Quantitative planar imaging of turbulent buoyant jet mixing

Abstract

Planar Rayleigh scattering provides quantitative mixing measurements in the developing region of axisymmetric turbulent helium jets issuing into air. The measurements focus on the relatively near field, in which the jets are primarily momentum driven. The imaging parameters are specified to ensure high spatial resolution. The mean jet fluid concentration fields attain self-similarity within the measurement region, though the forms of the mole fraction profiles indicate a reduction in turbulent transport at the jet outer boundary, arising from the reduced jet fluid density. Nevertheless, jet-like scaling pertains for the concentration fields. Mass fraction fluctuations on the jet centreline attain the expected asymptotic value of ≈23% of the centreline mass fraction values. The scalar dissipation rates, however, show an axial decay rate that is slower than theoretical predictions. The two-dimensional extent of the measurements also allows spatial filtering similar to that inherent in large-eddy simulations (LESs). The results confirm that fluctuation levels and scalar dissipation rates determined for the filtered fields are reduced as the effective resolution is reduced, but while the fluctuation profiles for the filtered fields are similar for the different filter sizes, the forms of the scalar dissipation profiles are highly dependent on filter size. These latter results in particular are of a form that will be useful for grid-dependent assessments of LES results.

On the Relationship Between the Statistics of the Resolved and True Rate of Dissipation of Mixture Fraction

“…Experimental work has shown that high levels of scalar dissipation at a stoichiometric contour are correlated with local flame extinction [5], making a correct model of scalar dissipation especially important in modeling turbulent combustion processes. Furthermore, recent experimental assessment indicates that the resolved scalar dissipation profiles are highly dependent on filter size [6]. In combustion, one must separate, at least conceptually, the variability of the scalar dissipation induced by the turbulent nature of the flow from other effects caused by the presence of finite heat release; i.e., variation of density throughout the flow.…”

Robust Outflow Boundary Conditions for Strongly Buoyant Turbulent Jet Flames

“…For the rate of spread, Chen and Rodi [1] proposedẎ = 0.11, which was recently also confirmed in experiments on a helium jet in air by Su et al [13]. A radial scalar profile associated with a half-width computed from (11) at the outflow boundary appears to be a very natural choice for the shape of the target profile.…”

“…They could show that including the advective/diffusive fluxes into the cross-stream direction, in particular the shear-stress term, finally leads to less perturbations of the pressure at the outflow, and hence, due to the elliptic nature of the pressure, to less contamination of the flow field upstream. To some extent the present approach follows the proposal of Fournier et al [15] in that it includes the shear stress term in (13) to improve the predictions for the streamwise velocity. The so extended convective boundary condition reads…”

Characterization of scalar mixing in dense gaseous jets using X-ray computed tomography

“…Gas-phase jet flows represent a classic test case for a wide variety of diagnostics, and for tomographic methods in particular, because they are relatively simple to set up while being theoretically, experimentally, and computationally well characterized (Birch et al 1978;Dowling and Dimotakis 1990;Emmerman et al 1980;Mi et al 2001;Su et al 2010;Watt and Vest 1990;Yip and Long 1986). Further, the jet geometry is useful in avoiding such requirements as confinement in a flow passage, which would complicate initial investigation of the diagnostic by introducing potential sources of interference or artifacts into the XCT scans.…”

“…In particular, scalar concentration measurements in turbulent gas-phase shear flows have yielded both quantitative and qualitative information that has significantly enhanced fundamental understanding of the underlying flow physics. While common methods such as Schlieren imaging (Crow and Champagne 1971;Meier 2002), Rayleigh scattering (Dowling and Dimotakis 1990;Pitts 1991;Richards and Pitts 1993;Su et al 2010), Raman scattering (Birch et al 1978), and planar laserinduced fluorescence (Hiller and Hanson 1988) reliably yield pointwise or two-dimensional results, advances in computational capabilities and design complexity have lent increased importance to the development of three-dimensional field data for transported scalars. Current options for three-dimensional visualization of scalar quantities are limited, and the majority of this work is accomplished via various types of optical tomography.…”