Investigation of Natural and Force Convection Losses from Cavity Receivers in Solar Dish Application

The present research is undertaking the numerical investigation of natural convection loss occuring in the variety of cavity receivers employed in the solar concentration dish. Flow and heat transfer simulations are carried out with the aid of commerical CFD software package, Fluent 6.0. Three different receiver geometries have been considered so far, one of which is the experimental model for validation. The other two are essentially the ones currently used in ANU 20 m2 and 400 m2 dishes. A good agreement between experimental and numerical results was obtained. Furthermore, the numerical results of all receivers were qualitatively comparable to the predictions by other available correlations hitherto. A new simplified correlation is proposed. The combined free-forced convection study, i.e. that includes the effect of wind speed and direction on convection loss, is also being undertaken.

Principal Investigator

Keith Lovegrove
Engineering, FEIT
Australian National University

Project

x51

Co-Investigator

Sawat Paitoonsurikarn
Engineering, FEIT
Australian National University

RFCD Codes

291802

Significant Achievements, Anticipated Outcomes and Future Work

The present report summarizes the most recent result of combined free-forced convection study for ANU 20 m2 dish receiver whose grid structure is illustrated in Figure 1. As an example, Figure 2 shows the cavity heat loss versus wind speed at different receiver inclinations. Note that the inclination of 0 degree represents the receiver at horizontal position whereas that of 90 degree represents the receiver at vertical position with cavity facing downward.

In a case of no wind, it is clearly shown that heat loss is drastically reduced with increasing inclination from 0 degree to 90 degree. This is intuitively obvious because when the cavity tips toward the vertical, greater amount of hot air should be trapped inside due to its lighter density compared to that of cooler air outside. So the heat loss decreases. However, with the existence of wind, the situation is much more complicated. At low wind speed, it is evident that heat loss decreases with increasing speed for the case of 0 degree inclination but not for other cases. This regime of low wind speed might be where magnitudes of free and force convections are comparable and their influences can either strengthen or weaken each other, which in turn may depends on cavity inclination. As wind speed increases, i.e. beyond 5 m/s, the effect of cavity inclination becomes diminished and all show essentially the same amount of heat loss at same wind speed. In this regime, the effect of force convection is dominated. The magnitude of heat loss at wind speed as high as 20 m/s is only 2 times higher than that of no wind for the case of horizontal cavity, whereas it is more than 90 times higher for the case of cavity facing downward. Note that heat loss of horizontal cavity is even slightly less than that of other inclinations at this high wind speed.

For a future plan, the complete information of heat loss dependency on cavity inclination, wind speed and direcion will be obtained for all cavity geometries considered. Then the universal correlation for predicting heat loss will be developed.

 

Computational Techniques Used

Computational Grids were made by Gambit 2.0 (Fluent Inc.). Each grid consists of several hexagonal cells interconnected to each other. The smallest cell is located near the boundary of interest, i.e., cavity wall where heat loss is to be monitored. The cell size increases gradually as it is away from the cavity. The exemplified grid for wind effect study of ANU 20 m2 dish receiver is shown in Figure 1. Essentially, it simulates an actual wind tunnel with the receiver placed at the center. The receiver's cavity is assigned an average temperature of about 900 K. A uniform wind at 300 K flows through the wind tunnel in negative x direction. The aperture plane of the receiver is parallel to the wind direction. An inclination of the receiver is adjusted by pointing a gravity vector to any desired direction.

Fluent 6.0 was employed to similate mass, momentum, and heat transfers by solving a set of non-linear governing equations in each cell. The steady mode is chosen which means that the time dependent terms in all equations are neglected and the final result would represent the steady-state solution. The 2nd order upwind scheme is used for the discretization of non-linear equations. The double-precision coupled solver is utilized with implicit method of advancing toward the solution. The Spalart-Allmaras one-equation turbulent model is incorporated in the present work to take into account the turbulence. Temperature-dependent properties of a working fluid are evaluated locally at each cell by polynomial fit equations derived from thermodynamic table.

The initial guesses for velocity, temperature, and turbulent viscosity fields were set to constant values over the entire computational domain. The solver undertakes iteration until the convergence criterion is satisfied, which employs scaled residuals of the modified variables in the governing equations as the measure. In addition, the averaged cavity wall heat flux was examined explicitly for convergence (to less than a 0.01% variation between iterations).

 

Publications, Awards and External Funding

External Funding and Awards

None.

Publications

S. Paitoonsurikarn , T. Taumoefolau, and K. Lovegrove, “Investigation of Natural Convection Heat Loss from a Solar Concentrator Open Cavity Receiver at Varying Angle of Inclination”, Proceedings of ISEC 2003, 2003 International Solar Energy Conference, Hawaii, 15-18 March 2003.
T. Taumoefolau, S. Paitoonsurikarn, G. Hughes, and K. Lovegrove, “Experimental Investigation of Natural Convection Heat Loss from a Model Solar Concentrator Cavity Receiver”, Journal of Solar Energy Engineering – Transaction of the ASME, submitted and accepted, in press.
K. Lovegrove et al, “Paraboloidal Dish Solar Concentrators for Multi-Megawatt Power Generation”, Proceedings of ISES 2003, ISES Solar World Congress 2003, Sweden, 14-19 June 2003.
S. Paitoonsurikarn, T. Taumoefolau, and K. Lovegrove, “On the Study of Convection Loss from Open Cavity Receivers in Solar Dish Applications”, Proceedings of 41st ANZSES Conference, Melbourne, 26-29 November 2003.