Heat transfer and friction factor inside elliptic tubes fitted with helical screw-tape inserts

Experimental investigation of heat transfer and friction factor characteristics of elliptic tubes with different twist ratios (Y) and pitch ratios (S) has been conducted under laminar flow condition. All data are obtained using cold water flowing in the tube side and hot water flowing in the annuli side of the heat exchanger. The experiments covered a range of Reynolds numbers, 5.7×102≤Re≤1.31×103. Heat transfer and friction factor analyses are presented for different conditions of Re, S, and Y. The results indicate that the helical screw element of different Y and S has an effect on the results of heat transfer coefficient and friction factor. The averaged Nusslet number (Nu) increases with an increase in Re and it decreases with an increase in Y and S. For a fixed Reynolds number, the friction factor (f) increases with a decrease in Y and S for the elliptic tubes. The thermal performance factor (η) is introduced to indicate the percentage increase in Nu over the percentage increase in f. The maximum percentage increase in Nu over the percentage increase in f (maximum increase in η) is 19.7% and 42.3% at the values of S and Y equal to 1 and 0.22, respectively. Correlations of the average Nusselt number and friction factor with Re, S, and Y are presented.