Evaporation heat transfer and pressure drop in micro-fin tubes before and after tube expansion

The objective of this study is to investigate the pressure drop and heat transfer characteristics of micro-fin tubes before and after the tube expansion process. Test tubes are single-grooved micro-fin tubes made of copper with an outer diameter of 9.52 mm before the tube expansion. The direct heating method is applied in order to make the refrigerant evaporate in the micro-fin tubes. The test ranges of the heat flux, mass flux, and saturation pressure are 5-15 kW/m, 100-200 kg/m(2.)s, and 540-790 kPa, respectively. The effects of the mass flux, heat flux, and saturation pressure of the refrigerant on the pressure drop and the heat transfer are presented for the refrigerant R22. In the test conditions of this study, the heat transfer coefficient for the micro-fin tube after the tube expansion is about 16.5% smaller than that before the tube expansion because the fin height of the micro-fins reduces and the fin shape becomes flatter. The micro-fin tube after the tube expansion has about 7.7% greater average pressure drop than before the tube expansion process.