Flow and Transfer Processes at Abrupt Expansions

Abstract

Variations in wall static pressure immediately downstream of abrupt expansions were determined for a range of expansion ratios (D,/D, < 7/1) in order to ascertain the rate of development of enclosed jets. Theoretical expressions for the recovery of static head from velocity head and the loss of pressure energy due to eddy turbulence were examined initially. Experimental data showed only slight deviations from predicted values. This discrepancy was not attributable to non-flat velocity profiles, but was shown to arise from the inherent approximations in the theory. At a given expansion ratio the static pressure distribution pattern could be represented by a single curve independent of velocity. The separate curves for each expansion ratio would not, however, reduce to a universal curve. The parameters of principal interest were the locations of the eye of the recirculating eddy and the point of reattachment of the jet. The position of each of these features was deduced from the changes in gradient of the pressure distribution curve. Local mass transfer coefficients downstream of abrupt expansions were determined by electrolysis of acidified copper sulphate solution under diffusioncontrolled conditions. Results were presented in the form of the dimensionless transfer factor JD: thus giving heat transfer factors by analogy. Peak local values were correlated in terms of the fully developed value <FORMULA> Recorded data were 10 - 25 per cent higher than values obtained by other researchers, and occurred closer to the plane of enlargement. Comparison of the results of each series of experiments suggested that peak transfer coefficients correspond to the eye of the recirculation eddy.