Direct numerical simulation of a spatially developing n-dodecane jet flame under Spray A thermochemical conditions: Flame structure and stabilisation mechanism

Abstract We present results from a three-dimensional (3D) direct numerical simulation (DNS) of a spatially developing n-dodecane round jet flame. The thermochemical conditions (i.e. pressure, temperature and oxidiser composition) correspond to those of Spray A, an experimental target flame of the Engine Combustion Network (ECN). To make the DNS computationally tractable, we consider a gas jet with a reduced Reynolds number of 17,000 and a shorter lifted length of 11 times the jet diameter. The flame structure and stabilisation mechanism of the statistically steady jet flame are discussed. Overall the flame structure is similar to that identified from experimental observations, i.e. a region of high formaldehyde (CH2O) concentration, which marks the low-temperature chemistry (LTC), is present upstream of the main flame and persists downstream in the central region of the jet where the mixture is rich. A high-temperature (HTC) nonpremixed flame marked by OH radicals shrouds the jet. Ignition kernels are observed upstream of the flame base, some of which are convected downstream and join with the main flame. At the flame base, a three-branch structure is observed, namely an LTC branch upstream of the flame, a rich HTC branch attached to the flame base and a trailing nonpremixed flame anchored at the stoichiometric mixture fraction. A detailed analysis of the flame stabilisation mechanism is reported, employing structural comparisons to two-dimensional (2D) reference cases, transport budgets, and flame speeds. It is concluded from these analyses that the flame is stabilised principally by flame propagation. The role of autoignition kernels is also analysed and found to be secondary.