CFD MODELLING AND EXPERIMENTAL INVESTIGATIONS OF AIR WATER FLOW THROUGH SERPENTINE CHANNEL

Abstract

Multiphase flows are ubiquitous in nature. Two-phase flow can be considered as the simplest example of multiphase flow. It can be solid-liquid flow, liquid-liquid flow, gas-solid flow, and gas-liquid flow. Gas-liquid flows displays a lot of complexities due to presence of a deformable interface. These flows are influenced by channel shape, channel orientation and the compressibility of one of the phases. The air water flow in mini channels finds application in compact heat exchangers, microelectronic cooling systems, fuel cell etc. Water removal from gas flow channels in Proton Exchange Membrane Fuel Cell (PEMFC) constitute one of its major applications in miniature systems. Often serpentine shape of these channels is preferred industrially. Despite the increasing applications, the literature on gas-liquid flow is relatively scarce and the present work attempts to investigate the physics of flow through extensive experimentation and numerical analysis. The experiments have been performed in a test section made up of acrylic plate. This plate has ten serpentine channels milled over and it is covered with another smooth acrylic plate at the top. There were three test sections with different hydraulic diameter. As orientation can play a significant role, hence experiments were performed to understand the effect of fluid flow rate and orientation. The test fluids used for entire range of experiments are zero air and deionised water. It is observed that as the hydraulic diameter of the channel reduced number of flow patterns observed decreased and the range of annular flow (most desirable flow pattern for PEMFC) increased. The vertical orientation in micro channel facilitated water removal from the channel owing to less maldistribution experienced by it. For micro channel vertical orientation exhibited four times of reduction in pressure drop in comparison to horizontal orientation of mini channel at similar combination of fluid flow rates. As water removal from serpentine channel is of importance attempts have been made to simulate suspended and adhered drop of water as well as slug flow using commercial CFD software ANSYS FLUENT 18. The parameters that are varied are drop size, air velocity and surface characteristics of channel wall. The surface characteristics is xviii altered by subjecting bottom wall of the channel to a wettability gradient As the pressure drop and water coverage ratio governs the performance of the PEMFC, the effect of wettability gradient on these parameters are estimated. A gradual hydrophobic Gas Diffusion Layer (GDL) surface resulted in lesser pressure drop as well as water coverage for suspended drop. In presence of a wettability gradient, the adhered drop shows very less water coverage of channel bottom surface, however, takes longer time to remove than suspended drop. The presence of wettability gradient enhances the chance of formation of slug flow into a more desirable film flow. Application of hybrid bottom surface proved beneficial with respect to pressure fluctuation in slug flow regime