A direct numerical simulation of cool-flame affected autoignition in diesel engine-relevant conditions

Abstract

In diesel engines, combustion is initiated by a two-staged autoignition that includes both low-and high-temperature chemistry. The location and timing of both stages of autoignition are important parameters that influence the development and stabilisation of the flame. In this study, a twodimensional direct numerical simulation (DNS) is conducted to provide a fully resolved description of ignition at diesel engine-relevant conditions. The DNS is performed at a pressure of 40 atmospheres and at an ambient temperature of 900 K using dimethyl ether (DME) as the fuel, with a 30 species reduced chemical mechanism. At these conditions, similar to diesel fuel, DME exhibits two-stage ignition. The focus of this study is on the behaviour of the low-temperature chemistry (LTC) and the way in which it influences the high-temperature ignition. The results show that the LTC develops as a "spotty" first-stage autoignition in lean regions which transitions to a diffusively supported cool-flame and then propagates up the local mixture fraction gradient towards richer regions. The cool-flame speed is much faster than can be attributed to spatial gradients in firststage ignition delay time in homogeneous reactors. The cool-flame causes a shortening of the second-stage ignition delay times compared to a homogeneous reactor and the shortening becomes

A conceptual model of the flame stabilization mechanisms for a lifted Diesel-type flame based on direct numerical simulation and experiments

“…This allowed (not shown here) to check that the imposed boundary conditions yielded satisfactory mean profiles in the area of interest where flame stabilization mechanisms were studied. Therefore, the chosen inflow boundary conditions allow to investigate the flame stabilization mechanisms, unlike temporally developing jets created by a mixing layer between fuel and air [20][21][22][23] . The initial condition for the DNS was a flow at rest at the initial temperature, pressure and composition known from the experiments.…”

“…The proportion of TF, L/R RZ is almost the same during Evolution B: 45 % TF and 55 % L/R RZ. It indicates that edge-flames must be taken into account to correctly model spray combustion under Diesel conditions as suggested in [10,17,[20][21][22][23] .…”

“…Published DNS have therefore restricted the computational domain to the gaseous part of the spray where chemical reactions essentially take place. A first approach was to perform temporal DNS of the turbulent mixing layer created downstream of the liquid part of the spray [20][21][22][23] . While this allowed addressing realistic Damköhler numbers, it may not account for spatial recirculation of hot burnt gases that have been found to possibly be of importance for the flame stabilization of ACDF [6] .…”

“…Low-temperature chemistry has been shown to play an important role in that dynamic process [19] . Reproducing these low-temperature chemistry phenomena, especially in the NTC (Negative Temperature Coefficient) regime, is impossible with global schemes [22] and requires more complex chemistry descriptions.…”

Characterisation of two-stage ignition in diesel engine-relevant thermochemical conditions using direct numerical simulation

“…However, by making appropriate simplifying assumptions, idealised DNS may be conducted that complement physical experiments and provide additional insight into diesel combustion. The growth of high performance computing resources has enabled several DNS investigations of diesel engine-relevant combustion at idealised conditions [33][34][35][36][37][38][39][40][41], the main findings of which are reviewed here. [33] conducted a three-dimensional (3D) DNS study of n-heptane ignition in decaying isotropic turbulence.…”