Hydrodynamics and Heat Transfer for a Two-Phase Flow in a Heated Vertical Minichannel at High Reduced Pressures

Abstract

The relevance of studies into hydrodynamics and heat transfer in minichannels is driven by the increased interest in high-pressure power systems and high-tech devices that employ compact and efficient heat exchangers with a high heat flux. The potential for application of small-diameter channels in various industries, including production of heat exchangers, in which various dielectric liquids or freons can be used as a coolant at moderate and high reduced pressures, is being actively investigated today. High heat fluxes should be removed by boiling as the most efficient heat removal mechanism. Proper designing of heat exchangers employing the boiling process requires reliable methods for calculating heat transfer and pressure drop in two-phase flows. The authors have tested the applicability of the known and most reliable methods for calculating pressure drops and heat-transfer coefficient, which have been developed for conventional channels and minichannels, under conditions of increased reduced pressures as high as p_r = p/p_cr = 0.7. A review of the best-known methods applicable to various diameter (0.16–32 mm) channels is presented, and the predictions by these methods are compared with experimental data. The experiments were performed at a reduced pressure of 0.43, 0.56, and 0.70 in the mass velocity range of G = 200–1000 kg/(m² s). The experimental setup, the test section, and the experimental procedure are described. The studies were done with R125 refrigerant in a 1.1 mm ID vertical round channel with a heated length of 50 mm. The comparison of the experimental data with predictions by the reviewed procedures demonstrated good performance of calculation methods that had been developed for conventional channels and for particular fluids under conditions close to those under which the experiments were carried out. The pressure losses predicted using the homogeneous model at high reduced pressures are in good agreement with the experimental data.