RÉSUMÉ

The characteristics of decaying swirling flows and forced convective heat transfer on the conditions of both laminar and turbulent flow in a narrow concentric annulus were simulated. The governing equations are solved numerically via a finite volume method. A uniform wall temperature at the inner wall and adiabatic conditions at the outer wall are considered as thermal boundary conditions. Solutions for the axial and swirl velocity distributions and the Nusselt number are obtained for different values of the inlet swirl number and the Reynolds number. Simulations show that the inlet swirl number have great influences on the heat transfer characteristics. Under both developing laminar and developed turbulent flow conditions, the increases of the inlet swirl number will enhance the heat transfer. When the inlet swirl number increases it increases the axial velocity near the wall and reduces it at the mid-gap to achieve the conservation of mass due to the existence of secondary flows in the annulus due to centrifugal forces. The increase of the near-wall velocity, in turn, produces larger temperature gradients and a higher heat transfer rate. The swirl velocity profiles decay gradually downstream as a result of friction which leads to damping of the tangential velocity. The swirl has a pronounced effect on the turbulent kinetic energy which is increased evidently with the swirl number. Obviously, a higher turbulence level leads to a considerable improvement in the heat transfer rate. Turbulence level improvement can be attributed to the high velocity gradients. Numerical results show that the turbulent kinetic energy is lower in the mid-gap and higher in the near-wall regions. Moreover, the turbulent structures near the outer wall are more activated than those near the inner wall. The comparison between predicted and experimental data of average Nusselt numbers was found to be in good agreement.