Experimental study on interfacial area transport in vertical upward bubbly two-phase flow in an annulus

In relation to the development of the interfacial area transport equation, axial developments of local void fraction, interfacial area concentration, and interfacial velocity of vertical upward bubbly flows in an annulus with the hydraulic equivalent diameter of 19.1 mm were measured by the double-sensor conductivity probe. A total of 20 data were acquired consisting of five void fractions, about 0.050, 0.10, 0.15, 0.20, and 0.25, and four superficial liquid velocities, 0.272, 0.516, 1.03, and 2.08 m/s. The obtained data will be used for the development of reliable constitutive relations, which reflect the true transfer mechanisms in subcooled boiling flow systems.     

Study on transport phenomena for flow film condensation in vertical mini-tube with interfacial waves

The Kelvin-Helmholtz instability of phase-change interface during flow film condensation in vertical mini-diameter tube was studied here by means of energy analysis. According to the interfacial boundary conditions, the film thinning effect and the phase-change area enlarging effect by interfacial waves on heat transfer enhancement were analyzed for flow condensation in tubes with different diameter. It is indicated that, in mini-diameter tube, more obvious heat transfer enhancement due to interfacial waves can be expected than that in normal-sized tube, and the interfacial waves enhance the heat transfer mainly by film thinning effect.     

Numerical studies of interfacial phenomena in liquid water transport in polymer electrolyte membrane fuel cells using the lattice Boltzmann method

Water management is an important issue in the polymer electrolyte membrane (PEM) fuel cell, which is considered as a promising alternative power source for future automotive applications. In this article, lattice Boltzmann simulations are conducted to examine the interfacial phenomena in liquid water transport in porous materials of a PEM fuel cell. Numerical results clearly indicate that large perforated pores through the porous diffusion layers can serve as a convenient liquid water transport pathway and thus assist in liquid water removal. An interconnected horizontal and vertical pore combination is especially beneficial to liquid water transport through the porous layers in flooding conditions. Therefore, liquid water transport in a PEM fuel cell may be effectively managed through well engineered interfacial structures in porous materials.     

Modeling of the condensation sink term in an interfacial area transport equation

A model for the condensation sink term in an interfacial area transport equation (IATE) was developed. In the model, a bubble nucleation due to a wall surface boiling and a bubble collapse due to a condensation were assumed to be symmetric phenomena. Based on this consideration the condensing region for a subcooled condition can be divided into two regions: the heat transfer-controlled region and the inertia-controlled region. In the heat transfer-controlled region, the condensation Nusselt number approach is appropriate. On the other hand, in the inertia-controlled region, the resultant mechanical force may be balanced through an interface between a bubble and an ambient liquid. The modeled condensation sink term in an IATE in this study was evaluated against existing data which had been obtained from a bubble condensation in a subcooled water flow through a non-heated annulus. The evaluation result showed that the present model could predict the axial distribution of the interfacial area concentration accurately.     

Modeling of bubble-layer thickness for formulation of one-dimensional interfacial area transport equation in subcooled boiling two-phase flow

In relation to the formulation of one-dimensional interfacial area transport equation in a subcooled boiling flow, the bubble-layer thickness model was introduced to avoid many covariances in cross-sectional averaged interfacial area transport equation in the subcooled boiling flow. The one-dimensional interfacial area transport equation in the subcooled boiling flow was formulated by partitioning a flow region into two regions; boiling two-phase (bubble layer) region and liquid single-phase region. The bubble-layer thickness model assuming the square void peak in the bubble-layer region was developed to predict the bubble-layer thickness of the subcooled boiling flow. The obtained model was evaluated by void fraction profile measured in an internally heated annulus. It was shown that the bubble-layer thickness model could be applied to predict the bubble-layer thickness as well as the void fraction profile. In addition, the constitutive equation for the distribution parameter of the boiling flow in the internally heated annulus, which was used for formulating the bubble-layer thickness model, was developed based on the measured data. The model developed in this study will eventually be used for the development of reliable constitutive relations, which reflect the true transfer mechanisms in subcooled boiling flows.     

