Heat transfer in a molten salt filled enclosure absorbing concentrated solar radiation

• Published in: International journal of heat and mass transfer Vol. 113; pp. 444 - 455
• Main Authors: Amber, I., O’Donovan, T.S.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.10.2017; Elsevier BV
• Subjects: Absorption; Adiabatic flow; Aspect ratio; Buoyancy; Computational fluid dynamics; Computer simulation; Convection; Finite element analysis; Finite element method; Fluid flow; Heat transfer; Heat transmission; High temperature; Molten salt; Molten salts; Natural convection; Navier-Stokes equations; Numerical simulation; Radiation; Rayleigh number; Salt; Simulation; Solar flux; Solar radiation; Stokes law (fluid mechanics); Surface chemistry; Numerical simulation; Natural convection; Molten salt
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2017.04.028

•Two dimensional numerical simulation of the transient heating of molten KNO3-NaNO3 presented.•Numerical formulation accounting for the volumetric heating source based on the SMARTS model.•Non linear temperature field and associated flow field described.•Effect of variable Air Mass on a nonlinear temperature field and flow field presented. Numerical simulations of the natural convection driven by the direct absorption of concentrated solar radiation by a high temperature molten salt filled enclosures for height to diameter ratios (H/D) of 0.5, 1 and 2 and Rayleigh numbers 107–1011 is presented. The domain of interest consists of a fluid cavity bounded by rigid adiabatic vertical walls, a heat-conducting bottom wall of finite thickness and an open adiabatic top surface, directly irradiated by a non- uniform concentrated solar flux. The salt volume is first heated non-uniformly by direct absorption of solar radiation and subsequently from the lower absorber plate which is heated by the absorption of the radiation transmitted through the salt. A Finite Element Method is used to solve the time dependent two dimensional Navier Stokes equations that includes a depth dependent volumetric heat source and temperature dependent thermophysical of molten salts. Numerical results presented in terms of isotherms and streamlines show a nonlinear temperature profile consisting of distinct layers where thermocapilarity and buoyancy effects are evident. Fluid flow development in the lower layer is found to vary significantly with time and exhibits an initial stage, transitional stage and quasi-steady stages. The magnitude of the natural convection and the duration of each stage is found to decrease as the aspect ratio increases from 0.5 to 2. Calculation of the average heat transfer reveals that the Nusselt Rayleigh number relationship is not uniformly linear and the average heat transfer over the lower boundary surface increased with increasing Ra.