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Daniel Guggenheim School of Aerospace Engineering
Daniel Guggenheim School of Aerospace Engineering
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ItemKinetic-hydrodynamic instability interactions in near blowoff bluff body flames(Georgia Institute of Technology, 2022-05-02) Manosh Kumar, RaghulThis work examines the dynamics of confined bluff body flames in the process of lean blowoff (LBO) using simultaneous stereo-PIV (particle image velocimetry), OH PLIF (planar laser induced fluorescence) and CH2O PLIF. Flames at high density ratios blow off in at least two distinct stages: stage 1, where intermittent extinction occurs along the flame front, but the flame and flow remain qualitatively similar to stable conditions, and stage 2, where there is permanent downstream flame extinction and large-scale changes in dynamic flow characteristics. This work particularly focuses on stage 2 processes, with the goal of understanding what ultimately leads to irrecoverable flame blowoff. A new test facility was developed with the operational flexibility to achieve two goals: (1) approach LBO by keeping the parameters that influence its hydrodynamic stability approximately constant, particularly flow velocity (u_bulk) and gas expansion ratio (σ), and (2) compare near-LBO dynamics under conditions where, well away from blowoff, the flame is globally stable (high σ case) and globally unstable (low σ case, where the Bénard-Von Karman (BVK) instability of the flow is present). The latter case was of particular interest as most prior detailed diagnostic studies of LBO have been performed at high σ, BVK-suppressed conditions. We find, however, that the transient blowoff process remains largely unchanged in the high and low σ cases, presumably because the BVK instability reappears in either case under conditions very close to LBO. In all cases, blowoff is preceded by permanent downstream extinction that moves progressively closer to the bluff body as LBO is approached. We find that blowoff dynamics are intrinsically 3D, due to both secondary instabilities of the shear layer and confinement effects associated with bluff body-wall interactions. These 3D structures often manifest themselves as burning reactant fingers which are caught in the backflow of the recirculation zone; under very near LBO conditions they impinge on the back of the bluff body and extinguish. At the very edge of blowoff, the recirculation zone is no longer composed of hot products and is unable to autoignite the oncoming reactant flow, leading to global extinction. The characteristic time associated with this feedback between downstream extinction and wake structure alteration causing blowoff is about 2 orders of magnitude larger than the characteristic flow time, D/u_bulk.
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ItemExperimental investigation of nitrogen oxide production in premixed reacting jets in a vitiated crossflow(Georgia Institute of Technology, 2019-09-03) Sirignano, Matthew DavisThe presented work describes the experimental investigation of nitrogen oxide (NOx) emissions from reacting jets in a vitiated crossflow (RJICF). It is motivated by interest in axial staging of combustion as an approach to reduce undesirable NOx emissions from gas turbine combustors operating at high flame temperatures (>1900K). In lean-premixed combustion, NOx levels are exponential functions of temperature and linear functions of residence time. Consequently, NOx production rates are high at such temperatures, and conventional combustor architectures are unable to simultaneously deliver low NOx and part-load operability. A RJICF is a natural means of implementing axial staging. Therefore, a fuller understanding of the governing processes and parameters regarding pollutant formation within this complex flow field is critical to the next generation of gas turbine technology advancement. It is clear that RJICF NOx production is a highly coupled process. A key challenge was decoupling the interdependent jet parameters in order to observe fundamental NOx production sensitivities. Data is presented for premixed jets injected into a vitiated crossflow of lean combustion products. The jets varied in: fuel selection (methane or ethane or a combination), equivalence ratio (0.8≤ϕjet≤9.0), momentum flux ratio (2≤J≤40), and exit geometry (pipe or nozzle). The crossflow temperatures ranged from 1350K – 1810K, and the reacting jets induced a bulk averaged temperature rise on the flow (ΔT) ranging from 75K – 350K. In addition, several data series were replicated with varied ethane/methane ratios at constant ϕjet to influence flame lifting independent of other parameters. Similarly, the jet exit geometry was varied to influence shear layer vortex growth rates. Overall, these data indicate that NOx emissions are largely determined by ΔT. However, significant variation was observed at constant ΔT levels. The data is consistent with the idea that this variation is controlled by the stoichiometry at which combustion actually occurs, referred to as ϕFlame. ϕFlame is influenced by ϕjet and pre-flame mixing of the jet and crossflow that, in turn, is a function of flame lift-off distance (LO), nozzle geometry, and crossflow temperature. The data highlights the importance of flame lifting as well as the potential importance of post-flame mixing effects. Both are complex problems and are not directly addressed in this work. Further work in these areas would significantly deepen understanding of the relevant phenomena in RJICF NOx production.
