Free-convective flows within a hot-water store, induced by a submerged, relatively cold heat exchanger

A coiled finned-pipe heat exchanger was employed to extract heat rapidly, at temperatures in excess of 30° C, from a 90-litre hot-water charged tank; the water being initially at a temperature of approximately 80° C. Free-convective buoyancy movements of the water around the outside of this coiled pipe (immersed in the store) occur as a result of initially-cold water (at ~ 20° C) being forced internally through the heat exchanger's pipe. For the most rapid rate of heat extraction, the axis of the heat exchanger coil should be oriented horizontally, and the heat exchanger's inlet arranged to be at the lowest level. Increasing the rate of water being passed through the heat exchanger's pipe resulted in a reduction of the heat exchanger's quality effectiveness, but a rise in the thermal store's recuperation effectiveness. The latter was due to the reduced level of mixing between the ascending warm streams and the descending colder streams. The development of the thermal boundary layer adjacent to the tank's wall was beneficial in deflecting the centrally descending stream of relatively colder water (which comes off the heat exchanger's pipe) to drive the ascending warmer water onto the heat exchanger, which is located near the surface of the water in the tank.