Experimental investigation on thermo-physical characteristics of fluid flow in circular tube enhanced with circular rings with different arrangements

This paper studies the effect of various arrangements of circular rings on thermal performance in a circular tube. A wall attached circular rings were configured with the different inner diameter to achieve flow blockage area of 30%, 40%, and 50%. These rings were fitted inside the tube to obtain the different arrangements as diverging sections, converging sections and converging-diverging sections. The effect of these various arrangements of the ring on heat transfer rate and fluid friction were studied. Parameters varied during the tests were the pitch-diameter ratio (PDR) and Reynolds number. Air with ambient temperature was used as working fluid in a test tube in which inner wall is maintained at uniform heat flux. The Reynolds number and pitch diameter ratio was varied from 6000 to 24,000 and 2 to 4 respectively. Significant enhancement in heat transfer rate up to 2.75 times that of the smooth tube was observed with these inserts along with an increase in friction loss. Also, inserts with diverging sections found effective than the inserts with converging and converging-diverging sections for a particular PDR. Insert with a diverging section with PDR 2 gives the highest overall performance factor in the range 0.94-1.17 for a tested range of Reynolds number.