Experimental data for heat transfer across air-filled inclined trapezoidal enclosures are reported for a Rayleigh number range of ∼2 × 103 to ∼5 × 107. The large side was cooled to uniform temperature while the opposing small side was electrically heated. The enclosures were completed by two aluminum sidewalls which simulated the reflective surfaces in moderately-concentrating solar collectors. The experiments show that conductive heat losses up the sidewalls can be very large even when the sidewalls are thermally insulated from the top and bottom surfaces. However, the conductivity of the sidewalls has only a small effect on convective heat transfer across the groove. Natural convection heat transfer in the groove can be correlated reasonably well by an equation of the form Nu = C Ra0.345.

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