Numerical study of cell performance and local transport phenomena in PEM fuel cells with various flow channel area ratios

Three-dimensional models of proton exchange membrane fuel cells (PEMFCs) with parallel and interdigitated flow channel designs were developed including the effects of liquid water formation on the reactant gas transport. The models were used to investigate the effects of the flow channel area ratio and the cathode flow rate on the cell performance and local transport characteristics. The results reveal that at high operating voltages, the cell performance is independent of the flow channel designs and operating parameters, while at low operating voltages, both significantly affect cell performance. For the parallel flow channel design, as the flow channel area ratio increases the cell performance improves because fuel is transported into the diffusion layer and the catalyst layer mainly by diffusion. A larger flow channel area ratio increases the contact area between the fuel and the diffusion layer, which allows more fuel to directly diffuse into the porous layers to participate in the electrochemical reaction which enhances the reaction rates. For the interdigitated flow channel design, the baffle forces more fuel to enter the cell and participate in the electrochemical reaction, so the flow channel area ratio has less effect. Forced convection not only increases the fuel transport rates but also enhances the liquid water removal, thus interdigitated flow channel design has higher performance than the parallel flow channel design. The optimal performance for the interdigitated flow channel design occurs for a flow channel area ratio of 0.4. The cell performance also improves as the cathode flow rate increases. The effects of the flow channel area ratio and the cathode flow rate on cell performance are analyzed based on the local current densities, oxygen flow rates and liquid water concentrations inside the cell.     

Numerical study on high-temperature diluted air combustion for the turbulent jet flame in crossflow using an unsteady flamelet model

The turbulent jet flame in a crossflow with highly preheated diluted air has been numerically investigated. The Favre-averaged Navier–Stokes equations are solved by a finite volume method of SIMPLE type that incorporates the flamelet concept coupled with the standard k–ε turbulence model. The NO formation is estimated by using the Eulerian particle transport equations in a postprocessing mode. For methane and propane with various conditions of inlet air temperature and oxygen concentration, the three-dimensional characteristics of the flame are successfully captured. The jet-flame trajectory is in remarkably good agreement with the existing cold-flow correlations. When the oxygen concentration is high, the maximum flame temperature becomes high and the two fuels show quite different characteristics in the downstream region. On the other hand, for low oxygen concentrations, the temperature difference between the two fuels is relatively small and remains fairly constant throughout the combustion chamber. The propane gives a higher NO formation compared to the methane especially when the oxygen concentration is high. A higher temperature, longer residence time of the combustion gases may be responsible for the higher thermal NO formation.     

A three-dimensional numerical study and comparison between the air side model and the air/water side model of a plain fin-and-tube heat exchanger

CFD is becoming an important heat exchanger research technique. It constitutes an inexpensive prediction method, avoiding the need of testing numerous prototypes. Current work in this field is mostly based on air flow models assuming constant temperature of fin-and-tube surface. The purpose of this paper is to present an enhanced model, whose innovation lies in considering additionally the water flow in the tubes and the conduction heat transfer through the fin and tubes, to demonstrate that the neglect of these two phenomena causes a simulation result accuracy reduction.3-D Numerical simulations were accomplished to compare both an air side and an air/water side model. The influence of Reynolds number, fin pitch, tube diameter, fin length and fin thickness was studied. The exchanger performance was evaluated through two non-dimensional parameters: the air side Nusselt number and a friction factor. It was found that the influence of the five parameters over the mechanical and thermal efficiencies can be well reported using these non-dimensional coefficients. The results from the improved model showed more real temperature contours, with regard to those of the simplified model. Therefore, a higher accuracy of the heat transfer was achieved, yielding better predictions on the exchanger performance.     

