Effect of heat release on turbulence and scalar-turbulence interaction in premixed combustion

Abstract

Stereoscopic particle image velocimetry and planar laser induced fluorescence measurements of hydroxyl radical are simultaneously applied to measure, respectively, local turbulence intensities and flame front position in premixed ethylene-air flames stabilized on a bluff body. Three different equivalence ratios, 0.55, 0.63, and 0.7, and three different Reynolds numbers, 14 000, 17 000, and 21 000, are considered. Laser measurements were made for five different flame configurations within the ranges above and in the corresponding cold flows. By comparing the measurements of the cold and the corresponding hot flows, the effect of heat release on the turbulence and its interaction with the flame front is studied. All the flames are in the thin reaction zone regime. Typical flow features forming behind the bluff body are observed in the cold flows, whereas in the reacting flows the mean velocities and thus the shape, size, and characteristics of the recirculating eddy behind the bluff body are strongly influenced by the heat release. The strong acceleration across the mean flame and the radial outward shift of the stagnation plane of the recirculating eddy yield negative radial velocities which are absent in the corresponding cold flow cases. The spatial intermittency of the flame front leads to an increase in the turbulent kinetic energy. Although a decrease in the mean and rms values of the strain rate tensor e ij components is observed for the reacting case as one would expect, the local flow acceleration across the flame front leads to a substantial increase in the skewness and the kurtosis of the probability density functions ͑PDFs͒ of e ij components. The turbulence-scalar interaction is studied by analyzing the orientation of the flame front normal with the eigenvectors of e ij . The PDFs of this orientation clearly show that the normals have an increased tendency to align with the extensive strain rate, which implies that the scalar gradients are destroyed by the turbulence as the scalar isosurfaces are pulled apart. This result questions the validity of passive scalar turbulence physics commonly used for premixed flame modeling. However, the influence of Lewis number on this alignment behavior is not clear at this time.

Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

“…negative) principal strain rate respectively and θ α , θ β and θ γ are angles between ∇c and the eigenvectors associated with e α ,e β and e γ respectively. It was demonstrated earlier [50][51][52] that ∇c aligns collinearly with the eigenvector direction corresponding to e γ in the region with weak heat release, but this alignment changes to the direction corresponding to e α in the region with strong heat release, in which case the strain rate induced by flame normal acceleration overcomes background turbulent straining [50,51]. According to Eq.…”

Simulation of the flame describing function of a turbulent premixed flame using an open-source LES solver

“…1. This is described in detail by Balachandran et al [5,6], and has featured in other studies of turbulent flames [41,42,43,44]. A long tube emerges from a plenum chamber, within which is a conical bluff body with diameter d = 25mm.…”

“…At the inlet, the velocity is imposed in the form of Eq.(11). In the experiments, the time-averaged bulk velocity entering the combustor was V b = 9.9m/s, giving a Reynolds number Re = dV b /ν = 17000 [44]. Forcing was via pulsing of the flow upstream of the combustor by loudspeakers, such that a single frequency harmonic velocity is superimposed on the mean flow.…”

Influence of Turbulent Scalar Mixing Physics on Premixed Flame Propagation

“…These situations are schematically shown in Figure 1, and this change is because of the fact that the dilatation due to heat release is strong compared to the turbulent strain rate. Evidence for such a behaviour has been found in direct numerical simulation (DNS) studies of statistically planar [16][17][18][19][20]22], spherically symmetric [25], and Bunsen [26] flames and also experimental bluff body stabilised flames [27]. As a consequence, the isoscalar surfaces are brought together by the compressive strain resulting in an increase of scalar gradient when the reaction is passive.…”