Thin-film resistance sensor for measuring liquid mass fraction in super-heated refrigerant

Abstract

A microfabricated thin-film resistance (TFR) sensor is presented for measuring the liquid mass fraction (LMF) of refrigerant in the evaporator exits of refrigerators, heat pumps, and air-conditioning systems. The objective for the TFR microsensor is to provide feedback for microvalves that control the amount of refrigerant passing through evaporators. Active control is needed to increase the liquid fill-factor in evaporators, while simultaneously reducing the amount of superheated vapor leaving the evaporator, thus saving on energy usage, weight, and capital costs for the refrigeration system. The design, fabrication, and two calibration methods of the TFR sensor are presented, along with the theory and method of operation. Experimental results showing the effect of LMF on the sensor temperature are given, as are results from a thermal model of the saturation regime. The results are compared with other detection methods, and the TFR sensor is shown to provide sensitive measurements of LMF below 1.5% in superheated vapor operating in a non-equilibrium, non-saturated operating regime. The dc and ac components of the TFR sensor signal provide the necessary control variables to maintain a small value of LMF at low superheat temperatures.