THE EDDY STRUCTURE MODEL OF TURBULENT FLAMELET PROPAGATION, THE EXPANDING SPHERICAL AND STEADY PLANAR CASES

A structural model of turbulence, composed of vortex tubes which are b ased on direct simulations of turbulence, is used to model the flame a rea enhancement found in direct simulations of passive flame propagati on. The resulting model produces a turbulent premixed flame speed S-T within a chamber which has the experimentally observed integral length scale and turbulence intensity behavior. The excess flame area is pro portional to (R(s)/lambda)root u'/S-L; where R(s) is the flame radius, lambda is the Taylor length scale and the diameter of the vortex tube , u' is the turbulence intensity and S-L is the burning velocity. This first model yields a linear dependence between S-T/S-L and u'/S-L and additionally, gives root u'Lambda/S-L lambda as the steady-state plan ar propagation, where Lambda is the integral scale. This planar relati on can also be expressed as root u'Re.(lambda)/15S(L), and this relati on has been shown to correlate experimental turbulent flame speed resu lts and direct turbulence simulations of passive flamelet propagation. A second propagation model uses a function that exhibits a maximum va lue of S-T/S-L to replace the root u'/S-L in the above excess area est imate. This eddy structure model yields departure from the linear u'/S -L, behavior of the first model, and this departure agrees with the ex perimental results. The maximum in S-T/S-L is caused by vortical struc tures which are not space filling, and depends upon the ratio of lambd a/Lambda. The two constants in this eddy structure model are determine d from experimental results, however, their values are close to those estimated from the spatial structure of vorticity. A model of flame pr opagation for internal combustion applications is proposed and the eff ect of the chamber geometry upon the turbulent flame speed is discusse d.