Abstract
Two-phase flow and boiling heat transfer in a micro-channel evaporator, to be integrated with a micro methanol reformer, designed with capillary channels, V-shaped micro diffusers at the inlet and localized heating area is investigated in this study. The evaporator with a dimension of 20 20mm2 machined on a silicon wafer with a thickness of 1.1 mm. The microchannel employed is diverging with inlet width of 100µm and outlet width of 150µm or 170µm, while the channel depth is uniformly at 100µm. The capillary channels at the inlet with width of 2µm and length of 500µm are designed to enable self-feeding of liquid methanol, while the V-shaped diffusers are designed to make flow distribution in the microchannels uniform. Two different length proportions of V-shaped diffuser to diverging channel, i.e. 1:1 and 1:2 have been tested. The heating area is localized below the diverging channels, resulting in the heating area of 1/2 and 2/3, respectively. A heated copper block or a film heater has been employed for localized heating. The experimental results indicate that the film heater performed better with less heat loss. Experimental results demonstrate relatively uniform flow distribution over all diverging channels with the inlet capillary channels and V-shaped micro diffusers. However, the uniform flow distribution is disturbed by randomized bubble nucleation in various channels. The flow pattern recorded by high speed camera shows that when the localized heating area is 2/3, it maintains a lower wall temperature at upstream near the inlet of evaporator than that under uniform heating over the whole test section. The lower wall temperature keeps the methanol subcooled or boiling with low quality. Such a situation makes methanol smoothly feed through the capillary channels and spread uniformly by V-shaped micro diffusers and distributed to the diverging channels relatively uniformly at the downstream. The methanol is then evaporated stably and finally dried out before leaving the diverging channel. Results shows that the critical heat flux increase nearly linearly with increasing the mass flux of methanol. For a volume flow rate of methanol at VMeOH =1.08sccm or mass flux of 20.7kgm-2s-1, the highest efficiency of heat to the methanol evaporation could reach about 88.7% and the critical heat flux can reach 248 kWm-2