R134a Flow Boiling Heat Transfer (FBHT) Characteristics in a Refrigeration System

Conventional micro tubes have been widely used in the last few decades in water and air for refrigeration heat exchanger applications during evaporation or condensation. Lowering the size possibility of micro tubes drives to a higher compact and heat exchanger performance and thus reducing the charge of system’s refrigerant. This research investigates the R134a pressure drop and flow boiling heat transfer measurement inside a small micro tube. In this work, a domestic refrigerator was simulated with real capacity and dimensions by designing and using a test rig refrigeration system of 310 W. A tube of 1000 mm in length made from copper horizontally oriented having 4.35 mm internal diameter representing the refrigeration’s system evaporator section is adopted as a test section. A total of 36 K-type thermocouples are installed in nine locations on the copper tube’s external round surface with 100 mm equal spacing. Two glass tubes are connected to the copper tube test section to visualize the R134a refrigerant in the inlet and outlet zones. A software of computational fluid dynamics (CFD) by (ANSYS Fluent 18) is employed to numerically simulate the flow boiling R134a refrigerant’s heat transfer in the evaporator. This study aimed to show the influencing factors of R134a refrigerant flow boiling heat transfer (FBHT) on the designed system’s evaporator experimentally and numerically. The research range is (-14 to -3) °C saturation temperature, (12.8 to 31.1) kW/m2 heat flux, (0.21 to 1) vapor quality, and (92, 160 and 187) kg/m2.s mass flux. Experimental results reveal an improvement of 32% at 31.1 kW/m2 in the local heat transfer coefficient and 77% at 187 kg/m2.s mass flux at constant operational testing conditions. An Enhancement of 68% in the local heat transfer coefficient when the temperature of saturation increased from (-7 to -3) °C. The average deviation between experimental and numerical results is 8%.