Numerical and experimental investigation on the effects of diameter and length on high mass flux subcooled flow boiling in horizontal microtubes

Abstract

High mass flux subcooled flow boiling was investigated both numerically and experimentally in horizontal microtubes. Microtubes with inner diameters of ∼600 and ∼900μm, and outer diameters of ∼900 and ∼1100μm, and heated lengths of 6 and 12cm were tested in order to investigate the effects of diameter and heated length on subcooled flow boiling at high mass and heat fluxes. In the experimental part, microtubes made of stainless steel were used, and deionized water was as the working fluid. In the numerical part, the two-phase Eulerian method was adopted using the finite volume approach. Numerical results showed a good agreement with experimental results. Heat transfer coefficients were higher in the microtubes with smaller diameters, while longer microtubes resulted in higher heat transfer coefficient. The results indicated that smaller pressure drops were achieved for shorter microchannels along with higher heat fluxes. Local heat transfer coefficients were presented along the microtube to provide an understanding on local flow boiling characteristics. As the vapor quality and void fraction increased, higher heat transfer coefficients were obtained. With the increase in mass flux, an enhancement in boiling heat transfer was observed implying convective heat transfer effects on flow boiling along with nucleate boiling. Furthermore, heat transfer coefficient increased with decreasing inlet subcooling.