Single-Phase Heat Transfer of R245fa + Lubricant Oil Mixtures Inside Horizontal Smooth and Microfin Tubes

Abstract

The effect of lubricant oil on the single-phase heat transfer of R245fa inside horizontal smooth and microfin tubes was experimentally investigated. Tests were conducted using a horizontal smooth tube with an inner diameter of 8.32[Formula: see text]mm and two microfin tubes with equivalent diameters of 8.9 and 8.7[Formula: see text]mm, fin height of 0.12 and 0.18[Formula: see text]mm, and number of fins of 65 and 85, respectively. The heat transfer coefficients were measured for pure R245fa and R245fa + lubricant oil mixture in the Reynolds number range of 2000–10000, heat fluxes of 5 and 10[Formula: see text]kWm[Formula: see text], and refrigerant temperatures of 30∘C and 45∘C at the test section inlet. Oil concentration was varied at 1, 2 and 5[Formula: see text]wt.%. The results showed that the heat transfer of pure R245fa agreed well with previous correlations. However, the heat transfer coefficients of the R245fa + lubricant oil mixtures were lower than those of the pure R245fa, and decreased with an increase in oil concentration for both smooth and microfin tubes. The reduction in heat transfer was more marked for the HF tube than for the smooth and LF tubes. For the HF tube, the decrease in heat transfer was apparently more pronounced at higher oil concentrations from 2 to 5[Formula: see text]wt.% owing to immiscibility between the oil and refrigerant.