Experimental study of heat release effects in exothermically reacting turbulent shear flows

Comparisons are presented from the first inner-scaled measurements of velocity gradient quantities in reacting and nonreacting versions of otherwise identical turbulent shear flows. Distributions of gradient quantities are obtained for outer-scale Reynolds numbers Redelta ≡ ucdelta/nu from 7200 to 200 000. The local outer length scale delta and velocity scale uc and associated inner scaling &parl0;6ui/6xj&parr0; n∼&parl0;n/l2n &parr0;n are used to identify the dominant physical mechanisms that produce heat release effects on the inner scales. In the nonreacting cases, classical inner scaling with the viscosity nu and inner (viscous) length scale lambda nu ∼ delta · Re-3/4d removes most differences in distributions measured at different Redelta, with remaining differences being due to incomplete resolution of lambdanu with increasing Re delta. Inertial and dissipation range spectra allow the measurement resolution scale Delta☆ and the proper resolution-corrected inner scaling to be determined, with the resulting scaling verifying near-perfect similarity for all Redelta. In the reacting cases, departures from this similarity reveal the true inner-scale changes due to heat release. Results clearly show that when inertial and body force effects on delta and uc are accounted for via the equivalent density, and viscous effects are accounted for via the mixture-fraction-averaged viscosity, the resolution-corrected inner scaling reveals remaining effects of heat release on turbulent shear flows to be remarkably small.; Using the same resolution-corrected inner scaling, further experiments were conducted under conditions of nonzero mean shear S≡&parl0;S&d1; ijS&d1; ij&parr0;1/2 . Accounting for the effects of classical external intermittency, results from the velocity gradient distributions indicate a small effect of nonzero mean shear S at the finest scales of the flow. This is corroborated by evaluation of the Corrsin-Uberoi criteria S⋆c ≡S&parl0;n/3&parr0;1/2 , where the present results satisfy the S⋆c ≪ 1 condition for all of the measured reacting and nonreacting cases. Use of the resolution-corrected inner scaling permits direct comparison between otherwise identical turbulent shear flows; the present results reveal that the combined effects of heat release and mean shear on the inner-scales of turbulent shear flows are relatively small.