ABSTRACT

A continuous two-phase flow apparatus was built to study the convective heat transfer in subcooled falling liquid films of water. Film Reynolds numbers in the range of 800 to 1560 and heat fluxes up to 35 kW/m2 were employed. An unheated calming section (82 cm long) preceded the heating surface (105 cm long, 2.2 cm OD). Heating surface temperature was measured at eight locations along the heater by internal thermocouples with an uncertainty of 0.05°C. The calming section allowed the hydrodynamic boundary layer to develop before the thermal boundary layer and the onset of the evaporative processes. The variation of the heat transfer coefficient along the heater was obtained as a function of the heat flux and the film Reynolds number. The separation of the influences of the hydrodynamic and the thermal entrance regions contributed to a better understanding of these effects on the heat transfer coefficient.