The convective regime and temperature distribution above a horizontal heated surface

The convective flow and temperature distribution above a horizontal heated surface in air have been examined by optical methods. Attention has been given both to the open system and to enclosed air layers. The observations show the existence close to the surface of a mode of convection similar to the cellular convective motion previously noted in shallow enclosed layers. This motion is found to be present both in the open system and in the lower part of an enclosed layer of wide separation, i.e., large Rayleigh number. The mean temperature profile in this region has been found for a range of surface temperatures and the results show that the logarithm of the mean absolute temperature (T) varies as the power (−1/2) of the height (Z) above the heated surface. This measured distribution has been compared with theoretical predictions in particular that of the Priestley similarity theory, which properly refers to convection above an infinite surface and in which the postulated conditions will apply more exactly in higher layers. Nevertheless the measured temperature profile in the surface region is of the form predicted by the Priestley theory with the experimental value (−1/2) in place of the theoretical (−1/3) as the power of the height (Z).