ABSTRACT

Combustion of fossil fuels remains the primary energy source in power generating plants, vehicle engines, furnaces, and boilers. It is a complex process that involves the inter-diffusion of a large number of chemical species and substantial heat release generated from the chemical reactions. In most applications the burning takes place within a turbulent flow that adds a fluctuating, time-dependent, three-dimensional aspect to the system. The objective of this project is to develop predictive tools that account for the relevant physics and chemistry. The focus is on the evolution of an outwardly expanding flame initiated from a small ignition source. The flame, which appears initially smooth and spherical, becomes highly corrugated due to instabilities inherent to the combustion process and to the turbulence. The increase in flame surface area results in enhanced fuel consumption and propagation speed, a process that may potentially transition to an explosion and/or detonation. Fundamental understanding of the mechanisms responsible for these complex flame properties will improve predictability of combustion systems and increase their safe and efficient operation. The proposed research has also a significant pedagogical value; the advocated physics-based approach will be used in the classroom and in outreach programs intended to extend the human-resources base of science and technology.

The complex dynamics of spherically expanding flames resulting from flame instabilities and turbulence will be investigated within the framework of the hydrodynamic theory, derived systematically from the general conservation laws using a multi-scale asymptotic approach. The flame, treated as a surface of density discontinuity, propagates into the fresh mixture in accordance to the procured flame speed relation which, in conjunction with the conditions across the flame front, mimic the influences of diffusion and chemical reaction occurring within the flame zone. The model is free of tuning parameters and ad-hoc sub-grid models that plague commonly used turbulent combustion models. Its numerical implementation uses an embedded manifold approach, wherein the flame surface is represented implicitly as the zeroth level set of a scalar field. Since the flame surface is determined unambiguously, all pertinent information to its propagation will be directly contained in the flame topology and in the flow field at the same location. The acquired knowledge on flame instabilities and flame-turbulence interactions will serve to guide the experimental studies in this area and improve large-scale computational efforts, by replacing ad-hoc lumped parameters with prototypical flame configurations that are fundamentally sound and based on physical first principles.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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