Title:	Direct numerical simulation study of reacting front propagation in homogenous turbulence
DNr:	SNIC 2015/1-457
Project Type:	SNIC Medium Compute
Principal Investigator:	Rixin Yu <rixin.yu@energy.lth.se>
Affiliation:	Lunds universitet
Duration:	2016-01-01 – 2016-07-01
Classification:	20306
Keywords:

Abstract

While premixed turbulent combustion has been widely utilized, the physics of this multiscale and highly nonlinear phenomenon requires a substantially deeper understanding, and the predictive capabilities of available models are still limited. Various approaches to theoretical, Reynolds-averaged Navier–Stokes (RANS) or large eddy simulation (LES) research into turbulent flames have been developed, they either address a hypothetical problem that cannot be investigated experimentally, or invoke closure relations obtained by considering a hypothetical problem. It is difficult to judge what particular simplifications are responsible for eventual disagreement between measured and computed data when testing the model against experiments. From this perspective direct numerical simulation (DNS) studies of substantially simplified problems appear to be of great interest and importance for assessing various theories or models. Recently we devised a DNS setup studying the basic problem of infinitely-thin interface self-propagation in homogenous turbulence. The interface is tracked by levelset equation and the fully periodic DNS setup allows a truly free propagation, without disturbance from any boundary. The DNS study was performed for a wide range of conditions covering various ratios of the interface speed (SL) to the r.m.s. turbulent velocity (U’) and various turbulent Reynolds number. The statistics computed from DNS has been used for (1) a thorough assessment of various existing models for mean brush thickness and mean flame speed ST (2) study of the counter-gradient/gradient transport based on turbulence scalar fluxes, (3) a detail examination of the difference between conditioned and canonical mean turbulence characteristics. The above work is a fundamental study of theoretical interest and are published in top journals in the field [1, 2]. In the current application we plan to extend the DNS study to examine finite-thickness front propagation. For finite thickness front, turbulence can wrinkle and may even distribute the reaction zones by intensive turbulence motion, the local flame speed may be significantly different than the undisturbed 1D laminar flame speed SL by factors such as the local stretch or flame curvature. The current project also provides new opportunities to study how turbulence affects the reaction zone and the preheat zone. This proposed study is at a long term series aimed at improving our understanding of the governing physical mechanisms of premixed turbulent combustion by investigating a set of basic problems starting from the simplest one addressed in the previous work. Subsequently, we plan (i) to complicate the problem step by step by allowing for density variations and, finally, complex combustion chemistry and (ii) to reveal the role played by each of these effects by straightforwardly comparing data obtained in two subsequent sets of the DNS series. 1. Yu, R., Bai, X.S., Lipatnikov, A.N. 2015 A direct numerical simulation study of interface propagation in homogeneous turbulence, J. Fluid Mech., 772, 127. 2. YU, R., LIPATNIKOV, A. N. & BAI, X. S. 2014 Three-dimensional direct numerical simulation study of conditioned moments associated with front propagation in turbulent flows. Phys. Fluids 26, 085104.