Numerical Simulations for Studies of Unsteady Combustion in Practical Combustion Devices
Sammanfattning: A major problem in the design of combustion equipment is to prevent the occurrence of combustion instabilities. Combustion instabilities appear as periodic oscillations in pressure and heat release through a feedback mechanism between the fluid dynamics and the combustion process. These oscillations may induce e.g. mechanical vibrations and increased heat transfer, leading to poor performance or even total loss of the system. The characteristic features of the unsteady combustion process observed for combustion instabilities can also be utilized advantageously in certain applications, e.g. pulse combustors. In pulsating combustion, the oscillations are utilized to enhance the efficiency and reduce emission of pollutants. The interaction between fluid dynamics and the chemical reactions play an important role in unsteady combustion processes. The mechanisms in the feedback process in terms of injection, mixing and ignition are of particular importance. The objective of this thesis is to utilize numerical simulations for studies of unsteady combustion. A physical or theoretical model for chemically reacting mixtures, based upon the theory of mixtures within continuum mechanics, is presented. A reduced physical model consisting of equations for conservation of mass for the mixture, balance of mass for the constituents and conservation of momentum and energy for the mixture is derived. With suitable constitutive relations a closed system of non-linear partial differential equations is achieved. A simulation model is derived from the reduced physical model. For numerical treatment of turbulent flow, the conservation and balance equations are spatially filtered according to the Large Eddy Simulation (LES) technique. The interaction from the small-scale fluctuations on the large-scale flow field is modelled by Sub Grid Scale models. The equations are discretized according to the Finite Volume Method. The model presented has been used for simulation of test rig assimilating the jet engine after burner, featuring unsteady flow in terms of a fluctuating vortex street. Additionally, the unsteady combustion process found in pulse combustors of Helmholtz type has been simulated. Results from the simulations have been validated against a number of experimental data, such as time resolved velocity fields, chemiluminescence from OH and CH, OH-LIF, as well as pressure and flue gas composition. The simulation model is found able to mimic most of the characteristic features of the unsteady combustion processes. Numerical simulations can serve as a useful tool for studies of unsteady combustion.
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