Enhancement of Heat Transfer in Laminar Flows Using a Toroidal Helical Pipe

We examine the effectiveness of forced convective heat transfer in a toroidal helical pipe (THP), as a compact process intensification device for use in heat exchangers. Earlier, we showed that mixing was enhanced significantly in such devices due to a continuously changing curvature along the length of the pipe while retaining its compactness. In this work, we examine six Reynolds numbers ranging from 1 to 200, two Prandtl numbers (1 and 7), and two thermal boundary conditions (fixed temperature and fixed flux). The heat transfer efficiency in THP is quantified mainly by the Nusselt number as a function of the downstream distance, and it is compared with that in straight pipes of identical setup. Thus, enhancement by the geometric factor alone is extracted. We found in THP that the Nusselt number is not monotonic but varies periodically around mean values that can be more than four times higher than straight pipes. The unique coupled-Dean vortex flow pattern, due to the varying curvature of the THP axis, significantly enhances the in-plane mixing and causes the higher and oscillatory Nusselt numbers.