Experimental quantification of transient stretch effects from vortices interacting with premixed flames

The understanding of complex premixed combustion reactions is paramount to the development of new concepts and devices used to increase the overall usefulness and capabilities of current technology. The complex interactions which occur within any modern practical combustion device were studied by isolating a single turbulent scale of the turbulence-chemistry interaction. Methane-air flame equivalence ratios (&phis; = 0.64, 0.90, and 1.13) were chosen to observe the mild affects of thermo-diffusive stability on the methane-air flame. Nitrogen was used as a diluent to retard the flame speeds of the &phis; = 0.90, and 1.13 mixtures so that the undisturbed outwardly propagating spherical flame kernel propagation rates, drf/dt, were approximately equal. Five primary propane equivalence ratios were utilized for investigation: &phis; = 0.69, 0.87, 1.08, 1.32, and 1.49. The choice of equivalence ratio was strategically made so that the &phis; = 0.69/1.49 and &phis; = 0.87/1.32 mixtures have the same undiluted flame propagation rate, drf/dt. Therefore, in the undiluted case, there are three flame speeds (in laboratory coordinates, not to be confused with burning velocity) represented by these mixtures. Three vortices were selected to be used in this investigation. The vortex rotational velocities were measured to be 77 cm/s, 266 cm/s and 398 cm/s for the "weak", "medium" and "strong" vortices, respectively. Ignition of the flame occurred in two ways: (1) spark-ignition or (2) laser ignition using an Nd:YAG laser at its second harmonic (lambda = 532 nm) in order to quantify the effect of electrode interference.;Accompanying high-speed chemiluminescence imaging measurements, instantaneous pressure measurements were obtained to give a more detailed understanding of the effect of vortex strength on the overall flame speed and heat release rate over an extended time scale and to explore the use of a simple measurement to describe turbulent mixing. Further local flame-vortex interface analysis was conducted using non-invasive laser diagnostics, such as particle image velocimetry and planer laser induced fluorescence of the OH radical. The dependence of heat release rate on temperature provides an estimation of the strain rate dependence of the reaction rate.;Findings include a direct effect of stretch rate on temperature along the flame reaction zone; however the trend depends on thermo-diffusive stability. For both fuels at a thermo-diffusively unstable mixture, by increasing the vortex strength, the stretch rate increases, while the temperature decreases. The thermo-diffusively stable flames (propane with &phis; ≤ 1.08) generally show an opposite trend to the unstable flames. By increasing the vortex strength, the thermo-diffusively stable flames tend to have a reduction in stretch rate, which allows an increase in temperature along the flame-vortex interface. Overall, an increase in stretch rate causes all flames to reduce the average reaction zone temperature regardless of thermo-diffusive stability. The formation of surface structure corrugation in thermo-diffusively unstable flames requires an external (to the flame) perturbation. This perturbation is generally in the form heat loss due to either a high stretch rate from an aerodynamic perturbation (vortex) or a localized thermodynamic heat sink (electrodes).