Direct numerical simulation of turbulent heat transfer in a radially-rotating circular pipe flow

The effects of radial system rotation on turbulent heat transfer in a circular pipe flow are studied using direct numerical simulations (DNS) over a wide range of rotation numbers varying from Roτ=0 to 1. To ensure that the pipe length used in DNS is sufficient for capturing the most energetic axial turbulent thermal-flow structures, a comparative study has been conducted based on five pipe lengths of Lz=2πR-20πR. In response to the radial system rotation imposed, large-scale secondary flow motions are induced, which appear as a pair of counter-rotating vortices in the cross-stream direction and significantly influence the transport of thermal energy within the pipe flow. It is observed that laminarisation occurs on the suction side and propagates towards the pressure side of the pipe as the rotation number increases. At the highest rotation number tested, the flow becomes completely laminarised, leading to a reduction of bulk mean temperature. The Coriolis force effects on the turbulent heat transfer is analysed through the statistical moments of the temperature field and transport of turbulent heat fluxes. The significant role of induced secondary flows in near-wall turbulent transport of thermal energy is investigated through joint probability density function (JPDF) and linear stochastic estimation (LSE). It is interesting to observe that the secondary flows induced by the radial system rotation are capable of enhancing the hot sweep events, resulting in a higher local turbulent scalar energy (TSE) level at relatively low rotation numbers. However, at relatively high rotation numbers, near-wall cold ejection and hot sweep events are significantly suppressed on the pressure side of the pipe.