Studying the Effects of a Backward Facing Step in Enhancing Fuel Mixing in Scramjets

This project aims to gauge the feasibility of using a backward facing step to induce mixture of a solid fuel grain in a scramjet. Solid fuel scramjets are a scarcely employed method of propulsion for hypersonic vehicles; the majority of work performed with scramjets emphasizes those which use liquid fuel mixtures to induce combustion. Solid fuel scramjets, however, can prove immensely useful in the hypersonic regime because of their high specific impulse, low complexity and, therefore, cost, and their high energy density. Backwards facing steps are a common method used in liquid fuel scramjets to induce better mixing of fuel and air within a scramjet (Seleznev 2019). The rapid expansion of the flow after the step creates a very strong recirculation zone, which contributes to a larger streamwise vortex structure (Seleznev 2019). This structure induces mixing of fuel and air and contributes greatly to the combustion stability at high speeds. The combustion stability of the engine is improved through the introduction of a flameholder region. This flameholder region utilizes the relatively lower speeds and higher temperatures within the recirculation zone created by the step to create a stable zone of combustion (Ben-Yakar 1998). Because of the importance of the flameholder region, it is important to understand the flow physics within so that statements can be made about the feasibility of the application of solid fuel scramjets to hypersonic flight. To this end, studies will be performed both experimentally and computationally to characterize the flow field within a scramjet combustion chamber with an inlet flow of Mach 2. Computational fluid dynamics (CFD) for the test setup will be performed with the goal of comparing the overall flow structure across turbulence models. CFD will be conducted using a multiphase solver to gain both qualitative and quantitative data on the effects of the backward facing step with and without the propellant mixture present. From this CFD, the effects of the recirculation zone on the volume fraction of propellant in the core flow will be measured. Experiments will be conducted in a supersonic blowdown wind tunnel with a planar nozzle designed to produce test conditions at incoming Mach number of 2. It is to be noted that the facility is currently a cold flow facility, which will limit the amount of realistic data that can be gathered from the experiments conducted. Z-type Schlieren Imaging will be performed to capture the flow-field with a focus on defining the scale and extent of the recirculation zone. The Schlieren datasets will be analyzed using data-driven methodologies such as Proper Orthogonal Decomposition (POD) to identify and isolate the recirculation zone. POD is a common tool used in fluid dynamics which finds the right bases of a set of data to extract energetic large-scale flow structures which are expected to be representative of the recirculation region. Overall, this work plans to utilize a cold, Mach 2 flow to capture the flow field within a solid fuel scramjet combustion chamber. Such work has scarcely been performed with only general comments on the flow field being made. By analyzing the recirculation zone, specifically, the formation of the shear layer and the reattachment following the recirculation zone, the structures which allow for the flameholding region to exist can be revealed. Literature has stated that the point where the recirculation zone reattaches is a point of very high fuel grain regression in ramjets (Senior 2021). The flameholding region is highly important to the function of the solid fuel scramjet since it greatly enhances the stability of the combustion processes. Without this region, the flame would extinguish, and the engine would not function. Results from this work will provide a better understanding of the flow field within a solid fuel scramjet and contribute to the scarce amount of literature on the topic. References 1 Seleznev, R. K., Surzhikov, S. T., and Shang, J. S., “A review of the Scramjet Experimental Data Base,” Progress in Aerospace Sciences, vol. 106, Apr. 2019, pp. 43–70. 2 Ben-Yakar, A., Natan, B., and Gany, A., “Investigation of a solid fuel scramjet combustor,” Journal of Propulsion and Power, vol. 14, Jul. 1998, pp. 447–455. 3 Senior, W., Evans, J., Gejji, R., Slabaugh, C., “Development and Investigation of an Optically Accessible Solid Fuel Ramjet Combustor”, AIAA SciTech 2021.