Sean P. Fitzgerald

Microchannel Chemical Reactors for Fuel Processing Applications. II. Compact Fuel Vaporization

A fuel processor is a critical element for the deployment of automotive fuel cell power systems. One component of the fuel processor, the compact gasoline vaporizer, was demonstrated at full-scale using commercial-grade gasoline. The full-scale process volume was less than 0.3 liters; it vaporized nearly 300 mL/min of gasoline, which is sufficient to support a 50-kWe fuel cell. The reduction in hardware volume was made possible using a microchannel reactor-based design; the compact process hardware is roughly an order of magnitude smaller than conventional technology.

Enhancing Two-Phase Flow Mixing and Mass Transfer in Microchannel With Surface Features

Slug or plug flow is generally considered as major flow pattern in microchannels in gas-liquid two-phase flow. A new microchannel design has enabled experimental interfacial surface area density exceeding 10,000 m2 /m3 based on the two-phase volume in bubbly flow, and mass transfer coefficients exceeding 10sec−1 . Numerical simulations as well as experiments are presented with the new microchannel design. The velocity components of secondary flow induced by specially designed angled microgrooves break the gas phase into small bubbles, where otherwise much larger gas pockets/slugs would dominate in flat or smooth wall microchannels. As such, mixing of the two phases and mass transfer are greatly enhanced as a results of increased interfacial surface area density and reduced average mass transfer distance. The Volume-Of-Fluid (VOF) method is used in the numerical computations for different surface feature patterns, gas and liquid flow rates, liquid viscosity and surface tension. In the experiments, nitrogen, carbon dioxide and water are used as the two phase media. The two-phase superficial velocity in the channel is in the range 0.45–2.75 m/s. The results show that the two-phase flow in the microchannel with the angled microgrooves leads to enhanced mass transfer relative to the flat microchannel. Higher flow rates and higher liquid viscosity lead to smaller gas bubbles and in turn enhanced mixing. Opportunities for additional improvement exist with increasing flow rates and optimized processing conditions.Copyright