Flow Structures and Frictional Characteristics on Two-phase Flow in Microchannels in Pem Fuel Cells

This experimental paper presents a study of gas-liquid two phase flow in rectangular channels of 500μm x 45μm and 23.7mm long with different wall conditions of hydrophilic and hydrophobic surface, in order to investigate the flow structures and the corresponding friction factors of simulated microchannels of PEMFC. The main flow in the channel is air and liquid water is injected at a single or several discrete locations in one side wall of the channel. The flow structure of liquid water in hydrophilic wall conditioned channel starts from wavy flow, develops to stable stratified film flow, and then transits to unstable fluctuating film flow, as the pressure drop and the flow velocity of air increase from around 10 kPa to over 100 kPa. The flow structure in hydrophobic channel develops from the slug flow to slug-and-film flow with increasing pressure drop and flow velocity. The pressure drop for single phase flow is measured for a base line study, and the fRe product is in close agreement with the theoretical value (fRe=85) of the conventional laminar flow of aspect ratio 1:11. At the low range of water injection rate, the gas phase fRe product of the two phase flow based on the whole channel area was not substantially affected by the water introduction. However, as the water injection rate increases up to 100 μL/min, the gas phase fRe product based on the whole channel area deviates highly from the single phase theoretical value. The gas phase fRe product with the actual gas phase area corrected by the liquid phase film thickness agrees with the single phase theoretical value. INTRODUCTION In the past decades, Proton Exchange Membrane Fuel Cells (PEMFC) have been studied as effective portable power sources due to many advantages such as low operating temperature, high power density, and good turn on-and-off capability. Recently, the relation between performance of PEMFC and the gas routing channel size was studied and it was found that sub-millimeter channels for gas routing improved overall fuel cell performance [Cha et al.(2004)]. However, the use of microchannels causes complicated water management issues because of the increasing importance of interfacial forces such as surface tension as the channel size decreases. Gas-liquid two phase flow in microchannels has been extensively studied in the scope of heat transfer including boiling and condensation, flow regime characterization, and pressure drop relations. Zhang et al (2002) characterized microchannel heat transfer phenomena in two phase flow. Flow regime and pressure drop in two phase flows was studied extensively by Kawahara et al (2002, 2003), Lee and Lee (2001), and Mishima and Hibiki (1996). They used liquid as the baseline transporting fluid, and gas was introduced to the liquid flow. Also, they tried to understand the flow phenomena of gasliquid two phase flow in terms of mixed two phase parameters such as two phase multiplier and Chisholm parameter. This approach may be appropriate for air-water mixed two phase flow, but there is little physical justification for this approach in the separated and stratified two phase flow regime. Gas liquid two phase flow in microchannels of PEM fuel cell can have diverse flow structures of gas liquid two phase flow. For example, near the downstream of the PEMFC microchannels the flow structure could be air water mixed flow due to the flooding of water inside the gas supply channels. However, farther upstream the flooding of gas supply channels does not happen and the two phase flow inside the channels shows different structure. To date, the study of two-phase flow in microchannels of PEMFC has mostly focused on transport phenomena in the Gas Diffusion Layer (GDL). Thus, there exists an urgent need for the study on two phase flow in PEMFC microchannels in order for a full model of a PEM fuel cell to be developed. For the development of predictive models of two-phase flow in microchannels, the characteristics of pressure drop and flow of gas and the liquid water transport phenomena should be studied under a broad range of conditions. 1 Copyright © 2005 by ASME This experimental work investigates the flow structures in microchannels under different surface conditions hydrophilic surface and hydrophobic surface, and then develops a model for two phase flow in microchannel under hydrophilic and hydrophobic surface.