Study of High Efficiency Micro Heat Exchanger and Refrigerant Distribution using Throttle Mechanism

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For the multi-path heat exchanger using small diameter tubes, a distributor supplies refrigerant to each passage of heat exchanger tubes. No evenness of refrigerant decreases the efficiency of the heat exchanger. In the present study, the distribution of refrigerant to parallel small tubes and the boiling heat transfer for refrigerant R-134a were investigated to provide useful data for the design of compact heat exchangers using small tubes. The distributor, which is connected to the parallel evaporator tubes, supplies refrigerant to each evaporator. Two orifice plates, which are three kinds of nozzle diameters(0.1mm, 0.15mm and 0.2mm), are installed in the distributor connected the front ends of the evaporator tubes. The experimental investigation was performed under the mass flux 100 to 250kg/m^2s, heat flux from 4.8 to 11.8kW/m^2 and inlet refrigerant vapor quality of the evaporator < 0. Results showed that 1) the distribution of refrigerant was not affected by nozzle diameter. 2) the refrigerant flow was steady with decreasing the nozzle diameter, which meant that the fluctuation of refrigerant flow decreased by throttle. 3) two-phase oscillating flow occurred in the evaporator was suppressed by throttle. For the effect of throttle for the boiling heat transfer, when mass fluxes were 100 and 150kg/m^2s, the boiling heat transfer coefficient for the nozzle diameter 0.1mm was better than for the diameter 0.2mm. But, in the case of the mass flux 250kg/m^2s, both the boiling heat transfer coefficients for the diameter 0.1mm and for 0.2mm were almost same values. The boiling heat transfer coefficient was improved at low mass flux. In previous study, it was known that the oscillating flow in the evaporator decreased the heat transfer coefficient. Throat on the entrance of evaporator suppressed flow instability and as a result, the boiling heat transfer coefficient was improved.