THE COMPUTATION OF TURBULENT HEAT TRANSFER IN HIGH PRANDTL NUMBER FLUIDS

Abstract

With increasing demand in industrial applications, high Prandtl number fluids such as molten salts are increasingly used in heat transfer equipment. A detailed investigation of the heat transfer mechanisms in high Prandtl fluids is needed to resolve problems arising in various heat transfer systems and improve their behaviours. As a part of this, it is necessary to investigate the stability and accuracy of a variety of turbulence model predictions for high Prandtl fluids. The present work addresses this by providing a detailed assessment of the performance of a number of standard and extended Reynolds-averaged Navier-Stokes (RANS) models through computations of forced, natural and mixed convection flows. The models tested include the high-Re k-epsilon, SST k-omega, the low-Re k-epsilon of B. E. Launder and Sharma (1974)(LS), the same model including the lengthscale correction of Yap (1987) (LS_YAP) and the damping term modification of Sarno et al. (2018) (LS_MOD). Based on their performance, further modifications have been developed to the damping term in the LS form and the lengthscale correction (LSMOD_YAPMOD) to improve the predictions in high Prandtl number cases. First, two-dimensional fully-developed turbulent forced convection channel flows were considered over a range of Reynolds and Prandtl numbers, employing two distinct thermal boundary conditions. Results showed that at high Prandtl numbers, the proposed modified model (LSMOD_YAPMOD) captured the correct effect on the thermal field near the wall, showing very good quantitative agreement with DNS data. Then, a two-dimensional vertical natural convection channel flow, driven by a constant wall temperature difference, and a three-dimensional time-dependent turbulent Rayleigh-Benard convection flow were considered over a range of Rayleigh and Prandtl numbers. Results revealed that the LSMOD_YAPMOD model accurately predicted both the dynamic and thermal fields in these natural convection flows across a range of moderate to high Prandtl numbers. Finally, a two-dimensional turbulent mixed convection channel flow, containing both buoyancy-opposed and buoyancy-aided flow regions, was analyzed for two different Prandtl numbers, resulting in different buoyancy parameter values. Results revealed that the key factor was the difference in dissipation caused by the modified or standard Yap correction (Yap (1987)), especially notable in low-turbulence cases such as partly laminarized flow. In buoyancy-opposed mixed convection, where turbulent kinetic energy dominated, the modified damping function term significantly affected the thermal field.

Date of Award	6 Jan 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Hector Iacovides (Supervisor) & Timothy Craft (Supervisor)