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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. 

Schlieren imaging is widely adopted in applications where fluid dynamics features are of interest. However, traditional Z-type schlieren systems utilizing on-axis mirrors generally require large system footprints due to the need to use high f-number mirrors. In this context, off-axis parabolic (OAP) mirrors provide an attractive alternative for permitting the use of smaller f-number optics, but well-documented methodologies for designing schlieren systems with OAP mirrors are lacking. The present work outlines a ray-tracing-based workflow applied to the design of a modified Z-type schlieren system utilizing OAP mirrors. The ray-tracing analysis evaluates the defocus and distortion introduced by schlieren optics. The results are used along with system size and illumination efficiency considerations to inform the selection of optimal optical components capable of producing high-quality schlieren images while minimizing the system footprint. As a step-by-step demonstration of the design methodology, an example schlieren system design is presented. The example schlieren design achieved an image resolution of 1.1 lp/mm at 50% contrast, with a 60% reduction in system length compared to traditional Z-type systems with f/8 mirrors; distortion characterizations of the designed schlieren system showed good agreement with ray-tracing predictions, and the distortion can be corrected through image post-processing. The current work provides a systematic approach for the design of compact schlieren systems with OAP mirrors and demonstrates the utility of this underutilized option. 

Observation of high-speed reactive flows using laser-absorption-based imaging techniques is of interest for its potential to quantitatively reveal both gas-dynamic and thermochemical processes. In the current study, an ultraviolet (UV) laser-absorption imaging method based on nitric oxide (NO) is demonstrated to capture transient flows in a shock tube. A tunable laser was used to generate a continuous-wave UV beam at 226.1019 nm to coincide with a strong NO absorption feature. The UV beam was expanded to a 20-mm diameter and routed through the shock tube to image the flow adjacent to the end wall. Time-resolved imaging was realized using a Lambert HiCATT high-speed UV intensifier coupled to a Phantom v2012 high-speed camera. Static absorbance measurements of 1.97% NO/Ar mixtures were first performed to validate the proposed imaging concept, showing good agreement with values predicted by a spectroscopic model. UV laser-absorption images of incident and reflected shock waves captured at 90 kHz temporal resolution are then reported. Translational temperature profiles across the incident and reflected shocks calculated from absorbance images show reasonable agreement with calculated values. After the passage of the reflected shock wave, the flow near the end wall was monitored to probe the development of the end-wall thermal boundary layer. Thermometry measurements across the thermal boundary layer show good agreement with analytical solutions. This study demonstrates the potential of UV laser-absorption imaging in high-speed flow fields, to be applied to more complex applications in the future. 

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. 

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.