An investigation into flow boiling heat transfer and pressure drop in a pin–finned heat sink

Abstract

Heat-transfer coefficient and pressure drop measurements are reported for a heat sink comprising pin–fins with a cross section of 1mm by 1mm and a height of 1mm. The pin–fins were manufactured on a 50mm square base plate in a square, in-line arrangement with a pitch of 2mm. The data were produced while boiling de-ionised water at atmospheric pressure. The mass flux range was 40–200kg/m2s and the heat flux range was 30–470kW/m2.The test section was heated from below by an electrical heating method that is normally associated with a constant heat flux boundary condition. However, because of the variation in single-phase liquid temperature in the entrance zone and the variation in the heat-transfer coefficient, the interceding aluminium and copper material is shown to produce a non-uniform heat flux with a near isothermal wall boundary condition. The heat conduction effect in the wall is taken into account in the analysis of the data and in the calculation of the heat-transfer coefficients.Heat-transfer coefficients and pressure drops are reported for single-phase and boiling flows, with subcooled and saturated boiling data obtained. The single-phase results are shown to be reasonably independent of position. The measured boiling heat-transfer coefficients are shown to be associated with flow pattern and to be confined, as the capillary length for water is much larger than the flow passage dimensions. This is true of the saturated and subcooled boiling data, as slugs were observed in subcooled liquid flows. The criterion for the onset of slug flow is not compatible with that associated with macro-scale flows. All of the boiling data were obtained at wall superheats substantially above the values associated with nucleate boiling. They therefore have a nucleate and convective component. The convective component is shown to be the most dominant.The water data are compared with R113 data obtained in a previous study. The R113 data are shown to be associated with slug and annular flow, and are, in the main, not confined, with its capillary length similar to the channel flow dimensions. The R113 flow pattern transition criteria are comparable to macro-scale values. Some consistency in behaviour is shown to exist in the slug heat-transfer coefficients obtained with water and R113.