Free convective turbulent flow within the trombe wall channel

A study of free convective turbulent heat transfer between parallel plates has been made. The initial flow is assumed to remain laminar until a combination of geometry, temperature, and flow rate conditions reach a pre-defined level. At this point the model used in this study assumes transition and permits laminar flow to gradually develop into fully turbulent flow. Turbulent flow characteristics are predicted by a mixing length model which incorporates empirical parameters used in the literature. Using air as the fluid, a wide range of channel geometries, relative surface temperatures, and flow rates have been examined. Guided by the very limited available experimental data, computations were made and several correlations were developed to enable important quantities to be estimated given the channel geometry, surface temperatures, and inlet air temperature.     

Numerical study of two-phase flow patterns in the gas channel of PEM fuel cells with tapered flow field design

In this study the air–water two-phase flow in a tapered channel of a PEMFC was numerically simulated using the volume of fluid (VOF) method. In particular, a 3D mathematical model of the fuel cell flow channel was used to obtain a reliable evaluation of the fuel cell performance for different taper angles and different temperatures and to calculate the total amount of water produced. This information was then used as boundary conditions to simulate the two-phase flow in the cell channel through a 2D VOF model. Typical operating conditions were assigned and the numerical mesh was constructed to represent the real fuel cell configuration. The results show that tapering the channel downstream enhances the water removal due to increased airflow velocity. In the rectangular channel no film formation is noted with a marked predominance of slug flow. In contrast, as the taper angle is increased the predominant two-phase flow pattern is film flow. Finally many contact angles have been used to simulate the effect of the hydrophobicity of a GDL surface on the motion of the water. As the hydrophobicity of a GDL surface is decreased the presence of film is more evident even for less tapered channels.     

Experimental investigations on overall cooling effect of ribbed channel with air bleeds

An experimental investigation on overall heat transfer performance of a rectangular channel, in which one wall has periodically placed oblique ribs to enhance heat exchange and cylindrical film holes to bleed cooling air, has been carried out in a hot wind tunnel at different mainstream temperatures, hot mainstream Reynolds numbers, coolant Reynolds numbers and blowing ratios. To describe the cooling effect of combined external coolant film with the internal heat convection enhanced by the ribs, the overall cooling effectiveness at the surface exposed in the mainstream with high temperature was calculated by the surface temperatures measured with an infrared thermal imaging system. The total mass flow rate of cooling air through the coolant channel was regulated by a digital mass flow rate controller, and the blowing ratio passing through the total film holes was calculated based on the measurements of another digital-type mass flow meter. The detailed distributions of overall cooling effectiveness show distinctive peaks in heat transfer levels near the film holes, remarkable inner convective heat transfer effect over entire channel surface, and visible conductive heat transfer effect through the channel wall; but only when the coolant Reynolds number is large enough, the oblique rib effect can be detected from the overall cooling effectiveness; and the oblique bleeding hole effect shows the more obvious trend with increasing blowing ratios. Based on the experimental data, the overall cooling effectiveness is correlated as the functions of Re_m (Reynolds number of hot mainstream) and Re_c (Reynolds number of internal coolant flow at entrance) for the parametric conditions examined.     

Temperature distribution on the MEA surface of a PEMFC with serpentine channel flow bed

Knowledge of the temperature distribution on the membrane electrode assembly (MEA) surface and heat transfer processes inside a proton exchange membrane fuel cell (PEMFC) is helpful to improvement of cell reliability, durability and performance. The temperature fields on the surface of MEA fixed inside a proton exchange membrane fuel cell with a serpentine channel flow bed were measured by infrared imaging technology under non-humidification conditions. The temperature distributions over the MEA surface under whole channel region were achieved. The experimental results show that the downstream temperatures are higher than the upstream. The hot region on the MEA surface is easy to locate from the infrared temperature image. The mean temperature on the MEA surface and the cell temperature both increase with the current density. Higher current density makes the non-uniformity of temperature distribution on the MEA surface worse. The loading time significantly affects the temperature distribution. Compared with the electrical performance of the cell, the MEA's temperatures need much more time to reach stable. The results indicate that isothermal assumption is not appropriate for a modeling of PEMFCs, and monitoring the temperature of external surface of the flow field plate or end plate cannot supply accurate reference to control the temperatures on MEA surface.     

