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The two important considerations in the design of a heat exchanger are — the total heat transfer rate and the allowable pressure drop. The allowable pressure drop defines the maximum flow rate through a single microchannel and economics drives the design towards this flow rate. Typically the flow rate in the microchannel is in laminar flow regime (Re < 2000) due to smaller hydraulic diameter. The laminar flow heat transfer in a smooth microchannel is limited by the boundary layer thickness. Commonly the heat transfer rate is enhanced by passively disrupting the laminar boundary layer using protrusions or depressions in the channel walls. More often these methods are best applicable at small range of Reynolds number where the heat transfer rate enhancement is more than the pressure drop increase and break down as the flow rate is changed outside the range. The benefit of a flow disruption method can be reaped only if it provides higher heat transfer enhancement than the increase in the pressure drop at the working flow rates in the microchannel. A heat transfer efficient microchannel design has been developed using wall features that create stable disrupted flow and break the laminar boundary layer in a microchannel over a wide range of flow rates. The paper experimentally investigates the developed design for the heat transfer enhancement and pressure drop increase compared to a smooth wall microchannel. A simple microchannel device was designed and fabricated with and without wall features. The experiments with single gas phase fluid showed promising results with the developed wall feature design as the heat transfer rate increase was 20% to 80% more than the pressure drop increase in the laminar regime. The wall feature design was an important variable to affect the magnitude of performance enhancement in different flow regime. A general criterion was developed to judge the efficacy of wall feature design that can be used during a microchannel heat exchanger design.

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.