Heat Transfer Coefficient, Two-Phase Flow Boiling of HFC134a

Experimental measurements of local heat tra nsfer coefficients and pressure drop are made for forced conv ection boiling and condensation for R134a-oil mixtures inside a horizontal tube. The results are compared with the experime ntal data of pure Rl34a. Average convection boiling and cond ensation heat transfer coefficients decreased about 20% at a 10\ o il mass fraction, and boiling and condensation pressure drop were 2. 5 and 1.4 times that of pure refrigerant flow respectively. INTRODUCTION Rl34a is most likely to be an alternative r efrigerant for Rl2 which is used as a refrigerant in automotiv e air conditioners and domestic refrigerators. Therefore, the heat transfer coefficient and in-tube pressure drop for Rl34a are desired in designing new systems that use Rl34a as the working fluid. Espec ially, the influence of oil on the heat transfer coefficient and th e pressure drop in heat exchangers have to be estimated, because lu bricants always circulate in actual vapor compression refrigeration s ystems. Eckels and Pate (1990) estimated the heat t ransfer coefficients , for R134a by using reliable correlations, a nd compared them to the heat transfer coefficients for R12. However . they did not take into account the oil effect on the heat transfer coefficients. An experimental study of the heat transfer coefficients and pressure drop for forced convection boiling and condensation of R134a-oil mixtures inside a horizontal tube are described in this paper. The results are compared with the ex perimental data for a R12oil mixture. FORCED CONVECTION BOILING HEAT TRANSFER COE FFICIENTS AND PRESSURE DROP Experimental Apparatus A schematic diagram of the apparatus is sho wn in Figure 1. This apparatus consists of a refrigeration cycle and an oil circulating loop. The main components of the refrigerat ion cycle are a hermetic compressor, oil separator, condenser, expan sion valve, test section and an evaporator. The oil is eliminated fr om the compressor discharge gas by the oil separator. The ref rigerant mass flow rates are controlled by using the expansion valve , and are measured with a turbine-type flow meter. The oil contained in the compressor shell i s taken out from the compressor shell bottom, and transferred to the test section inlet by a gear-type oil pump. The oil mass flow rates are changed by controlling the oil pump rotating speed. an d are measured with a oil flow meter. The refrigerant-oil mixtures flow into the test section where the heat transfer coefficients and pressure drop are measured. The