Simulations of post-filling process considering the cooling effect of the mold cooling systems

Simulation of the post-filling process has been developed and performed to study the compressible polymer melt flow during the packing phase and the pressure development inside the mold cavity for the entire post-filling process. A mixed finite-element/finite-difference numerical scheme is implemented to solve the non-isothermal, compressible viscous flow equations. The transient, non-isothermal mold cavity surface temperatures which depend on the mold cooling channel arrangement and coolant flow conditions are also incorporated during simulation using hybrid finite-element/shape-factor method. It has been found that the difference in the pressure profile variation during the post-filling stage is quite distinguished for cases assuming constant mold wall temperature and cases with the consideration of the cooling effect of the cooling system configuration.     

A mathematical model of a solar chimney

A simple mathematical model of a solar chimney is proposed. The physical model is similar to the Trombe wall. One side of the chimney is provided with a glass cover which with the other three solid walls of the chimney form a channel through which the heated air could rise and flow by natural convection. Openings provided at the bottom and top of the chimney allow room air to enter and leave the channel. Steady state heat transfer equations were set up to determine the boundary temperatures at the surface of the glass cover, the rear solar heat absorbing wall and the air flow in the channel using a thermal resistance network. The equations were solved using a matrix-inversion solution procedure. The thermal performance of the solar chimney as determined from the glass, wall and air temperatures, air mass flow rate and instantaneous heat collection efficiency of the chimney are presented. Satisfactory correlation was obtained with experimental data from other investigators. Further experimental investigation is currently under way.     

Theoretical analysis of film condensation heat transfer inside vertical mini triangular channels

An analytical model is presented for predicting film condensation of vapor flowing inside a vertical mini triangular channel. The concurrent liquid-vapor two-phase flow field is divided into three zones: the thin liquid film flow on the sidewall, the condensate flow in the corners, and the vapor core flow in the center. The model takes into account the effects of capillary force induced by the free liquid film curvature variation, interfacial shear stress, interfacial thermal resistance, gravity, axial pressure gradient, and saturation temperatures. The axial variation of the cross-sectional average heat transfer coefficient of steam condensing inside an equilateral triangular channel is found to be substantially higher than that inside a round tube having the same hydraulic diameter, in particular in the entry region. This enhancement is attributed to the extremely thin liquid film on the sidewall that results from the liquid flow toward the channel corners due to surface tension. The influences of the inlet vapor flow rates, the inlet subcooling, and the channel size on the heat transfer coefficients are also examined.     

Dynamic characteristics of local current densities and temperatures in proton exchange membrane fuel cells during reactant starvations

Reactant starvation during proton exchange membrane fuel cell (PEMFC) operation can cause serious irreversible damages. In order to study the detailed local characteristics of starvations, simultaneous measurements of the dynamic variation of local current densities and temperatures in an experimental PEMFC with single serpentine flow field have been performed during both air and hydrogen starvations. These studies have been performed under both current controlled and cell voltage controlled operations. It is found that under current controlled operations cell voltage can decrease very quickly during reactant starvation. Besides, even though the average current is kept constant, local current densities as well as local temperatures can change dramatically. Furthermore, the variation characteristics of local current density and temperature strongly depend on the locations along the flow channel. Local current densities and temperatures near the channel inlet can become very high, especially during hydrogen starvation, posing serious threats for the membrane and catalyst layers near the inlet. When operating in a constant voltage mode, no obvious damaging phenomena were observed except very low and unstable current densities and unstable temperatures near the channel outlet during hydrogen starvation. It is demonstrated that measuring local temperatures can be effective in exploring local dynamic performance of PEMFC and the thermal failure mechanism of MEA during reactants starvations.     

