Flow Structures and Frictional Characteristics on Two-Phase Flow in Microchannels in PEM Fuel Cells

Abstract

This experimental paper presents a study of gas-liquid two phase flow in rectangular channels of 500μm × 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.

Liquid-in-gas droplet microfluidics; experimental characterization of droplet morphology, generation frequency, and monodispersity in a flow-focusing microfluidic device

“…Although some early works have investigated the breakup of a liquid jet into droplets using a focusing air in unconfined nozzle-like geometries [16][17][18][19][20][21], gaseous generation of monodisperse drops within a microfluidic system is an entirely different process due to confinement-induced effects [22]. Interaction of liquid and gas inside confined microchannel geometries has also been widely studied in the operation of proton exchange membrane fuel cells (PEMFC) [23][24][25][26][27][28][29][30][31][32]. However, droplet generation in gaseous microfluidic systems that employ similar architectures as those used in conventional oil-based systems has been the subject of a very few studies during recent years.…”

Two-Phase Flow Pressure Drop Hysteresis under Typical Operating Conditions for a Proton Exchange Membrane Fuel Cell

“…This behavior occurs when the gas and liquid flow rates (determined by a given current density) are increased along a set path and then decreased along the same path with differing pressure drops. Additionally, flow regime hysteresis has been observed in minichannels bounded by a porous wall, where the transition between flow regime depended on whether the air flow rate was varied in an ascending or descending manner (7). A higher pressure drop represents a larger parasitic power loss, thus the hysteresis phenomenon requires additional study.…”