Convective heat transfer of molten salt-based nanofluid in a receiver tube with non-uniform heat flux

Abstract

To tackle thermal challenges of solar receiver tubes due to the non-uniform radiation heat flux, the heat transfer performance of molten salt-based nanofluid was numerically investigated. Parameters influenced on the thermal performance of the receiver tube and molten salt with or without Al2O3 nanoparticles were investigated, including particle concentrations, inlet velocities, and heat flux profiles. The simulation results showed that all nanofluid performed better heat transfer performance than pure molten salt under both cosine and Gaussian-cosine heat flux boundaries. The 0.063% nanofluid maximally enhanced the heat transfer coefficient by 7.29% and Nusselt number by 6.90% under cosine heat flux boundary, and 7.25% and 6.85% under Gaussian-cosine heat flux boundary. The temperature distributions of the tube wall surfaces and the heat transfer fluid were highly non-uniform and related to heat flux boundaries. The maximum temperature of the outer and inner surfaces could be reduced by nanofluid and the highest drop was achieved by the 0.063% nanofluid, which will protect the solar receiver and prohibit the decomposition of molten salt. According to the analysis of parameters, the specific heat capacity contributed to most of the change in heat transfer performance when the mass fraction of nanoparticles was smaller than 0.125%, while the thermal conductivity became an influencing factor as the mass fraction grew. The Gnielinski correlation was found to guide for designing molten salt-based nanofluid solar receivers exposed to non-uniform radiation heat flux, after comparing with simulation results. This work could be useful as a reference to the application of molten salt-based nanofluid in the absorber tube.