EXPERIMENT AND PREDICTION OF WATER-FILLED THERMOSYPHON HEAT PIPE HEAT EXCHANGER PERFORMANCE IN COUNTERFLOW AIR STREAMS

摘要

A laboratory study of air-to-air heat exchange was conducted on water-filled thermosyphon heat pipe heat exchanger (HPHE) with a fill ratio of 70 %. Two basic HPHE modules with 2 and 4 rows of staggered copper tubes and aluminium plate fins were fabricated so that tests could be carried out in four HPHE configurations, viz. a 2-row stand-alone module, a 4-row stand-alone module, a 4-row combination comprising of 2x2-row modules, and a 6-row combination comprising of 1x2-row and 1x4-row modules. For all tests, the condenser and evaporator air streams were in counterflow; ambient air was drawn through the condenser in a single pass while the inlet air to the evaporator was heated to a maximum temperature of around 100℃ and recycled in a closed loop duct system. Test results show that the effectiveness of a HPHE increases with increasing temperature difference between the air streams and tapers off asymptotically beyond 50-80 K depending on the number of rows. Also, a minimum temperature difference of 10 K is required for operation. Effectiveness tends to become minimum under equal air flow rates through the evaporator and condenser, suggesting that operation under unequal flow rates produces better thermal performance. Effectiveness increases with the number of rows; the increase is about 30 % from 2 to 4 rows and 10-20 % from 4 to 6 rows. Comparison between the 4-row and 2x2-row modules shows the latter has higher effectiveness especially at flow rate ratio > 1, suggesting that the gap between the 2-row modules serves to break up the thermal boundary layer in the air passages between the fins. Thermal performance of HPHE was predicted using the effectiveness-NTU method. Good agreement in the evaporator and condenser outlet air temperature is obtained for both the 4-row and 6-row HPHE while agreement for effectiveness is satisfactory.