Numerical investigation and thermal analysis of solar cavity receiver

The purpose of this thesis is to investigate steady, laminar, combined pure natural convection and surface radiation in a tilted 2D solar square cavity receiver, perform thermal analysis by using ANSYS Fluent, and compare the results with published ones and to be able to offer a new solution to the investigated reasons of thermal losses. This study is important to propose an optimum thermal design for solar concentrators. 2D solar square cavity receiver has a constantly heated left wall, and insulated top and bottom wall and an opening right wall. The temperature of the heated left wall is 350 K and temperature of the air coming from opening wall is 300 K, so the temperature difference investigated for the solar cavity receiver is 50 K. Different Rayleigh numbers from 102 to 106 were studied. Parameter changed for Rayleigh number depends on the factor investigated in the different cases for this study, such as it could be gravity or dimension. 2D, laminar, steady state flow, with SIMPLE scheme, presssure-based solver was studied and for air density, Boussinesq approach was applied. Operating temperature, density and pressure were defined as ambient airflow and for Radiation mode, S2S method was used. The default values for the relaxation factors were applied. The residual for continuity, x-momentum, y- momentum convergence criteria was taken as 10-3, on the other hand, for energy convergence it was taken as 10-6. In this thesis, in order to study the solar cavity receiver, six types of validation studies were performed for validating the mathematical model and boundary conditions used at both numerical calculations and ANSYS Fluent. The numerical and thermal analysis for both pure natural convection and combined natural convection and surface radiation were studied for finding the main reasons of thermal losses. In all the validation studies, the computed Nusselt numbers were validated with the published ones. The first validation study was for a pure natural convection analysis in a closed square cavity. The results indicated that, as Rayleigh number increases, the fluid velocity increases and fluid gets more close to the walls of the cavity and thermal boundary layer and velocity boundary layer thicknesses decrease. The heated fluid circulates in clockwise direction in the cavity because of the buoyancy effect. The second validation study was to investigate the pure natural convection heat transfer in a fully open square cavity. A fully open, 2D, square cavity thermal analysis was performed by just changing the opening wall boundary condition as pressure inlet. The velocity vector results indicated that fluid enters from bottom part of the cavity, reaches to the hot wall, gets heated, and exits from right side of the top wall of the cavity by buoyancy effect. There forms both thermal boundary layer and velocity boundary layer near the hot wall but as the Rayleigh number increases the thicknesses of these boundary layers' decrease. The third validation study was to investigate the tilt angle of open cavity effect on the pure natural convection heat transfer. The result showed that, the minimum natural convection heat loss occurs when φ= 180o, whereas, the maximum heat loss occurs when φ = 60o for the Rayleigh numbers of 103 and 104, and when φ = 75o for the Rayleigh numbers of 105 and 106. As the tilt angle increases up to φ = 60o for Ra= 103 and 104 and up to φ = 75o for Ra= 105 and 106, the heat loss increases slightly and then after that angles the heat loss decreases as dramatically. The fourth validation was to investigate the effect of opening ratio of cavity aperture on the natural convection heat transfer. The Nusselt numbers calculated for a partially open square cavity were compared with the ones for a fully open square cavity and then the optimum one was discussed for the heat loss reduction. In addition, isotherms and streamlines of a partially open square cavity were validated by comparing with the published results of study of Bilgen and Oztop. The result indicated that increasing the aperture size of the cavity increases the heat loss, so that using a partially open square cavity can reduce the heat loss. Finally, the last two validation studies were to validate the radiative model (S2S) used and to validate the Nusselt numbers found from combined heat transfer analysis by comparing with the published ones. The validation succeeded and the results indicated that, as the Rayleigh number increases, heat loss increases and radiation heat loss becomes dominant at high Rayleigh numbers. After validation studies, thermal losses in a solar cavity receiver were investigated by analyzing different parameters such as temperature difference, cavity dimension and cavity tilt angle. The temperature difference and dimension changing of cavity were studied just for Ra=106 and the results were discussed for investigating the heat loss. The results indicated that, as temperature difference increases, the heat loss increases mostly due to radiation heat loss. The Nusselt number for radiation increases dramatically as increasing the temperature of the hot wall, on the other hand, the Nusselt number for natural convection remains almost same. The increase in the length of the solar square cavity makes radiation heat loss dominant because the heat loss of convection decreases from % 97 to % 42. The results of the study performed for investigation of the tilt angle effect on the heat loss in solar cavity indicated that there is a nonlinear relationship with heat loss and tilt angle. To come up, background of a cavity such as a closed cavity and then an open cavity were studied. Different cases of open cavity were studied such as a fully open cavity, a partially open cavity, an inclined open cavity and all the results were compared with the published similar studies, and the best optimum cavity model at the highest thermal performance was proposed. Both pure natural and combined heat losses were studied and the dominant one was determined.