Numerical study on heat transfer enhancement and flow characteristics of double pipe heat exchanger fitted with rectangular cut twisted tape

In the present study, the heat transfer, friction factor and thermal performance factor of rectangular cut twisted tape (RCT) that are inserted in inner tube side of a double pipe heat exchanger (DPHE) are numerically examined. The obtained results have been compared with the performance of heat exchangers, which are equipped/unequipped by typical twisted tape (TT). The RCTs have constant twist ratio (Y = 3.0) and are used in different cut depth ratios (DR = 0.15, 0.24 and 0.33) and cut width ratios (WR = 0.15, 0.24 and 0.33). The investigated Reynolds number range is from 6000 to 18,000 and the working fluid is considered to be water. Hot water with a variable mass flow rate is flowing through the inner pipe but cold water has counter flow through annulus side with constant mass flow rate. For the selection of the turbulence model and validation, the obtained numerical results for plain double pipe heat exchanger (P-DPHE) and heat exchanger with TT are compared with empirical correlations. Then the more accurate model was used to simulate the RCTs cases. Comparing the simulation results for RCTs and TT, it was determined that Nusselt number, friction factor and thermal performance factor will have higher value when using RCTs. This behavior is directly and inversely proportional to the DR and WR respectively. In this study, the highest thermal performance factor is about 1.46 which is obtained for RCTs in DR = 0.33 and WR = 0.15 conditions. In addition, new empirical correlations have been offered for calculating a Nusselt number and friction factor for inserted rectangular cut twisted tape in a tube.