Yuki Minamoto

Reaction zones and their structure in MILD combustion

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.

Effect of flow-geometry on turbulence-scalar interaction in premixed flames

Turbulent combustion of stoichiometric hydrogen-air mixture is simulated using direct numerical simulation methodology, employing complex chemical kinetics. Two flame configurations, freely propagating and V-flames stabilized behind a hot rod, are simulated. The results are analyzed to study the influence of flame configuration on the turbulence-scalar interaction, which is critical for the scalar gradient generation processes. The result suggests that this interaction process is not influenced by the flame configuration and the flame normal is found to align with the most extensive strain in the region of intense heat release. The combustion in the rod stabilized flame is found to be flamelet like in an average sense and the growth of flame-brush thickness with the downstream distance is represented well by Taylor theory of turbulent diffusion, when the flame-brushes are non-interacting. The thickness is observed to saturate when the flame-brushes interact, which is found to occur in the simulated rod st...

A Fractal Dynamic SGS Combustion Model for Large Eddy Simulation of Turbulent Premixed Flames

ABSTRACT Direct numerical simulation of a turbulent hydrogen-air premixed plane jet flame is performed to investigate fractal characteristics and to evaluate the fractal dynamic subgrid scale (FDSGS) combustion model. The DNS results show that the fractal dimension of flame surfaces increases with the downstream distance, and the fractal dimension computed using a 3D box-counting method reaches about 2.54 in the region where turbulence is developed by mean shear. An inner cutoff representation employed in the FDSGS combustion model could be used in large eddy simulations (LES) of complicated combustion problems. Static tests show that the procedure applied in the FDSGS combustion model adequately predicts the fractal dimension, Kolmogorov length scale, and flame surface area despite the presence of strong mean shear. Dynamic model evaluations are also carried out by conducting a series of LES using the FDSGS and other combustion models. In the dynamic tests, the mean temperature distributions and peak positions of variations of a reaction progress variable fluctuation in the transverse direction obtained from the LES with the FDSGS combustion model show good agreement with the filtered DNS fields. The present evaluation also revealed that one of the strengths in the FDSGS combustion modeling approach is that the model does not require SGS turbulent velocity fluctuation, since modeling of this quantity is straightforward for neither homogeneous turbulence nor the turbulent shear flows.

A priori assessment of scalar dissipation rate closure for Large Eddy Simulations of turbulent premixed combustion using a detailed chemistry Direct Numerical Simulation database

ABSTRACT Algebraic and transport equation-based closures of Favre-filtered scalar dissipation rate (SDR) of the reaction progress variable, in the context of large eddy simulations, have been assessed using detailed chemistry direct numerical simulation (DNS) data of a stoichiometric H2-air turbulent V flame. The Favre-filtered SDR and the unclosed terms of its transport equation have been extracted by explicitly filtering the DNS data for different choices of the reaction progress variable. An algebraic closure of SDR, which was proposed previously using simple chemistry DNS data, has been found to predict the Favre-filtered SDR satisfactorily for detailed chemistry DNS data for different choices of the reaction progress variable. Similarly, the models of the unclosed sub-grid convection, density variation, scalar-turbulence interaction, reaction rate gradient, molecular dissipation, and diffusivity gradient terms of the Favre-filtered SDR transport equation, which have previously been proposed based on simple chemistry DNS data, have been found to satisfactorily predict both the qualitative and quantitative behaviors of these unclosed terms for a range of filter widths , for two different choices of reaction progress variable in the case of the detailed chemistry DNS dataset considered in this analysis.

Micro particle image velocimetry investigation of near-wall behaviors of tumble enhanced flow in an internal combustion engine

A micro particle image velocimetry has been performed to investigate tumble enhanced flow characteristics near piston top surface of a motored internal combustion engine for three inlet valve open timing (−30, −15, 0 crank angle degrees). Particle image velocimetry was conducted at 340, 350 and 360 crank angle degrees of the end of the compression stroke at the constant motored speed of 2000 r/min. The measurement region was 3.2 mm × 1.5 mm on the piston top including central axis of the cylinder. The spatial resolution of particle image velocimetry in the wall-normal direction was 75 µm and the vector spacing was 37.5 µm. The first velocity vector is located about 60 µm from the piston top surface. The micro particle image velocimetry measurements revealed that the ensemble-averaged flow near the piston top is not close to the turbulent boundary layer and rather has tendency of the Blasius theorem, whereas fluctuation root-mean-square velocity near the wall is not low. This result shows that revision of a wall heat transfer model based on an assumption of the proper characteristics of flow field near the piston top is required for more accurate prediction of heat flux in gasoline engines.

Effects of Turbulence on Ignition of Methane–Air and n-Heptane–Air Fully Premixed Mixtures

ABSTRACT We performed direct numerical simulations (DNS) for the two-dimensional (2D) turbulent ignition of ultra-lean methane–air and n-heptane–air mixtures with a high exhaust gas recirculation (EGR) rate at high pressure to determine the ignition criteria and ignition delay time. We defined an initial high-temperature region as an ignition kernel and conducted one-dimensional preliminary DNS to determine the ignition criteria in terms of the ignition source energy and the thermal conduction from the ignition kernel during the induction period. Additionally, we analyzed the 2D DNS results to clarify the influence of the turbulent strain rate on the ignition delay time and the mechanism by which the turbulence influences the establishment of the ignition kernel. We observed that the distribution of eddies and the strain rate in the high-temperature region influences the success or failure of the ignition process and, therefore, the ignition delay time. The ignition delay time increases proportionally to the square of strain rate averaged in the high concentration region of the intermediate species during the induction period. This suggests that the ignition in a turbulent field is based on the balance between the influence of a locally averaged strain rate in the preheating region and the chemical (flame) time scale. Based on these observations, a simple model for the ignition delay time was constructed based on the mean strain rate in the high concentration region of the intermediate species during the induction period. The strain rate averaged in the high concentration region of the intermediate species was normalized by using the laminar burning velocity and the laminar thermal flame thickness. Additionally, the ignition delay time was normalized by the ignition delay time of the corresponding laminar case, yielding the same ignition model/criterion for both examined fuels, which could be extended to other mixtures.

Flow-Geometry and Reynolds-Number Effects on Flame-Turbulence Interactions

Three-dimensional direct numerical simulation (DNS) with a detailed kinetic mechanism has been conducted for statistically-planar turbulent flame and turbulent V-flame of hydrogen–air mixture to clarify the effects of mean flow velocity on principal strain rates at flame front and on flame geometry. Reynolds numbers based on Taylor micro scale and turbulent intensity are selected to 60.8 and 97.1, and mean flow velocities for V-flame are 10 and 20 times laminar burning velocity. From results of DNS, eigenvalues and eigenvectors of strain tensor are evaluated to investigate characteristics of strain field near flame and flame normal alignments with the principal axes of strain in detail. It has been revealed that Reynolds number affects both magnitude of strain rates and alignment between flame normal and principal axis of strain, and that the magnitude of mean flow velocity affects flame normal alignments in turbulent V-flame.Copyright