radially lumped mass assumption. Radiative cavity redistribution thermal fluxes are considered to have a small effect on the overall performance, and therefore neglected; aperture loss is included as surface loss. With these assumptions, an energy balance on the differential receiver element shown in Figure 1 yields the following unsteady onedimensional equation where the solar source flux is represented as The magnitude of the absorbed concentrated solar flux typically rises rapidly to its maximum near the aperture end, followed by a gradual reduction to zero near the other end. as indicated in Figure 1. This axial distribution is represented by with the following parameter values, = 3.88 at Xj,, = 0.156, and = 0.181, which correspond well to flux distributions obtained with the CAV2 computer program, as used by Strumpf and Coombs (1987). To limit the scope of this analysis the above source-flux distribution is applied for the various designs considered, even though more comprehensive analyses would consider reflector design and source-flux shape variations, as well. The time-oscillation of the source flux, g('t), is shown in Figure 2(a), in terms of scaled time, X = t/P, for a typical orbit with period P = 1.56 hrs [5], The associated spectral content of this "flux signal" is obtained with the Fast Fourier Transform (FFT) algorithm [8], as shown in Figure 2(b) for nondimensional frequencies = 2nk, k = 1,2,3 ..., where G is the complex-valued FFT of g(x). This spectrum is utilized in the following analysis. By design, aperture and surface losses are typically each held to less than 5% of the captured energy; in this analysis these effects are modeled together (somewhat conservatively) through the effective emissivity, as occurs with multilayer, radiationshield insulation [10], Application of this loss model to the SSF phase-change receiver results in ee < 0.02. The associated boundary conditions are taken as the adiabatic tip condition
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