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1. Flame-drift velocimetry and flame morphology measurements with dual-perspective imaging in a shock tube

   Susa, Adam J; Hanson, Ronald K (May 2021, 12th U.S. National Combustion Meeting)

   null (Ed.)

   The application of simultaneous, dual-perspective, high-speed imaging to expanding flame experiments in a shock tube provides new opportunities to characterize the post-reflected-shock flow field. The shock-tube flame speed method has recently been demonstrated as an experimental approach to enable flame speed measurements at high unburned-gas temperatures inaccessible to previously established methodologies. The fidelity of these experiments are predicated on two underlying assumptions: quiescence of the unburned gas and symmetry of the expanding flames. While both are ubiquitous in the related literature, neither of these assumptions had been previously explicitly evaluated in relation to shock-tube flame experiments. This work reports the first measurements in which side-wall emission imaging, in addition to simultaneous end-wall imaging, is applied to expanding flame experiments in a shock tube. The fact that the burned gas within an expanding flame is nominally stagnant relative to the local flow field is leveraged to perform single-point, 3D velocimetry measurements of the core gas based upon the motion of the flame centroid, or “flame drift”. These measurements reveal that minimal motion is present in the radial directions, while the velocity of the core gas in the axial direction is larger in magnitude and displays strong temperature dependence. The 3D morphology of flames is also characterized for the first time. Side wall imaging reveals that, while the expected flame symmetry is observed under some conditions, it breaks down under others, particularly at increasing temperatures. These results shed new light on previously reported flame structure observed in shock-tube flame experiments, which can now be explained as the axial integration of emission from an axially distorted flame. These observations serve as a demonstration of a novel diagnostic application, provide new insight as to how future shock-tube flame experiments might be refined, and motivate the continued use of side-wall imaging to ensure the fidelity of future shock-tube flame speed measurements. 

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2. Schlieren-based measurements of propane flame speeds at extreme temperatures

   Susa, Adam J; Zheng, Lingzhi; Hanson, Ronald K (May 2021, 12th U.S. National Combustion Meeting)

   null (Ed.)

   Flame speed measurements of stoichiometric (f=1) propane in an oxygen-argon oxidizer (21% O2, 79% Ar) were conducted behind reflected shock waves at unburned-gas temperatures from 800 K to nearly 1,200 K. As in previous shock-tube flame speed experiments, non-intrusive laser-induced-breakdown is used to ignite an expanding flame in the nominally quiescent gas following the reflected-shock passage. In addition to the end-wall emission imaging employed in previous works, a schlieren imaging diagnostic is employed utilizing side-wall optical ports. The high temporal and spatial resolutions of the schlieren diagnostic allow for measurements to be made of small, curvature-stabilized flames (r < 7 mm) with short measurement times (t < 600 ms). Direct comparison of simultaneous emission- and schlieren-based measurements illustrates that measurements performed with the two techniques agree at comparable flame radii. The comparison further shows the schlieren-based measurements do not show evidence of flame acceleration as is seen in the emission based measurements at larger flame radii and longer measurement times. Extrapolated, zero-stretch flame speeds are compared with those calculated using detailed and reduced reaction mechanisms, accounting for auto-ignition chemistry effects in accordance with the recent literature. 

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3. End-wall Effects on Freely Propagating Flames in a Shock Tube

   https://doi.org/10.2514/6.2022-2346  

   Susa, Adam J; Zheng, Lingzhi; Nygaard, Zach D; Hanson, Ronald K. (January 2022, AIAA SCITECH 2022 Forum)

