The Structure Of Pre Mixed Flames In Isotropic And Shear Drive Turbulent Flows

Author : Ankit Tyagi
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (113 download)

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Download or read book Dynamics of Interacting Turbulent Flames written by Ankit Tyagi and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation investigates the physics of interactions between turbulent premixed flames. It is known that multiple flame configurations provide better stability characteristics compared to a large single-flame. However, the advantages of multiple flames are limited by flame proximity as flame-flame interactions tend to reduce the burning efficiency of the reactant gases. Previous studies have shown that interactions between multiple flames directly impact the flame structure and its propagation, resulting in reduced burning efficiency. Previous experimental studies of interacting flames addressed flame-flame interactions investigating their effects on combustor stability and efficiency from a global perspective. However, the local flame-flame interaction physics was not addressed comprehensively, in part because these studies were limited to specific flow and flame configurations. In particular, these studies focused on swirling flames in bluff-body configurations typical of modern combustor geometries. Furthermore, these studies lacked flowfield measurements and were limited to flame structure and heat release rate measurements due to the complex nature of the experimental configurations. Much of the work to date on understanding the local physics of interactions comes from direct numerical simulations (DNS), but these studies treated idealized configurations of limited practical utility.To bridge these two gaps, an experimental investigation of flame-flame interactions was performed using a dual-burner rig, composed of two flames, built to facilitate precise variations in flame spacing. This rig was designed to operate in different configurations. These facilitated the focus on local interaction physics. In particular, the rig was built to study interacting V-flames and Bunsen flames. Moreover, the design of the dual-burners permitted conducting studies of nonreacting flow interactions with flames to better understand local physics of the flame. Direct flame and flow measurements were performed to characterize the mutual interaction of flame and the local flowfield. In particular, flame structure and flow were characterized using synchronized OH-planar laser-induced fluorescence (OH-PLIF) and stereoscopic-particle image velocimetry (s-PIV). These measurements were performed at a sampling rate of 10 kHz to obtain converged statistics on flame-flame interactions. A novel image processing technique was implemented for robust detection and characterization of flame-flame interaction events from OH-PLIF images.Using this experimental approach, the following studies were conducted: i) effects of flame spacing on flame structure of interacting V-flames, ii) effects of multiple flames on frequency, topology, and orientation of local flame-flame interactions, iii) effects of high mean-shear flow on flame-flame interactions, and iv) effects of pocket formation on flame dynamics. In the first study, flame spacing variations in V-flames were found to directly impact flame attachment. For smaller flame spacings, recirculation of hot combustion products near the bluff-bodies facilitated a secondary flame branch attachment in the shear layers in the interaction regions. For larger flame spacing, the secondary attachment became intermittent, indicating that closer flame spacing resulted in better attachment and stability characteristics for these flames. In the second study, the presence of adjacent flames was found to directly impact the frequencies of flame-flame interaction events. Dual-flames showed lower reactant-side interactions rates and higher product-side interactions rates when compared with single-flames. For dual-flames, comparisons between interaction orientation and local strain rate orientation showed that compressive forces led to flame front merging or pinch-off. The third study, which focused on how mean shear affects the local flame dynamics, found that high-mean shear flows entrained the flame away from the center of the burner. This entrainment directly reduced interaction event frequencies along the flame branch closest to the high mean-shear flow, while interaction event frequency in the other branch increased. Finally, flame pocket formation was investigated and results showed that a majority of the reactant pockets burned-out, while a majority of the product pockets merged with the flame surface. These results suggested that pocket behavior in turbulent flames can change local flame dynamics and it is important to capture these effects to accurately predict flame behavior. Additionally, limitations of planar high-speed imaging techniques were explored and a statistical framework, using probabilistic models, was presented in the context of reactant pocket propagation. The outcome of this work provided improved uncertainty estimation for planar measurements in three-dimensional flows.This experimental investigation provided deeper insights into the local physics of flame-flame interactions, in practical configurations, using detailed flame and flow measurements. The presence of adjacent flames influenced the attachment characteristics and local flame structure that directly impacted the stability of these multiple flame configurations. Local compressive forces facilitated the occurrence of these events, highlighting the importance of changes to the flowfield due to adjacent flames. Pocket formation, which directly affected reactant gas burning efficiency, was found to occur frequently. Taken together, these results provided comprehensive insights into the effects of flame-flame interactions that enhance our understanding of the nature of interacting flames.