Identification of surrogate fluids for molten salt coolants used in energy systems applications including concentrated solar and nuclear power plants

Molten salts are of great interest in the energy industry as coolants and heat transfer fluids due to their superior heat transfer properties and high operating temperatures. Due to these high temperatures and the corrosive properties of many molten salts, it can be difficult to perform fluid flow and heat transfer experiments of molten salt components in laboratory settings. Instead, surrogate fluids that operate at lower temperatures and match relevant dimensionless parameters of molten salts are suggested for such experiments. In this work, we examine two well-known surrogate fluids, water and Dowtherm A, and introduce two heat transfer fluids, Freezium 60 and Zitrec S-25, as candidate molten salt surrogates. We compare these fluids to common fluoride-, chloride-, and nitrate-based molten salts by considering temperature ranges over which their Prandtl numbers match. A model example at matched Prandtl and Reynolds number is used to determine the effects of using such surrogates on friction factor, pressure drop, Nusselt number, and pumping and heating power in a theorized experiment, and potential distortions based on Prandtl number mismatches are discussed. This article describes a distortion calculation methodology and quantifies the largest Prandtl number distortions encountered for various molten salt and surrogate pairs using a linearly scaled technique. Distortions between the fluids are seen to be small, but uncertainties in salt thermophysical properties motivate further research to understand the magnitude of such distortions and their subsequent effects on experimental outcomes.