Free Heat Convection Through Enclosed Plane Gas Layers

Abstract Fifteen years ago, Mull and Reiher published elaborate experiments on heat convection through enclosed plane air layers. In the present paper it is shown that the representation and evaluation of the results of these experiments can be simplified, improved, and generalized when, as for a single surface in an extended medium, a laminar and turbulent range are distinguished. Defining slightly modified Nusselt and Grashof numbers with the layer thickness L as characteristic length, the experimental results for horizontal and vertical layers can be expressed by (NNu)L = Ch(NGr)Ln, or Cv(NGr)Ln(L/H)1/9, respectively, where n = 1/4 and 1/3 for the laminar and turbulent ranges, respectively, and H is the height of vertical layer. Comparison with tests of other authors indicates that, due to the experimental elimination of any influences from the edge of the enclosed layer, Mull and Reiher obtained relatively low values of the constants Ch and Cv, whereas without particular precaution these influences may considerably increase the heat transfer.