Buoyancy induced turbulence modulation in pipe flow at supercritical pressure under cooling conditions

A numerical investigation of cooling of a fluid at supercritical pressure has been performed by means of direct numerical simulations. The simulations were conducted with a uniform heat flux imposed at the wall at an inlet bulk Reynolds number of 5400. The aim of this work is to understand the role of buoyancy in modulating the turbulence in a flow where properties are spatially varying. Heat transfer deterioration followed by recovery is observed in the downward flow while enhancement occurs in the upward flow as compared to forced convection. The decomposition of the skin friction factor and the Nusselt number was performed. The major effects on the skin friction factor were brought by the nonuniform body force due to the gravity. The turbulent parts equally influence the Nusselt number as well as the skin friction factor in supercritical flows. Quadrant analysis and its weighted joint probability density function were analyzed to understand the role of sweep (Q(4)) and ejection (Q(2)) events. During the heat transfer deterioration, sweep and ejection events are decreased greatly, triggering the reduction in turbulence. The recovery in turbulence is brought by the Q(1) and Q(3) (also known as outward and inward interaction) events, contrary to the conventional belief about turbulence generation. The turbulence anisotropy of the Reynolds stress tensor is investigated showing that the turbulence structure becomes rod-like during the deteriorated heat transfer regime in the downward flow and disc-like for the upward flow. Published by AIP Publishing.