Mixing and scalar dissipation rate in a decaying jet

Abstract

The temporal development of the mixing field in a decaying jet (Re = 50,000) was quantified by measuring mole fraction and scalar dissipation rate (SDR) in a decaying, isothermal,turbulent gaseous jet. The 2D scalar field was measured by using planar laser induced fluorescence of acetone and, with appropriate image processing, this allowed estimation of the SDR using the two in-plane components within 16%error. The instantaneous and averaged distributions of the mole fractionare reported for downstream dimensionless distances up to 7 nozzle exit diameters and 35 exit flow time scales after end of injection. With advection of the last uniform exit concentration(UEC) profile coreaway from the nozzle exit, a region of weak concentration arises at the decaying jet’s trailing edge.Estimates made in a Lagrangian frame of reference show that the trailing edge of the jet becomes leaner,after the end of injection (AEI), faster than in the steady state, confirming the existence of an ‘entrainment wave’. The normalised probability density functions of the 2D SDR at various stations and times AEI differ from a lognormal distribution at both low and high SDR values with negative skewness and positive excess kurtos is. A pseudo 3DSDR, made by including an estimate for the out of plane component, showed reduced departure from log normal.The departure may be attributed to the disappearance of the strong shear layer associated with the absence of nozzle momentum AEI. To the authors’ knowledge, this study provides the first measurements of the SDR in a decaying, isothermal turbulent jet.