High-Speed Visualization of Two-Phase Flow in a Micro-Scale Pin-Fin Heat Exchanger

Flow regimes and bubble growth are observed in a pin-fin micro-scale heat exchanger with R-11 as the working fluid. The heat exchanger is machined in silicon and derived from a DNA micro-array consisting of 150 μm-square fins separated by 50 μm-square passages. The fins are staggered and oriented 45 degrees to the flow direction such that approximately 750 channel intersections occur within the volume of the exchanger. The purpose of the study is to determine if this multiply-connected geometry produces the flow blockage, reversal, and other instabilities observed in single and parallel micro-channel configurations. The upper surface of the exchanger is a glass plate that provides optical access. High-speed digital photography and microscope optics are used to obtain real-time images of the flow at a framing rate of 5 kHz. The lower surface is electrically heated and instrumented with a heat flux gage. Inlet and outlet temperatures and pressures, heater and wall temperatures, and volumetric flow rate are monitored. Nucleation is observed near the entrance of the heat exchanger. In the central section, developed vapor regions are composed of broad slug-like vapor fronts immediately followed by a slowly growing bubbly flow. An annular regime dominates the downstream section of the exchanger with drop-like liquid structures appearing at the downstream edge of fins. The heat transfer coefficient decreases with exit quality as in other micro-scale exchangers; however, the flow instability present in parallel channel exchangers is not observed in this configuration.     

An experimental study on convection heat transfer from an array of discrete heat sources

Convection heat transfer from an array of discrete heat sources inside a rectangular channel has been investigated experimentally for air. The lower surface of the channel was equipped with 8×4 flush-mounted heat sources subjected to uniform heat flux; the sidewalls and the upper wall were insulated and adiabatic. The experimental parametric study was made for an aspect ratio of AR=2, Reynolds numbers 864≤Re_Dh≤7955, and modified Grashof numbers Gr*=1.72×10⁸ to 2.76×10⁹. From the experimental measurements, surface temperature distributions of the discrete heat sources were obtained and effects of Reynolds and Grashof numbers on these temperatures were investigated. Furthermore, Nusselt number distributions were calculated for different Reynolds and Grashof numbers. Results show that surface temperatures increase with increasing Grashof number and decrease with increasing Reynolds number. However, with the increase in the buoyancy affected secondary flow and the onset of instability, temperatures level off and even drop as a result of heat transfer enhancement. This outcome can also be observed from the variation of the row-averaged Nusselt number showing an increase towards the exit.     

Effects of hydrophilic/hydrophobic properties of gas flow channels on liquid water transport in a serpentine polymer electrolyte membrane fuel cell

The droplet dynamics in the serpentine flow channel of a hydrogen fuel cell has been numerically investigated to obtain ideas for designing a serpentine channel with the aim of effectively preventing flooding. Three-dimensional two-phase flow simulations employing the volume of fluid (VOF) method have been performed. Liquid droplets emerging from four adjacent pores at the hydrophobic bottom wall are subjected to airflow in the bulk of the serpentine flow channel. The effects of contact angle variation of the channel walls on liquid water removal have been tested in terms of liquid water saturation and coverage of liquid water on the gas diffusion layer (GDL) surface. The numerical results show that the hybrid case, which consists of hydrophilic channel walls at the straight part and hydrophobic walls at the turning part of the serpentine flow channels, enhances water removal compared with two other cases in which the channel wall is homogeneously hydrophilic or hydrophobic. The three-dimensional visualization of liquid water droplets reveals that the hydrophobic wall at the turning part reduces the water saturation in the channel and the hydrophilic wall at the straight part prevents the liquid water from covering the GDL surface.     

Three‐dimensional simulation of water droplet movement in PEM fuel cell flow channels with hydrophilic surfaces

Water management is one of the critical issues in proton exchange membrane fuel cells, and proper water management requires effective removal of liquid water generated in the cathode catalyst layer, typically in the form of droplets through cathode gas stream in the cathode flow channel. It has been reported that a hydrophilic channel sidewall with a hydrophobic membrane electrode assembly (MEA) surface would have less chance for water accumulation on the MEA surface. Therefore, a comprehensive study on the effect of surface wettability properties on water droplet movement in flow channels has been conducted numerically. In this study, the water droplet movements in a straight flow channel with a wide range of hydrophilic surface properties and effects of inlet air velocities are analyzed by using three‐dimensional computational fluid dynamics method coupled with the volume‐of‐fluid (VOF) method for liquid–gas interface tracking. The results show that the water droplet movement is greatly affected by the channel surface wettability and air flow conditions. With low contact angle, droplet motion is slow due to more liquid–wall contact area. With high air flow velocities, increasing the contact angle of the channel surface results in faster liquid water removal due to lesser liquid–wall contact area. Copyright © 2010 John Wiley & Sons, Ltd.     

