Direct numerical simulation of a supercritical hydrothermal flame in a turbulent jet

The aim of this study is to establish a fundamental understanding of the flame structure and autoignition characteristics of supercritical hydrothermal flames in three-dimensional shear-driven turbulence. The study involves direct numerical simulation of a non-premixed flame (with fuel comprising a mixture of 10 % and 90 % in terms of mole fraction) at 25.0 MPa in a slot jet; detailed reaction mechanism and multispecies real-fluid properties are considered in the simulation. Qualitative transient inspection revealed that the flame undergoes a three-stage development process in the streamwise direction: sparse autoignition kernels in the upstream region, intense ignitions and establishment of a continuous flame surface in the middle region, and massive flamelets in the downstream region. Ignition kernels primarily form in the interior of large-scale shear-driven vortices featuring a low scalar dissipation rate. Probability density function (p.d.f.) analysis further confirmed that these kernels mainly form in the premixed combustion mode and on the fuel-lean side, in contrast to the authors' previous findings concerning autoignition in a two-dimensional mixing layer. Analysis of the preignition chemistry indicator (i.e. radicals) revealed that although the fuel-rich condition has a shorter homogeneous autoignition delay time, it does not exhibit any remarkable preignition chemistry or intense heat release in the upstream or middle regions because of its large-scale flow structure. A volume rendering of the dimensionless Damkohler number () reveals the distribution of autoignition spots and propagating flames. The joint p.d.f. of the mixture fraction and reveals the transition from sparse ignition to intense ignition and, finally, to flame propagation.

Automated summary

This study aims to establish a fundamental understanding of the flame structure and autoignition characteristics of supercritical hydrothermal flames in three-dimensional shear-driven turbulence. The qualitative transient inspection revealed a three-stage development process of the flame in the streamwise direction.