Some observations on the spatiotemporal orbits structure and heat transfer enhancement in pulsating flow

We describe in this work a numerical simulation of chaotic heat transfer by laminar flow in a twisted pipe that consists of three bends. Observations on the secondary flow topology changed by the variation of Reynolds number and tube wall temperature as well as their coincidence with heat transfer enhancement are discussed. Both steady and pulsating flows with constant wall temperature are considered. Numerical simulations are performed for Reynolds numbers range 300 ≤ Re ≤ 1000, velocity amplitude ratios (the ratio of the peak oscillatory velocity component to the mean flow velocity) 1 ≤ β ≤ 2, Womersley numbers 6 ≤ α ≤ 20, and wall temperatures 310 ≤ TW(K) ≤ 360. It is observed that the variation of the number of elliptic orbits (cells) and the area covered by cell centers in the pipe cross-section are two crucial factors in heat transfer enhancement. When the Reynolds number increases in the steady and pulsating flows, the number of elliptic orbits in the secondary flow patterns is increased and heat transfer is enhanced. However, heating uniformity is degraded with increasing Reynolds number that results in the reduction of heat exchanger performance. For the pulsating flows, small and moderate values of Womersley numbers (6 ≤ α ≤ 12) and high values of velocity amplitude ratios (β > 1) provide a more complex secondary flow and hence a better heat transfer. Pulsation energy consumption is higher than the energy needed for increasing Reynolds number of the steady flow to match the same heat transfer enhancement. However, a better heating uniformity is obtained in the pulsating flow, which is not the case in the steady flow.