Heat transfer characteristics of flow boiling in a micro channel array with various inlet geometries

Abstract

Flow boiling in micro channels is considered to be a promising solution for electronics cooling. However, its development is impeded by the lack of full understanding of mechanisms responsible for heat exchange and its susceptibility to instabilities. A common way of flow stabilization is employing restrictions at channel inlet, but their impact on micro channel thermal performance remains an open question. Therefore, this paper focuses on experimental determination of the dominant heat transfer mechanism during flow boiling of R245fa in 10 parallel rectangular channels with hydraulic diameter of 1 mm without inlet restrictions and with 0.5 mm wide and 2.5 mm long restrictions. Studied heat fluxes vary between 30 and 50 kW/m2, mass fluxes between 400 and 1000 kg/m2s, saturation temperatures between 49 and 82 ∘C, inlet subcoolings between 13 and 19.4 K. For the subcooled flow the dominant heat transfer mechanism was forced convection with heat transfer coefficient rising with mass flux, vapor quality and saturation temperature. For the saturated flow nucleate boiling was the dominant mechanism with heat transfer coefficient increasing with heat flux and saturation temperature and decreasing with mass flux and vapor quality. Geometry with inlet restrictions in general exhibited similar thermal behavior, but in the subcooled zone a more pronounced positive dependence on saturation temperature and a negative dependence on mass flux especially at lower temperatures were observed. This indicates that restrictions augment nucleate boiling contribution to the forced-convection-dominated heat exchange in the subcooled zone. Restrictions yielded similar or slightly lower heat transfer coefficients. Comparing the data against 24 heat transfer models showed that the model of Thiangtham et al. was the most accurate with MAPE 23.7% and 20.2% for geometry without and with restrictions respectively.