Convective and Radiative Heat Transfer in Molten Salts

Molten salts have been proposed as heat transfer media due to their superior thermal performance at elevated temperatures. A number of heat transfer correlations have been proposed in the literature for molten salts without explicitly considering the radiative heat transfer effect in the salts, which may not be negligible. This study therefore attempts to (1) quantitatively analyze the convective and radiative heat transfer of molten salts using an overall heat transfer model that includes a radiative heat transfer model developed in this research and an existing conventional convective heat transfer model/correlation, such as the Sieder-Tate or Hausen correlation, and (2) provide rationale on under what conditions it is necessary to consider the radiative heat transfer effect in salts. A parametric study was performed using the radiative heat transfer model developed to investigate the effects of various input variables, including the tube size (inner diameter 5 to 50 mm), salt temperature (500°C to 1000°C), salt and wall temperature difference (5°C to 100°C), and salt absorption coefficient (1 to 100 m^-1). Our study indicates that (1) the proposed overall heat transfer model reasonably predicts the salt convective and radiative heat transfer, (2) the radiative heat transfer is more important for laminar flows than transitional and turbulent flows, (3) the radiative heat transfer is more important in tubes of larger inner diameter, (4) the salt temperature affects the radiative heat transfer significantly while the temperature difference between the salt and wall has a slightly smaller effect for the range investigated (ΔT = 5°C to 100°C), and (5) the salt absorption coefficient significantly affects the salt radiative heat transfer.