Numerical Research On The Flow And Heat Transfer Of Molten Salt-based Nanofluids In A Receiver Tube

Concentrated solar power becomes more and more popular.However,the non-uniform radiation heat flux impinging on the wall surface of the receiver tube could worsen the flow and heat transfer characteristics of molten salt,causing some serious problems such as the degradation of molten salt and local hot burning of the tube.Therefore,this paper focuses on the convective flow and heat transfer of molten salt-based nanofluids?the mixture of molten salt and nanoparticles?inside the receiver tube.A solar power tower plant was first modeled geometrically and optically,and the wall heat flux distribution of the receiver was obtained.According to the results,the variation of the solar radiation heat flux received on the wall of different receiver tubes was related to the annular distribution of the heliostat field.For a single receiver tube,the wall heat flux distribution was non-uniform.The heat flux distribution of the receiver at noon of Winter Solstice was the lowest,and the minimum value appeared on the wall surface of the southward receiver tube with the maximum value dropped by about 0.50 MW/m².Then,characteristics of flow and heat transfer of molten salt-based nanofluids inside the receiver tube were numerically investigated.Simulation results showed that the temperature of the nanofluids decreased compared with the temperature distribution of pure molten salt.The heat transfer performance of molten salt suspended with Al₂O₃nanoparticles was analyzed.It is indicated that the nanofluids with mass fraction of 0.063%performed the largest heat transfer enhancement with its heat transfer coefficient in the fully developed zone increased by 6.5%.The specific heat capacity was found as the main reason for the enhancement of heat transfer performance.The numerical study of the turbulent flow and heat transfer of Al₂O₃/HITEC nanofluids was subsequently conducted.According to simulation results,it is proved that the 0.063%nanofluids achieved the largest average heat transfer coefficient with 7.29%and average Nusselt number?Nu?with 6.90%under cosine heat flux boundary,and 7.25%and 6.85%under Gaussian heat flux boundary.The maximum temperatures of the outer and inner surfaces were dramatically reduced by nanofluids,which would protect the solar receiver and prohibit the decomposition of molten salt.The relative parameters analysis verified that the specific heat capacity contributed to the heat transfer variations of molten salt-based nanofluids.The simulation results for all nanofluids agreed well with the Gnielinski correlation,indicating this correlation could be used to guide the design of molten salt-based nanofluids solar receivers.Finally,a new combination technique of molten salt-based nanofluids and porous inserts was presented to improve the thermo-hydraulic performance of the receiver tube.The effects of porous inserts configurations,porosities,materials,nanoparticle concentrations,and Reynolds numbers?Re?were discussed in terms of the Nu number,the pressure drop,and the performance evaluation criterion?PEC?.The results showed that molten salt-based nanofluids had a better heat transfer performance inside the tube with a porous medium,compared with pure molten salt flow inside a smooth tube.The enhanced tube inserted with wall coherent type porous medium achieved much better heat transfer performance.Among the four kinds of materials of the porous medium,the best one was copper,followed by aluminum,nickel,and steel in order.The optimal combination for the best thermo-hydraulic performance was the molten salt-based nanofluids with a mass fraction of 0.125%and the receiver tube with copper metal foam with the porosity of 0.9005.This combination accomplished the best heat transfer enhancement with 12.44 times of Nu number higher than pure molten salt inside the smooth tube at the Renumber of 10000.The peak value of PEC?5.98?reached at the Re number of 60000.