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Large Eddy Simulation of Supercritical Swirling Flow of Injector in a Combustion Chamber

| 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 55624 (45)
  4. University: Sharif University of Technology
  5. Department: Aerospace Engineering
  6. Advisor(s): Farshchi, Mohammad
  7. Abstract:
  8. In the present thesis, the extensive study of reactive and non-reactive supercritical and transcritical flows has been discussed in shear-axial and swirling forms. Considering the need for combustion models with the application of detailed chemistry for accurate simulation of the field, the Laminar Flamelet Models (LFM) are suitable in order to reduce computational expensive. For this purpose, it is necessary to produce thermodynamic tables specific to real gas conditions for the flamelet model. The tables related to the flamelet model in real gas conditions are extracted from the open source software Cantra. The difference point in the Flamelet tables of real gas and ideal gas is for near-extinction and near-equilibrium conditions. In near-equilibrium conditions, the results obtained from real gas and ideal gas conditions are not seen, but in close to extinction point, the results of the extinction state and the equilibrium state are different from each other, which is important in simulating the ignition and extinction conditions of supercritical flames. . In the following, the flow dynamics of transcritical axial shear flames of methane-liquid oxygen and hydrogen gaseous/liquid oxygen have been investigated by using flamelet tables of real gas. In order to validate, the results of numerical simulation have been compared with experimental results. The standard k-ε model has been used to simulate turbulence. In order to interact combustion and turbulence, the flamelet model has been used. By examining the behavior of axial shear transcritical flame, it can be seen that the major effect on the flame shape is caused by changes in the mass flux ratio of fuel and oxidizer. By increasing the mass flux ratio of the fuel to oxidizer the turbulent viscosity increases too and as a result of the heat transfer to the oxygen dense core, the pseudo-boiling phenomenon occurs at a higher rate. Regarding to this condition the flame length decreases. The phenomenon of pseudo-boiling is one of the important cases in trans-critical flows. In the present simulation, the flame spread, the creation of eddy flow, changes in the density of the central oxygen core and changes in the specific heat capacity of the trans-critical region are well predicted. By examining different turbulence models, the role of pseudo-boiling phenomenon in the behavior of transcritical flames is well revealed. Considering that different turbulence models predict different mixing rates in the shear layer, the amount of energy transfer at the boundary of the dense layer and the surrounding environment is different. In sub-critical and super-critical flames, due to the absence of the pseudo-boiling phenomenon, this difference in the amount of energy transfer between layers does not have a significant effect on the behavior of the flow and the shape of the flow. However, in transcritical flames, due to the fact that with the smallest difference in energy transfer, the pseudo-boiling process will take place with a different rate and strength, the role of turbulence models is very key, and choosing the right model is far more important than Sub and Supercritical conditions.. Supercritical turbulent flows also have their own unique characteristics. The structure of instability in supercritical swirling flows is more based on PVC dynamics, however, due to the very high computational expensive (the density is 100 times higher than that of subcritical flows), the three-dimensional solution is associated with many problems. However, due to the importance of identifying the structures related to the flow instability, in this study, it has been tried to put the three-dimensional solution of the supercritical twisting flow on the agenda. It can be seen that the structure of PVC is visible with a certain frequency in the flow field. Also, in predicting the central core of the injector, the behavior of RANS, DES and LES models is completely different. In the case of a bi-propellant injector, it can be seen that the structure of the injector geometry has a significant effect on the amount of fuel and oxidizer mixing.
  9. Keywords:
  10. Supercritical Combustion ; Computational Fluid Dynamics (CFD) ; Cryogenic Fluids ; Swirl Injector ; Combustion Chamber ; Transcritical Injection