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Direct Numerical Simulations of Turbulent Lean Premixed Hydrogren Flames with Detailed Chemistry

Turbulent premixed combustion in practical devices is surely a complex phe- nomenon combining chemical kinetics, molecular transport and hydrody- namic turbulence in di cult geometrical con gurations. Increasing regu- lative and competitive demands require improved physical understanding and predictive modeling capability for combustion phenomena including ig- nition, quenching and pollutant formation. For example, many governments impose limits on emissions of oxides of nitrogen, carbon monoxide and un- burnt hydrocarbons from gasoline sparked-ignited automotive engines. In this prospective, hydrogen combustion is a promising option and in recent years it found interesting applications in several technologies as fuel cells, hydrogen infrastructures and vehicles. One of its main advantages is the ab- sence of many pollutants as carbon oxides or particulates. In addition, oper- ating at fuel-lean condition minimizes combustion exhaust gas temperature, which in turn reduces the formation of nitrogen-based emissions downstream of the ame. However, lean premixed ames, and hydrogen-air mixtures in particular, are subject to a variety of hydrodynamic and combustion insta- bilities that lead to di culties in the robust stabilization of the ame front. These di culties are considerably increased when turbulence interacts with combustion, because both phenomena are intrinsically complex processes involving a large range of times and lengths. In fact when a ame interacts with a turbulent ow, turbulence is strongly modi ed and, on the other hand, even the ame structure itself is altered, enhancing chemical reactions or, in extreme cases, completely inhibiting them, leading to quenching e ects. Hy- drogen combustion is anyway simple and well-understood, allowing a clear overview of some peculiar features that involve even hydrocarbons, since they are mainly constituted of hydrogen and carbon. Therefore the main aim of this thesis is the simulation of turbulent lean premixed hydrogen Bun- sen ame using DNS with a signi cant detailed chemistry, chosen among the GriMech database. However, in order to perform a three-dimensional simulations, several parameters have to be carefully calibrated so to min- imize the gap between numerical results and experiments. This operation was e ectuated through one-dimensional and two-dimensional codes, which do not require excessive processing capacities and whose results can be easily comprehended and compared with a wide literature. In addition, they set up reference values for three-dimensional ames, enhancing the evaluation of typical turbulent uctuations and the comprehension of non-negligible di usional processes. Nevertheless these considerations are not enough to characterize properly a turbulent ame, whose description must take into account a computation of the averaged elds, thus it is essential to perform a statistical analysis capable of highlighting the relations among thermody- namics, uid-dynamics and chemistry. Results showed a strong dependence of OH radicals on the progress variable and the magnitude of its gradient, called usually \Surface Density Function" (SDF); in addition the dynamics of the ame results deeply a ected by strain e ects and curvature, which help to understand peculiar behaviors as the increase of the local equiva- lence ratio in some zones or the quenching e ects. At last, even the heat release has a deep in uence on combustion, in fact it was observed to be an excellent quantity to summarize e ciently many phenomena.

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Introduction Turbulent premixed combustion in practical devices is surely a complex phe- nomenon combining chemical kinetics, molecular transport and hydrody- namic turbulence in difficult geometrical configurations. Increasing regu- lative and competitive demands require improved physical understanding and predictive modeling capability for combustion phenomena including ig- nition, quenching and pollutant formation. For example, many governments impose limits on emissions of oxides of nitrogen, carbon monoxide and un- burnt hydrocarbons from gasoline sparked-ignited automotive engines. In this prospective, hydrogen combustion is a promising option and in recent years it found interesting applications in several technologies as fuel cells, hydrogen infrastructures and vehicles. One of its main advantages is the ab- sence of many pollutants as carbon oxides or particulates. In addition, oper- ating at fuel-lean condition minimizes combustion exhaust gas temperature, which in turn reduces the formation of nitrogen-based emissions downstream of the flame. However, lean premixed flames, and hydrogen-air mixtures in particular, are subject to a variety of hydrodynamic and combustion insta- bilities that lead to difficulties in the robust stabilization of the flame front. These difficulties are considerably increased when turbulence interacts with combustion, because both phenomena are intrinsically complex processes involving a large range of times and lengths. In fact when a flame interacts with a turbulent flow, turbulence is strongly modified and, on the other hand, even the flame structure itself is altered, enhancing chemical reactions or, in extreme cases, completely inhibiting them, leading to quenching effects. Hy- drogen combustion is anyway simple and well-understood, allowing a clear overview of some peculiar features that involve even hydrocarbons, since they are mainly constituted of hydrogen and carbon. Therefore the main aim of this thesis is the simulation of turbulent lean premixed hydrogen Bun- sen flame using DNS1 with a significant detailed chemistry, chosen among the GriMech database. However, in order to perform a three-dimensional 1 it consists in solving the fluid-dynamics fields without any turbulent model 6

Laurea liv.II (specialistica)

Facoltà: Ingegneria

Autore: Gabriele Rocco Contatta »

Composta da 156 pagine.

 

Questa tesi ha raggiunto 172 click dal 12/03/2010.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.