Data for: RADIATIVE HEAT TRANSFER IN FLIBE MOLTEN SALT PARTICIPATING MEDIUM IN A VERTICAL HEATED TUBE UNDER FORCED AND MIXED CONVECTION LAMINAR FLOWS

Published: 17-09-2020| Version 1 | DOI: 10.17632/ztfggjjp4n.1
Contributors:
Mario Trujillo,
Mohamed Abou Dbai

Description

The contribution of radiative heat transfer (RHT) to convective heat transfer in a heated vertical pipe with laminar flow and constant wall temperature, for FLiBe molten salt, is investigated computationally. The combined effects of conduction in the fluid, forced convection, buoyancy, temperature-dependent physical properties, and thermal radiation are investigated. The P1 approximation is employed in the discretization of the Radiative Transfer Equation (RTE). The COMSOL Multiphysics software is used to generate the numerical solutions. Theoretical analysis demonstrates that only for intermediate values of the optical thickness, i.e. τ_D~O(1), the participating media effects are expected to be important. This analysis is confirmed computationally, where a value of τ_D≈4 is shown to lead to the highest increase in overall heat transfer behavior. Under forced convection, the Nusselt ratio Nu_total/ Nu_(no-rad) reaches a peak value of 1.76 at τ_D=4 and z/D=200, and is less than 1.1 for τ_D<0.1 and τ_D>60. Under aiding-flow mixed convection, Nu_total/ Nu_(no-rad) reaches a peak of 1.34 at τ_D=4.2 and is less than 1.1 for τ_D<‍0.4 and τ_D>36. Under opposing-flow mixed convection, Nu_total/ Nu_(no-rad) reaches a peak of 1.81 at τ_D=4.2 and is less than 1.1 for τ_D<0.2 and τ_D>50. RHT effects on the overall heat transfer are most pronounced in opposing mixed convection where peak Nu_total/ Nu_(no-rad) is 2.26 observed at z/D=50. The sensitivity to wall emissivity is evaluated; in forced convection, the peak Nu_total/ Nu_(no-rad) is 1.66 at ε=0 (reflective wall) and 2.04 at ε=1 (absorptive wall). The sensitivity to pipe diameter is also discussed. Overall, for a vertical heated tube, radiative heat transfer effects lead to enhancement of heat transfer by as high as a factor of two, and they depend on the optical thickness of the flow, mixed convection environment (Gr\/Re^2 and direction of flow relative to the gravitational force), surface emissivity, and entrance effects.