Estudio experimental y modelado cfd del proceso de combustión de mezclas combustibles de hidrógeno, metano y gas de síntesis en una bomba cilíndrica con acceso óptico

Resum

This doctoral thesis is part of the research on alternative energy sources, which allow to make the world of combustion a more efficient and environmentally friendly environment. The work has been carried out in the Research Group Engines and Renewable Energy at the University of Valladolid. The objective is to characterize alternative gaseous fuels (methane, hydrogen, carbon monoxide and their mixtures) in a constant volume cylindrical combustion bomb. This bombhas quartz glass windows in its bases, which allows recording images of the flame front by a high-speed camera, at the same time as the pressure evolution is recorded. The study of the combustion process and in particular the obtaining of the laminar burning velocity can be carried out by two methods: analysis of the pressure recorded by means of a two-zone diagnostic model, and from the images recorded with the Schlieren methodology, using an automated procedure. In parallel to this, CFD simulations have been carried out that allow predicting the development of the combustion process, to have additional information to that obtained in the experiments. In addition, a new methodology has been developed that allows characterizing the flame front, including its deformation due to the effect of the cylindrical geometry of the bomb. In this way, it is possible to study how this deformation of the flame influences the calculation of the laminar burning velocity and other important parameters of the combustion process. The experiments consist of in-air combustion of H2–CH4 and H2–CO mixtures (syngas). For the H2–CH4 mixtures, experiments are carried out varying the H2 content in the mixture from 0, 20, 50, 80 and 100% with initial conditions of pressure of 0.1 MPa and temperature of 300 K, for equivalence ratios 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0. For the 100% H2 mixtures, a study was also carried out on the effect of the initial pressure and temperature and experiments were carried out with initial pressures of 0.1, 0.2 and 0.3 MPa and initial temperatures of 315 and 373 K for an equivalence ratio of 0.7. For H2-CO mixtures, experiments were carried out to see the influence of the H2 content in the mixture with proportions of 0, 6.7, 25, 50, 80 and 100% H2, with initial conditions of 0.1 MPa, 300K and an equivalence ratio sweep of 0.5 and 1.0. From the results obtained for the H2–CH4 mixtures, it can be seen that, below 50% of H2, the laminar burning velocity increases linearly with respect to that of methane with the increase in H2 content. However, for each equivalence ratio the laminar combustion velocity increases very strongly when an H2 content of the order of 80% is passed. In addition, from the analysis of the Schlieren images obtained, it can be seen that the increase of hydrogen in the mixture favors the growth of instabilities, resulting in a cellular flame, in accordance with what is published in other studies on growth of combustion instability. As in the previous mixtures, in the H2-CO mixtures, the laminar burning velocity values increase linearly with the hydrogen content and the increase of equivalence ratio. Regarding the analysis of the Schlieren images, it was obtained that the increase of hydrogen contents has a destabilizing effect on the flame, causing cellularity on the surface of the flame front. The results obtained from the laminar burning velocity by means of the pressure increase method, for the mixtures of 100% H2, 100% CH4 and the H2-CO mixtures, were first compared with those obtained in previous studies in a spherical bomb existing in the Engine Laboratory. Additionally, comparisons were made between the velocities obtained by both methods and with other authors cited in the bibliography, to validate the results obtained. As for the CFD simulation, it is a first approximation in that field within the work of the research group, whose most important result is to verify the deformation of the flame front, moving away from the spherical geometry. From a radius of the front of approximately 60% of the radius of the cylindrical pump, the spherical symmetry is lost, with different growth in the main directions of the cylindrical pump (radial and axial). The developed algorithm allows to follow the evolution of the flame front geometry calculated by the CFD simulation, assimilating it to a revolution pseudoelipsoid. A relevant contribution of the thesis is the calculation of important theoretical parameters for the study of hydrodynamic and thermodifusive instabilities, such as: Markstein length, Markstein number, Lewis number, Peclet number, Zel’dovich number, and the rates of growth of instabilities.