Effect of lubricant oil on boiling heat transfer and flow patterns of R32 inside a multiport tube

This study experimentally investigated the boiling flow patterns, local heat transfer coefficients, and pressure drops of R32/lubricant oil mixtures flowing inside a horizontal multiport extruded tube having rectangular minichannels. The experiments were carried out in a mass flux range of 50-400 kg m(-2) s(-1), heat flux range of 5-40 kW m(-2), and oil circulation up to 3.7 w% at an evaporation temperature of 15 degrees C. The boiling flow patterns were visualized by using a setup that consisted of horizontal multiple square minichannels, each with a hydraulic diameter of 1.0 mm. The heat transfer coefficients of the R32/oil mixtures were lower compared with those of pure R32 and decreased with increasing oil concentration at lower heat fluxes, while they were higher for conditions of higher heat flux and low vapor quality. In addition, the heat transfer coefficients of the mixtures significantly decreased at a lower mass flux and in a higher vapor quality region. The frictional pressure drops of the R32/oil mixtures was higher compared with that of pure R32 and increased with increasing mass flux, vapor quality, and oil concentration. The effect of oil concentration on the pressure drop was significant at the higher vapor quality region. Furthermore, the pressure drops increased with increasing heat flux for lower to middle vapor quality, in contrast to pure refrigerants.