Modified similarity prediction of jet and buoyant jet entrainment in the transition region

Abstract

The current study examines the transition region of axisymmetric isothermal and bouyant jets of low Reynolds numbers, directed vertically upward into a stagnant ambient. Velocity profiles were measured at axial distances ranging from 5 to 28 nozzle diameters downstream for Reynolds numbers from 850 to 7405 in an isothermal jet air. Profiles at the same locations were measured in a buoyant jet for Reynolds numbers from 540 to 3200. The Gaussian constant, m, is commonly used in characterizing jet, buoyant jet, and plume velocity profile width in the fully developed similarity zone. Results indicated that this parameter can also be used in the transition region if modified to incorporate the effects of Reynolds and Froude numbers. Correlation equations are presented that predict the profile width in the transition region as a function of the nozzle exit conditions. Assumption of some average value of profile width without consideration of Reynolds number, Froude number, or downstream distance would result in large error in estimation of volumetric flow rate, and, thus, entrainment at a location.