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Flow boiling heat transfer of water and sugar solutions in an annulus



The subcooled flow boiling heat-transfer characteristics of water and sugar solutions in an annulus have been investigated under operating conditions similar to those found in the sugar mill evaporators. The variations in the effects of heat flux, fluid velocity, and subcooling on the observed heat-transfer coefficients over a range of sugar concentrations implied an enhanced contribution of nucleate boiling heat transfer in the partial flow boiling regime, where both forced convection and nucleate boiling heat transfer occurred. Increasing the sugar concentration led to a significant drop in the observed heat-transfer coefficient because of a mixture effect, which resulted in a local rise in the saturation temperature of sugar solution at the vapor-liquid interface. The reduction in the heat-transfer coefficient with increasing sugar concentration is also attributed to changes in the fluid properties (for example, viscosity and wettability) of solutions with different sugar content. The experimental data were compared with predictions by various flow boiling models and a modified Chen model incorporating the effect of sugar concentration. The new model provided a better prediction of heat-transfer coefficients of sugar solutions in an annulus. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1119–1128, 2004