Thermal transients in a U-Bend

TL;DR

This study uses large eddy simulations to analyze how a hot thermal transient propagates through a turbulent U-bend, revealing the dominance of baroclinic vorticity and effects of buoyancy on vortex behavior and flow losses.

Contribution

It provides new insights into the thermal and flow dynamics in a U-bend under turbulent mixed convection with conjugate heat transfer, highlighting vortex reversal effects.

Findings

Baroclinic vorticity dominates during the transient.

Buoyancy causes reversal of vortex pairs.

Low frequency flow modes are suppressed in the reversed vortex state.

Abstract

We study numerically the propagation of a hot thermal transient through a U-bend via an ensemble of wall-resolved large eddy simulations. Conjugate heat transfer between fluid and solid domains is accounted for. The flow is in a fully turbulent mixed convection regime, with a bulk Reynolds number of $10,000$, a Richardson number of $2.23$, and water as the working fluid (Prandtl number = $6$). These conditions lead to strong thermal stratification, with buoyancy-induced secondary flows, and the generation of a large and persistent recirculation region. The evolution of Dean vortices as the thermal transient passes is studied. It is found that baroclinic vorticity generation dominates over a large period of the transient, due to the thermal inertia of the wall. Gravitational buoyancy leads to a reversal of the counter-rotating vortex pair. The impact of this reversal on the swirl-switching and secondary-current losses is assessed. It is found that low frequency modes are suppressed in the reversed-vortex state.