Investigation of free convective heat transfer for the uniform heat flow (UHF) state from the upper and lower surfaces of a double-sided heated horizontal plate

This paper presents the results of an experimental study on free convection in air, focusing on heat transfer from the upper and lower surfaces of a Type II horizontal plate heated on both sides. This arrangement contrasts with the more common Type I plate, where one surface is heated, and the other is thermally insulated to minimize reverse heat loss. Type II plates are characterized by minimal heat loss from the lateral surface, which is proportional to the plate thickness. In this study, a plate with a minimal thickness of 2.1 mm was used, chosen for its technical and manufacturing feasibility. This design reduced the ratio of convective heat loss through the lateral edges to the total heat flux of the plate to Q loss / Q total = 0.042, thereby enhancing the accuracy of the balance method. Two methods were used to analyse heat transfer: the balance method, which calculates the total heat flux through the plate ( Q total = Q low + Q up ​), and the gradient method, which determines the proportion of flux components ( Q low / Q up ​). Unlike a vertical plate, where the total heat flux is symmetrically divided ( Q up = Q low = Q total /2), no such symmetry is expected for a horizontal plate arrangement. Using a thermal imaging camera equipped with a gradient detection mesh, the temperature distributions T ( y ) and local temperature gradients d T /dy| y=0 ( x, z ) were analysed in air in the cross-section perpendicular to the upper and lower surfaces of the plate. The local heat transfer coefficients α ( x,z ) and Nusselt numbers Nu( x,z ) were also determined. By averaging these values over the heating surfaces, the relationship Nu = C . Ra n was established for both surfaces of the plate. The main result of this study is the demonstration that the ratio of the free convective heat transfer constants C up and C low , determined using three independent methods (gradient, balance and numerical) for a uniform heat flow (UHF) from a horizontal Type II plate, is C up / C low = 0.891. In the case of the well-studied Type I plates however, this relationship is reversed. This is because the heat transfer intensity in these plates (heating side facing upwards) is 1.48 to 2.33 times greater than when the heating side is facing downwards.