Abstract:
The present work represents a 2-D and 3-D numerical investigation of forced convection heat transfer over circular and oval tube banks consisting of seven rows of tubes in an inline arrangement with rectangular longitudinal vortex generators (LVG) placed at the bottom of the channel. At first, the effects of common flow up and common flow down pairs of vortices produced by vortex generators (VG) with rectangular winglets restudied for a range of Reynolds numbers. The results show that effect of LVGs could effectively enhance the heat transfer of the heat exchanger in case of common flow up configuration of the winglets. Subsequently, the effects of Reynolds number (from 550 to 1775 in 2-D /from 500 to 850 in 3-D), the number of vortex generators (1, 3, and 7) and the angle of attack (30º and 45ºin 2D, and 15º and 25º in 3D) of rectangular VGs on the heat transfer and fluid flow characteristics are examined. The characteristics of average Nusselt number, associated pressure drop, friction factor, area goodness factor, streamline distribution and temperature contours etc. are studied numerically by the aid of the computational fluid dynamics. In case of the 2-D analysis, heat transfer coefficient is increased by80.5% for 7VGs and 45º angle of attack configuration compared to the baseline case. At the same time, circular tube banks give a nearly three times more pressure loss than that of oval tubes under the same operating conditions.
The 3-D cases are also evaluated and the circular tube (7VG) is found to exhibit an enhancement of heat transfer coefficient of 45% with an associated pressure drop penalty of 127%. In the case of oval tubes, an aspect ratio of 1.24 gives a remarkable heat transfer enhancement of 26.5% at a mere 69% increase in the pumping power than for oval tubes having aspect ratio of 2.34. For the Reynolds number ranging from 500 to 850, for the case of 1-RWP (Rectangular Winglet Pair), the increment of j/ f ratio is 27% over 7-RWP case. As number of VG increases, the area goodness factor reduces to ~0.18 from~ 0.23.From the viewpoint of “area goodness,” the1-RWP and 3-RWP case will require a smaller frontal area than the other cases and would be more efficient in compact heat exchanger design. However, in both cases, the streamline plot shows an excellent recirculation region near the VG‟s position indicating that the addition of vortex generators intensifies the mixing between the hot and cold fluids and thereby enhances heat transfer significantly. The performance of the oval and circular tube banks with vortex generators under the same operating conditions is compared with a focus on finding the preferable configuration of vortex generators.