   The dynamics of flame propagation at high unburned-gas temperatures are of critical importance to the performance and operability of modern engine systems but have long existed beyond the temperature regimes accessible to controlled laboratory study. The shock-tube flame speed method has been demonstrated to enable the study of premixed, freely propagating flames over a wide range of previously unachievable engine-relevant unburned-gas temperature conditions. This study reports the first systematic investigation of end-wall-induced effects on the propagation and stability of flames subject to asymmetric flow confinement in a shock tube. Through the flexibility afforded by newly available optical access, the axial position of flame ignition was varied over a range spanning from 3.3 to 15.5 cm from the driven end wall. Experiments performed under static conditions isolated the effect of asymmetric end-wall confinement and provided an opportunity to measure the flow velocity induced by the confinement effect; results show the expected functional scaling exists between flame radius, distance from the end wall, and flow velocity, but the velocity scaling deviates from that predicted. Experiments performed behind reflected shock waves are then used to probe the interplay between the confinement and gas-dynamic effects in the post-reflected-shock environment. In a break with intuition, the post-shock results show a non-monotonic relationship between position and flame stability, with one particular distance (6.4 cm) producing significantly more severe distortion than flames ignited either nearer or farther from the end wall. Finally, experiments demonstrating the generation of hemispherically expanding flames in the shock tube are reported, providing a baseline to inform the consideration of such flames as an alternative basis for flame speed measurements. The experimental measurements reported in this work provide valuable new validation targets against which detailed modeling of confinement and gas-dynamic effects can be compared, while the side-wall observations reaffirm that spherically expanding flames suitable for use in reliable laminar flame speed measurements can be generated in a post-reflected-shock environment. 

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4. Acceleration of Premixed Flames in Obstructed Pipes with Both Extremes Open

   https://doi.org/10.3390/en13164094  

   Adebiyi, Abdulafeez; Abidakun, Olatunde; Akkerman, V’yacheslav (August 2020, Energies)

   Premixed flame propagation in obstructed channels with both extremes open is studied by means of computational simulations of the reacting flow equations with a fully-compressible hydrodynamics, transport properties (heat conduction, diffusion and viscosity) and an Arrhenius chemical kinetics. The aim of this paper is to distinguish and scrutinize various regimes of flame propagation in this configuration depending on the geometrical and thermal-chemical parameters. The parametric study includes various channel widths, blockage ratios, and thermal expansion ratios. It is found that the interplay of these three critical parameters determines a regime of flame propagation. Specifically, either a flame propagates quasi-steady, without acceleration, or it experiences three consecutive distinctive phases (quasi-steady propagation, acceleration and saturation). This study is mainly focused on the flame acceleration regime. The accelerating phase is exponential in nature, which correlates well with the theoretical prediction from the literature. The accelerating trend also qualitatively resembles that from semi-open channels, but acceleration is substantially weaker when both extremes are open. Likewise, the identified regime of quasi-steady propagation fits the regime of flame oscillations, found for the low Reynolds number flames. In addition, the machine learning logistic regression algorithm is employed to characterize and differentiate the parametric domains of accelerating and non-accelerating flames. 

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5. Laminar Flame Speed Measurements of Primary Reference Fuels at Extreme Temperatures

   https://doi.org/10.1115/ICEF2022-90501  

   Susa, Adam J.; Zheng, Lingzhi; Nygaard, Zach D.; Ferris, Alison M.; Hanson, Ronald K. (October 2022, ASME 2022 ICE Forward Conference (ICEF2022))

   Abstract Experimentally measured values of the laminar flame speed (SL) are reported for the primary reference fuels over a range of unburned-gas temperatures (Tu) spanning from room temperature to above 1,000 K, providing the highest-temperature SL measurements ever reported for gasoline-relevant fuels. Measurements were performed using expanding flames ignited within a shock tube and recorded using side-wall schlieren imaging. The recently introduced area-averaged linear curvature (AA-LC) model is used to extrapolate stretch-free flame speeds from the aspherical flames. High-temperature SL measurements are compared to values simulated using different kinetic mechanisms and are used to assess three functional forms of empirical SL–Tu relationships: the ubiquitous power-law model, an exponential relation, and a non-Arrhenius form. This work demonstrates the significantly enhanced capability of the shock-tube flame speed method to provide engine-relevant SL measurements with the potential to meaningfully improve accuracy and reduce uncertainty of kinetic mechanisms when used to predict global combustion behaviors most relevant to practical engine applications. 

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