SMALL CIRCULAR-CHANNEL AND RECTANGULAR-CHANNEL BOILING WITH 2 REFRIGERANTS

Boiling heat transfer experiments were performed in a small circular c hannel (d = 2.46) and a small rectangular channel (d(h) = 2.40 mm) wit h Refrigerant 12. The channel sizes are representative of flow passage s in compact evaporators. An experimental technique minimized test sec tion end effects which can be appreciable relative to the heat transfe r in these small channels. Local heat transfer results were obtained o ver a range of qualities up to 0.94, a mass flux range of 44-832 kg/m( 2)s, and a heat flux range of 3.6-129 kW/m(2). Saturation pressure was nearly constant, averaging 0.82 MPa for most of the testing, with som e tests performed at a lower pressure of 0.51 MPa. Local heat transfer coefficients were determined experimentally as a function of quality along the length of the test section. Heat transfer rates were compare d with results of previous experiments in which Refrigerant 113 in a 2 .92-mm diameter small circular tube was used. The effects of channel g eometry and fluid properties on heat transfer, as well as insights rel ative to heat transfer mechanisms in small channels, are presented. Re sults are presented for both nucleation- (wall superheat above 2.75 de grees C) and convection-dominant boiling heat transfer regimes and com pared with large-channel predictions. The heat transfer data for the t wo fluids were successfully correlated in the nucleation-dominant regi on. Copyright (C) 1996 Elsevier Science Ltd.