AN EXPERIMENTAL INVESTIGATION OF ENTRAINED LIQUID CARRY-OVER FROM A SERPENTINE EVAPORATOR

The quantity of liquid emerging as droplets entrained in superheated vapour at a serpentine evaporator exit is reported. Experiments were conducted under widely varying conditions representative of current and anticipated domestic refrigerator operation using oil-free R134a. A fractional factorial matrix was used to investigate the effects of exit superheat, inlet quality and heat and mass flux on the entrained mass fraction (E(f)) at the exit. Droplet measurements were made with a laser-based phase/Doppler particle analyser. Time-averaged E(f) varied with operating conditions by several orders of magnitude, decreasing with increasing superheat level (due to lower entrainment rates near the exit and more rapid droplet vaporization) and mass flux and inlet quality (due to reduced slug formation near the evaporator inlet). The occurrence of slug flow caused substantial variations in E(f), superheat and operating pressure with time, even under nominally steady-state conditions.