Numerical study on heat transfer performance of nanofluid in circular pipe with combined rib and rotating twisted tape

Abstract

Enhancement of heat transfer is important for many engineering applications. Numerous researchers investigated the enhancement of heat transfer through turbulence by a swirl flow. The secondary flow (swirl flow) generated by twisted tape promotes greater mixing and higher heat transfer coefficient. Till now numerous experimental and numerical researches have been conducted with twisted tape to obtain an optimal solution. However, in most of the numerical research stationary twisted tape was used. This study is performed numerically by rotating a twisted tape insert in a circular pipe with ribs inside the pipe surface considering the turbulent swirl flow. The realizable k-epsilon (2 equations) model for incompressible Newtonian fluid is used. The velocity profile of the flow obtained from the numerical study is compared with the theoretical velocity profile for turbulent flow. The comparison is satisfactory and thus the code is used for the present investigation. Uniform heat flux is considered on the outer surface of the pipe. The investigation reveals the effect of turbulence, Reynolds number and different sizes of swirling ribs on heat transfer performance. Moreover, the effect of nanofluid in heat transfer is also evaluated.

The results show that heat transfer performance of a circular pipe with ribs inside the pipe surface increases about 8.4 % than the smooth circular pipe. With the increase of rotating speed of twisted tape heat transfer increases.However, the increment of heat transfer is not linear, and can be observed that high rotational speed results in high increment of heat transfer. The implementation of rotating twisted tape at the center of the pipe increases heat transfer rate about 13.5 % than the smooth circular pipe. With the decrease of twist ratio high heat transfer can be observed. It can be pointed out that decrease of twist ratio means increase of turns in the twisted tape which creates more turbulence. In smooth pipe using high-volume fraction of nanofluid gives 15.4% more heat transfer rate than water. The high-volume fraction of nanofluid increases the viscosity of the fluid which leads to the high pumping power.Heat transfer rate per unit pumping power decreases with the increase of Reynolds number.For lowest Reynolds number (about 3800) heat transfer per unit pumping power increases about 14 % for rib with aspect ratio 2. On the other hand, for high Reynolds number the increment of heat transfer is not significant for high volume fraction of nanofluid. However, the result is encouraging for low Reynolds number.