Effects of Turbulence and Temperature Fluctuations on Knock Development in an Ethanol/Air Mixture

Abstract

The effects of turbulence on knock development and intensity for a thermally inhomogeneous stoichiometric ethanol/air mixture at a representative end-gas autoignition condition in internal combustion engines are investigated using direct numerical simulations with a skeletal reaction mechanism. Two- and three-dimensional simulations are performed by varying the most energetic length scale of temperature, lT, and its relative ratio with the most energetic length scale of turbulence, lT/ le, together with two different levels of the turbulent velocity fluctuation, u′. It is found that lT/le and the ratio of ignition delay time to eddy-turnover time, τig/ τt, are the key parameters that control the detonation development. An increase in either lT or le enhances the detonation propensity by allowing a longer run-up distance for the detonation development. The characteristic length scale of the temperature field, lT, is significantly modified by high turbulence intensity achieved by a large le and u′. The intense turbulence mixing effectively distributes the initial temperature field to broader scales to support the developing detonation waves, thereby increasing the likelihood of the detonation formation. On the contrary, high turbulence intensity with a short mixing time scale, achieved by a small le and a large u′, reduces the super-knock intensity attributed to the finer broken-up structures of detonation waves. Either τig/ τt less than unity or le= lT even with a large u′ is found to have no significant effect on super-knock mitigation. Finally, high turbulent intensity may induce high-pressure spikes comparable to the von Neumann spike. Increased temperature and pressure by combustion heating, noticeably after the peak of heat release rate, significantly enhance the collision and interaction of multiple emerging autoignition fronts near the ending combustion process, resulting in localized high-pressure spikes.