Analysis of the Eddy Dissipation Concept formulation for MILD combustion modelling

Abstract

Performance of the Eddy Dissipation Concept (EDC) in the regime of Moderate and Intense Low-oxygen Dilution (MILD) combustion is investigated. The special MILD features, where chemical and turbulence time scales are comparable (Damköhler number close to unity), have led several researchers to suggest modifications of EDC, mainly by changing model constants. EDC with standard and modified constants are compared, and the importance of each effect is outlined. Different fine-structure reactor models and their inflow/initial conditions are discussed and found to play a significant role. The reacting fraction of fine structures, which in virtually all other numerical studies is set to unity, is also discussed and found to be important. We observe better agreement with experiment when the reacting fraction is reduced below unity, which is also described by the original EDC. The results obtained with the variable reacting fraction are found to improve both the temperature distributions and the lift-off height predictions. The calculations are carried out with the use of open source software OpenFOAM. The main test case was the Delft Jet-in-Hot-Coflow burner emulating MILD regime at three different flow conditions (jet Reynolds numbers of 2500, 4100 and 8800).

The Eddy Dissipation Concept—Analysis of Different Fine Structure Treatments for Classical Combustion

“…Moreover, they can be approximated by γ 2 L or γ 3 L if γ L << 1, respectively. However, since the fine structure residence time (τ * ) should be smaller than or equal to a characteristic mixing time (τ mix ), the fine structure fraction should be limited in order to ensure a ratio (R τ ) of the fine structure to the mixing time below unity, e.g., for the original EDC [16][17][18]28]:…”

“…Moreover, Cantera tutorials inspired the authors for the periodic oscillation example. A convenient way to get rid of the instabilities and to reduce the numerical effort, is to neglect the mixing term between the fine structures and the surroundings [9,[16][17][18]. This approach is referred to as the plug flow reactor (PFR) approach, since back-mixing is ignored in the species conservation equations and only chemistry is integrated over time.…”

“…The fine structure state is determined by solving the PSR to steady-state, which is numerically expensive due to the strong nonlinearity in the reaction source terms. Some EDC implementations, therefore, treat the fine structures as plug flow reactors (PFR) [7,[16][17][18][19]. These PFRs are solved for the fine structure residence time, implying that the fine structures are spatially isolated structures in the fluid only evolving in time.…”

“…However, the latter approach is not in line with the EDC theory. Although the PFR fine structure treatment was employed in many published research works, e.g., [16][17][18][19][20][21], only De et al [17], Li et al [16], and Lewandowski and Ertesvaag [18] commented on the consequences of the PFR simplification. However, none of them performed an in-depth analysis of species profiles and species consumption rates.De et al [17] state that the PSR and PFR results are only similar for slow reactions having little heat tone.…”

“…An in-depth analysis of the results is missing. Lewandowski and Ertesvåg [18] concluded that the PSR and PFR approaches should give comparable results, since the steady-state solution of the PSR equations is of a similar form to the PFR solution. However, no numerical data to prove this conclusion was shown.…”