Pressure Gain Combustion: Fuel Spray and Shockwave Interaction

Pressure gain combustion can attain higher thermodynamic cycle efficiency in gas turbine power systems, resulting in the reduction of specific fuel consumption/fuel burn and Carbon dioxide emissions.There are many ways to achieve pressure gain and the present research investigates pressure gain through shock bubble (gas and liquid bubble) interaction (SBI) using computational fluid dynamics (CFD) simulations. The numerical simulations have been performed in 2D and 3D representations of the shock tube to depict the interaction of a planar shock wave with distinct gas and liquid inhomogeneities. The three scenarios considered cover the interaction of a planar shock wave in air with: spherical helium bubble (Mach number, Ma = 1.25); cylindrical helium bubble (Ma = 1.22) and cylindrical water bubble (Ma = 1.47). To perform these simulations, the Unsteady Reynolds-Averaged Navier-Stokes (URANS) mathematical model and the coupled level set and VOF method within the commercial CFD code, ANSYS FLUENT, have been applied. A finite volume method (FVM) is also employed to solve the governing equations. For the spherical and cylindrical gas bubble cases, various quantitative analyses are presented and compared to the experimental work of Haas and Sturtevant (1987). These include: refracted wave, transmitted wave, upstream interface, downstream interface, jet, vortex filament, non-dimensional bubble, and vortex velocities. The predicted non-dimensional
bubble and vortex velocities have also been compared with experimental data, a simple model of shock- induced Rayleigh Taylor (RT) instability and other theoretical models. Comparisons are also shown between the predicted bubble length/width and the experimentally measured results to elucidate changes in the shape and size of the 2D and 3D bubbles. Additional quantitative analyses are also presented for the spherical bubble involving the size estimation of the vortex pair as well as their spacing. For the shock cylindrical water bubble interaction case, the quantitative predictions include: displacement/drift, acceleration, distortion in the lateral direction, distortion in flow direction, area variation from bubble distortion, as well as drag coefficient and are compared to the experimental measurements of Igra et al. (2002). It has been demonstrated that 3D simulations compare very well with the experimental data, suggesting that 3D simulations are necessary to capture SBI process accurately. Finally, comprehensive flow visualization has been used to elucidate the shock-bubble interaction (SBI) process from bubble compression to the formation of the vortex filaments (cylindrical helium bubble), vortex rings (spherical helium bubble), vortices (cylindrical water bubble) as well as the production and distribution of vorticity. It is demonstrated for the first time that turbulence is generated at the early phase of the SBI process, with the maximum turbulence intensity reaching about 20%
around the vortex filaments/vortex rings regions for the cylindrical/spherical helium bubble cases respectively and about 22% for the cylindrical water bubble case at the later phase of the interaction process.