3D measurements of ignition processes at 20 kHz in a supersonic combustor

Reliable ignition in high-speed flows represents a significant scientific problem with a wide range of practical applications. Scientifically, the ignition processes involve complicated interactions among various aspects of chemical reaction and turbulence, which are not fully understood yet [1]. Reliable ignition in propulsion and power devices is further complicated by practical factors such as the relatively long chemical timescales of practical hydrocarbon fuels and the nonideal geometry of practical devices [1, 2]. As a result, a significant amount of research has been invested for a better understanding of the ignition processes at a fundamental level and also for the design of practical devices [1–3]. This work reports an experimental study of the ignition processes in supersonic (Mach 2) flows. Nonintrusive techniques are usually desired or required for experimental study in supersonic flows, and a range of optical diagnostics has been employed in past efforts ranging from chemiluminescence [4] and schlieren [3] imaging to planar laser-induced fluorescence (PLIF) [5, 6] and particle image velocimetry (PIV) [2]. Results from these past efforts all reveal highly transient and 3D flow and flame structures during the ignition processes, but the diagnostics employed are not capable of fully resolving transient, 3D events. Therefore, there is a need for measurements that can resolve the ignition processes with the required temporal resolution, spatial resolution, and also dimensionality. Based on the above understanding, this work reports 3D measurements of the ignition processes in a supersonic combustor at 20 kHz. The measurements were made in the Research Cell 19 supersonic wind tunnel facility housed at the Air Force Research Laboratory (AFRL) [7]. The measurements were obtained using a combination of tomographic chemiluminescence (TC) and fiber-based Abstract The ignition dynamics in a Mach 2 combustor were investigated using a three-dimensional (3D) diagnostic with 20 kHz temporal resolution. The diagnostic was based on a combination of tomographic chemiluminescence and fiber-based endoscopes (FBEs). Customized FBEs were employed to capture line-of-sight integrated chemiluminescence images (termed projections) of the combustor from eight different orientations simultaneously at 20 kHz. The measured projections were then used in a tomographic algorithm to obtain 3D reconstruction of the sparks, ignition kernel, and stable flame. Processing the reconstructions frame by frame resulted in 4D measurements. Key properties were then extracted to quantify the ignition processes, including 3D volume, surface area, sphericity, and velocity of the ignition kernel. The data collected in this work revealed detailed spatiotemporal dynamics of the ignition kernel, which are not obtainable with planar diagnostics, such as its growth, movement, and development into “stable” combustion. This work also illustrates the potential for obtaining quantitative 3D measurements using tomographic techniques and the practical utility of FBEs.