A study of multi-phase flow through the cathode side of an interdigitated flow field using a multi-fluid model

This work presents a study of multi-phase flow through the cathode side of a polymer electrolyte membrane fuel cell employing an interdigitated flow field plate. A previously published model has been extended in order to account for phase change kinetics, and a comparison between the interdigitated flow field design and a conventional straight channel design has been conducted. It is found that the parasitic pressure drop in the interdigitated design is in the range of a few thousand Pa and could be reduced to a few hundred Pa by choosing diffusion media with high in-plane permeability. The additional compressor work due to the increased pressure loss will only slightly increase, and this may be offset by operating at lower stoichiometries as the interdigitated design is less mass transfer controlled, which means that the overall efficiency of the interdigitated arrangement will be higher. In the interdigitated design more product water is carried out of the cell in the vapor phase compared to the straight channel design which indicates that liquid water management might be less problematic. This effect also leads to the finding that in the interdigitated design more waste heat is carried out of the cell in the form of latent heat which reduces the load on the coolant. Finally we see that the micro-porous layer might help keep the gas diffusion layer substrate dry due to a potentially higher evaporation rate caused by a combination of the Kelvin effect and a larger specific surface area compared to the diffusion layer substrate.     

Experimental and modeling study on water dynamic transport of the proton exchange membrane fuel cell under transient air flow and load change

Water management is important in the proton exchange membrane fuel cell (PEMFC) operations, especially for those cells based on sulfonic acid polymers due to the depending of the conductivity on water. This paper aims at illustrating the effect of the change in membrane water content on cell potential response. For this purpose, the cell potential response has been investigated experimentally and computationally under transient air flow and load change of a PEMFC. From the experimental and computational results, an undershoot behavior of cell potential as well as the great influence of the relative humidity on the magnitude of undershoot is observed. It is found that the magnitude of cell potential undershoot increases as the relative humidity decreases. By carrying out a transient simulation on the water content of the membrane, the undershoot phenomena could be well explained. It is also found from the computational prediction that the time scale for the cell potential to reach its steady state is about 20 s, in agreement with experimental results. The model prediction also suggests that the dynamic behavior of PEMFC is critically dependent on the water content in the membrane.     

Prediction of autoignition in a lifted methane/air flame using an unsteady flamelet/progress variable model

An unsteady flamelet/progress variable (UFPV) model has been developed for the prediction of autoignition in turbulent lifted flames. The model is a consistent extension to the steady flamelet/progress variable (SFPV) approach, and employs an unsteady flamelet formulation to describe the transient evolution of all thermochemical quantities during the flame ignition process. In this UFPV model, all thermochemical quantities are parameterized by mixture fraction, reaction progress parameter, and stoichiometric scalar dissipation rate, eliminating the explicit dependence on a flamelet time scale. An a priori study is performed to analyze critical modeling assumptions that are associated with the population of the flamelet state space.For application to LES, the UFPV model is combined with a presumed PDF closure to account for subgrid contributions of mixture fraction and reaction progress variable. The model was applied in LES of a lifted methane/air flame. Additional calculations were performed to quantify the interaction between turbulence and chemistry a posteriori. Simulation results obtained from these calculations are compared with experimental data. Compared to the SFPV results, the unsteady flamelet/progress variable model captures the autoignition process, and good agreement with measurements is obtained for mixture fraction, temperature, and species mass fractions. From the analysis of scatter data and mixture fraction-conditional results it is shown that the turbulence/chemistry interaction delays the ignition process towards lower values of scalar dissipation rate, and a significantly larger region in the flamelet state space is occupied during the ignition process.     

