Twisted tape is a widely used technique for heat transfer enhancement. In the present study, the influences of centre-trimmed twisted tapes (CTTT) on heat transfer rate, friction factor and thermal enhancement efficiency were numerically investigated. The centre-trimmed twisted tapes were used as swirl flow generators inside the circular duct. Investigations were performed in the Reynolds number range of 100 -5,000 with four different centre-cleared twin twisted tapes at twist ratio (TR) of H/W = 1.0, 2.0, 3.0 and 4.0. The governing equations are solved with a finite-volume-based numerical method. A three-dimensional non uniform grid was generated, in order to critically examine the flow and heat transfer. Results show that the centre-cleared twin twisted tape causes considerable enhancement of heat transfer when compared with plain twisted tape (TT). The enhancement over standard twisted tape increases with the increase of Reynolds number. The improved parameter is observed in the range of Reynolds number below 800 and also above Re 4000 for all TR, though the best performance is found at TR = 1.0 and 3.0

Experimental investigations on heat transfer enhancement by swirl generators are presented. The thermo hydraulic performance has been evaluated. The experiments were conducted over a wide range of Reynolds number from 993 to 12,034 by employing air (Pr = 0.7) as the working fluid. The major findings of this experimental investigation are that the circular grooved tube with different diameter ratio (h = d/D = 0.04 and 0.06) and pitch ratio (y = p/D = 1.5 and 3.0) perform better than the smooth duct. In evaluation of thermal performance, it is seen that the bluff cylinder of (y = 1.5, h =0.06) are thermodynamically advantageous than the other roughness used. The result is useful for heat exchanger, solar air-heater, etc.