EXPERIMENTAL INVESTIGATION ON BEHAVIORS AND HEAT TRANSFER IN SHEAR-DRIVEN LIQUID FILM FLOW

Abstract

Experimental investigation on heat transfer characteristics of shear-driven liquid film in co-current gas flow is presented at different heat fluxes up to 300 kW/m2, gas Reynolds numbers up to 3175, liquid film Reynolds numbers of 27.6 − 75.7 and inlet liquid temperatures of 25, 40, and 80° C. Water and nitrogen are used as test liquid and gas, respectively. The heated section is a rectangular duct with 30 mm in width and 5 mm in height. The heated length is 100 mm divided into 10 segments in the flow direction by the structure of thermally insulated, which makes possible the evaluation of local heat transfer coefficients. The unexpected initiation of liquid film rupture at the upstream edge of heated section is observed under some combinations of heat fluxes and inlet liquid temperatures. In addition, the decrease in the width of dry area towards the downstream is observed with increasing interfacial temperature. The local heat transfer coefficient is increased towards the downstream because of the reduction of liquid film thickness by the enhanced interfacial shear stress exerted by the increased gas flow rate due to the evaporation, while it is decreased by the extension of dry area with increasing gas Reynolds number. A simple analysis for the interfacial force balance of the shear stresses in liquid and gas phases and the thermocapillary stress is performed to confirm the importance of thermocapillary force on the behaviors of liquid film. The variation of surface tension with temperature plays an important role in the liquid film behaviors and the corresponding heat transfer.