Some experimental studies on heat transfer augmentation for flow of liquid through circular tubes using twisted angles and tapes

Abstract

Heat exchangers have several industrial and engineering applications. The designprocedure of heat exchangers is quite complicated, as it needs exact analysis of heat transfer rate and pressure drop estimations apart from issues such as long-term performance and the economic aspect of the equipment. Whenever inserts are used for the heat transfer enhancement, along with the increase in the heat transfer rate, the pressure drop also increases. This increase in pressure drop increases the pumping cost. Therefore any augmentation device should optimize between the benefits due to the increased heat transfer coefficient and the higher cost involved because of the increased frictional losses. The present paper includes various heat transfer augmentation techniques. A literature review of heat transfer augmentation using passive techniques has been included.Experimental work on heat transfer augmentation using twisted tape and a new kind of insert called twisted angles is carried out. Inserts when placed in the path of the flow of the liquid, create a high degree of turbulence resulting in an increase in the heat transfer rate and the pressure drop. The work includes the determination of friction factor and heat transfer coefficient for various twisted tapes and twisted angles having different twist ratios. The results of twisted tapes and twisted angles having different twist ratios have been compared with the values for the smooth tube. Five twisted angles(y=∞,y=2.915, y=3.612, y=4.105 & y=5.07) and three twisted tapes(y=2.149, y=3.127 & y=4.705) having different twist ratios are used in the study.For twisted angles it was observed that the heat transfer coefficient could vary from 1.16 to 2.87 times the smooth tube value but the corresponding friction factor increases by 4 to 9.6 times the smooth tube values. Similarly for twisted tapes it was observed that the heat transfer coefficient varied from 1.28 to 2.48 times and the friction factor increased by 3.19 to 9.1 times the smooth tube value. It was also observed that with an increase in Reynolds number (Re), the heat transfer coefficient increases where as the friction factor decreases.