用变频调速技术改造纺纱传统空调设备

介绍用变频调速技术改造纺织厂纺纱车间传统空调设备的经验、方法、获得的高节能率、技改中发生的问题及解决问题的途径。     

Numerical study of water management in the air flow channel of a PEM fuel cell cathode

The water management in the air flow channel of a proton exchange membrane (PEM) fuel cell cathode is numerically investigated using the FLUENT software package. By enabling the volume of fraction (VOF) model, the air–water two-phase flow can be simulated under different operating conditions. The effects of channel surface hydrophilicity, channel geometry, and air inlet velocity on water behavior, water content inside the channel, and two-phase pressure drop are discussed in detail. The results of the quasi-steady-state simulations show that: (1) the hydrophilicity of reactant flow channel surface is critical for water management in order to facilitate water transport along channel surfaces or edges; (2) hydrophilic surfaces also increase pressure drop due to liquid water spreading; (3) a sharp corner channel design could benefit water management because it facilitates water accumulation and provides paths for water transport along channel surface opposite to gas diffusion layer; (4) the two-phase pressure drop inside the air flow channel increases almost linearly with increasing air inlet velocity.     

A further study of the effects of water vapor concentration on the rate of combustion of an artificial graphite in humid air flow

Test specimes of graphite (disk-shaped, 10 mm in diameter and 1 mm in thickness) were burned in the stagnation region of an impinging oxidizer flow under atmospheric pressure; the velocity gradient of the impinging oxidizer flow was 100 s⁻¹. The surface temperature of the test specimen was kept constant during each experimental run by an external heat source in a range of 1200–1700K. Three groups of oxidizers containing water vapor were used. The first group consists of humid air having six H₂O concentrations (0.0681, 0.641, 3.13, 3.63, 5.15, 7.41 mol m⁻³ at 0.101 MPa and 320–339K); the second group, O₂-recovered humid air having a fixed O₂ concentration and five H₂O concentrations equal to, respectively, the higher five of those for the aforementioned humid air; the third group, N₂-enriched air having a fixed H₂O concentration and five O₂ concentrations equal to, respectively, the higher five O₂ concentrations for the aforementioned humid air. The following results have been obtained. (1) When the water vapor concentration in the oxidizer flow is low, water vapor actively suppresses the combustion rate. (2) As the water vapor concentration exceeds a certain value, the effect of water vapor to suppress actively the combustion rate diminishes. (3) When the water vapor concentration is high, water vapor actively augments the combustion rate in the high surface temperature range.     

Determination of transport parameters for multiphase flow in porous gas diffusion electrodes using a capillary network model

The changes of relative permeability and capillary pressure as a function of liquid water phase saturation, two key parameters in two-phase PEMFC models, are investigated using a capillary network model incorporating an invasion percolation algorithm with trapping. The two-dimensional capillary network accounts for capillary dominated drainage and cluster formation. It is shown that relative permeability is constant for low saturation, but follows a power law of saturation for high saturations, with an exponent of about 2.4 that is independent of network size or heterogeneity. An increase of the network size and reduction in heterogeneity tend to reduce the relative permeability, and relative permeabilities of much less then unity are obtained even for saturations as large as 0.8. Capillary pressure on the other hand does not vary with saturation and network size, but is influenced by heterogeneity only. This suggests that regardless of the interface shape and size, the capillaries at the interface maintain a constant average radius causing the capillary pressure to remain constant. It is finally shown that with appropriate scaling and for a given network heterogeneity, the normalized capillary pressure, single-phase permeability and relative permeability can be deduced for other choices of porous medium physical scales without requiring a new set of simulations.     

Measurements of minimum ignition energy in premixed laminar methane/air flow by using laser induced spark

