CRAFT Tech (Combustion Research and Flow Technology, Inc.) is a small and well-established CFD consulting firm that specializes in addressing unique and challenging problems for both government and commercial customers. The focus of two recent projects was the accurate modeling of flame extinction and blow-out effects in aircraft propulsion and power generation systems. Within these applications there are strong interactions of flow turbulence with the flame dynamics.
These turbulence — flame, or turbulence — chemistry interactions are difficult to accurately model and are the origin of extinction and blow-out effects for these applications. To simulate these dynamic and complex environments, large-eddy simulation (LES) is often used. Within LES, the time evolution of the large turbulent scales in the flow are directly simulated, while the small scales are removed through a spatial filtering process. As a consequence of this filtering process, the effects of turbulent transport and turbulence-chemistry interactions at the small scales must be modeled. The accurate modeling of turbulence-chemistry interactions at these small scales is of critical importance to capturing flame extinction and blow-out effects.
CRAFT Tech was recently tasked by its customers to develop an LES turbulence — chemistry interaction subgrid model to capture local extinction and blow-out effects that satisfied the following requirements:
- the model should be portable to commercial CFD flow solvers, and
- the model should be tractable for routine application within a system design cycle environment.
To satisfy the first requirement CRAFT Tech partnered with ANSYS to develop a user-defined-function (UDF) interface to implement the new model within Fluent. To satisfy the second requirement, CRAFT Tech adopted a database driven modeling approach. In this approach statistics from an advance turbulent combustion model are pre-computed and stored within a database. At run-time, the LES solver simply retrieves the required statistics in an efficient manner. Thereby, the formulation is tractable for routine application to industrial scale problems.
Within this modeling approach, CRAFT Tech chose to use the linear-eddy model/counter flow (LEM-CF) solver as the underlying combustion model to generate the LES subgrid model statistics. The LEM-CF formulation is an advanced stochastic combustion model coupled with a 1-D counter flow solver. This formulation separately treats molecular diffusion and turbulent convective transport while resolving the flame structure to the DNS limit. The counter flow implementation allows large scale strain to be applied to this stochastic formulation. As a result, the LEM-CF model may be used to directly predict turbulent flame extinction limits. This formulation may also be used to generate turbulent counter flow flame statistics as a function of large scale strain rate.
Flame statistics and extinction limits from the LEM-CF formulation have been used to develop a database driven LES subgrid combustion model. This subgrid model includes flame properties, an extinction criterion, and an ignition/blow-out criterion based on pre-computed LEM-CF simulation data. This new formulation has recently been applied to predict local extinction effects and global blow-off in laboratory flames relevant to gas turbine combustion.
The model was implemented into ANSYS Fluent® as a user-defined-function (UDF) to simulate the Sandia Flame C – F series of piloted, partially premixed flames, including the blow-out condition. This study demonstrated the accuracy of the formulation in capturing flame temperature suppression due to local flame extinction, and in predicting of the global blow-out limit. This study also identified an oscillating flame mode just prior to global extinction. A slight increase in the jet velocity beyond this condition resulted in global blow-out.
The Fluent UDF and model generation software for this unique modeling formulation are available from CRAFT Tech. Contact us for more information.
EDITOR’s Note: To discover more about ANSYS CFD Combustion solutions visit Solving Complex Combustion Challenges.
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