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In recent years, conditioned particle image velocimetry (CPIV) has attracted much attention for flame front extraction. It is an economical and straightforward technique because the flame front can be obtained simply from Mie-scattering images. In the current work, Mie-scattering and hydroxyl planar laser-induced fluorescence (OH-PLIF) measurements were taken simultaneously to study the applicability of CPIV under conditions with varying equivalence ratios, and the reliable working range of the CPIV method and the source of bias were clarified quantitatively. The premixed dimethyl ether (DME)/air flames stabilized on a two-dimensional bluff body were tested. It is found that the accuracy of the CPIV method worsens as the equivalence ratio decreases. The bias of CPIV is supposed to be related to the flame structure and aerodynamics. The extraction deviation in the upstream region for the stable flames is more significant than that in the downstream area due to the intensified interaction between the shear layer and flame near the bluff body. However, for the flames approaching the lean blow-off (LBO), the bias in the upstream part is smaller than that in the downstream area, which is related to the "back-support" of the hot recirculation zone (RZ). In addition, the difference between the results obtained by CPIV and OH-PLIF is usually the preheat layer of flame and concave-wrinkled location of OH-PLIF filled with CH2O radicals, especially under conditions close to the LBO, which can be used to estimate the spatial distribution of CH2O. [ABSTRACT FROM AUTHOR]

• Increasing the bluff-body temperature by 200 K can extend the lean blowoff (LBO) limit by up to 6.5 %. • The higher bluff-body temperature has limited effects on the velocity field. However, the flame brush thickness, the flame wrinkling ratio, and turbulent flame speed will be changed, leading to a more stable flame. • Based on the probability density functions (PDFs) of the vorticity and strain rate on the flame front, the flame front in the case of higher bluff-body temperature can survive with larger flame strain rates, and the occurrence of the local extinctions can be reduced. In flames stabilized by a two-dimensional (2D) bluff body, the lean blowoff (LBO) limit can be extended by increasing the bluff-body temperature, which is verified experimentally in premixed dimethyl ether (DME)/air flames in this work. Two bluff-body temperatures of 573 K and 773 K are tested, respectively. High-speed OH* chemiluminescence (CL) imaging is applied to record the spatial distribution of the heat release. Particle image velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) are performed simultaneously to explore the flow field and the combustion characteristics. Based on the results of OH*-CL, the heat release of the area near the bluff body is strengthened in the case of higher bluff-body temperature, and the enhanced chemical reactivity will further influence the downstream area. The turbulent flame speed obtained by OH-PLIF is increased in the entire area of interest. Due to the higher velocity gradient adjacent to the bluff body in the upstream region (y < 35 mm), the flame surface density (FSD), brush thickness, and flame wrinkling ratio are not sensitive to the bluff-body temperature. However, in the downstream region (y > 35 mm), a noticeable change can be found in these three parameters due to a higher turbulent flame speed and a much lower velocity gradient. Based on the PIV results, a limited effect on the velocity field by increasing the bluff-body temperature by 200 K. Furthermore, the probability density functions (PDF) of the vorticity and strain rate on the flame front are computed. The results indicate that the enhanced flame stability leads to a more pronounced overlapping between the flame front and the high-vorticity region, further increasing the values of these two parameters. [ABSTRACT FROM AUTHOR]

A detailed experimental study of ethanol spray swirling flames was performed in an axial bluff body stabilized burner. The characteristics of the non-reacting and reacting sprays were recorded by particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) of the OH radical. A few typical flames with different structures (outer-side-flame-lifting, stable, and near-blow-off) were compared and analyzed. The parameters of the spray, including the spray half-angle (α) and droplet number density (nd), are quantified, and it has been found the flame structure and stability were strongly correlated with the droplet distribution. Several parameters of the flow field, such as velocity magnitude ( | U | vorticity (ωz), and turbulent kinetic energy (TKE), are quantitively analyzed, and it is observed that the local strain rate rose as the air flow rate increased, which is not conducive to local flame stability. Regarding the flame, quantities such as progress variable (), flame height (Lf), lift–off height (hlf), and symmetry factor (Snd and S) are calculated, and it can be observed that the flame symmetry keeps worsening when approaching blow–off, and the inner flame branch exhibits a worse stabilization than the outer one. Our comprehensive investigations offer a deeper understanding of stable combustion in such two–phase flames. [ABSTRACT FROM AUTHOR]