Convection in a radiating gas

Abstract

We have made a theoretical and experimental study of the effect of radiative transfer on the onset of convection by means of comparative experiments in dry air and ammonia contained between horizontal aluminium plates maintained at different temperatures.The first effect of radiative transfer is to distort the linear, diffusive profile in the initial, pre-convective state. A non-grey theory is developed and is shown to predict the non-linear profiles profiles with satisfactory accuracy. The change of radiative flux with pressure of ammonia is also predicted and confirmed. This constitutes the first successful attempt to confirm the methods used in atmospheric studies in controlled experiment.A non-grey theory of the onset of convection is developed, separating effects associated with the distortion of the static state and the dissipative effect of radiative transfer on temperature fluctuations.Experiments indicate that the critical Rayleigh number in ammonia can be greatly increaesd over the best value for air (Ra_c = 1786); the maximum observed being Ra_c = 4870. Theory and experiment are shown to agree within reasonable limits.This work confirms the validity of methods which are of great importance to astrophysics and atmospheric physics.