D.A. Santavicca

Surface properties of turbulent premixed propane/air flames at various Lewis numbers

Abstract Surface properties of turbulent premixed flames including the wrinkled flame perimeter, fraction of the flame pocket perimeter, flame curvature, and orientation distributions have been measured for propane-air flames at Lewis numbers ranging from 0.98 to 1.86 and u′ S L = 1.42–5.71 . The wrinkled flame perimeter is found to be greater for the thermodiffusively unstable Lewis number (Le u′ S L tested, and show a much stronger dependence on the turbulence condition than on the Lewis number. Similarly, the flame orientation distributions show a trend from anisotropy toward a more uniform distribution with increasing u′ S L at a similar rate for all Lewis numbers. Thus, for turbulent premixed propane/air flames for a practical range of Lewis number from 0.98 to 1.86, the effect of Lewis number is primarily to affect the flame structures and thereby flame surface areas and flame pocket areas, while the flame curvature and orientation statistics are essentially determined by the turbulence properties.

A Turbulent Flow System for Studying Turbulent Combustion Processes

The purpose of this communication is to describe a new turbulent flow system for the study of turbulent combustion processes. The system produces very high levels of relative turbulence intensities and allow for the generation of an unstabilized , propagating flame

Local response and surface properties of premixed flames during interactions with Kármán vortex streets

Premixed flames interacting with Karman vortex streets have been experimentally investigated, in which local flame responses consistent with the results of stretched laminar flame theories are observed in that the OH LIF intensity increases when the local flame curvature becomes positive (negative) for thermodiffusively unstable (stable) flames. Departure of the peak OH LIF intensity for hydrogen flames ranges from 20% to 150% of the value at zero flame curvature for flame curvature ranging from −1.5 to 0.7 mm−1, while for propane/air flames the variation is within ±20% of the value at zero curvature. Variation in the averaged peak OH LIF intensity is nearly linear with respect to the variation in flame curvature from −1.2 to 0.8 mm−1. The flame area during interactions with Karman vortex streets increases as a relatively weak function of the vortex velocity, while the vortex size affects the flame area increase in that smaller vortices are found to be less effective in generating flame area. The effect of Lewis number on the flame front is to enhance (suppress) the amplitude of the wrinkles generated by vortices for thermodiffusively unstable (stable) flames, thus resulting in larger (smaller) flame area. The flame curvature pdfs for flames interacting with Karman vortex streets exhibit a bias toward positive flame curvature due to the large area of positively-curved flame elements that develop downstream along the V-flame. A decrease in vortex size tends to increase the flame curvature and thus broaden the pdfs, while the vortex velocity and Lewis number have relatively small effects on the flame curvature pdfs. The flame orientation distribution is peaked near the normal direction of flame propagation for small vortex velocity, while an increase in vortex velocity results in broadening of the flame orientation distribution and a shift toward larger flame angle due to the increased distortions in the flame front and increases in the effective flame propagation speed, respectively.

Flame stretch measurements during the interaction of premixed flames and Kármán vortex streets using PIV

Flame stretch measurements in a premixed, lean methane-air, V-flame interacting with a Karman vortex street have been made using a particle image velocimeter. The flame curvature and strain rate along the flame have been measured over the range of −2.0–1.0 mm−1 and −175-175 s−1, respectively, which corresponds to a range of flame stretch from −475-200 s−1. Results indicate that the curvature and strain rate are statistically independent. Flame stretch statistics based on individual flames are also considered since each flame represents a particular phase within the regular and periodic interaction cycle studied. These results indicate that for a particular flame the average stretch rate becomes independent of the average flame strain rate. A comparison among PIV measurements, OH planar laser-induced fluorescence (OH-PLIF) measurements, and numerical simulation reveals similar flame stretch statistics. Qualitative measurements concerning flame shape and the relative location of vortices within the reactant flow are also discussed.

Oxidation of Propane at Elevated Pressures: Experiments and Modelling

Abstract The oxidation of propane in air at elevated pressure was investigated in a chemical flow reactor and modelled with a comprehensive chemical kinetic model. Results are presented for pressures of 3.6. and 10 atmospheres, temperatures near 850 and 900 K, and equivalence ratio of 0.3. Gas samples were analyzed using gas chromatography with aldehydes additionally sampled using a dinitrophenylhydrazine/acetoni-trile(DNPH/ACN) procedure. Major product species observed include C3H6, C2H5. and CO: trace amounts of CH4 and C02 were detected, as well as H2 and oxygenated species including CH2O, CH3CHO, C3H60, and C2H5CHO, Fuel conversion was increased with increased pressure and temperature, and the product distribution was significantly shifted in favor of C3H6 over C2H4 with increased pressure and decreased temperature. Comparison between modelling and measured results for ethylene concentrations supported the use of Tsangs recent values for the rate of propyl radical decomposition. The model compared we...

