MULTI-CHANNEL R134A TWO-PHASE FLOW MEASUREMENT TECHNIQUE FOR AUTOMOBILE AIR-CONDITIONING SYSTEM

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1 Proceedings of ASME FEDSM 3 4 th ASME_JSME Joint Fluids Engineering Conference Honolulu, Hawaii, USA, July 6-1, 23 MULTI-CHANNEL R134A TWO-PHASE FLOW MEASUREMENT TECHNIQUE FOR AUTOMOBILE AIR-CONDITIONING SYSTEM Junjie Gu Department of Mechanical and Aerospace Engineering Carleton University, Ottawa, Ontario K1S 5B6 Canada jgu@mae.carleton.ca F E DSM Masahiro Kawaji Department of Chemical Engineering & Applied Chemistry, University of Toronto, Toronto, Ontario M5S 3E5, Canada kawaji@ecf.utoronto.ca Tracey Smith-Pollard, James Cotton Long Manufacturing Ltd., Oakville, Ontario L6K 3E4, Canada Tracey_Smith-Pollard@longmfg.com ABSTRACT 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. Keywords: two-phase flow, evaporator, flow distribution, flow quality, measurement technique INTRODUCTION Two-phase flow rate distributions in parallel channels of a compact heat exchanger may be non-uniform depending on how the vapour-liquid mixture enters the heat exchanger. Our previous measurements of air-water flow rate distributions indicated strongly non-uniform conditions (Rong et al., 1995, 1996), however, the actual flow rate distributions of a refrigerant vapor-liquid mixture could be somewhat different due to a different density ratio between the vapour and liquid, and other fluid properties. Thus, there is a need to investigate the two-phase flow rate distributions in parallel flow channels of an evaporator for a refrigerant commonly used in car air-conditioning systems. This paper deals with the design and set-up of a prototype test station, which contains a multi-channel two-phase flow measurement system and a pumped freon flow loop. The first trial experiment has been performed with a 3-channel test evaporator, and the vapour and liquid flow rates in each channel were determined separately. MULTI-CHANNEL TWO-PHASE FLOW LOOP In order to measure the two-phase flow rate distribution in parallel channels of compact heat exchangers, a freon flow loop was constructed as shown in Figure 1, equipped with hot-film anemometer probes and an ultrasonic flow meter (Gu and Kawaji, 22). The measurement methods adopted are based on the stratification of two-phase flow in horizontal tubes with relatively large diameters (Kawaji et al., 21; Hwang, 21). 1

2 Figure 1 Schematic of a Freon Flow Loop Glass tubes of 1, 1.5 and 2 diameter were tested and initially they were placed horizontally, but that resulted in a stagnant liquid pool to remain in the glass tube under no or low flow conditions. Thus, to ensure there is no stagnant liquid in the glass tube under low flow rates, the glass tubes were slightly inclined downward at an angle of 2~5 degrees such that the elevation of the inlet was higher than that of the outlet. MEASUREMENT The two-phase flow was stratified in each glass tube, and the flow rate of each phase was measured separately using a hot-film anemometer (FMA-94-V, Omega) and an ultrasonic flow meter (DUFX2-D1-NT, Omega) at a measurement station located 1.75 m from the glass tube inlet as shown in Fig. 2. A scale was taped around the bottom of the glass tube to measure the width of the liquid stream, from which the liquid level and the cross sectional areas of both phases were determined. Validation of hot-film anemometer Different single-phase vapour flow rates could be achieved in the glass test section by boiling the refrigerant at a given mass flow rate at different power levels in the heating section (Gu et al., 21). Figure 3 shows the actual velocities of R-134a vapour and those measured using the hot-wire anemometer for different total liquid-vapor flow rates. The hot-film anemometer can be seen to successfully measure the velocities of R134a vapour. Actual, m/s Measured, m/s.5 l/min 1I/min 2 l/min Figure 3 Validation of hot-wire anemometer Validation of ultrasonic flow meter The ultrasonic flow meter uses the principle of a Doppler effect, in which the frequency of ultrasonic pulses reflected from tiny impurities (as reflectors) in a moving liquid will be altered from the frequency of the transmitted ultrasonic waves. The magnitude of frequency change is directly proportional to the velocity of the liquid. Figure 4 shows the application of a commercial ultrasonic flow meter to the measurement of the liquid R134a velocity in two-phase stratified flow in a slightly inclined tube. Figure 2 Measurement of two-phase stratified flow 2

