평활관 및 마이크로핀관 내 초임계 이산화탄소의 냉각열전달 특성

Abstract

As the environmental concern is being increased, the use of CFCs and HCFCs is suppressed. In response to environmental problem, the use of the newly developed HFCs or natural refrigerants is discussed. But, HFC refrigerants are listed together with five other gases by the Kyoto Protocol as greenhouse gases. The other natural refrigerants have a zero ozone depletion potential(ODP), and most of them also have zero global warming potential(GWP). Among natural refrigerants, CO2 is not a new refrigerant and has a successful history of the use as a refrigerant. It has many advantages as a working fluid. Namely, the most relevant characteristics of CO2 are no toxicity, inflammability, no ODP and no GWP. The gas cooling process of CO2 system is different with the existing process. Due to low critical temperature(31.1℃) and critical pressure(7.38 MPa) of CO2, the CO2 cycle takes place at transcritical state when the ambient temperature is near or higher than the critical temperature. Therefore, the system needs attention of the stability, efficiency and durability. In the present study, the local cooling heat transfer characteristics of CO2 are investigated experimentally for smooth tube with inner diameter of 4.95 mm and micro-fin tube with inner diameter of 4.6 mm. The experiment consists of three parts; a refrigerant loop, a test section and cooling water loop. The main components of the refrigerant loop are a receiver, a sight-glass, a mass flow meter, an expansion valve, an evaporator, a compressor, a relief valve, an oil separator and a gas cooler(test section). The test section consists of 12 subsections. Each subsection is a tube-in-tube type and a counterflow heat exchanger. The CO2 flows in the inner tube and water flows in the annulus. The gas cooler contains 12 subsections with 200 mm in length of each subsection. In the inlet and outlet of each subsection, T-type thermocouples are used to measure the CO2 temperature. The experiment of the CO2 cooling heat transfer at a supercritical condition is conducted as respectively varied mass fluxes and inlet pressure of the gas cooler. Mass fluxes are controlled at 1200, 1600 and 2000 kg/m2s by a compressor and an expansion valve. The inlet pressure is varied from 8.0 to 10.0 MPa. The cooling water loop for the test section consists of circulation pump, water flowmeter and constant temperature bath. The main results are summarized as follows; the variation of the heat transfer coefficient tended to decrease as cooling pressure of CO2 increased. The heat transfer coefficient with respect to mass flux increased as mass flux increased. In comparison with the heat transfer coefficient of test sections for smooth and micro-fin tubes at the same mass flux and cooling pressure, the cooling heat transfer coefficients of the micro-fin tube are about 12～39% higher than those of the smooth tube. Also, as the experimental data compared with the existing correlations for the supercritical heat transfer coefficient, which generally underpredicted the measured data. However, the experimental data showed a relatively good agreement with correlations by Pitla et al. except for pseudo critical temperature.