Heat transfer behavior of laminar flow inside circular duct with centre-cleared twin twisted tape (CTTT) swirl generator was investigated numerically. The governing equations were 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. The centre-cleared twin twisted tapes were tested in the current work; investigations were performed in the Reynolds number range of 100- 1,000 with four different CTTTs at twist ratio (TR) of H/W = 1.0, 2.0, 3.0 and 4.0. Water is used as working fluid (Pr = 7). The effects of Reynolds number, Nusselt number, friction factor, convective heat transfer coefficient were examined and discussed. The use of CTTT was found to increase the heat transfer considerably when compared with plain tube. The Nusselt number increased with the increase of Reynolds number. The overall enhancement ratio has been calculated in order to discuss the overall effect of CTTT and the working parameters. The present findings would be useful inputs for the design of solar thermal heaters and heat exchangers.