Large Eddy Simulation of combustion instabilities in a lean partially premixed swirled flame

Abstract

a b s t r a c tThis paper investigates one issue related to Large Eddy Simulation (LES) of self-excited combustion instabilities in gas-fueled swirled burners: the effects of incomplete mixing between fuel and air at the combustion chamber inlet. Perfect premixing of the gases entering the combustion chamber is rarely achieved in practical applications and this study investigates its impact by comparing LES assuming perfect premixing and LES where the fuel jets are resolved so that fuel/air mixing is explicitely computed. This work demonstrates that the perfect premixing assumption is reasonable for stable flows but is not acceptable to predict self-excited unstable cases. This is shown by comparing LES and experimental fields in terms of mean and RMS fields of temperature, species, velocities as well as mixture fraction pdfs and unsteady activity for two regimes: a stable one at equivalence ratio 0.83 and an unstable one at 0.7. IntroductionThe instabilities of swirled turbulent flows have been the subject of intense research in the last ten years. One important issue has been to identify the possibilities offered by simulation and especially Large Eddy Simulation (LES) to predict self-excited combustion oscillations. The specific example of swirled combustors where flames couple with acoustic modes has received significant attention [1-4] because such oscillations are often found in real gas turbines [5,6]. An important question in swirled unstable flames is the effect of mixing on stability. In most real systems, combustion is not fully premixed and even in laboratories, very few swirled flames are truly fully premixed. The effects of equivalence ratio fluctuations on flame stability in combustors have been known for a long time [7,8]: changes in air inlet velocity induce variations of the flow rate through the flame but may also induce mixing fluctuations and the introduction into the combustion zone of nonconstant equivalence ratio pockets. These pockets create unsteady combustion and can generate instabilities.In many experiments, LES is performed assuming perfect mixing mainly because the computational work is simpler: there is no need to mesh the fuel injection holes or to resolve the zone where these jets mix with air. However, this assumption totally eliminates fluctuations of equivalence ratio as a mechanism of instability, thereby limiting the validity of the LES. One specific example of such limitations is reported in the experiment of [9][10][11] which has been computed by multiple groups [12][13][14][15][16]. This methane/air swirled combustor was especially built to study combustion instabilities in such systems and for all computations up to now, perfect mixing has been assumed by LES experts because methane was injected in the swirler, far upstream of the combustor, suggesting that perfect mixing is achieved before the combustion zone. Interestingly, all computations performed with perfect mixing assumptions have failed to predict the unstable modes observed in the experiments. Moreover, recent...

Bistable swirled flames and influence on flame transfer functions

“…This scheme was developed and fitted to match the full mechanism's behavior for the considered equivalence ratios [50]. The Arrhenius rate parameters are all given in [50] and were used without any change.…”

“…The Sub-grid stress tensor is modeled by the classical Smagorinsky model [49]. Chemistry is computed using a two-step mechanism for methane/air flames [50] which includes two reactions and six species (CH 4 , O 2 , CO 2 , CO, H 2 O and N 2 ). The first reaction is irreversible and controls the oxydation of CH 4 , while the second reaction is reversible leading to an equilibrium between CO and CO 2 [6].…”

“…The Arrhenius rate parameters are all given in [50] and were used without any change. To capture flame/turbulence interactions, the dynamic thickened flame model is used [51][52][53] and is well suited for all flames studied here which correspond to premixed or partially premixed regimes.…”

Large Eddy Simulation of an industrial gas-turbine combustion chamber using the sub-grid PDF method

“…Previous studies have included, for example, LES of a swirl burner, [4] using a two parameter flamelet combustion model and LES of combustion instabilities in a lean-partially premixed combustor, [5] using a two-step global mechanism. More detailed chemical reaction mechanisms have also been incorporated, for example in the LES of a non-premixed, temporally evolving, syngas/air flame using an 11-species, 21-step mechanism in conjunction with an artificial neural networks approach, [6].…”

Large Eddy Simulation of the PVC Behavior in both Non-Reacting and Reacting Flows with Different Reynold Numbers

“…A parallel LES solver is applied to solve the fully compressible Navier-Stokes equations on the structured grids of the computational domain [28]. A Favre filter is applied to the conservation equations of mass, momentum and energy yields [29].…”