Flow boiling heat-transfer in micro to macro transition flows

摘要

Heat-transfer coefficients and pressure drops are reported for a test section containing 25, 1 mm by 1 mm, parallel channels. The channels were 50 mm long and had a glass top plate to allow visual observations. The data were produced while boiling R113 at atmospheric pressure. The mass flux range was 200-600 kg/m~2 s and the heat flux range was 5-80 kW/m~2. 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 significant variation in the single-phase, heat-transfer coefficient in the entrance zone, the interceding aluminium and copper material is shown to produce a near isothermal wall boundary condition. The heat conduction effect is taken into account in the data analysis. Heat-transfer coefficients and pressure drops are reported for single-phase and boiling flows, with sub-cooled and saturated boiling data obtained. The single-phase results are shown to be position dependent, consistent with a developing laminar flow. All of the measured boiling heat transfer coefficients are shown to be reasonably independent of mass flux and vapour quality. However, some are shown to be independent of heat flux while others are not. This is true of the saturated and subcooled boiling data, which are all substantially above the values associated with nucleate boiling. The boiling data therefore have a nucleate and convective component. The convective boiling component is shown to have a heat-transfer coefficient that is reasonably independent of heat flux, mass flux, vapour quality and liquid sub-cooling. The heat-transfer coefficients are compared to macro-scale and micro-scale correlations, neither of which adequately explains the data. Measured two-phase pressure drops are in reasonable agreement with a macro-scale correlation.