outlet, even when using dry hydrogen. The water transport is a strong function of current density but it also depends on stoichiometry and humidity level. In a series of tests with dry nitrogen on one side and humid nitrogen on the other side, the membrane's water permeability coefficient is determined to be 2.4 × 10⁻¹³ mol s⁻¹ cm⁻¹ Pa⁻¹ at 160 °C which is more than an order of magnitude higher than the values previously reported in the literature. Also, the results indicate that the permeability coefficient might be relative humidity dependent and could even be somewhat higher than the value reported here, but further investigation is needed. The experimental findings are reproduced and explained with a 2D steady state computational fluid dynamics (CFD) model. Internal water transport profiles across the membrane and along the gas flow channels are presented and discussed.     

Disclosure of the internal transport phenomena in an air-cooled proton exchange membrane fuel cell—Part I: Model development and base case study

In this study, the internal transport phenomena and mechanism inside an air-cooled proton exchange membrane fuel cell (PEMFC) are investigated. It helps to understand the factors that affect the performance of an air-cooled PEMFC and optimize the design of Membrane Electrode Assembly (MEA) and the flow field. This series article contains two parts. In this paper, i.e., Part Ⅰ of this series, a three-dimensional, two-phase flow, non-isothermal, steady-state Computational Fluid Dynamics (CFD) model is established to investigate the liquid water generation mechanism and the species distributions inside an air-cooled PEMFC single cell with a Base Case flow field design. Dry hydrogen and ambient air (the relative humidity and the stoichiometry are 60% and 150 separately) are considered for the simulation and validation. It is found that the liquid water appears mostly inside the cathode electrode underneath the cathode rib. Inside the anode gas diffusion layer (GDL), the mass fraction of H₂ underneath the cathode ribs is lower than that underneath the cathode channels, while the mass fraction of H₂O shows the opposite. The distributions of O₂ mass fraction and H₂O mass fraction inside the cathode GDL have the same trend as those of H₂ mass fraction and H₂O mass fraction inside the anode GDL. The membrane water content is periodically distributed from channel to channel and its value underneath the cathode rib is much larger than that underneath the cathode channel. The current density distribution is affected by the distribution of water content, i.e., the part underneath the cathode rib shows a larger current density than that underneath the cathode channel.     

Novel hybrid serpentine-interdigitated flow field with multi-inlets and outlets of gas flow channels for PEFC applications

In our previous study, a novel flow field design for a polymer electrolyte fuel cell (PEFC) called “Hybrid Serpentine-Interdigitated (HSI)” had been proposed. Although it was very promising in terms of performance and pressure drop, it still had a major drawback of the low oxygen concentration area. To improve its design and performance, three HSI configurations with different numbers of gas inlet and outlet, namely one inlet and one outlet HSI (1-IO HSI), one inlet and two outlets HSI (1I-2O HSI) and two inlets and two outlets HSI (2-IO HSI) were numerically investigated and compared with the conventional single channel serpentine (1S). The investigation on the cell performance and other transport behaviors has been carried out using CFD techniques via ANSYS FLUENT software. At a practical operating potential of about 0.6 V of 50 cm² PEFCs, the 2-IO HSI offered the best distributions of oxygen, current density and water due to the shorter channel length. More importantly, the 2-IO HSI could contribute to a reduction in cathode pressure drop by 90%, as compared with the 1S, resulting in the enhancement in the net power output by 6%, approximately.