Condensation of vapor in the presence of non-condensable gas in condensers

This paper presents a set of differential and algebraic equations that model heat and mass transfer in condensers in which a mixture of water vapor and non-condensable gas is cooled. The model has been used to predict the condensation rate, the bulk temperatures of the coolant and the gas–vapor mixture, and the surface temperatures of the condenser wall. The predicted results for counter flow tube condensers are compared with three sets of published experimental data for system in which air is the non-condensable gas. It is found that the predicted condensation rates and coolant bulk temperatures agree very well with all the three sets of experimental data, the predicted wall temperatures agree reasonably well with the experimental results, and the agreement between the predictions and the experimental results on the bulk temperature of the air–vapor mixture is excellent for one set of the experimental data, reasonable for the second set of experimental data, but poor for the third set of experimental data. It is suggested that the poor agreement between the predicted and measured bulk temperatures of the mixture for the third set of experimental data arises from the experimental errors. The results from this study show that when modeling vapor condensation in the presence of a non-condensable gas, a simple model for the mixture channel alone may not be sufficient since neither the temperature nor the heat flux at the wall can be assumed to be constant. The results also show that the wall temperature in the coolant channel can be quite high, and careful modeling of the heat transfer in the coolant channel is needed in order to achieve good agreement between the model predictions and the experimental results.     

Boiling flow characteristics in microchannels with very hydrophobic surface to super-hydrophilic surface

Boiling flow process plays a very important role to affect the heat transfer in a microchannel. Different boiling flow modes have been found in the past which leads to different oscillations in temperatures and pressures. However, a very important issue, i.e. the surface wettability effects on the boiling flow modes, has never been discussed. The current experiments fabricated three different microchannels with identical sizes at 105 × 1000 × 30000 μm but at different wettability. The microchannels were made by plasma etching a trench on a silicon wafer. The surface made by the plasma etch process is hydrophilic and has a contact angle of 36° when measured by dipping a water droplet on the surface. The surface can be made hydrophobic by coating a thin layer of low surface energy material and has a contact angle of 103° after the coating. In addition, a vapor–liquid–solid growth process was adopted to grow nanowire arrays on the wafer so that the surface becomes super-hydrophilic with a contact angle close to 0°. Different boiling flow patterns on a surface with different wettability were found, which leads to large difference in temperature oscillations. Periodic oscillation in temperatures was not found in both the hydrophobic and the super-hydrophilic surface. During the experiments, the heat flux imposed on the wall varies from 230 to 354.9 kW/m² and the flow of mass flux into the channel from 50 to 583 kg/m²s. Detailed flow regimes in terms of heat flux versus mass flux are also obtained.     

Study of Heat Transfer Distribution in a Channel with Inclined Target Surface Cooled by a Single Array of Staggered Impinging Jets

An experimental investigation has been carried out to study the heat transfer characteristics in a channel with a heated target surface inclined at an angle, cooled by a single array of staggered impinging jets. The work encompasses the effect of three feed channel aspect ratios (5, 7, 9) and three exit outflow orientations (coincident with the entry flow, opposed to the entry flow, and both), and three Reynolds numbers (9400, 14,400, 18,800) on heat transfer. Results show that increasing the Reynolds number increases the heat transfer on the inclined target surface. The outflow orientations affect significantly the local heat transfer charactracistrics, through influencing the jet flow together with the crossflow in the impingement channel. The outflow orientation coincident with the entry flow and the outflow from both sides show better averaged Nusselt number values compared to outflow orientation opposed to the entry flow. The inclined surface affects the local Nusselt number distribution especially for the outflow orientation opposing the entry flow at the narrow region of the impingement channel. In general, the feed channel aspect ratio does not affect the Nusselt number distribution, except for outflow coincident with the entry flow. The local Nusselt number for aspect ratio 9 has been found to be greater than the Nusselt number for aspect ratio 5 by 11%. Additionally, for a given jet-orifice plate with staggered holes, the heat transfer is almost the same throughout the target surface for the outflow exiting in both directions.     

