Combustion in thermonuclear supernova explosions

Type Ia supernovae are associated with thermonuclear explosions of white dwarf stars. Combustion processes convert material in nuclear reactions and release the energy required to explode the stars. At the same time, they produce the radioactive species that power radiation and give rise to the formation of the observables. Therefore, the physical mechanism of the combustion processes, as reviewed here, is the key to understand these astrophysical events. Theory establishes two distinct modes of propagation for combustion fronts: subsonic deflagrations and supersonic detonations. Both are assumed to play an important role in thermonuclear supernovae. The physical nature and theoretical models of deflagrations and detonations are discussed together with numerical implementations. A particular challenge arises due to the wide range of spatial scales involved in these phenomena. Neither the combustion waves nor their interaction with fluid flow and instabilities can be directly resolved in simulations. Substantial modeling effort is required to consistently capture such effects and the corresponding techniques are discussed in detail. They form the basis of modern multidimensional hydrodynamical simulations of thermonuclear supernova explosions. The problem of deflagration-to-detonation transitions in thermonuclear supernova explosions is briefly mentioned.Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A. Alsabti and P. Murdin, Springer. 24 pages, 4 figure

Analysis of Flame Topology and Burning Rates

Datasets generated using Direct Numerical Simulation (DNS) are used to investigate the influence of local flame surface topology on global flame propagation. A mathematical framework based on Morse theory is presented and is shown to lead to a classification of all possible types of flame surface topology. A similar mathematical approach is shown to provide insight into the behaviour of the surface density function (SDF) and the displacement speed in the vicinity of flame pinch-off and pocket burnout events. DNS data for a pair of colliding premixed turbulent hydrogen–air flames is used to identify and locate topological points of interest and to determine their frequencies of occurrence on the flame surface. Further analysis of the dataset is carried out to evaluate terms of the SDF balance equation and the displacement speed in the presence of flame–flame interactions. Considerable insight is gained into the underlying mechanisms of flame propagation