Quantification of contact resistance of metal foam heat exchangers for improved, air-cooled condensers in geothermal power application

Two types of aluminum foam-wrapped cylindrical heat exchanger, with foam layer thickness of 5mm, are being tested for heat transfer performance and pressure drop characteristics. The first heat exchanger, HX1, has the core tube made of S316 stainless steel with the tube external and internal diameters measure 32.0mm and 28.3mm, respectively. The foam layer of HX1 is bonded to its core tube using highly conductive thermal glue. The second heat exchanger, HX2, has the core tube made of AA6061, the same material as the foam layer covering it, and the two parts are bonded together by a brazing technique. The core tube of HX2 has nearly identical dimension to that of HX1 with a small variation in its thickness. The tests are carried out on each heat exchanger, installed horizontally in a cross-flow arrangement inside a wind tunnel, one at a time with air velocity varying between 1.5 to 5 m/s. Heat transfer from 75C hot liquid, circulating through the core tube, to external air is evaluated using the concept of total thermal resistance where the effects of tube material and the two bonding methods are quantified as a series of smaller resistant components. The results show that, within the range of designated air velocity, thermal contact resistance (TCR) of HX1 is 0.015 ± 0.001K/W larger than that of HX2; essentially constant with the air flow rate. This TCR however contributes between 10% to 19% of HX1 total thermal resistance from lowest to highest air velocity, respectively. On the other hand, pressure drop results show very close figures between HX1 and HX2. They are steadily increased from 2.0 Pa at lowest air velocity to 19 Pa at highest velocity