Mixing characteristics of cracked hydrocarbon fuels in scramjets
Creators: name: RAVINDRAN, Magesh
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400106
subject: Hypersonic propulsion and hypersonic aerothermodynamics
classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)

description: Supersonic Combustion Ramjets (Scramjets) encounter severe structural thermal loads at high Mach number flights. To date, hydrogen-fuelled scramjets have been favoured due to their superior combustion performance and structural cooling (heat-sink) capacity, as compared to hydrocarbon-fuelled scramjets which exhibit inferior combustion and cooling performance. Although hydrogen-fuelled scramjets are the ideal choice for low-cost access-to-space missions, they are not effective considering long-range intercontinental cruise missions. This is because hydrogen has very low volumetric energy density as compared to hydrocarbon fuels, which makes the hydrogen-fuelled scramjets larger in size as they must carry a greater volume of fuel onboard than a hydrocarbon equivalent. Conversely, hydrocarbon-fuelled scramjets have significantly smaller overall vehicle design as the vehicle integration becomes easier and are suitable for long-range intercontinental flights. 

Hydrocarbon fuels are endothermic at high-temperatures, and their specific heat-sink capacity could be increased through a process called cracking (catalytic or thermal). Cracking breaks the long-chain heavy hydrocarbon fuel into smaller chains, and the resulting shorter chain molecules may enhance the fuel diffusivity and shorten ignition delay time leading to enhanced mixing processes which are critical for hydrocarbon-fuelled scramjet flights. In effect, cracking reduces the heat loads and increases the maximum Mach number of a scramjet. This motivates the need for research on energy-dense hydrocarbon fuels while considering the effects of cracking. Endothermic heavy hydrocarbon fuels such as JP-10 have high energy density and generally used for regenerative cooling technology in advanced supersonic airplanes. This thesis aims to investigate the mixing performance of uncracked and cracked hydrocarbon fuels in scramjets. Additional parametric studies conducted include the effects of injection and wall conditions, constant fuel mass flux and shock-boundary layer interactions (SBLI¿s) on the overall mixing characteristics. 

CFD simulations are conducted with steady state compressible Reynolds-averaged Navier-Stokes equations using a scramjet combustor geometry with conventional wall porthole injection and an axisymmetric circular scramjet geometry with inlet porthole injection. The effects of both catalytic and thermal cracking of hydrocarbon fuel on mixing are investigated in this thesis. The catalytic cracked fuel compositions are adapted from literature, whereas the thermal cracked fuel compositions are obtained from modelling a reactor using Cantera (Chemical Kinetics software). The fuel compositions are modelled for various degrees of cracking (0% - 100%). The simulations are conducted with Reynolds-Averaged Navier-Stokes (RANS), Shear-Stress Transport (SST) turbulence model. 

The results show that the mixing efficiency increases with cracking in all the investigated cases. Maintaining constant injectant mass flux shows opposite mixing trends. The best injection condition for achieving higher mixing is found at high temperature and low pressure. The mixing efficiency increases significantly while injecting fuel in the scramjet inlet as compared to the conventional wall porthole injection. Moreover, the reacting simulations are not conducted in this thesis because of the lack of reaction model for cracked JP-10. The reaction models are only available for uncracked JP-10. Although the results presented in this thesis are for specific scramjet geometries, but the fundamental knowledge and insights gained on the mixing characteristics of uncracked and cracked hydrocarbon fuels applies to any hydrocarbon-fuelled scramjet design.