EXPERIMENTAL INVESTIGATION OF THE EFFECT OF FLOW BLOCKAGES ON HEAT TRANSFER AND FLUID FRICTION IN A ROUND TUBE USING WALL-ATTACHED CIRCULAR RINGS

This paper studies the flow blockage effect on thermal performance in a round tube. The experiments were carried on a heat exchanging tube which is fitted with wall-attached circular rings (with no gap between the ring and inner wall of the tube) as a flow blockage device. The aim of this investigation is to introduce a flow blockage area (FBA) as a new parameter to evaluate the performance of different turbulators (turbulator is a passive device which induces secondary/reverse flow within the flow field). The wall-attached circular rings were selected as flow blockage geometry and configured with different inner diameters to achieve flow blockage area of 30%, 40%, and 50%. The parameters varied during the tests were FBA, the pitch-to-diameter ratio (PDR), and the Reynolds number. Air with ambient temperature was used as a working fluid in a test tube in which the inner wall is maintained at a uniform heat flux. The Reynolds number and pitch-to-diameter ratio were varied from 6000 to 24,000 and from 2 to 4, respectively. Significant enhancement in heat transfer rate up to 3.04 times that of the smooth tube was observed with inserts of 50% FBA and a smaller pitch-to-diameter ratio, i.e., 2 at a higher Reynolds number. An insert, i.e., a circular ring which offers flow blockage area of 30% and pitch-to-diameter ratio 2, gives the highest overall performance factor (1.26) at a lower Reynolds number.