Experimental Performance Evaluation of a Two-phase Condenser Coil with Plain-Finned Oblique Tubes for Residential Air-conditioning

This paper presents the performance evaluation of a two-phase condenser coil with plain-finned oblique tubes for residential air-conditioning (AC) applications. An internal rib designed to increase the tube strength allows for increased internal pressure even at 5 MPa. Thermal and hydraulic performance of the test coil were assessed in a 300 x 300 mm recirculating wind tunnel, providing the air-side inlet temperature of 35°C with a velocity of 1-3 m/s. Generated by a single-split-type 2.5 kW AC unit, the R410A two-phase flow entered the test coil at 45.5°C (27.9 bar) and exits at 44.6°C (27.3 bar). The original condenser piping network was modified and added with new valves to control the R410A mass flow rate which effectively delivered the desired and constant condensation temperature into the test coil. The detailed manufacturing of the test coil including fin forming, tube manufacturing, safety-driven design, tube expansion technique and test leak is provided. In comparison with a plain-finned circular-tube coil over the velocity of 1-3 m/s, the oblique-tube coil has 7-19% lower air-side pressure drop penalty due to the more streamlined tube profile which reduces the air recirculation zone behind the circular tubes and 10% higher overall heat transfer coefficient. At the same fan power, the coil heat transfer capacity can be enhanced up to 9.5-13.8%. Experimental results also showed that the pressure drop of the R410A two-phase flow caused lower condensation temperature, thus reducing the log mean temperature difference (LMTD). The modification to the widely used LMTD relationship is discussed and implemented to accurately obtain the coil’s overall heat transfer coefficients. The comparison of the two designs with plain fins clearly shows the contributions of the oblique tubes on the coil performance while other patterned fins can be adopted in future for higher performance.