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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. 

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. 

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.