Propagation of wrinkled turbulent flames in the context of hydrodynamic theory

Abstract

We study the propagation of premixed flames in two-dimensional homogeneous isotropic turbulence using a Navier-Stokes/front-capturing methodology within the context of hydrodynamic theory. The flame is treated as a thin layer separating burnt and unburnt gases, of vanishingly small thickness, smaller than the smallest fluid scales. The method is thus suitable to investigate the flame propagation in the wrinkled flamelet regime of turbulent combustion. A flow-control system regulates the mean position of the flame and the incident turbulence intensity. In this context we study the individual effects of turbulence intensity, turbulence scale, thermal expansion, hydrodynamic strain and hydrodynamic instability on the propagation characteristics of the flame. Results are obtained assuming positive Markstein length, corresponding to lean hydrocarbon-air or rich hydrogen-air mixtures. For stable planar flames we find a quadratic dependence of turbulent speed on turbulence intensity. Upon onset of hydrodynamic instability, corrugated structures replace the planar conformation and we observe a greater resilience to turbulence, the quadratic scaling being replaced by scaling exponents less than one. Such resilience is also confirmed by the observation of a threshold turbulence intensity below which the propagation speed of corrugated flames is indistinguishable from the laminar speed. Turbulent speed is found to increase and later plateau with increasing thermal expansion, this affecting the average flame displacement but not the mean flame curvature. In addition, turbulence integral scale is also observed to affect the propagation of the flame with the existence of an intermediate scale maximizing the turbulent speed. This maximizing scale is smaller for corrugated flames than it is for planar flames, implying that small eddies that will be unable to significantly perturb a planar front could be rather effective in perturbing a corrugated flame. Turbulent planar flames, and more so corrugated flames, were observed to experience a positive mean hydrodynamic strain, which was explained in terms of the overwhelming mean contribution of the normal component of strain. The positive straining causes a decrease in the mean laminar propagation speed which in turn can decrease the turbulent speed. The effect of the flame on the incident turbulent field was examined in terms of loss of isotropy and vorticity destruction by thermal expansion. The latter can be mitigated by a baroclinic vorticity generation which is enhanced for corrugated flames

Effect of folds and pockets on the topology and propagation of premixed turbulent flames

“…The numerical implementation of the hydrodynamic model is carried out using a hybrid NS/front-tracking scheme, with a pre-generated homogeneous, isotropic, turbulent flow swept at the inflow with velocity v in , as described in [46]. The NS Eqs.…”

“…In our earlier work [46], where the first application of the hydrodynamic model to turbulent flames was presented, we have identified two modes of propagation that were termed sub-and super-critical, in analogy with the bifurcation property of a laminar flame that results from the DL instability. Under laminar conditions, a planar flame transitions beyond criticality to a stable large-scale conformation with a sharp peak protruding into the burned gas that propagates steadily at a speed U L > S L .…”

Linear and weakly nonlinear instability of a premixed curved flame under the influence of its spontaneous acoustic field

“…A general hydrodynamic theory of flames pertaining to low-Mach-number flows was presented by Matalon & Matkowsky (1982). As the resulting system, consisting of the Euler equations governing the flow coupled with a flame-front equation, is highly nonlinear, numerical solutions appeared only recently (Helenbrook & Law 1999;Rastigejev & Matalon 2006;Creta & Matalon 2011;Altantzis et al 2012). It has been found that a slightly perturbed flat flame develops wrinkles owing to DL instability.…”

“…For small H, the flame evolves into steady cellular structures consisting of cusped crests, Instability of a curved flame under the influence of its spontaneous sound 183 whereas for large H, a steady state is never reached, and instead, wrinkles develop in the region near a trough and propagate towards the crests. Application of the theory to cases with a turbulent oncoming flow indicates that DL instability modulates flame propagation substantially, and thus plays a significant role in turbulent combustion (Creta & Matalon 2011;Fogla, Creta & Matalon 2013). All these studies excluded acoustics from consideration since their main focus was on flame-flow interactions in open space.…”

Research and Analysis of Engine Combustion Based on Independent Test System

“…[1][2][3][4][5][6][7][8][9] Using a variety of sensors, such as cylinder pressure sensors, data acquisition cards, high-performance filters, combustion analysis software and more mature analytical tools, databases, etc. [10] Through real-time monitoring of each cylinder pressure and other working conditions, we let more continuous combustion measurement information processing optimization, so we could get more accurate and reliable data to promote the development of image analysis of engine combustion.…”