Thermal entrance lengths in a subcooled falling liquid film were studied experimentally on a vertical cylindrical heater preceded by an adiabatic section. A wavy-laminar flow regime close to the transition to turbulent flow was achieved. Water at atmospheric pressure was used as the test fluid. Heating surface temperatures were measured at eight axial locations along the flow direction and the local heat transfer coefficients were obtained as functions of heat flux and mass flow rate. Heat transfer coefficients were observed decreasing along the heater and reaching a minimum. The influences of the Reynolds number and heat flux on the thermal entrance length at low heat fluxes were weak. Both the Reynolds number and the heat flux strongly influenced the thermal entrance length at relatively high heat fluxes.
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