Mixed convective heat transfer of medium-Prandtl-number fluids in horizontal circular tubes

Abstract

Studies on mixed convective heat transfer or mixed convection of laminar flows in horizontal circular tubes are normally restricted to fluids of low or high Prandtl numbers, such as air or oil, while few of them focus on fluids of medium Prandtl numbers, such as molten salts for liquid-fuel and solid-fuel Molten Salt Reactors (MSRs). Therefore, this paper attempts to investigate mixed convection of medium-Prandtl-number fluids. A three-dimensional model is built in STAR-CCM+ for a molten salt, LiF-BeF2-ThF4-UF4 (67.5–20.0–12.0–0.5 mol%), flowing in horizontal circular tubes uniformly cooled. Correlations are developed to predict the entrance length for laminar mixed convection with ± 20% uncertainties. In addition, the Nusselt number predicted by STAR-CCM+ deviates significantly from conventional correlation results by up to 85%. An accurate mixed convective heat transfer correlation is therefore developed with ± 15% uncertainties and recommended for medium-Prandtl-number fluids in horizontal circular tubes, which is applicable for Re¯=2.0×102to1.8×103, Gr¯=9.2×101to1.3×105 (or Gr*¯=1.7×103to1.8×106), Pr¯=3.9to3.2×101, and L/Di=2.0×102to1.0×103. The proposed correlation is also applicable for Pr¯=7.0×10−1to1.8×104 with ± 40% uncertainties by comparing with the experimental data. Furthermore, flow regime maps are proposed for forced and mixed convection by presenting Nu¯m/Nu¯f in a plot with the ordinate of Gz¯ (=Re¯Pr¯Di/L) and abscissa of Gr¯Pr¯Di/L or Gr*¯Pr¯Di/L, which help visibly identify under what conditions it is necessary to consider the buoyancy effect. A correlation is also developed with ± 15% uncertainties to quantitatively evaluate the buoyancy effect in laminar mixed convection of medium-Prandtl-number fluids in horizontal circular tubes.