Mixed convective and radiative heat transfer of LiF-BeF2-ThF4-UF4 in a vertical circular tube

Abstract

Molten salts are promising fluids for applications involving heat transfer and thermal energy storage. The buoyancy effect and radiative heat transfer effect are frequently ignored in previous studies that concentrate on forced convective heat transfer of molten salts. However, these effects might not be negligible, particularly for laminar flows of optically semi-transparent fluids, such as molten salts. Therefore, this study tries to understand the heat transfer characteristics of molten salts, including mixed (forced and natural) convective heat transfer and radiative heat transfer. A two-dimensional axisymmetric model is created in STAR-CCM+ for assisting (buoyancy-assisted) laminar flows of LiF-BeF2-ThF4-UF4 (67.5–20.0–12.0–0.5 mol%) in a 5-m long, 15-mm-inner-diameter vertical circular tube. The effects of several non-dimensional parameters on the velocity, temperature, and Nusselt number are examined, including the Reynolds number (Re¯=2.0×102to1.8×103), Grashof number (Gr¯=3.1×103to5.4×104), optical thickness (τ=0.1τ0to10τ0, τ0 = 0.27, 2.11, and 446.47 for the three bands λI = 0.10 to 3.36 µm, λII = 3.36 to 5.42 µm, and λIII = 5.42 to 100.00 µm, respectively), and emissivity (ε=0.0to1.0). A flow regime map is also suggested to assess the radiative heat transfer effect in molten salts. Our analysis demonstrates that: (1) the predicted results in this study deviate from published experimental data by 14.5 to 45.1 % without taking the buoyancy effect and radiative heat transfer effect into account, however, these differences are dramatically decreased to 10.1 to 17.4 % while considering the buoyancy effect or 7.5 to 14.7 % while considering the buoyancy effect and radiative heat transfer effect; (2) the radiative heat transfer effect tends to mitigate the buoyancy effect, but their net effect enhances heat transfer; (3) the buoyancy effect factor fbuoy and thermal radiation effect factor frad are, respectively, up to 38.5 % and 19.5 % under the conditions investigated, illustrating the need to evaluate the two effects for molten salts; (4)fbuoy becomes larger for smaller values of Re¯, τ, and ε, and larger values of Gr¯ and non-dimensional axial location x/Di. frad becomes larger for smaller values of Re¯ and larger values of Gr¯, τ, ε, and x/Di; and (5) the flow regime map provided in this study aids in determining whether the radiative heat transfer effect in molten salts is negligible.