Turbulent Combustion Modelling and Experiments: Recent Trends and Developments

Abstract

The development of better laser-based experimental methods and the fast rise in computer power has created an unprecedented shift in turbulent combustion research. The range of species and quantities measured and the advent of kHz-level planar diagnostics are now providing great insights in important phenomena and applications such as local and global extinction, pollutants, and spray combustion that were hitherto unavailable. In simulations, the shift to LES allows better representation of the turbulent flow in complex geometries, but despite the fact that the grid size is smaller than in RANS, the push towards realistic conditions and the need to include more detailed chemistry that includes very fast species and thin reaction zones emphasize the necessity of a sub-grid turbulent combustion model. The paper discusses examples from current research with experiments and modelling that focus on flame transients (self-excited oscillations, local extinction), sprays, soot emissions, and on practical applications. These demonstrate how current models are being validated by experimental data and the concerted efforts the community is taking to promote the modelling tools to industry. In addition, the various coordinated International Workshops on non-premixed, premixed, and spray flames, and on soot are discussed and some of their target flames are explored. These comprise flames that are relatively simple to describe from a fluid mechanics perspective but contain difficult-to-model combustion problems such as extinction, pollutants and multi-mode reaction zones. Recently, swirl spray flames, which are more representative of industrial devices, have been added to the target flames. Typically, good agreement is found with LES and some combustion models such as the progress variable-mixture fraction flamelet model, the Conditional Moment Closure, and the Transported PDF method, but predicting soot emissions and the condition of complete extinction in complex geometries is still elusive.

Detailed chemistry LES/CMC simulation of a swirling ethanol spray flame approaching blow-off

“…where D is the molecular diffusivity (D = µ/(ρSc) with Sc = 0.7), µ t is the sub-grid scale viscosity and C N is a model constant taken equal to 42.0. This value, obtained through calibration against scalar dissipation rate measurements in the Sandia D flame [6], has been recently applied to simulations of both gaseous non-premixed [8,9,22] and spray flames [10,11] giving reasonable predictions of the local extinction behaviour compared to experiments. The term Π|η, representing conditional source terms due to spray evaporation, was modelled as discussed in [10].…”

“…According to the experiment [2], the injected droplets show a dispersion droplets crossing each cell [11]) and allows us to analyse the location and dispersion of the spray. Furthermore, in Fig.…”

“…[1], predicted with reasonable accuracy both the degree of local extinction, described in terms of lift-off height statistics [8], and the experimental blow-off limits for a wide range of air flow bulk velocities [9]. However, similar capability for spray combustion has not been demonstrated yet, except for two LES/CMC attempts [10,11]. In both these studies, the simulations were performed using a modified one-step chemistry model [12], that was tuned so as to give a reasonable prediction of the extinction strain rate of a laminar non-premixed flamelet.…”

“…Some attempts to include spray evaporation effects in the computation of the sub-grid mixture fraction variance were done in Refs. [10,11] where a formulation based on the equilibrium model proposed by Pera et al [17] was used. However, as pointed out in [17], such model presents some difficulties in predicting a proper level of variance even for simple homogeneous flows suggesting the use of a transport equation to improve the prediction capability.…”

“…The experimental condition E1S1, characterized by an air flow bulk velocity U b equal to 17.1 m/s (79.2% of the blow-off velocity) and a fuel mass flow rate of 0.27 g/s, was considered. Measurements include Mie scattering from the spray droplets, mean axial droplet velocity and Sauter Mean Diameter (SMD), OH* chemiluminescence that provides qualitative information of the heat release, and qualitative OH-PLIF.The air inlet in the numerical domain was located immediately down-stream of the swirler[11]. A hexahedral mesh of about 5 million cells and with a minimum grid size of 0.2 mm in the vicinity of the bluff body was used for the LES, whereas a hexahedral mesh of 45,000 cells was used for the CMC equations.…”