Quantification of fuel chemistry effects on burning modes in turbulent premixed flames

Abstract

The present work quantifies the impact of fuel chemistry on burning modes using premixed dimethyl ether (DME), ethanol (EtOH) and methane flames in a backto-burnt opposed jet configuration. The study considers equivalence ratios 0 ≤ Φ ≤ 1, resulting in a Damköhler (Da) number range 0.06 ≤ Da ≤ 5.1. Multi-scale turbulence (Re 19,550 and Re t 360) is generated by means of a cross fractal grid and kept constant along with the enthalpy of the hot combustion products (T HCP = 1700 K) of the counterflow stream. The mean turbulent rate of strain exceeds the laminar extinction rate for all flames. Simultaneous Mie scattering, OH-PLIF and PIV are used to identify reactants, mixing, weakly reacting, strongly reacting and product fluids. The relative balance between conventional flame propagation and auto-ignition based combustion is highlighted using suitably defined Da numbers and a more rapid transition towards self-sustained (e.g. flamelet type) combustion is observed for DME. The strain rate distribution on the reactant fluid surface for methane remains similar to the (non-reactive) mixing layer (Φ = 0), while DME and EtOH flames gradually detach from the stagnation plane with increasing Φ leading to stabilisation in regions with lower compressive rates of strain. The study further provides information on the conditions leading to burning mode transitions via (i) multi-fluid probabilities, (ii) structural flow field information and turbulence-flame interactions delineated by means of conditional (iii) velocity statistics and (iv) the rate of strain along fluid iso-contours.

Quantification of PAH concentrations in premixed turbulent flames crossing the soot inception limit

“…The soot layer thickness increases significantly with a decreasing rate of strain and increasing equivalence ratio with the flames becoming bright yellow/orange at ϕ U N = 2.2 and a T = 255 s −1 . Hampp and Lindstedt [52] have shown that the impact of the HCP temperature is comparatively modest for fuel-lean self-sustained flames that detach from the stagnation plane. The influence on soot formation is investigated below.…”

Particle size distributions in turbulent premixed ethylene flames crossing the soot inception limit

“…1, has advantages for the study of soot formation including the control of chemical and flow timescales [31]. The back-to-burnt (or fresh-to-burnt) configuration offers aerodynamic flame stabilisation [32], accurate boundary conditions [33] and a turbulent flowfield that is free of bulk instabilities [34,35]. The nozzle separation is set to the burner diameter for (close to) optimal flow stability [36].…”