The Effect of Turbulent Mixing on Compression Ignition of a Lean Hydrogen/Air Mixture

The influence of a turbulent spectrum of the temperature field on compression ignition at constant volume under homogeneous charge compression ignition engine conditions is studied by direct numerical simulation with complex chemistry. In particular the dependence of overall ignition progress on initial mixture conditions and turbulence parameters is determined. The propagation speed of ignition fronts that emanate from ‘hot spots’ given by the temperature spectrum is monitored by using the displacement velocity of a progress variable that tracks the location of maximum heat release. The evolution of the front velocity is compared for different initial temperature distributions and statistics of the ignition front speed, stretch, and curvature are presented. 1 Corresponding author: [email protected] Associated Web site: http://www.ca.sandia.gov/crf/staff/jhchen.html Proceedings of the Western States Section of the U.S. Sections of The Combustion Institute, 2003 Introduction Lean premixed autoignition of fuel-lean hydrocarbon mixtures at high pressure in the presence of spatial inhomogeneities in temperature or mixture composition has received much attention recently. Understanding inhomogeneous autoignition may lead to the development of control strategies for a mode of combustion being considered for compression ignition automotive engines known as homogeneous charge compression ignition combustion (HCCI). By operating under overall fuel-lean conditions, and hence, at lower temperatures, HCCI can potentially achieve high engine efficiencies comparable to diesel combustion without producing NOx and soot. Because the fuel is sufficiently lean that the normal deflagrative flame propagation found in spark ignition engines may not be possible, the primary mode of combustion in this regime is thought to occur by volumetric autoignition. Therefore, HCCI combustion is primarily kinetically driven. A major challenge posed by this method of combustion is controlling the rate of heat release, and in particular, developing a strategy to spread it out over several crank angle degrees to minimize the generation of damaging engine knock. One possible control strategy is to introduce inhomogeneities in the temperature or mixture composition, tailored to produce the desired heat release rate [1]. In practice, incomplete turbulent mixing and temperature stratification between the bulk gases and the cylinder wall leads to spatial inhomogeneities that also contribute to a range of autoignitive combustion modes distinct from homogeneous autoignition. The objective of the present numerical study is to understand the influence of temperature inhomogeneities on the evolution of the different ignition modes of combustion [2,3]. As described by Zel’dovich [3], there are at least three modes of propagation: deflagration, ignition front propagation and detonation. The parameters of the present study are such that the ignition propagation mode is dominant. Ignition propagation is fast relative to deflagration in this regime, but slow relative to the sound speed. The evolution of autoignition from an initial spectrum of ‘hot’ spots is numerically simulated using twodimensional direct numerical simulations (DNS) with complex hydrogen/air chemical kinetics. Due to the computational expense of resolving extraordinarily thin ignition fronts at high pressure and tracking their temporal evolution, the chemical kinetics considered are necessarily over simplified, corresponding to detailed hydrogen/air systems. Hydrocarbon fuels, which build on hydrogen as an important subset of the mechanism, will be considered in future studies. To keep the problem tractable, a further assumption is that the problem is twodimensional, although turbulence is inherently threedimensional. It is the opinion of the authors that many of the chemical induction and ignition front propagation characteristics can be captured qualitatively in two dimensions.