Modeling and Analysis of Different Aspects of Finite-rate Chemical Kinetics in Hypersonic Combustors

Hypersonic air-breathing propulsion systems offer more efficient access to space compared to traditional rocket based propulsion systems due to their higher specific impulse and reusability. However, actual implementation of hypersonic air-breathing engine is hampered by a number of technical challenges. First, a vehicle operating at high speeds (Mach 5 and above) will experience extremely high temperatures especially at the combustor walls. One proposed concept is to use the on board fuel itself for active cooling of the hot surfaces which results in significant reduction of the weight of the vehicle by eliminating the need for carrying cooling fluids and heat exchanger.

When hydrocarbon fuels are exposed to high temperatures in cooling channels, they not only absorb heat physically (sensible heat), but also crack to smaller hydrocarbons and absorb chemical heat. This process is called endothermic pyrolysis which has two main benefits. First, it increases the cooling capacity of the fluid beyond its sensible heat by nearly a factor of two. Second, the cracked fuel components produced from the pyrolysis reduce the induction time of the fuel-air mixture in the combustor, which is an important parameter in hypersonic flights with short flow residence times.

In the present work, endothermic pyrolysis of different jet fuels occurring in such cooling channels is explored under experimental conditions performed in the UVa high-pressure flow reactor. The analysis presented includes modeling of finite rate chemical kinetics and multidimensional transport effects in representative cooling channels. The present results show significant differences exist between the currently available finite-rate chemical kinetic models. The sensitivity analysis, reaction path flux analysis, and uncertainty quantification of the chemical kinetic models in this work, provide guide lines for improvement and optimization of kinetic model parameters.

The second major challenge is maintaining combustion at hypersonic flow conditions where the available time for the fuel injection, mixing, ignition, and complete combustion is less than a millisecond.
University of Virginia Supersonic Combustion Facility (UVaSCF) is currently exploring parametric conditions using a cavity flame holder to address certain aspects of these challenges. The complexity of the turbulent reacting flow in the combustor makes parametric study with full 3D CFD modeling computationally expensive. In this work a multiple reactor network including partially stirred reactor, perfectly stirred reactor, and closed homogeneous reactor model is developed to address the key effects of both flow residence time and turbulence mixing time scales on the flame holding and stabilization limits.

The third challenge that is addressed in this work, is the thermal choking in dual-mode hypersonic combustor. A novel computational model is developed in this work to predict the thermal choking in a variable area combustor. This tool provides a computationally efficient model to predict the operability limits of hypersonic combustors at different flight conditions without the need for time consuming multi-dimensional reacting flow simulations.

As part of the present work, reduced chemical kinetic models are also developed for reacting flow simulations in hypersonic applications. A reduced kinetic model for ethylene combustion and a reduced kinetic model for n-dodecane pyrolysis were extracted using Principal component analysis. In addition, a one-step lumped reaction model is developed for the pyrolysis of JP-8 aviation fuel to major smaller hydrocarbon species. The accuracy of these reduced order models were tested by comparing their predictions with experimental data and detailed chemistry mechanisms.

The fundamental analysis performed to understand various rate controlling reactions contributing to fuel pyrolysis in cooling channels and ignition phenomena in turbulent flame holding regimes are expected to advance design of future hypersonic vehicles.