Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G05: Turbulent Flames |
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Chair: Peter Hamlington, University of Colorado Boulder Room: 204 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G05.00001: Particle Pair Dispersion in a Turbulent Premixed Flame Ryan Darragh, Colin Towery, Peter Hamlington Turbulence greatly enhances the mixing of scalar quantities in a wide range of both natural and engineering flows. In flames, mixing can be especially important to understanding the transport of chemical species concentrations and temperature. Here, we study this mixing by considering the dispersion of fluid particle pairs in a turbulent premixed methane-air flame at a Karlovitz number of $Ka \approx 100$. Particle pairs are seeded such that their centroid lies on a temperature isosurface, providing temperature-resolved results and allowing for the study of dispersion as particles travel through the flame. Our results are compared to non-reacting turbulent flows to determine what differences exist between high Karlovitz number flames and non-reacting turbulence. Particular attention is given to recovering Richardson's scaling and determining how the scaling changes through the flame. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G05.00002: Direct Numerical Simulation of Turbulent Hydrogen-Air Premixed Flames at Preheated and Diluted Donditions Wonsik Song, Francisco E. Hernandez Perez, Efstathios-Al. Tingas, Hong Im Moderate or intense low-oxygen dilution (MILD) combustion is a promising concept to achieve high-performance and low-emission combustion simultaneously. In this study, we investigate the front propagation and burning characteristics in the MILD mode through high-fidelity direct numerical simulation (DNS). As a baseline condition, a lean hydrogen-oxygen mixture at an equivalence ratio of 0.7 is considered, which is diluted with nitrogen and preheated. The turbulence parameters such as the integral length scale and RMS velocity are set to ten and five times larger than the laminar flame thickness and flame speed ($l_\mathrm{T}/l_\mathrm{f}$ = 10, $u’/S_\mathrm{L} = 5.2$), respectively, such that the turbulence condition falls under the thin reaction zone in the so-called Borghi diagram of premixed combustion. Fundamental turbulent combustion characteristics including burning rate, flame structure and topology are examined, together with intrinsic hydrodynamic and diffusive-thermal instabilities. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G05.00003: Effects of Lewis number on turbulent kinetic energy transport at high Karlovitz numbers. Hsu Chew Lee, Peng Dai, Zheng Chen, Minping Wan Three-dimensional Direct numerical simulation (DNS) with detailed chemical kinetics scheme was used to investigate the statistical behavior of turbulent kinetic energy (TKE) transport in dodecane (Lewis $=$ 4.2) and hydrogen (Lewis $=$ 0.4) flames at Karlovitz numbers ranging from 4 to 150 spanning thin and broken reaction zones. The behavior of the terms in the TKE transport equation is analyzed and scaling terms proposed for the thin reaction zone is examined for a broken reaction zone and high Lewis number. The resulting normalized TKE transport equation involves only a small set of parameters. The Lewis number is found to affect only the mean velocity dilatation term and the velocity-pressure gradient term, while other terms in the TKE balance behaved identically to fuels with unity Lewis number. The TKE transport due to velocity fluctuations term is found to be insignificant when compared to other terms in the TKE transport equation regardless of the Lewis and Karlovitz numbers. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G05.00004: Multiscale Geometry and Fractal Scaling of Spherically Expanding Turbulent Premixed Flames Tejas Kulkarni, Fabrizio Bisetti The burning rate in combustion devices operating under turbulent conditions is typically dictated by the flame surface area. A turbulent flame is stretched, folded, and wrinkled on a multitude of length scales ranging from the Kolmogorov scale to the integral scale. Consistently with studies on interfaces in isothermal turbulence, it has been postulated that turbulent flame surfaces exhibit scale-invariant or fractal properties. In this study, we investigate the fractal nature of the surface of several turbulent spherical flames subject to decaying turbulence at varying Reynolds number. We find that the flame surface has a fractal dimension that varies in time, remaining close to 2.4, although the limited size of the inertial range makes a more specific attribution difficult. The inner cut-off length is found to be equal to about 10 Kolmogorov lengths and about the same size as the Taylor miscroscale. The cut-off length can be interpreted as the characteristic scale of surface wrinkling processes and is found to be be nearly independent of the Reynolds number. Our results are compared with others in the literature. