Flow Boiling in Minichannels of Copper, Brass, and Aluminum Round Tubes

The past work on flow boiling heat transfer in minichannels ranging one to three millimeters of hydraulic diameter has indicated that the local heat transfer coefficients are largely independent of mass flux and vapor quality, but mainly a function of wall heat flux. The present work is a revisit of flow boiling in minichannels by conducting experiment using 1.67 mm inner diameter tubes of three different materials; aluminum, brass, and copper, to investigate an effect of the tube inner surface conditions with the focus on an effect on nucleate boiling. Tests were conducted for R-22, a fixed mass flux of 600 kg/m²s, 5∼30 kW/m² of wall heat flux, 0.0∼0.9 of local vapor quality. The present experimental data confirmed that the flow boiling heat transfer coefficient in a minichannel varies only by heat flux, independent of mass flux and vapor quality. The effect of tube material was found small for the tubes used in the present work. The present data were well predicted by the correlation proposed by Tran et al. (1996).