3 From heat balace, f/s Measured, f/s.5 l/min 1 l/min 2 l/min Measured flow rate, kg/s Actual flow rate from heat balance, kg/s Figure 4 Validation of ultrasonic flow meter The data show satisfactory performance of the ultrasonic flow meter to measure the velocity of the liquid phase under stratified flow conditions. Surface waves In liquid-vapour two-phase stratified flow, surface waves often form because of the interfacial shear stress induced by the fast traveling vapour phase. The waves makes the flow measurements more difficult. To avoid waves, large diameter tubes were chosen to reduce the vapour velocity. A small downward inclination (~ 3º) of the tubes in the flow direction also helped in obtaining a smooth liquid flow. Liquid and vapour mass flow rates Figure 5 shows a comparison of the measured and actual liquid mass flow rates. The actual liquid flow rate was calculated from the total mass flow rate measured by a turbine flow meter minus the vapour mass flow rate calculated from a heat balance applied to the heating section. The measured vapour mass flow rates are compared with the actual flow rates as shown in Figure 6. The actual vapour flow rate was calculated from the total mass flow rate and electric heater power using a heat balance applied to the heating section. In both cases, the measurements are seen to be consistent with the actual liquid and vapour mass flow rates. Total mass flow rate Figure 5 Liquid mass flow rate The measured and actual total mass flow rates are compared in Figure 7. The actual total flow rate is the flow rate measured by a turbine flow meter. The sum of the liquid and vapour mass flow rates measured conform to the actual total flow rate. Vapour quality Vapor quality is the ratio of the vapor mass flow rate to the total mass flow rate. The combination of an ultrasonic flow meter and a hot-film anemometer probe could yield satisfactory flow quality measurements, as shown in Figure 8. Measured flow rate, kg/s Actual flow rate from heat balance, kg/s Figure 6 Vapour mass flow rate 3

4 Measured total flow rate, kg/s Actual total flow rate, kg/s Figure 7 Total mass flow rate.8 Measured quality, Figure 8 Quality from heat balance, - Flow quality measurements The validation tests described above have successfully demonstrated the feasibility of using an ultrasonic flow meter and a hot-film anemometer probe to measure the flow rates of vapour and liquid separately, in a refrigerant R134a two-phase flow loop. MULTIPLE-CHANNEL EVAPORATOR TEST The freon loop was modified to enable testing of multiple-channel evaporators supplied by Long Manufacturing Ltd. Liquid and vapour flow rates in up to seven separate channels need to be measured at the same time, thus an outlet manifold and a seven parallel glass-tube assembly were designed and constructed as shown in Figures 9 and 1, respectively. The outlet manifold is used to connect up to seven channels to a single inlet tube of a condenser. For each channel, the fluid temperature is measured using a thermocouple. The pressure is also measured in the outlet manifold so that the densities of freon vapour and liquid can be determined for each channel assuming saturated conditions. In Figure 1, the parallel glass tube assembly containing three tubes is shown connected to the outlet manifold and a three-channel evaporator. Figure 9 Outlet manifold connecting the parallel glass tubes to a condenser Figure 11 shows the hot-film anemometer probes installed in three glass tubes, along with the readout device of an ultrasonic flow meter for liquid flow measurement. The liquid level in each glass tube could be measured by visual observation and using a scale taped around each glass tube. Although the spacing between the glass tubes was tight, it was possible to observe the liquid level by viewing each glass test section from the top at a certain angle, and locating the side edge of the liquid layer on the taped scale. Figure 1 Freon two-phase flow loop with a three-channel evaporator 4

