Vortex Tracking Algorithm for Hypersonic Flow in Scramjets

Hypersonic airbreathing propulsion for access to space has several advantages over conventional rockets. However, to achieve the theoretical propulsion efficiencies, many technological challenges, such as fast and efficient mixing of the fuel with air, still need to be resolved. Several scramjet inlet geometries naturally generate vortices that could be used for mixing enhancement. Thorough characterization of these vortices is required for studying their effect on mixing and to determine the optimum way to utilize these naturally occurring vortical flow structures. For this purpose, an algorithm that tracks and extracts data along a vortex from a computational fluid dynamics (CFD) flow field solution was implemented. It performs a search in slices of the computational solution, based on Galilean-invariant criteria for vortices. The algorithm has been used to analyse the vortices generated by compression corner and within a real scramjet inlet. Results are presented showing the insight in vortex formation and evolution provided by the algorithm. Introduction By using the oxygen in the atmosphere, hypersonic airbreathing engines such as scramjets are not required to carry both fuel and oxidizer. This presents an increase in specific impulse and payload mass fraction compared to rocket based launchers [1] [2]. However, the high supersonic flow velocities inside the engine and thus low residence times (< 1ms) pose several challenges. One of them is fast and efficient fuel mixing, a pre-requisite to fast and efficient combustion. In order to have short combustors, required to keep skin friction losses and heat transfer at a minimum, fast mixing and burning is essential [3]. Therefore, mixing enhancements for hypersonic engines are an active topic of research. Currently, different injection strategies such as hypermixers or utilisation of inlet generated vortices are studied for this purpose. In the later case, the interaction between the vortices generated by scramjet inlet and the porthole fuel jets, situated towards the end of the scramjet inlet, are expected to significantly improve mixing efficiency. Currently, investigation on this process is ongoing at University of Queensland (UQ). One or more of the following mechanisms of mixing enhancement as a result of the vortex-fueljet interaction are expected by the authors: • Convection of the fuel away from the injector location by the rotating flow of the stream-wise vortex. • Increased penetration of the fuel due to the reduced pressure in the vortex core. • Bursting of the vortex, provoked by the interaction between stream-wise vortex fuel plume bow-shock. Such bursting is documented to produce a high turbulence region [10] and this increases turbulent mixing. To study the mixing enhancement, the characterisation of the stream-wise vortical structure that interacts with the fuel jet is essential. Specifically the size and intensity of the vortex, and the pressure and velocity fields at its core are of interest. Mach P0 T0 Enthalpy Simple 10 89.5MPa 5140.5K 5.33MJ kg−1 REST 12 75.0MPa 6487.5K 6.5MJ kg−1 Table 1: Flow boundary conditions Figure 1: Flat plate and fin vortex generation. This paper reports the development of an algorithm developed to analyse stream-wise vortex structures. In addition the results obtained from vortex analysis performed for a simplified geometry which generates an inlet-like vortex, and for the Rectangular-to-Elliptical Shape Transition (REST) inlet developed by Smart [8] are presented. Test geometries The development of the vortex characterisation algorithm is explained based on two test geometries which are presented next. First, a simple geometry for testing the algorithm in a relatively simple flow-field. Second, the REST inlet geometry to show the potential of the code to track and extract valuable information from realistic complex flow-fields. Flow boundary conditions used for the two geometries are given in table 1. Simplified test geometry The simplified geometry is a flat plate with a sharp fin at a 15o compression angle. The viscous-shock interaction between the flat plate boundary layer and the fin shock-wave generates a corner vortex (Figure 1). The flow-field present a conical flow-field a certain distance downstream of the fin leading edge, named “inception zone” in [11]. This simple geometry generates a vortex in a similar fashion and with a similar velocity field in the vortex core as those generated by real scramjet inlets. REST inlet The REST inlet, developed by Smart [8] is currently under development at UQ. Here the rectangular capture area, that transitions to an elliptical cross-section (Figure 2), allows efficient airframe integration of the engine while maintaining an efficient elliptical combustor cross-section. For the current investigation, RANS CFD results generated by Barth et al. [9] are used.