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A restrictive analysis based on a new model is presented for the prediction of thermal contact resistance in a vacuum environment. It is demonstrated that, for many surfaces commonly encountered in engineering practice, the macroscopic constriction has a commanding influence. Extensive results are given for brass, magnesium, stainless steel, and aluminum surfaces which show the effects of material properties and the degree of conformity of mating surfaces under load. Limited results are presented to show the influence of surface films, surface roughness, creep, additional interstitial material, mean interface temperature, etc. Good agreement between the measured and predicted values of the thermal contact resistance was found over wide ranges of the applied load and other system variables.

Cavity solar receivers are generally believed to have higher thermal efficiencies than external receivers due to reduced losses. A simple analytical model was presented by the author which indicated that the ability to heat the air inside the cavity often controls the convective loss from cavity receivers. Thus, if the receiver contains a large amount of inactive hot wall area, it can experience a large convective loss. Excellent experimental data from a variety of cavity configurations and orientations have recently become available. These data provided a means of testing and refining the analytical model. In this manuscript, a brief description of the refined model is presented. Emphasis is placed on using available experimental evidence to substantiate the hypothesized mechanisms and assumptions. Detailed comparisons are given between analytical predictions and experimental results. Excellent agreement is obtained, and the important mechanisms are more clearly delineated.

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An experimental investigation of heat transfer by natural convection from a smooth, isothermal cubic cavity with a variety of side-facing apertures is described in this paper. The study was motivated by the desire to predict the convective loss from large solar thermal-electric receivers and to understand the mechanisms which control this loss. Hence, emphasis is placed on the large Rayleigh number, Ra, regime with large ratios of the cavity wall temperature Tw to the ambient temperature T∞. A cryogenic wind tunnel with test section temperatures which are varied between 80 K and 310 K is used to facilitate deduction of the influences of the relevant parameters and to obtain large temperature ratios without masking the results by radiative heat transfer. A 0.4-m cubic cavity, which is mounted in the side wall of this tunnel, is used. The area of the aperture Aa and its location are key variables in this study. The data which are presented cover the ranges: 1 < Tw/T∞ < 3, L2/18 ≤ Aa ≤ L2, and 3 × 107 < Ra < 3 × 1010.