Two-phase evaporative heat transfer coefficients of refrigerant HFC-134a under forced flow conditions in a small horizontal tube

In this study, the two-phase heat transfer coefficient characteristics of HFC-134a evaporating under forced flow conditions inside a smooth horizontal tube are experimentally investigated. Different from most previous studies, the present experiments have been performed with lubricating oil in the refrigerant loop at high flow rate and high heat flux conditions. The test section is a 1.8 m long counterflow double tube heat exchanger with refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tube is made from smooth horizontal copper tubing of 9.52 mm. outer diameter and 7.2 mm. inner diameter. The test runs were done at average saturated evaporating temperatures ranging between 4 and 25 °C. The inlet dryness fractions were between 0.1 and 0.25. The mass fluxes were between 160 and 470 kg/m2s. and the heat fluxes were between 8 and 55 kW/m2. The inlet dryness fraction of the refrigerant in the test section was calculated using the temperature and pressure obtained from the experiment. The exit dryness fraction and heat transfer coefficient of the refrigerant were determined by applying an energy balance based on knowledge of the energy going into the test section. The effects of heat flux, mass flux, evaporation pressure and lubricating oil on the convection heat transfer coefficient are also discussed. The results from the experiment are compared with those calculated from correlations reported in the literature. Moreover, new correlations for the convection heat transfer coefficient are proposed for practical applications. The results of this study are of technological importance for the efficient design of evaporators when systems are assigned to utilize HFC-134a.