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1. 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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2. 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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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. Dissimilar cavitation dynamics and damage patterns produced by parallel fiber alignment to the stone surface in holmium:yttrium aluminum garnet laser lithotripsy

   https://doi.org/10.1063/5.0139741  

   Xiang, Gaoming; Li, Daiwei; Chen, Junqin; Mishra, Arpit; Sankin, Georgy; Zhao, Xuning; Tang, Yuqi; Wang, Kevin; Yao, Junjie; Zhong, Pei (March 2023, Physics of Fluids)

   Recent studies indicate that cavitation may play a vital role in laser lithotripsy. However, the underlying bubble dynamics and associated damage mechanisms are largely unknown. In this study, we use ultra-high-speed shadowgraph imaging, hydrophone measurements, three-dimensional passive cavitation mapping (3D-PCM), and phantom test to investigate the transient dynamics of vapor bubbles induced by a holmium:yttrium aluminum garnet laser and their correlation with solid damage. We vary the standoff distance ( SD) between the fiber tip and solid boundary under parallel fiber alignment and observe several distinctive features in bubble dynamics. First, long pulsed laser irradiation and solid boundary interaction create an elongated “pear-shaped” bubble that collapses asymmetrically and forms multiple jets in sequence. Second, unlike nanosecond laser-induced cavitation bubbles, jet impact on solid boundary generates negligible pressure transients and causes no direct damage. A non-circular toroidal bubble forms, particularly following the primary and secondary bubble collapses at SD = 1.0 and 3.0 mm, respectively. We observe three intensified bubble collapses with strong shock wave emissions: the intensified bubble collapse by shock wave, the ensuing reflected shock wave from the solid boundary, and self-intensified collapse of an inverted “triangle-shaped” or “horseshoe-shaped” bubble. Third, high-speed shadowgraph imaging and 3D-PCM confirm that the shock origins from the distinctive bubble collapse form either two discrete spots or a “smiling-face” shape. The spatial collapse pattern is consistent with the similar BegoStone surface damage, suggesting that the shockwave emissions during the intensified asymmetric collapse of the pear-shaped bubble are decisive for the solid damage. 

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5. Ultrafast time-resolved 2D imaging of laser-driven fast electron transport in solid density matter using an x-ray free electron laser

   https://doi.org/10.1063/5.0130953  

   Sawada, H.; Yabuuchi, T.; Higashi, N.; Iwasaki, T.; Kawasaki, K.; Maeda, Y.; Izumi, T.; Nakagawa, Y.; Shigemori, K.; Sakawa, Y.; et al (March 2023, Review of Scientific Instruments)

   High-power, short-pulse laser-driven fast electrons can rapidly heat and ionize a high-density target before it hydrodynamically expands. The transport of such electrons within a solid target has been studied using two-dimensional (2D) imaging of electron-induced Kα radiation. However, it is currently limited to no or picosecond scale temporal resolutions. Here, we demonstrate femtosecond time-resolved 2D imaging of fast electron transport in a solid copper foil using the SACLA x-ray free electron laser (XFEL). An unfocused collimated x-ray beam produced transmission images with sub-micron and ∼10 fs resolutions. The XFEL beam, tuned to its photon energy slightly above the Cu K-edge, enabled 2D imaging of transmission changes induced by electron isochoric heating. Time-resolved measurements obtained by varying the time delay between the x-ray probe and the optical laser show that the signature of the electron-heated region expands at ∼25% of the speed of light in a picosecond duration. Time-integrated Cu Kα images support the electron energy and propagation distance observed with the transmission imaging. The x-ray near-edge transmission imaging with a tunable XFEL beam could be broadly applicable for imaging isochorically heated targets by laser-driven relativistic electrons, energetic protons, or an intense x-ray beam. 

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