Two-phase flow and heat transfer in microchannels.

摘要

Flow boiling in microchannels has been investigated broadly over the last decade for electronics cooling applications; however, the implementation of microchannel heat sinks operating in the two-phase regime in practical applications has lagged due to the complexity of boiling phenomena at the microscale. In the current study, extensive experimental work has been conducted to systematically determine the effects of important geometric and flow parameters on flow regimes and heat transfer in microscale flow boiling. Local heat transfer measurements obtained with simultaneous, detailed flow visualizations lead to a better understanding of boiling phenomena and the governing heat transfer mechanisms in microchannels.;Based on the experimental results obtained with microchannel test pieces encompassing a wide range of channel dimensions and operating conditions, a new transition criterion is developed which predicts the conditions under which microscale confinement effects are exhibited in flow boiling. This criterion depends on the value of a parameter termed the convective confinement number in this study, Bo0.5xRe, which depends not only on the channel dimensions and fluid properties, but also on the mass flux. It is shown that physical confinement exists in the microchannels for Bo0.5xRe 160. In this case, thin-film evaporation contributes to heat transfer in addition to nucleate boiling and results in larger values of heat transfer coefficient compared to those cases in which no confinement is observed. For the larger convective confinement numbers where physical confinement does not occur and nucleate boiling is dominant, the heat transfer coefficient is independent of channel dimensions.;A comprehensive flow regime map for flow boiling of a perfluorinated dielectric liquid (FC-77) is developed based on the experimental data. Using the convective confinement number and a nondimensional form of heat flux as coordinates, the flow regime map reveals four distinct regions of confined slug, bubbly, churn/confined annular, and churn/annular/wispy-annular flow regimes separated by two quantitative transition lines.;Models are proposed for prediction of the heat transfer coefficient in each of the four regions in the flow regime map. Also, regime-based prediction of pressure drop in microchannels is discussed by evaluating pressure drop of each flow regime along the microchannels separately.