Reynolds-number effects and anisotropy in transverse-jet mixing

Abstract

Experiments are described which measured concentration fields in liquid-phase strong transverse jets over the Reynolds-number range 1.0 × 10 3 6 Re j 6 20 × 10 3 . Laserinduced-fluorescence measurements were made of the jet-fluid-concentration fields at a jet-to-freestream velocity ratio of V r = 10. The concentration-field data for far-field (x/d j = 50) slices of the jet show that turbulent mixing in the transverse jet is Reynoldsnumber dependent over the range investigated, with a scalar-field PDF that evolves with Reynolds number. A growing peak in the PDF, indicating enhanced spatial homogenization of the jet-fluid concentration field, is found with increasing Reynolds number. Comparisons between transverse jets and jets discharging into quiescent reservoirs show that the transverse jet is an efficient mixer in that it entrains more fluid than the ordinary jet, yet is able to effectively mix and homogenize the additional entrained fluid. Analysis of the structure of the scalar field using distributions of scalar increments shows evidence for well-mixed plateaux separated by sharp cliffs in the jet-fluid concentration field, as previously shown in other flows. Furthermore, the scalar field is found to be anisotropic, even at small length scales. Evidence for local anisotropy is seen in the scalar power spectra, scalar microscales, and PDFs of scalar increments in different directions. The scalar-field anisotropy is shown to be correlated to the vortex-induced large-scale strain field of the transverse jet. These experiments add to the existing evidence that the large and small scales of high-Schmidt-number turbulent mixing flows can be linked, with attendant consequences for the universality of small scales of the scalar field for Reynolds numbers up to at least Re = 20 × 10 4 .

Time-averaged characteristics of a reacting fuel jet in vitiated cross-flow

“…Note that these jet Reynolds numbers all substantially exceed the value of 10 3 beyond which the trajectory is insensitive to Reynolds number, following Shan and Dimotakis [26]. Buoyancy effects are negligible even at the lowest J value (J = 1.5), where the Froude number…”

“…The coefficients, A and B, typically vary between the ranges 1.2 < A < 2.6 and 0.28 < B < 0.34, depending on such parameters as the velocity profile of the jet exit [24,25], the thickness of the boundary layer [24], and the specific definition used to identify the jet trajectory. Shan and Dimotakis [26] showed that jet trajectory is effectively independent of Reynolds number for 1.0 Â 10 3 6 Re d (tested up to Re d = 20.0 Â 10 3 ). The limited data for reacting jets indicates that the time-averaged jet trajectory is quite close to the nonreacting jet [27,28].…”

Molecular Mixing and Flowfield Measurements in a Recirculating Shear Flow. Part I: Subsonic Flow

“…Another flow configuration that has been widely studied is the jet in crossflow [17,18]. The jet in crossflow has a higher entrainment rate than a jet into a quiescent reservoir, resulting in increased mixing [18].…”

“…The jet in crossflow has a higher entrainment rate than a jet into a quiescent reservoir, resulting in increased mixing [18]. For both free-shear layers and jets in crossflow, as the Reynolds number is increased, the strain rates experienced by a fluid element within the mixing zone will exceed the extinction strain rates of hydrocarbon fuels, for example, and result in flame-out with little recourse for reignition [19].…”

Numerical simulation of a passive scalar transport from thermal power plants

“…A detailed overview of recent work on the current cross-flow stream can be found in [3]. Previous authors have investigated numerically the velocity field [4][5][6][7][8][9], and the passive scalar field of concentration considered in [10][11][12]. As well as numerical simulation of the velocity field were considered in papers [13][14][15][16][17].…”