A Study On Flamelet Model In Combustion Simulation

Now and in a foreseeable future, combustion of fossil fuel seems to be the main source for human to obtain energy. With the rapid development of electronic technology, numerical simulation has become one of the most important approaches in combustion science research and engineering practice. In reality, combustion takes place under complex three-dimensional circumstances. The description of this phenomenon requires the combination of mass conservation equation, momentum equation, energy equation, species equation and state equation. It is the coupling of stiff chemistry with large reaction mechanism and multi-scale flow that makes the solving process of equations abovementioned very difficult and time-consuming.Howerver, the so-called flamelet model, by simplifying real flames to one-dimensional structure and at the same time virtually decoupling chemistry calculation from flow, is able to successfully predict many kinds of combustion phenomenon with decent computing cost, which makes itself rather promising to be applied in engineering practice. So, the exactness, limitations as well as the potential improvements of the flamelet model well worth researching.In many practical combustion equipments, reactants do not burn in typical premixed or non-premixed conditions, but rather in partially-premixed ways. Furthermore, auto-ignition, propagation and stabilization of even non-premixed flames always involve partially-premixed combustion. All the above drives us to embark on specific research in partially-premixed combustion area. The building of flamelet database is very important a step in improving flamelet model. It includes improving the exactness and completeness of combustion states as well as extinction limit capturing. First, we introduced different diffusion and radiation effects in mixture fraction space flamlet equations and improved the exactness and completeness of combustion states as well as extinction limit. Then, we took a close investigation on scalar dissipation models and proposed the new method of calculating flamelet equations in physical space and greatly improved the results. After that, we used a special method and achieved the whole calculation of"S"curve and in this way improved the completeness of flamelet database. Besides, we investigated the influence of fuel dilution of combustion products on flamelet model. Next, we developed direct calculation solvers and flamelet solvers based on OpenFOAM and used them to study and analyzed exactness and limitations of flamelet model in multi-dimensional CFD.On the respect of partially-premixed combustion research, we systematically studied the structure of partially-premixed flames in counter-flow and coaxial jet flow by using direct calculation and flamelet modeling. We proposed to use equivalent ratio and stoichiometric scalar dissipation rate to describe partially-premixed flames and compared it with non-premixed flames.In a word, through our study, it can be concluded that although not a new one, flamelet model, under careful refinement, is proved to be a valid, widely-applicable, time-saving and after all a good combustion model.