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G05.00005: Experimental Investigation of the turbulence-flame interaction using POD based Finite Time Lyapunov Exponents (FTLEs) Sina Rafati, Noel Clemens Rare events are defined as the excursions of a dynamical system toward unwanted conditions with possible catastrophic consequences. To that end, the focus of this study is to investigate the interaction of turbulence with a jet flame to better understand the occurrence of rare events in combustion such as flashback, extinction or blowout. Kraichnan (1965) has shown that there is a strong correlation between the existence of rare events and a fluid's memory. As a consequence, the persistence of an initiated perturbation in a dynamical system for time-scales comparable to the large-scale flow time-scales might lead to rare events. In this study, 20 kHz Particle Image Velocimetry (PIV) has been used for velocity measurement of lifted methane-air turbulent flames. Two Coherent Evolution-90 diode-pumped Nd:YLF lasers were used to create 527nm pulses for PIV. The proper orthogonal decomposition (POD) method was utilized to obtain a compact representation of the velocity field. Then, the high-dimensional velocity field was projected into a lower-dimensional space for Lower Order Reconstruction (LOR) of the flow field with the aim of bringing new insight to the contribution of various scales in the chaotic development of the flow. Finally, Lagrangian coherent structures (LCSs) are obtained as ridges of FTLE maps to study the flow topologies as a function of space and time. Our results are representing how LCSs interact with flame as they are approaching to the flame front. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G05.00006: Compressible Dynamics of Fast Turbulent Flames Rachel Hytovick, Jonathan Sosa, Jessica Chambers, Kareem Ahmed, Alexei Poludnenko, Vadim Gamezo h $-abstract-$\backslash $f1The research characterizes the dynamics of compressible flame-turbulence interactions for propagating fast flames. A Turbulent Shock Tube with a series of turbulence inducing plates has a large viewing area to capture the flame dynamics with various optical diagnostics, including high-speed PIV and schlieren. The experimental results show that the turbulent Mach number, $M_{T}$, within the flame increases non-linearly relative to the flame propagation Mach number, $M_{f}$, and grows quickly for flames propagating faster than Chapman-Jouguet deflagrations ($M_{f}$\textgreater 1). This relationship shows that turbulence is self-generated by fast turbulent flames. Furthermore, the flames with $M_{f}$\textgreater 1 are intrinsically unsteady. They tend to accelerate and generate shocks. This acceleration is accompanied by the fast increase of $M_{T}$ and continues until shocks become strong enough to ignite a detonation. Slower flames with $M_{f}$ \textless 1 show, little or no self-generated turbulence, and do not produce shocks. The results are highly relevant for hypersonic scramjet propulsion engines and compressible shock-laden turbulent reacting flows in rotating detonation engines.$\backslash $f2-/abstract-$\backslash $\tex [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G05.00007: Kinetic Energy Backscatter in High-Speed, Compressible Reacting Turbulence Arnab Moitro, Ashwath Sethu Venkataraman, Alexei Poludnenko Previous studies have shown that in certain reacting flow regimes, near the flame region, the direction of kinetic energy cascade reverses compared to the non-reacting turbulence, and is primarily directed from small scales to large scales on average. Studying this phenomenon (often termed backscatter) is important for developing large-eddy simulation (LES) models for turbulent combustion. Previous studies, however, were limited to relatively low-Mach number flows in idealized geometries. In the present work, we study the backscatter in highly-compressible regimes characterized by large Reynolds numbers. In particular, we present direct numerical simulations (DNS) of the flow in a Turbulent Shock Tube facility designed at the University of Central Florida with the goal of probing the dynamics of turbulent flames in such fast regimes. We quantify the backscatter by low-pass filtering the primitive variables in the DNS at various scales, and evaluating the the sub-filter scale terms in the equation for the transport of kinetic energy. Finally, we discuss the implications of these results for the development of the new generation of LES models for high-speed, compressible reacting flows. [Preview Abstract] |
Sunday, November 24, 2019 5:19PM - 5:32PM |
G05.00008: Direct Numerical Simulation of Flame-Wall Interaction in a Constant Volume Vessel with a Crevice Yuki Minamoto, Andrea Gruber, Mamoru Tanahashi Understanding of flame-wall interaction phenomena is important for further reduction of pollutant formation and enhancement of efficiency of various combustors. Many combustion devices involve crevice regions which yields much smaller length scale than the size of a combustor. In such crevice regions, the wall heat loss and turbulent mixing could be modified due to the geometrical effect of the crevice, resulting in more complex flame-wall interaction, which is not fully understood. In this study, a DNS of turbulent methane-air flame-wall interaction in a constant volume vessel with a crevice has been performed to understand the combustion physics near the crevice region. The present DNS configuration is chosen based on a typical IC engine combustion in terms of domain and crevice sizes and turbulent combustion condition on the combustion diagram, although methane-air combustion at atmospheric pressure was considered. The visual examination of DNS results shows that the mixture in the main domain is entrained and mixed with the mixture in the crevice. On the other hand, the mixture in the crevice is cooled as thermal boundary layer develops. The DNS results also revealed several interesting thermochemical and fluid dynamic aspects relevant to near-wall crevice combustion. [Preview Abstract] |
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