Heat Transfer Characteristics of Carbon Dioxide During Evaporating and Cooling Process in a Horizontal Tube

摘要

The heat transfer coefficient during evaporating and cooling process of CO_2(R-744) in a horizontal tube were investigated. The experiments are conducted without oil in the refrigerant loop. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, an evaporator and a gas cooler. The evaporator and gas cooler were made of horizontal stainless steel tubes with I.D. of 7.75 mm. The length of evaporator is 5000 and the length of gas cooler 6000 mm. The evaporation experiments were conducted at saturation temperatures of-5 to 5℃, heat flux of 10 to 40 kW/m~2, and mass flux of 200 to 500 kg/m~2s, and the gas cooling experiments were conducted at mass flux of 200 to 400 kg/m~2s and inlet temperature of 90 to 100℃. The evaporation test results showed the heat transfer coefficients of CO_2 were much more dependent than conventional refrigerants on the vapor quality, heat flux and saturation temperature. In comparison with CO_2 and Freon refrigerants, the heat transfer coefficient of CO_2 is much higher than that of R-22 and R-134a at the same given conditions. As comparing with the data and previous correlations, the best fit of evaporation data is Jung et at. s correlation and gas cooling data agrees well with the correlation by Bringer-Smith. However, the gas cooling data indicates higher values than the Bringer-Smith correlation at the region near Pseudo-critical temperature.