Detection of liquid mass fraction at the evaporator exit of refrigeration systems

Abstract

The article presents four methods of detecting droplets in the stream of superheated vapor at the evaporator exit of refrigeration systems: (a) an energy balance, (b) a thin-film resistance (MEMS) sensor developed for this project, (c) a laser and photodiodes for measuring light scattered by entrained droplets, and (d) an exposed beaded thermocouple. Three configurations of the plate evaporator are examined, with refrigerant flow controlled by: (1) a thermostatic expansion valve, (2) a manual valve, and (3) a separate liquid injector used to examine the detection of controlled flow of droplets. The signals were recorded at both slow (0.5 Hz) and fast (40 Hz) rates in order to characterize the unsteady flow exiting the evaporator. The presence of liquid in superheated vapor at the evaporator exit indicates nonequilibrium conditions and maldistribution of two-phase refrigerant within the evaporator. The performances of the MEMS sensor and the beaded thermocouple when exposed to small liquid mass fractions in a superheated vapor stream are examined to assess the feasibility of using each instrument to detect and control liquid mass fraction instead of superheat at evaporator exit and improve distribution among evaporator plates. Results show that the MEMS sensor is more sensitive to liquid mass fraction (LMF) in superheated vapor than the thermocouple at lower values. Both instruments in current versions exhibit a saturation point beyond which they can no longer detect increases in LMF.