Experimental investigation of turbulent scalar flux in premixed stagnation-type flames

Joint velocity/scalar imaging measurements are performed in turbulent premixed stagnation-type flames to characterize the turbulent scalar flux, (rho) over bar(u '' c '') over tilde. Simultaneous two-dimensional measurements of the flow field and relative OH concentration are obtained by particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), respectively. Conditional processing of the flow-field based on qualitative OH concentration provides ensemble averaged product and reactant velocities which may be used to infer the behavior of the turbulent scalar flux according to the Bray-Moss-Libby (BML) formalism. Stagnation-type flames, being subject to an external mean pressure gradient because of the impinging flow, are examined to test the effect of external pressure gradients on the behavior of the scalar flux. The results indicate that external pressure gradients affect the scalar flux by either inhibiting or enhancing the expansion of combustion products away from the instantaneous flame front, depending on orientation of the pressure gradient relative to the flame brush. Additionally, experimental results are used to derive a modified version of the Bray number which is able to predict the behaviour of the scalar flux as gradient or counter-gradient for all flames for which data are available. (C) 2002 by The Combustion Institute.