Transported scalar PDF calculations of a swirling bluff body flame (‘SM1’) with a reaction diffusion manifold

Abstract

The modeling of a reacting swirling flow behind a bluff-body burner (SM1) in the framework of RANS and transported scalar PDF is presented. The EMST mixing model is applied and the composition space is reduced to mixture fraction (Z) and a progress variable (CO 2 mass fraction, Y CO 2 ) by means of a Reaction Diffusion Manifold (REDIM). With an ad hoc adjustment of the turbulent Schmidt number, the mean flow and mixing fields obtained are comparable to LES results from the literature. The REDIM reduction of the composition space to (Z, Y CO 2 ) is discussed and its validity for the present swirling flame is first considered by an a priori comparison with experimental data. The (Z, Y CO 2 )-scatter plots from the transported PDF calculation show the capacity to reproduce the mixing between fresh air and hot products in the recirculation zone above the bluff-body. However, too little scatter is observed. The study of tracer trajectories helps to better understand the capacities and limitations of the modeling approach. Zones where mixing competes with reaction can be identified, and coincide with the highly rotating collar region where local extinction is expected to take place. However, in our modeling, the competition between mixing and reaction is not enough to lead to local extinction. An important modeling deficiency is claimed to be the use of a mean time scale in the EMST mixing model, which limits the possibilities to model high scalar dissipation rate events.

A priori investigation of PDF-modeling assumptions for a turbulent swirling bluff body flame (‘SM1’)

“…The latter is a measure for the deviation from the mentioned flamelet type lines or, equivalently, for the amount of local extinction. However, flame SM1 is different from jet type flames in that transported PDF simulations indicate that, close to the burner inlet, the deviations from flamelet type lines in composition space are not due to slower chemistry, caused by turbulence-chemistry interaction, but rather due to 'large scale' mixing of hot combustion products with air or fuel in the recirculation region behind the bluff body [11]. This motivates the present work.…”

“…Figure 2 shows the conditional PDFs P (Y CO 2 |Z), P (c|Z) and P (λ|Z) at two points in first recirculation zone behind the bluff body (x/D = 0.4): in the shear zone where recirculated combustion products mix with air from the swirling annulus (r/R = 0.85) and in the inner recirculation zone (r/R = 0.49). A rich flamelet branch predominantly characterized by the mixing between combustion products at stoichiometry and pure fuel is observed at r/R = 0.49, while a mixing line representing mixing of fresh air with rich combustion products [11], caused by the recirculation of combustion products, is observed at r/R = 0.85.…”

“…The study at hand concerns a direct analysis of experimental data in the sense of RANS modeling. As mentioned, the scatter in the experimental data of flame SM1 is primarily caused by 'large scale' mixing of hot combustion products with air or fuel in the recirculation region behind the bluff body [11], not by local extinction (or lack in 'progress' of the reaction). As such, the case resembles 'multiple stream mixing' [30], where the combustion products act as third stream.…”

“…Therefore none of the proposed definitions can ensure independence of Z with the progress variable. Note that in this case, Y CO 2 , c and λ do not really indicate the progress of reaction as it would in a reacting homogeneous mixture, but rather the mixing between fresh air and recirculated combustion products without any significant reaction (see also [11]). This recirculation zone 'creates' a third stream [30] of rich combustion products, resulting in a mixing of air with rich combustion products.…”

Transported PDF Modeling of Ethanol Spray in Hot-Diluted Coflow Flame

“…However, many studies [28,55] already pointed out that due to the presence of droplet evaporation, the β shape PDF is no longer valid for mixture fraction in spray combustion. For the PDF of progress variable, even more ambiguities exist, both β-function and δ-function have been reported in the literatures [9,14], and further studies are required. Alternatively, in this study, the transport equation of the joint PDF of gas phase properties is directly modeled and solved, such that the turbulence-chemistry interaction is considered in a more precise manner.…”