Numerical investigation of turbulent aided mixed convection of liquid metal flow through a concentric annulus

• Published in: International journal of heat and mass transfer Vol. 105; pp. 479 - 494
• Main Authors: Marocco, L., Alberti di Valmontana, A., Wetzel, Th
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.02.2017; Elsevier BV
• Subjects: Aerodynamics; Boussinesq equations; Buoyancy; Computational fluid dynamics; Concentric annulus; Convection; Dissipation; Fluid flow; Heat exchangers; Heat flux; Heat transfer; Liquid metals; Mixed convection; Navier-Stokes equations; Physical properties; Pipe flow; Pipes; Prandtl number; Reynolds averaged Navier-Stokes method; Reynolds number; Stokes law (fluid mechanics); Studies; Temperature; Turbulence; Turbulence models; Turbulent flow; Turbulent mixing; Vortices; Liquid metals; Concentric annulus; Mixed convection; Turbulence models
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2016.09.107

•Aided turbulent mixed convection to liquid metal in concentric annulus.•Two radius ratios and heat flux applied on inner, outer or both walls.•Assessment of RANS models and heat flux models.•Influence of Reynolds number on onset and magnitude of heat transfer impairment.•Different behavior of liquid metals from ordinary fluids due to different heat transfer mechanism. Turbulent aided mixed convection of a liquid metal with Pr=0.021 in a concentric heated annulus is investigated by solving the Reynolds-Averaged-Navier-Stokes equations. This geometry approximates rod bundles heat exchangers at high pitch-to-diameter ratios. Two inner-to-outer radius ratios of 0.13 and 0.5 are considered and a constant uniform heat flux is applied only to the inner wall, only to the outer wall or to both walls. Constant thermo-physical properties are assumed and buoyancy is accounted for in the momentum equation using the Boussinesq assumption. Four different eddy-viscosity models are first assessed against the few available experimental data for a pipe flow. The turbulent heat fluxes are modeled with the Simple-Gradient-Diffusion-Hypothesis and the turbulent Prandtl number is locally evaluated either with a correlation or by solving one additional transport equation for the temperature variance and one for its dissipation rate. The first approach gives a better agreement with the experimental data. It is found that, compared to medium-to-high Prandtl number fluids, the Reynolds number has a much greater influence on the onset and magnitude of heat transfer impairment. Its extent and degree are less than for ordinary fluids. It is shown that, contrarily to a pipe flow where liquid metals with Pr≈0.025 behave similar to air or water, in the concentric annulus big differences exist. The reason is the considerable contribution of molecular heat transfer in liquid metals that compensates the reduced turbulent mixing due to buoyancy.