An experimental study of turbulent convection in air

An experiment was performed in a 3·5 by 3·5 m variable-height, closed convection box, with conditions ranging from a Rayleigh number of 4 × 10 4 up to 7 × 10 9 , using air as the working fluid. Heat-flux measurements made at Rayleigh numbers up to 7 × 10 9 yielded a Nusselt number Nu = 0·13 Ra 0·30 . Velocities and temperatures were measured up to Ra = 1·7 × 10 7 , and Fourier spectra calculated to find the predominant horizontal scales of the motion midway between the boundaries. The predominant scale at Ra ∼ 10 5 was approximately four times the distance between plates, changing to six as Ra increased to 10 6 . With side walls introduced so that the transverse aspect ratio was equal to five, Fourier spectra indicated considerable smaller scale motions, approximately equal to the layer depth. These motions decreased in size as Ra was increased. The results are discussed in relation to previous experimental and theoretical work.