Experimental verification of natural convective heat transfer phenomenon from isothermal cuboids

The paper presents results of experimental investigations of the convective heat transfer from isothermal cuboid with relatively big dimensions (i.e. 1.5 m × 1 m × 0.5 m). The aim of this research was to obtain results, which could be comparable to those obtained for the relatively small cuboid, presented in our previous paper [E. Radziemska, W. Lewandowski, Natural convective heat transfer from isothermal cuboids, Int. J. Heat Mass Transfer 46 (2003) 2169–2178]. The analytical solution was performed, taking into account complete boundary layer length and the manner of its propagation around the isothermal cuboid. As the characteristic linear dimension L in Nusselt–Rayleigh theoretical and experimental correlations, we proposed the ratio of six volumes to the cuboids surface area, which allowed performing the experimental results independently from the orientation of the block. The experiment was then carried out for three possible positions of the tested cuboid with dimensions 0.2 m × 0.1 m × 0.045 m.In present the investigations the Rayleigh numbers based on the proposed characteristic length ranged from 108 to 109, due to the cuboids size. The Nusselt number, describing the intensity of convective heat transfer from the cuboid, was expressed by Nu = X · Ra1/5 +Y · Ra1/4, where X and Y are coefficients dependent on the cuboids dimensions. For the range of provided experiment the theoretical Nusselt–Rayleigh relation can be presented in the form: NuL=0.452·RaL1/5+0.516·RaL1/4that is adequate to NuL=0.743·RaL1/41. A better correlation is obtained for the experimental results: NuL=0.90·RaL1/4 has a 10% deviation to the value previously obtained, for the cuboid with small dimensions, results (NuL=0.818·RaL1/4).