Numerical study of heat transfer losses by mixed convection and surface thermal radiation in an open cavity receiver for a solar tower system

The thermo solar central tower power plants are complex systems that consist of a heliostats field which provide a high solar concentrated flux to a thermal receiver located in the top of a tower. With this type of technology, a fluid moving in the thermal receiver can be heated up to 800 to 1200 K, so a conventional thermodynamic cycle can be operated to generate electricity. In the city of Hermosillo, in the northern state of Sonora, Mexico, the National Autonomous University of Mexico in agreement with the University of Sonora is developing this type of technology for a plant of 2 MWt with an array of 80 heliostats (36 m2 each one) and a tower of 32 m height. Therefore, an appropriated thermal receiver has to be designed. Considering above, in this work the numerical results of heat transfer losses by mixed convection and surface thermal radiation in an open cavity receiver considering variable fluid properties are presented. Numerical calculations were performed in a cavity of 1 m width, 2 m height and 2 m depth, considering (a) only natural convection and (b) mixed convection, both with surface thermal radiation. The temperature difference between the hot wall and the bulk fluid (ΔT) was 600 K. The kt-εt standard turbulence model was solved for the turbulent convection and for the surface thermal radiation the discrete ordinate method was applied. The simulations were conducted in steady state and the fluid properties were considered as a function of temperature. The software of computational fluid dynamics FLUENT 6.3 was used. The velocity, temperature fields and heat transfer coefficients were obtained. The total heat transfer losses increases 37.5% when the mixed convection is considered.