Investigation of Strain/Vorticity and Large-Scale Flow Structure in Turbulent Nonpremixed Jet Flames

Our study will use the microgravity environment to investigate the underlying flow structure of turbulent nonpremixed round jet flames. In particular, we aim to investigate the large-scale turbulent structure using planar laser Mie scattering (PLMS), and the strain rate and vorticity fields using particle image velocimetry (PIV). This work is motivated by recent studies in our laboratory that have led to several interesting observations of nominally momentum-driven turbulent nonpremixed planar flames. First of all, the organized large-scale turbulent structures that are observed in nonreacting planar jets may be substantially modified or suppressed in nonpremixed planar jet flames. Furthermore, a recent study using PIV and planar laser-induced fluorescence of OH has shown that in transitional and turbulent nonpremixed planar jet flames the presence of the flame seems to greatly influence the underlying vorticity and strain fields, as compared to nonreacting jets. For example, the reaction zones in the jet flames are strongly correlated with regions of high vorticity. A related study has demonstrated that vorticity is not correlated in the same way with either iso-scalar surfaces or scalar dissipation layers in nonreacting planar jets. Furthermore, the relationship between strain and the reaction zone appears to be modified by the presence of high levels of heat release. In particular, the strain rate field in planar jet flames exhibits a preferred direction of principal compressive strain that apparently is related to strong shear across the reaction zone. This preferred direction of strain was not observed in nonreacting jets. One of the major problems encountered when conducting these types of studies is that it is difficult to know to what extent buoyancy influences the results. Therefore, the microgravity environment provides us with an excellent opportunity to explore these issues without the complicating effects of buoyancy. This is particularly the case when studying flames that are transitional between laminar and turbulent states. For example, the strong correlation of vorticity with the reaction zone (discussed above) was observed in both transitional and turbulent planar flames, but the effect was stronger for the transitional case. To date, the reason for the presence of the vorticity-reaction zone correlation is not known, although vorticity production via baroclinic torque is a likely cause. The microgravity environment will allow us to specifically determine whether the vorticity is produced by baroclinic torque resulting from the flame density gradient acting with the hydrostatic pressure gradient.AAdditional details of the planned experiments are provided.