Swaying motion in thermal plume above a horizontal line heat source

The swaying spectrum, the swaying frequency, and several characteristics in the swaying motion of a laminar thermal plume of air arising from a horizontal line heat source were experimentally investigated in an enclosure of 800 x 800 mm square cross section and 1000 mm high. Temperature waves of swaying motion are apparently irregular. However, a certain kind of regularity exists in the probability density function and the autocorrelation coefficient of swaying temperature waves. Therefore, the spectrum analysis is carried out on the swaying temperature. As a result, the swaying spectrum has the slope values of —4.5 and —8.0 in the power spectrum densityfrequency diagram. These slopes values -4.5 and -8.0 are the characterizing values of the laminar plume. Nondimensional formulas are obtained for swaying spectrum with the slope values of —4.5 and —8.0. These equations are useful for determination of the laminar region of the plume. The swaying frequency is proportional to /3 power of the heat rate from the line heat source. Nondimensional formulas of the swaying frequency are obtained with two kinds of representative lengths, A and B, as follows. Re/A = 0.333/fau » Re/B = 0.219/fai. Here, Ref and Ra are the frequency Reynolds and the Rayleigh numbers, respectively. By using these relations, it is possible to predict the swaying frequency of a plume in an enclosure with any dimensions.