Experimental Measurement of Oil Hold-Up During Refrigerant Condensation and Evaporation in Two Phase Flow

This paper presents the effects of heat transfer on oil hold-up in smooth, axially finned, and 18° helically finned 8.915mm I.D. horizontal tubes under evaporation and condensation conditions with a R134a /ISO32 polyol ester refrigerant oil mixture. Heat fluxes ranging from 4000 to 6000 W/m2 were applied through a water jacket. The tests were conducted at a saturation temperature of 25° C, at mass fluxes of 100 to 300 kg/m2 -s, and a range of average qualities from 20-95%, oil flow concentration varied from 3-5.5%. Oil hold-up is observed to reach a minimum value in the mid-range qualities, but increases at higher qualities. In general, the enhanced tubes exhibit higher oil hold-up compared to the smooth tube, but enhanced tube trends differ between condensation and evaporation based on enhancement. The addition of oil is found to decrease condensation heat transfer while increasing evaporative heat transfer at lower and mid-range qualities.