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

Abstract

With the goal of providing a more detailed fundamental understanding of ignition processes in diesel engines, this study reports analysis of a direct numerical simulation (DNS) database. In the DNS, a pseudo turbulent mixing layer of dimethyl ether (DME) at 400 K and air at 900 K is simulated at a pressure of 40 atmospheres. At these conditions, DME exhibits a two-stage ignition and resides within the negative temperature coefficient (NTC) regime of ignition delay times, similar to diesel fuel. The analysis reveals a complex ignition process with several novel features. Autoignition occurs as a distributed, two-stage event. The high-temperature stage of ignition establishes edge flames that have a hybrid premixed/autoignition flame structure similar to that previously observed for lifted laminar flames at similar thermochemical conditions. A combustion mode analysis based on key radical species illustrates the multi-stage and multi-mode nature of the ignition process and highlights the substantial modelling challenge presented by diesel combustion.

Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study

“…2(g). The cool flame can be considered as a fourth branch of the edge-flames, resembling the tetrabrachial flames as observed before under similar thermochemical conditions in the DME-air mixing layer [24,25]. The edge flames gradually collide with each other and their branches are eventually connected.…”

“…The doubly conditioned statistics revealed a two-stage autoignition. Krisman et al [24,25] recently investigated the two-stage ignition in a turbulent mixing layer of dimethyl ether (DME)-air mixture by DNS. It was found that LTC can strongly affect the timing and location of the second stage autoignition.…”

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] .…”