Impact of Infinite Thin Flame Approach on the Evaluation of Flame Speed using Spherically Expanding Flames

Combustion is an important part of most current and future overall energy-conversion systems, especially if using renewable fuels in energy-storage concepts. Therefore, the laminar flame speed, which is a key parameter for the design of combustion systems, needs to be known for a growing multitude of different thermodynamic conditions and fuels. The spherically expanding flame method is one of the few techniques that enables the flame speed to be measured under particular conditions such as elevated pressure and temperature as well as under turbulent conditions, which are important for energy-conversion applications. The radius of a spherically propagating flame is tracked and used for evaluation of the flame speed. Usually, the flame is assumed to be infinitely thin. To assess the influence of this assumption, direct numerical simulations were conducted for the experimental setup and compared with measurements and correlations from the literature. The flame speed determined by the consumption rate of fuel, which takes a finite thickness of the flame into account, was found to be always larger than the flame speed computed by assuming an infinitely thin flame. The difference between these flame speeds was observed to be as large as approximately 10-20% in the evaluation range of the measured flame radii, which decreases with growing flame radius. This gives rise to the discrepancies in the flame speeds obtained from different measurement methods. An analytical estimation for this difference was developed as a function of the flame radius, which showed quantitatively good agreement with the simulation results and may be used for experimental validations of the flame speed. Both premixed H-2/air and CH4/air flames with equivalence ratios ranging from lean to rich conditions were studied.