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, l T , and its relative ratio with the most energetic length scale of turbulence, l T ∕l e , together with two different levels of the turbulent velocity fluctuation, u ′ . It is found that l T /l e and the ratio of ignition delay time to eddy-turnover time, ig ∕ t , are the key parameters that control the detonation development. An increase in either l T or l e enhances the detonation propensity by allowing a longer run-up distance for the detonation development. The characteristic length scale of the temperature field, l T , is significantly modified by high turbulence intensity achieved by a large l e 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 l e and a large u ′ , reduces the super-knock intensity attributed to the finer broken-up structures of detonation waves. Either ig ∕ t less than unity or l e = l T 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.

Knock propensity in a thermally inhomogeneous DME/air mixture: a DNS study

“…The KAUST Adaptive Reacting Flow Solver (KARFS) [2,3,41,49] is used to solves the fully compressible Navier-Stokes, species and energy equations for gaseous mixtures. An eighth-order finite-difference scheme was employed for the spatial discretization of the diffusive terms.…”

“…Modern internal combustion (IC) engines operate under elevated pressure enabled by downsized boosted technologies to achieve higher thermal efficiency, higher power-density output per volume, and ultra-low emission. However, being operated at elevated pressure and high-load conditions leads to a higher propensity of undesired preignition, knock, and even superknock [1][2][3][4][5][6][7][8]. Superknock is characterized as a developing detonation process featuring excessive pressure oscillations and extremely high-pressure amplitudes that can damage combustion-chamber components [7,[9][10][11][12][13][14][15][16][17][18][19][20]20].…”

“…Improved understandings of the superknock propensity and a reliable criterion to predict it are needed to prevent destructive operation of combustion devices [2,4,5,7,[21][22][23][24][25][26][27]. Therefore, first-principle high-fidelity direct numerical simulations (DNS) with the capability of fully resolving all temporal and spatial scales and the complex interaction of thermochemistry and turbulence under extreme high-load operating conditions are employed to unravel the mechanism of abnormal combustion phenomena in combustion devices such as IC engines, shock tubes, and rapid compression machines (RCM) [2,3].…”

“…The 𝜉 − 𝜀 diagram has been widely adopted in one-dimensional (1-D) DNS studies [11,14,[35][36][37][38][39][40][41][42][43][44][45], while a few multi-dimensional DNS studies have been conducted due mainly to computational expense [2][3][4][46][47][48].…”

Simulation of mild auto-ignition initiated by a hot spot

“…Luong et al [13] used two-dimensional (2D) and threedimensional (3D) DNS to study the effects of turbulence on knock characteristics using a stoichiometric ethanol/air mixture. They found that once the most energetic length scale associated with the fluctuation of temperature or velocity starts to increase, the detonation becomes more likely, because this allows a larger distance for detonation to develop.…”