Experimental study of heat transfer and friction factor in a double pipe heat exchanger using twisted tape with dimple inserts

Abstract

ABSTRACT The present experimental work deals with heat transfer and friction factor analysis of water flowing in double pipe heat exchanger with dimpled twisted tape inserts. The investigated parameters are twist ratio of 5.5, Reynolds number (Re) of 6000–14,000, dimple diameter (D) of 3 mm, 5 mm, and 7 mm and dimple diameter-to-depth ratio (D/H) of 1.5, 3, and 6. Nine dimpled twisted tapes were tested and a comparison was made between these twisted tapes and plain twisted tape along with plain tube. Nusselt number (Nu) is found to be maximum (113.68 at Re 14,000 and 59.56 at Re 6000) for D = 5 mm and D/H = 3. This highest value of Nu is 1.50–1.82, 1.37–1.46, 1.24–1.19 and 1.18–1.10 times greater than the values for Nu for plain tube, twisted tape without dimple, dimpled twisted tape with D = 3 mm (D/H = 3) and dimpled twisted tape with D = 7 mm (D/H = 3) respectively. Based on the analysis, the friction factor (f) is found directly proportional to the depth and diameter of the dimple, consequently, maximum value of friction factor (f) is found at 7 mm diameter and D/H 1.5. In addition, it is observed that maximum value of performance evaluation criteria (PEC) is found at dimple diameter of 5 mm (D/H = 3). The correlations for heat transfer and friction factor have been established in terms of dimple geometry (D and D/H) and Reynolds number (Re).