Effect of wettability on water removal from the gas diffusion layer surface in a novel proton exchange membrane fuel cell flow channel

Effective water removal from the proton exchange membrane fuel cell (PEMFC) surface exposed to the flow channel is critical to the operation and water management in PEMFCs. In this study, the water removal process is investigated numerically for a novel flow channel formed by inserting a hydrophilic needle in the conventional PEMFC flow channel, and the effect of the surface wettability of the membrane electrode assembly (MEA) and the inserted needle on the water removal process is studied. The results show that the liquid water can be more effectively removed from the MEA surface for larger MEA surface contact angles and smaller needle surface contact angles. The pressure drop for the flow in the channel is also examined and it is seen to be indicative of the liquid water flow and transport in the flow channel, suggesting that pressure drop is a useful parameter for the investigation of water transport and dynamics in the flow channel.     

Numerical Simulation of Boundary Conditions and the Onset of Instability in Natural Convection due to Protruding Thermal Sources in an Open Rectangular Channel

ABSTRACT

Two-dimensional natural-convection heat transfer to air from multiple identical protruding heat sources, which simulate electronic components, located in a horizontal channel which is open on both sides, has been studied numerically. Of particular interest is the accurate simulation of realistic boundary conditions in such a channel. The effects of source temperatures, channel dimensions, openings, boundary conditions, and source locations on the heat transfer from and flow above the protruding sources are investigated, and the onset of instability is studied. The results indicate that the channel dimensions and the presence of openings have significant effects on the fluid flow. However, their effects on heat transfer are found to be relatively small. The increase in channel height is seen to lead to a less stable flow and, consequently, a decrease in the critical Grashof number. Numerical simulation of the flow at the openings is investigated and found to be crucial to an accurate modeling of the transport.     

An experimental study on thermal mixing in a square body inserted inclined narrow channels

An experimental study has been performed on thermal mixing phenomena in a narrow channel by twin-jets at different temperatures. Water was used as working fluid and it is supplied by hot and cold taps. The channel has a circular exit hole to supply continuity of mass. An adiabatic square shaped object, which in the thickness of the channel, is inserted into the channel to control thermal mixing as a passive technique. Other parameters in experiments are ratio of flow rate of inlet fluid, inclination angle of the channel, jet diameter and jet velocities. Finally, a thermal mixing index was calculated from measured values of temperatures for different parameters. Temperature distribution is obtained for whole channel and isotherms are plotted. The obtained results indicated that higher thermal mixing efficiency is observed for ? = 60^o and inserted body can be a control parameter for thermal mixing for the same geometrical parameters.     

Experimental study of heat transfer enhancement in electrohydrodynamic conduction pumping of liquid film using flush electrodes

Electrohydrodynamic conduction pumping of free surface dielectric liquid film, using flush electrodes, has been studied experimentally for various film temperatures. Volume flow rate, heat transfer and power consumption ratio and conduction pumping efficiency of free surface liquid film in different film thicknesses and temperatures have been investigated and then the best operating conditions have been presented. Also, the heat transfer coefficient on free surface liquid film passing on flush electrodes is compared with similar liquid film in absence of flush electrodes in different temperatures. Results show that as applied voltage increases, significant differences in volume flow rates have been observed by changing the temperature. Applied voltage related to the highest percentage of heat transfer coefficient enhancement demonstrates the reverse relation with temperature. Results confirm that there is a direct relationship between film thickness and the applied voltage related to the maximum heat transfer per pumps power consumption.     

The effect of flow maldistribution on the evaluation of axial dispersion and thermal performance during the single-blow testing of plate heat exchangers

A single-blow transient test technique based on axial dispersion model is proposed for the determination of both heat transfer coefficient and axial dispersion coefficient in plate heat exchangers, characterized by NTU and dispersive Peclet number respectively. The present experimental analysis deals with the effect of flow maldistribution on the transient temperature response for U-type plate heat exchangers. The experiments are carried out with uniform and non-uniform flow distributions for various flow rates and two different numbers of plates. Special effort has been made to differentiate the deviation from plug flow due to flow maldistribution and fluid backmixing. The fluid axial dispersion is used to characterize the backmixing and other deviations from plug flow. Due to unequal distribution of the fluid, the velocity of the fluid varies from channel to channel and hence the heat transfer coefficient variations are also taken into consideration. The computed outlet fluid temperatures are compared with experimental outlet temperatures, and the values of dispersive Peclet numbers are estimated. The results indicate that in order to get parameters independent of the number of plate used in single-blow experiment, it is essential to isolate flow maldistribution from backmixing. This paper has brought out a practical way in which this isolation can be done in the process of data reduction through suitable computational model.