The dynamics of premixed flames is paradigmatic of highly nonlinear physical phenomena. Their multiscale nature and the complex chemistry of the oxidation of common fuels contribute significantly to this complexity. Nevertheless, this multiscale nature can be judiciously exploited, using mathematical asymptotic techniques, to simplify problems and uncover the essential physics. These ideas will be used to analyze two problems recently solved: the extremely nonlinear dynamics of pulsating flame fronts, and the chemical structure of H2 and CH4 flames and its relation to the flame´s chemiluminescence.
The pulsating instability of premixed flames is well known since its discovery in the early 70´s in the self-propagating high-temperature synthesis of materials (SHS). It is important in applications, for instance in the flames typically found in modern jet engines. In this mode, the flame propagation assumes the form of extremely nonlinear relaxational-type pulsations, each characterized by a long stage with front velocities lower than the steady flame velocity, followed by a very short stage with very large front velocities. This large disparity of time- dependent spatial and temporal scales characterizing the dynamics makes the numerical solution extremely challenging. However, it can be advantageously used in the theoretical analysis as will be shown.
As a second example, the kinetics of excited or chemiluminescent species in premixed flames is studied. Chemiluminescence refers in literature to the electromagnetic radiation emitted by flames due to the chemical activity. It has been used extensively since the early 50’s as a nonintrusive diagnostic technique to simply reveal chemical activity, but also to indirectly measure, through purely phenomenological laws, a number of flame properties such as the equivalency ratio, the rate of fuel consumption or the heat release rate. However, a theoretical quantitative analysis is still missing. Recent excitation-de-excitation kinetic schemes for OH* and CH* are used to understand the dynamics of excited species production and their interaction with the base flame chemical structure. Again, asymptotic techniques are used to elucidate this interaction and to suggest novel approaches to the chemical structure of premixed flames.
About the Speaker
José Graña-Otero has Aeronautical Engineer and Doctor degrees from the School of Aeronautics of the Polytechnic University of Madrid. He developed his PhD thesis under the supervision of Professor Amable Liñán, with whose group he still collaborates. He also worked during a Postdoc with Professor Paul Clavin and his group at Marseille, France. Since 2014 he is Assistant Professor of the Department of Mechanical Engineering of the University of Kentucky.
His research interests are related to the field of fluid mechanics, in which he has paid special attention to two main lines of research: two-phase flows, with and without phase changes, and the analysis of combustion phenomena. Currently he is working on the dynamics of boiling fronts in superheated liquids, and in problems related to the simplified description of the chemical structure of premixed flames and its application to study the dynamics of unstable and unsteady premixed flames.
Host: Professor Kelly Stephani