Experimental analysis of a bubble pump operated H2O–LiBr vapour absorption cooler

Experimental analysis of a H₂O–LiBr vapour absorption cooler based on bubble pump technique and hydrostatic principle are presented. Heat input to the bubble pump and boiler, pump lift/driving head and weak solution concentration are varied to analyze the system performance. The cooler is tested at different load conditions, i.e., at no load, pull down load and steady load conditions. Results indicate that coefficient of performance of about 0.50 could be attained for the boiler temperature of 85 °C and condenser and absorber temperatures in excess of 40 °C.     

Comparison of methanol-based working fluid combinations for a bubble pump-operated vapour absorption refrigerator

Miniaturization of an alcohol-based absorption refrigerator requires an air-cooled absorber and condenser and the replacement of customary solution pumps by the bubble pump. Evaluation of such a refrigerator requires thermodynamic (specific heat and heat of mixing) and thermophysical (vapour pressure, density, viscosity, surface tension and solubility) properties of refrigerant–absorbent solution. These property correlations for five alcohol-based working combinations, majority of them obtained by curve fitting, have been complied and presented in this paper along with their validity ranges and percentage of error. The working fluids have been analyzed and compared with reference to the solution density governing the hydrostatic height, viscosity and specific heat affecting the heat and mass transfer and solubility to avoid crystallization. Further the variations of performance parameters like cut-off temperature, circulation ratio, coefficient of performance and efficiency ratio of these working fluids with respect to various operating conditions are discussed. © 1998 John Wiley & Sons, Ltd.     

Effect of GDL compression on pressure drop and pressure distribution in PEMFC flow field

Improving reactant distribution is an important technological challenge in the design of a PEMFC. Flow field and the Gas Diffusion Layer (GDL) distribute the reactant over the catalyst area in a cell. Hence it is necessary to consider flow field and GDL together to improve their combined effectiveness. This paper describes a simple and unique off-cell experimental setup developed to determine pressure as a function of position in the active area, due to reactant flow in a fuel cell flow field. By virtue of the experimental setup being off-cell, reactant consumption, heat production, and water generation, are not accounted as experienced in a real fuel cell. A parallel channel flow field and a single serpentine flow field have been tested as flow distributors in the experimental setup developed. In addition, the interaction of gas diffusion layer with the flow distributor has also been studied. The gas diffusion layer was compressed to two different thicknesses and the impact of GDL compression on overall pressure drop and pressure distribution over the active area was obtained using the developed experimental setup. The results indicate that interaction of GDL with the flow field and the effect of GDL compression on overall pressure drop and pressure distribution is more significant for a serpentine flow field relative to a parallel channel flow field.     

Effect of pressure ratio on transient flow in hydrogen circulating pump

The pressure ratio has an important influence on the performance and internal flow characteristics of the positive displacement pump. In this paper, the influence of the four pressure ratios 1.1/1.2/1.3/1.4 on the internal flow characteristics of the hydrogen circulating pump is studied, the internal relationship between the change of pressure ratio and the flow pattern in the pump are clarified, and the leakage flow pattern and its coupling mechanism in each gap are revealed. The results show that the gap leakage flow induced by pressure difference is an important reason for flow disorder in the pump, however, the generation and growth of gap leakage flow will be affected not only by the pressure difference, but also by the shear drive. The scale and influence of axial gap leakage are far greater than the other two types of gap leakage. The existence of gap leakage flow makes the rate of flow and pressure presents a large amplitude high-frequency pulsation characteristic. The research results of this paper provide a reference for the efficient and stable operation of hydrogen circulating pump in fuel cell system.     

Estimates of pressure gradients in PEMFC gas channels due to blockage by static liquid drops

Numerical analyses are presented to explain the effect of drop size and contact angle on local pressures inside small channels. These pressures and channel characteristics are of interest when water condenses in the gas channels of Proton Exchange Membrane Fuel Cells and hence the study uses Reynolds numbers consistent with as typical utilization of reacting gases in 200 cm² flow fields (i.e., 200 < Re < 1500 and stoichiometries of 1.2–2.0 at 1.0 A/cm²). The analyses were performed using three-dimensional computational fluid dynamic techniques and the results show that pressure drops are minimal until the blockage was greater than 50%. As blockage increased further, due to larger drops or increased hydrophobicity, pressure drop increased. The results of a stagnant drop are supported by visualization experiments, which show minimal distortion of the drop for these low flow rates, small ratios, and hydrophobic contact angles. Proper scaling parameters and design criteria for microchannels validation experiments are presented.     

Effect of porous coating on two-phase pressure drop of water during up-flow boiling in tubes

This paper focuses on the effects of porous coating on two-phase flow pressure drop during up-flow boiling of water in vertical tubes. The experiments were carried out under subcooled fluid–inlet conditions (5–73 K) for different mass fluxes (200–400 kg/m² s) and pressures (0.11–0.69 MPa). The measured pressure drops were compared first with each other, and then with correlations from literature. It was found that the best agreement between predicted and measured values is obtained by the method of Thom [3] for a smooth tube and by Müller-Steinhagen and Heck's method [4] for a porous coated tube respectively.     

Effect of high compression ratio on improving thermal efficiency and NOx formation in jet plume controlled direct-injection near-zero emission hydrogen engines

The Plume Ignition and Combustion Concept (PCC) developed by the authors significantly reduced nitrogen oxide (NOx) emissions in a direct-injection hydrogen engine under high-load operation. With PCC, a rich fuel plume is ignited immediately after completion of injection in the latter half of the compression stroke to reduce NOx formation. Simultaneously, high thermal efficiency was also achieved by mitigating cooling losses through optimization of the jet configuration in the combustion chamber. This basic combustion concept was applied to burn lean mixture in combination with the optimized hydrogen jet configuration and the application of supercharging to recover the power output decline due to the use of a diluted mixture. As a result, a near-zero-emission-level engine has been achieved that simultaneously provides high thermal efficiency, high power output and low NOx emissions at a single-digit ppm level [1]. In this study, a high compression ratio was applied to improve thermal efficiency further by taking advantage of the characteristics of hydrogen fuel, especially its diluted mixture with a high anti-knock property. As a result, NOx emissions at a single-digit ppm level and gross indicated thermal efficiency of 52.5% were achieved while suppressing knocking at a compression ratio of 20:1 by optimizing the excess air ratio and injection timing, and increasing power output by supercharging.