Fundamental Processes in Combustion of Stratified Mixtures
Stratified combustion, combustion of fuel/air mixtures with temperature and/or mixture-composition stratification, is present in many combustion-related phenomena and applications such as forest wildfires, mining explosions, vessel ruptures, gas turbines, and reciprocating engines to name a few. A new generation of highly efficient internal combustion (IC) engines capable of satisfying stringent emission requirements, including modern direct-injection gasoline engines and gas turbines with lean premixed pre-vaporized (LPP) combustors, requires more comprehensive understanding and control of stratified combustion. Fundamentally, stratification of temperature or mixture composition affects a wide range of combustion characteristics such as flame speed, flammability, mode of combustion, instability, and others.
This dissertation aims to identify, analyze and evaluate fundamental processes in the combustion of stratified mixtures, using theoretical analysis and advanced numerical simulation tools. ASURF-Parallel, a transient numerical solver of compressible reacting flow, is developed on the basis of the original A-SURF and exploited for stratified combustion simulations. A domain-decomposition parallelization scheme using Message Passing Interface (MPI) is developed and implemented in ASURF-Parallel to speed up the otherwise time-consuming numerical simulations. A significant speedup with the speed-up factor up to 10 is achieved on lab-scale servers.
Effects of stratification on flame speeds, lean flammability limit, and modes of combustion are numerically investigated and studied. For flame speeds, laminar flame speeds of stratified flames propagating from rich mixtures to lean mixtures are generally faster than those of the corresponding homogeneous flames, primarily due to the preferential diffusion of lighter species and radicals such as H2, H and OH, i.e., the chemical effect. The degree of enhancement in flame speeds can be correlated to the degree of stratification, leading to the development of a transient local stratification level (LSL) model which is able to determine the stratified flame speeds incorporating both chemical effect and memory effect. For lean flammability limits, the extension introduced by stratification is very weak due to reduced overall reactivity and reduced degree of stratification. For modes of combustion, different modes can be realized by specific reactivity gradients, regardless of the sources of such gradients. Pressure waves introduced by ignition in a closed chamber can also lead to different modes of reaction front propagation and end-gas combustion. A transient reactivity gradient method is proposed to identify the onset of detonation.