An Experimental Study on Heat Transfer and Entropy Generation in Circular Tube Fitted with Trapezium-Nozzles

Abstract

Heat transfer, friction factor and entropy generation from the inside surface of a horizontal circular tube fitted with trapezium - nozzle have been investigated experimentally. The heat transfer test section is heated electrically imposing axially and circumferentially constant wall heat flux. Three different pitch ratios (PR) of trapezium -nozzle arrangements in the test tube are introduced with PR=2, 4, and 7. The experiments covered a range of Reynolds numbers from 8000 to16000. Heat transfer and friction factor analyses are presented for different conditions of pitch ratios (PR) and Reynolds number. The results indicate that the trapezium - nozzle of different pitch ratios has a great effect on the results of heat transfer coefficient and friction factor. The Nusslet number increases with an increase in Reynolds number and it decreases with an increase in pitch ratios. It is found that using the trapezium -nozzle results in increasing the heat transfer rate compared with the plain tube. The maximum gain in Nusslet number is obtained for the smallest pitch ratio used, PR=2. This indicates that the effect of the reverse/re-circulation and surface flows can improve the heat transfer rate in the circular tube. For fixed Reynolds number, the friction factor increases with the decrease in pitch ratio for the circular tubes with trapezium -nozzle. The entropy generation number increases with increase Reynolds number at all cases, whereas the entropy generation number shows its highest value at pitch ratio of 2. From these results, it was found that the average enhancement in Nusselt number for circular tube fitted with trapezium - nozzle at pitch ratio (PR=2) is in the range of 202% to 257% compared with the plain circular tube for all tested conditions. Correlations of the Nusselt number and friction factor with Reynolds number and pitch ratio are presented. Key words : Enhancement heat transfer; Circular tube; Re-circulation/Reverse flow; Turbulator; Entropy generation; Trapezium –nozzle