Thermal transients in a U-bend

Abstract

We study numerically the propagation of a hot thermal transient through a Ubend 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 buoyancyinduced 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.

Transient Thermal Flow in a U-Bend and Its Influence on Wall Thermal Stress

“…To investigate the effects of the stratification of the thermal fluid on the stress of the U-bend, the solid domain is focused on in the following sections. A thermal lag of the U-bend wall has been observed due to the thermal inertia as reported by Skillen et al (2020). At 𝑡 ̃= 15, the thermal front has travelled to the near-horizontal section, while, as can be seen from Figure 7 (a), the upstream vertical section is still far below 𝑇 1 .…”

“…The URANS results at 𝐹𝑟 = 0.670 are compared with reference Large Eddy Simulation (LES) results (Skillen et al, 2020) for the wall temperature and the velocity and has been presented…”

“…These secondary flows occur in many industrial applications and can have a significant impact on heat and mass transfer (Vashisth et al, 2008, Noorani et al, 2013, Giannakopoulos et al, 2017. Skillen et al (2020) also indicated that there is a significant impact on the thermal stratification from the secondary flow, such as Dean vortices. The dynamics of a hot thermal transient propagating through pipe bends is particularly relevant to the nuclear industry.…”

“…In mixed convection conditions (relevant to the passive safety case of nuclear reactors), thermal transients can significantly affect the Dean vortices, potentially leading to a reversal of the secondary flow (Viollet, 1987). Skillen et al (2020) proposed a mechanism by which this Dean vortex reversal occurs; the thermal inertia of the wall causes near-wall fluid to remain at a lower temperature than that away from the wall. This generates a baroclinic torque, resulting in a buoyancy-induced secondary flow of the opposite direction to that of the Dean vortices.…”