Enhanced flow boiling performance in high-aspect-ratio groove-wall microchannels

Abstract

The dry-out phenomenon easily occurs on the side walls in high aspect ratio (channel depth to width ratio, AR) microchannel due to the decreasing of liquid film thickness. It is desirable to design better microchannels to address this issue. In this study, we proposed a new high-aspect-ratio groove-wall microchannel (Height=200 μm, Width=80 μm, Length=10 mm, AR=2.5), which features a series of rectangular grooves etched on the plain side walls. Flow boiling experiments were conducted in the novel groove-wall microchannel and conventional plain-wall microchannel at two inlet subcoolings of 40 and 70 °C, three mass fluxes (G) of 446, 630 and 815 kg/m2·s and wall heat fluxes of 3.8–139.5 W/cm2, using deionized water as the working fluid. The outlet of the channel is connected to the atmosphere, maintaining a saturation temperature of 100 °C. Flow visualization was captured by a high-speed camera to better understand the flow and heat transfer characteristics. Compared with plain-wall microchannel, the groove-wall microchannel demonstrates earlier boiling incipience, higher heat transfer coefficient and lower pressure drop in the two-phase flow region. It was found that the groove-wall microchannel obtains better heat transfer coefficient (HTC) at higher inlet subcooling. In comparison with plain-wall microchannel, the HTC enhancements of 25.4%, 73.0% and 55.0% were obtained respectively for the mass fluxes of 446, 630 and 815 kg/m2·s at ΔTsub=70 °C, while the corresponding HTC enhancements are 25.0%, 36.5% and 56.7% at ΔTsub=40 °C, respectively. Since the groove walls characterize the thin film evaporation especially at high heat flux, an enhanced critical heat flux (CHF) was obtained. At G = 446, 630 and 815 kg/m2·s, the groove-wall microchannel enhances the CHF respectively by 26.5%, 31.1% and 45.0% at ΔTsub= 40 °C, and the corresponding enhancements are 5.3%, 10.3% and 10.0% at ΔTsub=70 °C, respectively. Furthermore, flow reversal and boiling instability (pressure drop and wall temperature fluctuations) were drastically suppressed. Boiling instabilities are more severe at high inlet subcooling. The maximum pressure drop reductions of 27.4%, 42.0% were achieved in groove-wall microchannels for the mass fluxes of 630 and 815 kg/m2·s respectively at ΔTsub= 40 °C, and the corresponding maximum reductions are 9.07%, 23.6% at ΔTsub=70 °C, respectively. Almost no difference of the pressure drop was found in two microchannels at G = 446 kg/m2·s.