LOCAL HEAT TRANSFER CHARACTERISTICS OF HORIZONTAL IN-TUBE EVAPORATION

Local heat transfer characteristics along the perimeter of horizontal tubes for evaporating R-12 were investigated with various parameters of different tube wall thickness, mass flux, heat flux, and quality. Plain copper tubes with outside diameter of 9.5mm, and thicknesses of 0.8mm and 0.4mm were tested with indirect electrical wire heating. Circumferential and axial wall temperatures were measured, and exit flow visualization was carried out to understand the local heat transfer mechanism. Because of significant heat conduction for the present tubes, the circumferential wall superheat profile was quite flat and the wall superheat function was insensitive to the local heat transfer coefficient. A three-step model for predicting the circumferential heat transfer coefficient at the partially wetted flow is proposed. It is based upon a liquid film distribution that consists of the wavy film and the base film. The five parameters that characterize the predicted wall superheat were obtained by regression. The liquid film distribution predicted by the present model qualitatively agreed with flow visualization. Although a large variation in the circumferential heat transfer coefficient is predicted, the average heat transfer with and without considering the circumferential heat conduction was within 10% for a mass flux of 50kgl(m²·s) and a heat flux of 5kW/m². The characteristics of the circumferentially averaged heat transfer coefficient were explained mainly by the liquid film wetting in separated flows.