This paper describes an analytical study of the laminar heat transfer in cylindrical, flat and fin-tube absorber ducts subjected to variable wall irradiation and external heat losses. The study addresses two collector design aspects. The first is the effect of the wall thermal boundary condition on the collector efficiency factors. The second aspect is the efficiency improvement which can be achieved by redistributing the incident radiation in a nonuniform manner. Analysis shows that the use of the average Nusselt numbers for the constant wall temperature boundary condition in a radially lumped solution give results for the efficiency factors which agree within about 6 percent with the distributed solution. The efficiency improvement which may be achieved by using stepwise increases in radiation with conventional absorbers is less than about 8 percent. The solution procedure developed in this study can be extended to study other flow regimes when the constant wall temperature solution is known.

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