Experimental study of heat transfer and friction factor inside elliptic tube fixed with helical coils

The circular and rectangular helical-coils were sited in elliptic tube section with four various pitches of 20, 45, 135 and 225 mm. The cold air Re number was ranged from (11 x 10(3) <= Re <= 3.375 x 10(4)). The effect of reverse-flow for different pitches and inserted helical wire cross-sections can improve the heat transfer rate. Also, comparing with previous studies, the use of circular and rectangular wire cross-sections inserted in elliptic tube at various helical-coil pitches was useful over the smooth tube, and rectangular cross-section helical-coil is more potent in heat transfer enhancement rather than circular one. Nu, f and eta for rectangular helical wire cross-section inserted in elliptic tube are higher than that is for circular one for all values of helical-coil pitches by about 15%, 3% and 14% respectively. eta for the current elliptic tube fixed with circular and rectangular helical coil are higher than that of Sharafeldeen et al. (2017) for all helical-coil pitches by about 24.5% in average. Correlated data results of Nu, f and eta for helical-coil with different wire cross-sections are obtained for the considered Re range.