Reducing engine pollutant emissions and fuel consumption is an important challenge. Lean-burning engines are a promising development; however, such engines require high-energy ignition systems for typical working conditions (equivalence ratio, Φ < 0.7). Laser-induced ignition is envisaged as a way to obtain high-energy ignition as a result of progress that has been made in laser beam technology in terms of stability, size, and energy. This study investigated the minimum energy necessary to ignite a laminar premixed methane air mixture experimentally. A parametrical study was performed to characterize the effects of the flow velocity, equivalence ratio, and lens focal length on the minimum energy required for ignition. Experiments were conducted using a premixed laminar CH₄/air burner. Laser-induced breakdown was achieved by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser with an anti-reflection-coated lens. Mixture ignition and the early stages of flame propagation were studied using a high speed Schlieren technique. Despite the stochastic characteristic of the laser breakdown phenomena, good reproducibility in the minimum energy required for the ignition measurements was observed. The cases in which the CH₄/Air mixture flow ignites are defined as those with a laminar flame front propagation visible in the Schlieren images 10 ms after the energy deposition. The same minimum ignition energy (MIE) versus equivalence ratio (Φ) type of curves were obtained with a laser-induced spark and with a spark plug. Due to the threshold of energy required to obtain breakdown and the stochastic character of the energy absorption by the spark, a constant value was obtained (corresponding to the breakdown threshold) when the minimum ignition energy was lower than the breakdown threshold. As already noticed by several authors, MIE values higher than those observed using spark plugs were obtained. However, these differences tended to disappear at the lean and rich fuel limits.     

In situ investigation of water transport in an operating PEM fuel cell using neutron radiography: Part 2 – Transient water accumulation in an interdigitated cathode flow field

An interdigitated cathode flow field has been tested in situ with neutron radiography to measure the water transport through the porous gas diffusion layer in a PEM fuel cell. Constant current density to open circuit cycles were tested and the resulting liquid water accumulation and dissipation rates with in-plane water distributions are correlated to measured pressure differential between inlet and outlet gas streams. The effect of varying the reactant gas relative humidity on liquid water accumulation is also demonstrated. These results provide evidence that the reactant gas establishes a consistent in-plane transport path through the diffusion layer, leaving stagnant regions where liquid water accumulates. A simplified permeability model is presented and used to correlate the relative permeability to varying gas diffusion layer liquid water saturation levels.     

Investigation of liquid water heterogeneities in large area proton exchange membrane fuel cells using a Darcy two-phase flow model in a multiphysics code

Proper management of the liquid water and heat produced in proton exchange membrane (PEM) fuel cells remains crucial to increase both its performance and durability. In this study, a two-phase flow and multicomponent model, called two-fluid model, is developed in the commercial COMSOL Multiphysics® software to investigate the liquid water heterogeneities in large area PEM fuel cells, considering the real flow fields in the bipolar plate. A macroscopic pseudo-3D multi-layers approach has been chosen and generalized Darcy's relation is used both in the membrane-electrode assembly (MEA) and in the channel. The model considers two-phase flow and gas convection and diffusion coupled with electrochemistry and water transport through the membrane. The numerical results are compared to one-fluid model results and liquid water measurements obtained by neutron imaging for several operating conditions. Finally, according to the good agreement between the two-fluid and experimentation results, the numerical water distribution is examined in each component of the cell, exhibiting very heterogeneous water thickness over the cell surface.     

Enhancing solar energy collection by using curved flow technology coupled with flow in porous media: an experimental study

In this study two solar energy collectors were designed and built. To enhance the heat transfer characteristics, flow in curved channel technology is used. Porous media (with 0.1453 porosity) composed of coarse aluminum chips fill the flow channels to provide for further increase in heat transfer performance and for extra energy storage capability.Measured data were recorded water flow rates that range between 50 and 400 l/h. The results show that the enhancement of heat transfer characteristics associated with the existence of porous media does not improve the calculated collector daily efficiency. The collector daily efficiency reduces during sunlight relative to that for that of clean collectors by approximately 1.0% and 2.0% at flow rates of 300 and 200 l/h, respectively. The daily efficiency at 300, 200 l/h flow rates are 60%, 56%, respectively, for the collector without porous media and 59%, 54%, respectively for the collector packed with porous media.It is noticed that using porous medium significantly decreases the rate of decline in water temperature to approximately half its value for the case without porous medium for flow rate of 300 l/h during the absence of sunlight. Also using porous medium will decrease the temperature rise across the collector during sunlight.The maximal outlet temperature reached was 73 °C for the collector without porous media at 70 l/h flow rate and 60 °C for the collector backed with porous medium at 50 l/h flow rate.