Rate of heat recovery from a hot-water store: Influence of the aspect ratio of a vertical-axis open-ended cylinder beneath a submerged heat-exchanger

A coiled finned-pipe heat-exchanger was employed to extract heat rapidly, 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. The axis of the heat-exchanger coil is orientated vertically, and the heat-exchanger's inlet is arranged to be at its lowest level. The influence of a smooth, vertical-axis cylindrical PVC baffle located symmetrically beneath the heat-exchanger upon the rate of heat recovery via the heat-exchanger was investigated. Irrespective of the flow rate of water through the heat-exchanger's pipe, the greatest rate of heat recovery was achieved using the baffle index (a function of the height, diameter and depth of the baffle within the thermal store as well as the latter dimensions) equals 1·70±0·05. The improvement in the heat-exchanger's effectiveness is approximately 2·5 to 3%. Increasing the rate of water flow 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 amount of mixing between the ascending warm-streams and the descending colder-streams of water in the tank. The experimental, two-dimensional radial temperature-distribution can be employed to predict (to ±5·0%) the cumulative amount of heat recovered from the thermal store.