Flamelet/Progress Variable Modeling for a Dual-Mode Scramjet Combustor

Hypersonic air-breathing propulsion technologies hold great promise for revolutionizing America’s means of accessing space. However, the successful design of hypersonic propulsion systems is hindered by simulation and modeling capabilities. These limitations are most problematic for dual-mode scramjet engines, for which a single engine flowpath may support both dual-mode (i.e. primarily subsonic combustion) and scram-mode (i.e. primarily supersonic combustion) operation. For these engines, the isolator and combustor flowfields may contain a complex shock train, shock-boundary-layer interactions, and large regions of separated flow. Combustion processes are tightly-coupled to the fluid mechanics and are often mixing-limited. It is especially important that designers understand the effect of these phenomena on the flow and combustion physics in order to guarantee vehicle operability. Experimental investigations of these combustors are constrained by technology and affordability limitations, and as a result, computational studies must serve an integral role in the engine design and analysis process, at both the conceptual level and the detailed, flight-experiment level. The current dissertation addresses the problem of accurately and affordably simulating the combustion physics for a dual-mode scramjet combustor of engineering-complexity.

To significantly reduce the computational costs, the state-of-the-practice in combustion modeling for practical devices is to reduce the detailed chemical kinetics to a single step mechanism, such as through the use of a global reaction or the eddy dissipation concept. While these approaches require low computational overhead, these models could be at most relied upon to produce only qualitative representations of scramjet flowfields. Alternatively, state-of-the-art approaches typically involve closing the governing transport equations using a reduced chemical kinetics reaction mechanism requiring the transport of tens of chemical species for which the reaction kinetics are derived from a detailed reaction mechanism. While more accurate, the computational cost associated with these direct approaches increases substantially with both the number of transported species and the reaction mechanism complexity and associated numerical stiffness. One approach for capturing detailed kinetics effects while also maintaining low numerical stiffness is through the use of a flamelet/progress variable (FPV) model. FPV models currently available in the literature, however, are typically only valid in the low Mach number limit, and recent attempts at extending these models to compressible flows fail to include adequate corrections for compressibility and flamelet boundary condition variability.

The current dissertation used an experimental dual-mode scramjet combustor, referred to as the Hypersonic International Flight Research and Experimentation (HIFiRE) Direct Connect Rig (HDCR), as a testbed for a priori analysis and a posteriori testing of several compressible FPV model formulations. First, Favre-averaged RANS simulations of the HDCR for test points characterizing both dual- and scram-mode operation were performed using the Viscous Upwind aLgorithm for Complex flow ANalysis and Computational Fluid Dynamics (VULCAN-CFD) with a 22-species finite-rate kinetics reaction mechanism for the simulation of a JP-7 fuel surrogate. These baseline RANS solutions were validated against available experimental data and were then used for a priori analysis of the HDCR combustion dynamics and subsequent investigation into the applicability of FPV models. Based on this analysis, a new compressible FPV model was proposed, hereafter referred to as the CFPVX model, which utilized a four-dimensional flamelet manifold incorporating compressibility effects on composition via parameterization on static pressure and effects of flame reactants variability via parameterization on static enthalpy. The CFPVX model, a standard incompressible FPV (IFPV) model, and most recent compressible FPV (CFPV) models were then evaluated using the baseline RANS data with an a priori flamelet-modeled-RANS (APFM-RANS) analysis method. These FPV models were subsequently implemented in the VULCAN-CFD solver, and a posteriori testing supported the findings of the a priori analysis.