5 Figure 11 Hot-film anemometers and ultrasonic flow meter TRIAL EXPERIMENTS USING A 3-CHANNEL EVAPORATOR Following a leak test, the completed multi-channel test facility was charged with R-134a, and the first trial experiment was carried out with the three-channel evaporator as shown in Figure 12. As summarized in Table 1, these preliminary tests were performed at two different freon flow rates of 1. and 2. litre/min, corresponding to mass flow rates of 1.24 and 2.48 kg/min. For each flow rate, the heater power was set to, 1 W, and 2 W to have either single-phase liquid or a vapour-liquid mixture of different quality flowing into the three-channel evaporator. From a heat balance, the inlet mass qualities could be determined as shown in Table 1. Figure 12 Test evaporator with three outlets For each channel, the hot-film anemometer and ultrasonic flow meter readings as well as liquid levels were obtained. Surprisingly, the flow rate distribution for this top feed configuration and conventional inlet header design was relatively uniform, unlike in the previous air-water tests. The main reason for this result is suspected to be the long (about 2-m) and thin (1/4 ) copper tubing used to connect each evaporator outlet to the glass tube as shown in Figure 13. This copper tubing also had two small loops near the ends to provide enough flexibility for connection to the fragile outlet tubes of the evaporator unit. A high resistance to flow at the exit of an evaporator may have created enough back pressure for all the channels to equalize the freon flow rates. A modification is underway to shorten this connection and eliminate any loop in order to reduce the flow resistance at the evaporator outlet. Flow Rate, l/min Table 1: Experimental Data Sheet Heater Power Temperature Hot-film Ultrasonic W Channel # C (reading) (reading) Level, mm , , , , Note: The channels are numbered from the position near the evaporator inlet, and the evaporator is seated in a vertical top-feed orientation. 5

6 Kawaji, M., Hwang, D.H., Karimi, G., Dickson, T. R., Smith-Pollard, T., 21, Flow quality measurement based on stratification of flow in air-water and refrigerant two-phase flow systems. 4th International Conference on Multiphase Flow, New Orleans, Louisiana, USA. May 27 - June 1, 21. Hwang, D.H., 21. Flow quality measurements based on stratification of flow in nitrogen gas water and HFC-134a refrigerant PAG oil two-phase flow systems, M.A.Sc. Thesis, Department of Chemical Engineering & Applied Chemistry, University of Toronto. Figure 13 The long tubing from evaporator outlets to glass tubes SUMMARY An in-line two-phase flow measurement system was developed and tested in an R-134a flow loop. A hot-film anemometer and an ultrasonic flow meter were used to measure the vapour and liquid flow rates separately in a horizontal glass tube at the exit of a multiple-channel evaporator. The freon loop was also modified to enable measurements of two-phase flow rate distributions from three up to seven outlets of a commercial evaporator. The preliminary test results showed a relatively uniform flow rate distribution in all three channels in contrast to the air/water experiment results, in which the first channel received most of the liquid under the top-feed condition. This result may have been caused by the high flow resistance presented by the long and thin tubing (2.2 m long, 1/4" diameter), used to connect each evaporator outlet to the glass tube. In order to eliminate the influence of the outlet flow resistance, a further modification of the evaporator outlet connection to the glass tube is underway. REFERENCES Gu, Junjie and Kawaji, M., 22. Two-phase Flow Measurements of R134a in a Pump-driven Flow loop, a Final Report submitted to Long Manufacturing Ltd., January 22. Rong, X.Y., Kawaji, M. and Burgers, J.G., Two-phase header flow distribution in a stacked plate heat exchanger. ASME, FED - Vol. 225, pp Rong, X.Y., Kawaji, M. and Burgers, J.G., Gas-liquid and flow rate distributions in single end tank evaporator plates. A paper presented at the SAE Annual Congress, Detroit, February 26-29, 1996, SAE SP-1175, pp