Numerical investigation of upstream cavity flows in scramjets

Wall-mounted cavities are commonly used in scramjet combustors to provide both flameholding and mixing enhancement. The characteristically short residence time of air inside a scramjet combustor necessitates the use of flow devices to enhance both mixing and combustion to produce positive thrust and cavities can provide both, at a relatively low drag penalty relative to other devices. At high supersonic combustor Mach numbers the acoustics-based mechanism driving cavity mixing enhancement breaks down however, limiting their operational scope. This work numerically examines a novel cavity arrangement where a cavity is placed directly upstream of the fuel injection point in a high supersonic Mach number combustor flow. Cavities are commonly placed downstream of the fuel injection point to ensure a supply of fuel to the cavity for flameholding purposes, however this limits interaction between the cavity and the fuel jet interaction, the latter of which drives mixing performance. By facilitating interaction between the cavity and the jet interaction mixing enhanced could be achieved through an alteration of the combustor flow physics, a mechanism that is robust across a wide range of Mach numbers. Two types of upstream cavity are studied in this work: spanwise infinite rectangular cavities and novel finite width crescent-shaped cavities. The first type of cavity is used to provide a proof of concept of the upstream cavity geometry and to gain detailed understanding of the upstream cavity flowfield and the mechanisms that drive its performance, while the second type of cavity attempts to optimise upstream cavity performance using the findings of the rectangular cavity study by enhancing streamwise vorticity in the flowfield, a proven mixing enhancement technique. For both cavity designs both mixing (in chemically frozen flow) and combustion (in chemically reacting flow) performance is compared to a flat-plate baseline case, something that is rarely done, and the cavity flowfield is studied in detail. In the rectangular cavity study the performance of eight cavity geometries with cavity aspect ratios L/D ranging from 2.5-30 and using three different wall treatments is studied, to determine how cavity geometry and wall treatment affect performance. The cavity with L/D=15 is found to perform best, enhancing mixing by up to 9% and combustion by up to 8.7% and heat release by up to 10.2% while increasing total pressure loss by up to 2%. All cavities were found to perform better at higher wall temperatures, especially in terms of their mixing performance. The best performing cavities were also observed to add heat to the flow more efficiently (i.e. higher heat addition per unit total pressure loss). Examination of the flowfield reveals that in the upstream cavity geometry the cavity recirculation region rises out of the cavity, shielding the fuel jet from the freestream and enhancing streamwise vorticity. In chemically frozen flow the recirculation was clearly largest and tallest for the L/D=15 cavity, creating a geometric optimum in terms of mixing performance, while in chemically frozen flow the size of the cavity recirculation was more similar for cavities with the same length and as a result there was no geometric optimum in combustion performance. Some fuel is also drawn upstream into the cavity through the interaction between the cavity recirculation region and the fuel jet and because of this the rectangular cavities are observed to maintain the flameholding and ignition enhancement ability of conventional cavities, demonstrating that this attractive feature of wall-mounted cavities is not lost by placing the cavity upstream of the fuel injection point. The cavity flowfield is also observed to be steady in nature because the feedback mechanism driving self-sustained oscillations in conventional cavity flow is interrupted. The findings from the rectangular cavity study are then used to improve the upstream cavity design and a range of novel, crescent shaped cavity geometries is proposed, a subset of which employs a hybrid fuelling strategy where fuel is injected into both the freestream and the cavity. The crescent cavities are found to enhance mixing by up to 22% without hybrid fuelling and 90% with hybrid fuelling, with no or a limited increase in total pressure loss and lower viscous drag. The crescent cavity flowfield is studied in detail and it is found that the cavity geometry forces the cavity recirculation vortex to wrap around the injector, as the design intended, significantly enhancing streamwise vorticity and increasing fuel-air contact area, enhancing mixing. The enhanced vorticity does draw more fuel to the wall in the cavity cases than in the baseline however so jet penetration is observed to be lower in the crescent cavity cases. The performance enhancement offered by the crescent cavity is also found to be sustained in chemically reacting flow, where combustion and heat release are enhanced by up to 90% and 143%, respectively, when hybrid fuelling is used and up to 47.4% and 54.7%, respectively, without hybrid fuelling. Ignition is also found to occur faster in the cavity cases and flow losses and integrated heat flux are lower than in the baseline case, mitigating the primary drawbacks of using cavities in scramjet combustors. The vortex reorientation process observed in chemically frozen flow is also still present in chemically reacting flow. The crescent cavity flowfield is also found to be oscillatory in nature in both chemically frozen and chemically reacting flow and they are observed to be driven by the curvature of the cavity walls and the secondary fuel jet, if present. Unlike conventional cavity flow the oscillations are lateral in nature and the harmonic frequencies are not predicted accurately by traditional analytical relations. While this work is one of the first efforts to investigate upstream cavities and much is still unknown about the upstream cavity flowfield, the results of this work demonstrate that upstream cavities can effectively enhance combustor mixing and combustion performance at high supersonic Mach numbers while incurring minimal flow losses. The flameholding and ignition enhancement ability of conventional cavities is also maintained, which is especially relevant if fuels with longer ignition delay times are used. The crescent cavity designs employing hybrid fuelling are especially promising, providing the greatest performance increase at little to no drag penalty.