Flame-Turbulence Interactions in a Freely-Propagating, Premixed Flame

Abstract Previous experimental investigations of turbulence measurements in conventional laboratory premixed flames are extensively reviewed. Because of numerous disadvantages of these configurations, primarily associated with flame stabilization mechanisms, a freely-propagating flame configuration is proposed for the study of flame-turbulence interactions. The unperturbed, unrestrained flow field in this configuration is ideal for measuring the effect of the flame on turbulence properties. Temporally resolved, ensemble averaged measurements of turbulence intensity, integral length scale, integral time scale, and energy spectrum have been measured both normal and parallel to the mean flame surface with laser Doppler velocimetry (LDV) in a propane-air flame at an equivalence ratio of 1.0, in a flow with incident turbulence intensity of 25 cm/s and length scale of 8 mm. The following changes in turbulence parameters were observed across the flame: a factor of five to six increase in the normal turbulence in...

Lewis number effects on premixed flames interacting with turbulent Kármán vortex streets

The effects of Lewis number on the global and local structure of premixed flames interacting with turbulent Karman vortex streets are experimentally investigated using OH planar-laser-induced fluorescence (PLIF). The OH PLIF results show that over the range of Lewis numbers studied, i.e., Le = 0.21, 0.94 and 1.79, the flame area increases and the flame front is oriented more randomly as Lewis number decreases, while the flame curvature pdfs are unchanged. The relationship between the local flame structure and the local flame curvature is found to be consistent with the results of stretched laminar flame theory. The correlation between the local maximum OH fluorescence intensity and the local curvature tends to level off for large positive curvature (H > 0.5 mm−1) as UθSL increases, indicating that the response of the flame to large flame stretch may be non-linear at high UθSL. The pdfs of peak OH LIF intensity suggest that the mean burning rate of the H2/He/air flame at UθSL = 3.3 is increased approximately by 10% in comparison to the undisturbed laminar flame. The present results imply that even though the local flame curvature may strongly influence the local structure and burning rate of nonunity Lewis number flames through the effect of flame stretch on the local burning rate, these variations tend to cancel in the mean due to the linear relationship between local burning rate and curvature for the most probable values of curvature (−0.5 mm−1 < H < 0.5 mm−1) and due to the symmetry and zero mean of the curvature distribution. Therefore, the main effect of turbulence and Lewis number is to wrinkle the flame and produce flame area, while increasing the mean burning rate per unit surface area by relatively small amount through flow strain effects.

Flame Front Geometry and Stretch During Interactions of Premixed Flames with Vortices

Abstract Flame front geometry and flame stretch during interactions of premixed flames with vortices at various configurations are computationally investigated using a kinematical relationship between vortex-induced velocity and flame propagation velocity, in conjunction with Lagrangian coordinates to represent the flame front. The maximum flame stretch that can occur during an interaction with a single vortex is found to be very close to Uθ/rm + SL/2rm, while the mean flame stretch has a maximum that asymptotes to 18-23% of this value for vortex radii between 1.25 and 5.0 mm as UθSL is increased to 2. For flames interacting with a counter-rotating vortex pair, the flame stretch near the flow symmetry axis increases when the spacing between the vortices increase to align the velocity vectors with the flame front; while the flame stretch away from this region is relatively independent of the action of the opposite vortex and is determined primarily by the vortex velocity and length scale similar to the sin...

The Dynamic Response of Turbulent Dihedral V Flames: An Amplification Mechanism of Swirling Flames

The authors analyze the dynamic response of swirl-stabilized flames submitted to upstream acoustic perturbation. Extensive measurements were performed in an optically accessible single-nozzle gas turbine combustor operating on natural gas-air at inlet temperatures of 200 and 300 °C over a range of equivalence ratios from 0.55 to 0.70, a range of inlet velocities from 60 to 100 m/s, and swirl angles of 30° and 45°. Temporal oscillations of inlet velocity and heat release rate in the whole flame were measured using the 2-microphone method and global OH*, CH*, and CO2* chemiluminescence emission intensities, respectively, whereas spatially resolved measurement of heat release rate was made using time-averaged CH* chemiluminescence flame images. For the dihedral V flames, amplification characteristic of the flame transfer function was observed. This effect is, unlike the amplification mechanism of a small laminar flame, controlled by the relative ratio of the two length scales, disturbance convective wavelength and flame length. The measured transfer functions show resonance-like behavior when a nondimensional number, the ratio of half the convective wavelength to flame length, approaches unity. It was found that the flame geometric properties, specifically flame angle, also play a crucial role in the flame transfer function. The frequency-dependent behavior of swirl-stabilized flames is closely related to eigenfrequency selection processes at limit cycle pressure oscillations.