Effect of Distributor Design on Gas-Liquid Distribution in Monolithic Bed at High Gas/Liquid Ratios

Experiments were carried out to investigate the liquid flow distribution at high gas/liquid ratios in a cold model monolith bed of a 0.048 m diameter with 62 cells per cm2.Three types of distributor for the liquid distribu-tion were used to evaluate their distribution performance.Local liquid saturation in individual channels was meas-ured using 16 single-point optical fiber probes mounted inside the channels.The results indicate that 1) The optical fiber probe technique can measure phase distribution in the monolith bed;2) Liquid saturation distribution along the radial direction of the monolith bed is not uniform and the extent of non-uniformity depends on the distributor de-sign and phase velocities;and 3) The tube array distributor provides superior liquid distribution performance over the showerhead and nozzle distributors.

Effects of Cracks on the Mass Transfer of Polymer Electrolyte Membrane Fuel Cell with High Performance Membrane Electrode Assembly

A two-dimensional geometric model is developed for a polymer electrolyte based on the liquid water penetration mechanism in the membrane electrode assemblies under the action of capillary pressure.The effects of the diameter,number,and distribution of cracks in the micro-pore layers(MPLs)of the modeled MEA on the performance of the PEMFC are simulated to investigate the influence of mass transfer across the membrane.The results indicate that liquid water in the catalyst layer(CL)of the MEA can be discharged to gas channels through the cracks in MEA under the action of capillary pressure,thereby alleviating the flooding in the CL and enhancing the diffusion of oxygen to the CL.When the proportion of the total area of cracks in the active area of the MEA was 8%-12%,crack diameter was 20-30μm,and cracks were distributed uniformly.MEAs with and without cracks were prepared,fuel cells were assembled,and their performance was measured.The effects of cracks on mass transfer were then verified.This study helps prepare MEAs with controllable cracks.

Simulation of the Internal Transport Phenomena for PEM Fuel Cells with Different Modes of Flow

A numerical model for proton exchange membrane (PEM) fuel cell is developed, which can simulate such basic transport phenomena as gas-liquid two-phase flow in a working fuel cell. Boundary conditions for both the conventional and the interdigitated modes of flow are presented on a three-dimensional basis. Numerical techniques for this model are discussed in detail. Validation shows good agreement between simulating results and experimental data. Furthermore, internal transport phenomena are discussed and compared for PEM fuel cells with conventional and interdigitated flows. It is found that the dead-ended structure of an interdigitated flow does increase the oxygen mass fraction and decrease the liquid water saturation in the gas diffusion layer as compared to the conventional mode of flow. However, the cathode humidification is important for an interdigitated flow to acquire better performance than a conventional flow fuel cell.

Critical Heat Flux in Forced Convective Boiling with a Wall Jet

The critical heat flux (CHF) in the forced convective boiling with a wall jet has been investigated.The experiments of CHF with a wall jet have been performed over a wide range of ρ l/ρ g=6.6-1 603 and ΔT sub =0-60 K. The mechanism on CHF is discussed and a CHF model based on heat balance in sublayer can provide a good clue for analyzing and deriving CHF.Finally,a generalized correlation is presented, which can predict CHF for saturated and subcooled conditions.

A New Approach to Investigation of vdW Type of Equations of State

A new approach to the investigation of vdW type of equations of state (EOS) is developed by embedding a vapor pressure equation and a saturated liquid volume equation into vdW type EOS, which results in a new function AS(T). The AS(T) possesses the properties of an attractive parameter A(T), and if an EOS is accurate in the whole PVT space, then its numerical value equals A(T). As a useful tool for investigating EOS, the As(T) has been used to make comparisons among RKS, PRSVII, PT and ALS EOS, and to indicate where the shortcomings of the EOS are coming from. Based on the AS(T), a possible way to develop a real predictive equation of state is also suggested.

Numerical investigation of water and temperature distributions in a proton exchange membrane electrolysis cell

A three-dimensional, non-isothermal, two-phase model for a PEM water electrolysis cell(PEMEC) is established in this study.An effective connection between two-phase transport and performance in the PEMECs is built through coupling the liquid water saturation and temperature in the charge conservation equation. The distributions of liquid water and temperature with different operating(voltage, temperature, inlet velocity) and physical(contact angle, and porosity of anode gas diffusion layer) parameters are examined and discussed in detail. The results show that the water and temperature distributions, which are affected by the operating and physical parameters, have a combined effect on the cell performance. The effects of various parameters on the PEMEC are of interaction and restricted mutually. As the voltage increases, the priority factor caused by the change of inlet water velocity changes from the liquid water saturation increase to the temperature drop in the anode catalyst layer. While the priority influence factor caused by the contact angle and porosity of anode gas diffusion layer is the liquid water saturation. Decreasing the contact angle or/and increasing the porosity can improve the PEMEC performance especially at the high voltage. The results can provide a better understanding of the effect of heat and mass transfer and the foundation for optimization design.

An Improvement on the Method for Calculating the Capillary Limit of Axial-Grooved Heat Pipes

A new model has been developed to predict the capillary limit of axial-grooved heat pipe.In the model the concepts of liquid saturation or liquid fraction of the cross-sectional area of groove,the modified relative permeability,absolute permeability of groove and Leverrt’s function are used.The Leverrt’s function is well represented by the function f（s）=(1/51/2)（1/s-1）0.175.In the model the effects of gravitational force,capillary force and viscous force are considered.The calculated results are in good agreement with existing experimental data reported in the literature.