Effect of hemispherical turbulators in a double-pipe heat exchanger for heat transfer augmentation

In a double-pipe heat exchanger, heat transfer enhancement can be carried out by active or passive methods. In the present study, heat transfer characteristics are studied by the addition of hemispherical turbulators placed in the annulus region in a double-pipe heat exchanger. The turbulators are placed in the annulus region on the inner pipe separated at 90° to each other around the circumference. Studies have been carried out for various diameter ratios of 0.29, 0.44, and 0.58 and different pitch ratios of 1.47, 4.41 and 7.35. Water is taken as the fluid flowing in the inner pipe and air in the annulus. The numerical simulation is carried out to study various thermal parameters such as the Nusselt number (Nu), friction factor and thermohydraulic performance index. It is observed that the addition of the hemispherical turbulators in the double-pipe heat exchanger enhanced the air turbulence. The Nusselt number and the heat transfer rate increased but caused an undesirable increase in the friction factor compared to the bare pipe. Decreasing the pitch ratio as well as increasing the diameter ratio led to an increase in the heat transfer rate and Nu.