Optimization of Metallic Multi-Microchannel Array Evaporators

Pulsation and maldistribution effects (hot spot formation) with characteristic frequencies below 100 Hz occurring in electrically powered microchannel array heat exchangers used as evaporators for water were investigated primarily with the aid of visualisation techniques. Pulsation at subaudio frequencies was found to be dominated by the consequences of boiling in the inlet plenum, creating large vapour bubbles that intermittently entered the microchannel array, pushing liquid water at velocities too high to achieve complete evaporation. A new design minimising the residence time in the inlet and comprising an intermediate void was found to produce a two phase mixture that could be evaporated in an array of 68 microchannels, each 200 μm wide, 100 μm deep, and 20mm long, at a mass flux of 60 kgm⁻² s⁻¹ at an average surface temperature of 220°C. The redesign led to a change in characteristic flow patterns in the microchannel arrays from plug and slug flow to film flow and drops moving along the walls of the microchannels, as evident from high speed (10³ fps) video sequences. By means of infrared thermography of the surface of a metallic device comprising four layers of microchannel arrays, maldistribution between the microchannel array layers leading to hot spot formation was observed when the device was operated in constant power mode. The formation of these hot spots could be avoided by the use of simple temperature control electronics operating at characteristic frequencies below 10⁻¹ Hz.