Multi-Channel R134a Two-Phase Flow Measurement Technique for Automobile Air-Conditioning System

This paper presents an improved on-line measurement technique developed to study two-phase flow rate distributions of refrigerants in parallel flow channels of compact heat exchangers and evaporators used in automotive air-conditioning systems. A prototype test station containing a refrigerant flow loop and a multi-channel two-phase flow measuring system, was designed and constructed based on the stratification of two-phase flow in horizontal tubes with relatively large diameters. In this work, glass tubes of 1”, 1.5” and 2” diameter were tested. Upon entering the glass tube, a vapour-liquid refrigerant mixture would readily stratify and the mean velocities of vapour and liquid phases could be measured separately using a hot film anemometer and an ultrasonic flow meter, respectively. Scales taped onto the glass tubes were also used to determine the mean liquid levels, from which the flow area of each phase could be calculated. The product of the flow area, phase density and mean phase velocity would then yield the mass flow rate of each phase. Validation experiments have been performed with R-134a as the working fluid and a 3-channel evaporator test section, designed with three separate outlets and kept under an adiabatic condition. The sum of the flow rates in the glass tubes for each phase was compared with the vapour or liquid flow rate determined from the total mass flow rate measured at the evaporator inlet and a heat balance in the pre-heater section of the pump-driven refrigerant flow loop. Validation tests yielded satisfactory results for both vapor and liquid phases, indicating the soundness of the measurement system based on the stratification tubes as well as the use of an ultrasonic flow meter and hot-film anemometer probes for phase average velocity measurements. The present measurement system has been equipped with seven glass tubes and thus can be used to study liquid and vapor flow rate distributions in commercial compact heat exchangers and improve their performance in automobile air conditioning systems.