Reaction Zones and Their Structure in MILD Combustion

Abstract

Three-dimensional direct numerical simulation (DNS) of turbulent combustion under moderate and intense low-oxygen dilution (MILD) conditions has been carried out inside a cuboid with inflow and outflow boundaries on the upstream and downstream faces, respectively. The initial and inflowing mixture and turbulence fields are constructed carefully to be representative of MILD conditions involving partially mixed pockets of unburned and burned gases. The combustion kinetics are modeled using a skeletal mechanism for methane-air combustion, including non-unity Lewis numbers for species and temperature-dependent transport properties. The DNS data is analyzed to study the MILD reaction zone structure and its behavior. The results show that the instantaneous reaction zones are convoluted and the degree of convolution increases with dilution and turbulence levels. Interactions of reaction zones occur frequently and are spread out in a large portion of the computational domain due to the mixture non-uniformity and high turbulence level. These interactions lead to local thickening of reaction zones yielding an appearance of distributed combustion despite the presence of local thin reaction zones. A canonical MILD flame element, called MIFE, is proposed, which represents the averaged mass fraction variation for major species reasonably well, although a fully representative canonical element needs to include the effect of reaction zone interactions and associated thickening effects on the mean reaction rate.

Extension of the Eddy Dissipation Concept for turbulence/chemistry interactions to MILD combustion

“…(13), a characteristic Reynolds number Re à ¼ 4 is obtained, which indicates that the reacting structures in MILD combustion have larger characteristic dimensions than in traditional combustion systems. This was recently confirmed by the analysis of Minamoto et al [34], who pointed out that reacting regions in MILD combustion are distributed over a good portion of the computational domain and the interaction between reaction zones leads to an appearance of distributed reaction, resulting in relatively uniform temperature distribution. We assume that the MILD combustion happens in the so-called Distributed Reaction Regime, to base our revision of the standard cascade model.…”

“…However, in MILD combustion, there is no longer a clear separation between large and small scales of turbulence, and reaction can occur over a wide range of scales [34]. Therefore, the chemical reactions proceed in a thick reaction zone, comparable to the integral length scale, leading to a modification of the characteristic scales of the reaction structures, due to the transfer of energy to higher frequencies than those of the reacting structures in the spectrum.…”

“…In MILD combustion, the dilution and preheating of the reactants generate a unique ''distributed" reaction zone [34] system evolves towards perfectly mixed conditions and the reaction process is characterised by a Damköhler number approaching unity. As pointed out in the introduction, this has led several research groups to modify the classic EDC model coefficients to achieve better predictions of experimental data.…”

Ignition Characteristics in Spatially Zero-, One- and Two-Dimensional Laminar Ethylene Flames

“…4,12 At the microscale, however, recent turbulent direct numerical simulations (DNS) of a well mixed region of MILD combustion have indicated small-scale regions of autoignition with interconnecting flamelets. 21,22 This microscale explanation of the structure of MILD flames does not account the mixing processes during fuel injection, with a focus on a well-mixed environment similar to many previous experimental investigations. Much of the fundamental research on the MILD combustion regime has been undertaken in premixed plug reactor, well-stirred reactor (WSR) or perfectly-stirred batch reactor (batch PSR) environments 23-27 rather than non-premixed configurations.…”

Distributed Low Temperature Combustion: Fundamental Understanding of Combustion Regime Transitions

“…Burning modes with distributed reactions show a strong degeneration of the laminar species / reaction progress correlation [41] and a bimodal two-fluid description (reactants and products) with a negligible probability of encountering chemically active states becomes problematic [42]. Spalding [25] suggested a multi-fluid approach that permits the identification of various intermediate fluid states.…”