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ItemEnsemble-averaged dynamics of premixed, turbulent, harmonically excited flames(Georgia Institute of Technology, 2017-04-07) Humphrey, LukeIncreasing awareness of the negative impacts of pollutant emissions associated with combustion is driving increasingly stringent regulatory limits. In particular, oxides of nitrogen, generally referred to as NOx, now face strict limits. These restrictions have driven development of cleaner burning combustion systems. Because NOx formation increases significantly at elevated temperatures, one method to reduce NOx emissions is to burn the fuel at lower temperatures. By premixing the fuel and oxidizer prior to combustion significantly lower flame temperatures can be achieved, with corresponding reductions in NOx emissions. Unfortunately, premixed combustion systems are generally more prone to potentially problematic feedback between the unsteady heat release from the flame and unsteady pressure oscillations. This self-excited feedback loop is known as combustion instability. Because these oscillations are associated with unsteady pressure fluctuations they can degrade system performance, limit operability, and even lead to catastrophic failure. Understanding combustion instability is the primary motivation for the work presented in this thesis. The interaction of quasi-coherent and turbulent flame disturbances changes the spatio-temporal flame dynamics and turbulent flame speed, yet this interaction is not fully understood. Therefore, this thesis concentrates on identifying, understanding, and modeling these interactions. In order to address this topic, two primary avenues of research are followed: development and validation of a flame position model and experimental investigations of predicted ensemble-averaged flame speed sensitivity to flame curvature. First, a reduced order modeling approach for turbulent premixed flames is presented, based on the ensemble-averaged flame governing equation proposed by Shin and Lieuwen (2013). The turbulent modeling method is based on the G-equation approach used in laminar flame position and heat release studies. In order to capture the dependence of the ensemble-averaged turbulent flame speed on the ensemble-averaged flame curvature, the turbulent flame model incorporates a flame speed closure proposed by Shin and Lieuwen (2013). Application of the G-equation approach in different coordinate systems requires the inclusion of time-varying integration limits when calculating global flame area. This issue is discussed and the necessary corrections derived. Next, the reduced order turbulent modeling approach is validated by comparison with three-dimensional simulations of premixed flames, for both flame position and heat release response. The reduced order model is the linearized, allowing development of fully analytical flame position and heat release expressions. The use of the flame speed closure is shown to capture nonlinear effects associated with kinematic restoration. Second, the development of and results from a novel experimental facility are described. This facility has the capability to subject premixed flames to simultaneous broadband turbulent fluctuations and narrowband coherent fluctuations, which are introduced on the flame using an oscillating flame holder. Mie scattering images are used to identify the instantaneous flame edge position, while simultaneous high speed PIV measurements provide flow field information. Results from this experimental investigation include analysis of the ensemble-averaged flame dynamics, the ensemble-averaged turbulent displacement speed, the local ensemble-averaged area and consumption speed, and the dependence of both the displacement speed and consumption speed on the ensemble-averaged flame curvature. Finally, the flame speed sensitivity to curvature is quantified through calculation of the normalized turbulent Markstein displacement and consumption numbers. The results show that the amplitude of coherent flame wrinkles generally decreases with both downstream distance and increasing turbulence intensity, providing the first experimental validation of previous isothermal results. The average displacement and consumption speeds increase with downstream distance and turbulence intensity, reflecting the increasing wrinkled flame surface. The ensemble-averaged, phase dependent displacement and consumption speeds demonstrate clear modulation with the shape of the ensemble-averaged flame. Specifically, these turbulent flame speeds increase in regions of negative curvature. For both the displacement and consumption speed, the magnitude of the normalized turbulent Markstein length increases with ratio of the turbulent flame wrinkling length to the coherent wrinkling length when u'/SL0 >2.5 . For u'/SL0 < 2.5 the trends are less clear due to the presence of convecting disturbances which introduce additional fine scale wrinkles on the flame. Together the results presented in this thesis provide a foundation for modeling turbulent flames in the presence of quasi-coherent disturbances. The flame position can be modeled using the ensemble-averaged governing equation with the dynamical flame speed closure, and the corresponding heat release can be calculated from the turbulent consumption speed closure. The turbulent Markstein numbers and uncurved flame speed may be extracted from experimental or numerical data.