Subcooled flow boiling heat transfer in narrow passages

Subcooled flow boiling heat transfer coefficients for refrigerants R I I and HCFC123 in smooth copper tubes of small diameter have been investigated experimentally. The parameter ranges examined are: tube diameters of 0.92 and 1.95 mm; heat fluxes 11-170 kW m(-2); mass fluxes 110-1840 kg m(-2) s(-1). The range of liquid Reynolds numbers encompassed by the data set is 450 to 12,000. The data in the subcooled and saturated regions are well represented by the simple addition of convective and nucleate boiling heat transfer contributions h = h(conv) + h(pb)(q(nb)) DeltaT(sat)/DeltaT(mean) where the nucleate component is found to be represented best by the Gorenflo pool boiling correlation using q(nb) = q(tot) - q(conv). The convective component is calculated as for single-phase liquid-only heat transfer, with due allowance for laminar entrance effects. There is no evidence that convection suppresses the nucleate term nor that nucleation events enhance the convective term, even in laminar and transitional flows. However, the laminar flows, in particular, are prone to enhancement by unknown mechanism, possibly by outgassing of the fluid as it is heated through the subcooled region. (C) 2003 Elsevier Ltd. All rights reserved.