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End-Wall Effects on Freely Propagating Flames in a Shock Tube

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 ex- isted 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 tempera- ture 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 con- finement 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. Experi- ments 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 demon- strating 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 valu- able 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. C

Susa Adam J.
Zheng Lingzhi
Nygaard Zach D.
Hanson Ronald K.
AIAA Scitech 2022 Forum
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