An experimental study on evaporative heat transfer in micro-fin tubes before and after tube-expansion

Evaporative heat transfer characteristics of R-22 in micro-fin tubes before and after tube-expansion have been investigated. Single-grooved micro-fin tubes with outer diameter of 9.52 mm were used as test sections, and they were uniformly heated by applying direct current to the test tubes. The saturation temperature of refrigerant was calculated from the inlet and outlet refrigerant pressures by using an equation of state. Inner wall temperature was calculated from the outer wall temperature which was measured using T-type thermocouples and thermopiles, accounting for heat generation in the tube and heat conduction through the tube wall. Mass quality of refrigerant was calculated by considering energy balance in the preheater and the test section. Experiments were conducted at mass flow rates of 20 and 30 kg/hr and mass fluxes of 100 and 150 kg/m(2)s. For these mass flow rates or mass fluxes, evaporation temperature was set at 5 and 15 and heat flux was changed from 6 to 11 kW/m(2). The enhancement of evaporative heat transfer of micro-fin tubes compared to plain tubes was caused by the higher convective boiling effect induced by the promotion of turbulence due to the existence of micro-fins and by the increased heat transfer area. Therefore, the evaporative heat transfer coefficient of micro-fin tubes after tube-expansion decreased because of the change of fin shape and the enlargement of inner diameter compared to that before expansion. Results show that the differences of the heat transfer coefficient in micro-fin tubes before and after expansion were greater for higher quality range. The evaporative heat transfer coefficient of the micro-fin tube after expansion was 19.9% smaller on the average than that before expansion.