A systematic modeling strategy for non-premixed flame under resonant condition was investigated experimentally and numerically.
In order to achieve the resonant condition, a non-premixed type dump combustor having a rearward step was constructed. Deliberating the application of acoustic excitation (AE), acoustic drivers composed of a loud speaker and an audio amplifier were installed to the combustor. The mixing between fuel and oxidizer was guaranteed by a set of coaxial nozzles where the oxidizer nozzle was located inside of the fuel nozzle and the diameter of the oxidizer nozzle was increased as large as possible.
Detailed measurements on acoustic characteristics, turbulence, temperature distribution and NOx emission were carried out with and without AE in the combustor.
The combustor was governed by one major resonant frequency and the frequency is that of the longitudinal mode. The resonant frequency is a function of overall equivalence ration and supplying flow rate.
Direct photographs showed the flame structure; the flame is confined within the shear layer and has the shape of a hollow cylinder. The AE shifts the flame upstream.
Phase-resolved CH chemiluminescence images enabled to observe transient behavior of the flame and the role of high temperature burnt gas on the sustenance of the resonance.
Velocity measurement based upon triple decomposition on instantaneous velocity revealed the effects of the AE and chemical reactions. The AE increases the turbulence intensity and decreases the centerline velocity, which enhance the mixing and the consumption of the fuel. An oscillation of large amplitude at the exit of the combustor with the chemical reactions is due to the acoustic condition of the combustor.
Temperature contours by using R-type thermocouple showed the monotonic decrease of temperature after the main reaction zone and the complex structure just above the dump plane. However temperature distribution along the centerline of the shear layer showed the consistency.
Deliberating the experimental results, homogeneous properties in the shear layer and a fully developed flow after the main reaction zone were confirmed. As the consequence, a proper modeling strategy was proposed; One-dimensional modeling on the centerline of the shear layer.
The one-dimensional code was constructed including linear eddy model (LEM) for turbulence modeling, 2 step overall reaction with modified eddy break-up (EBU) model for chemical reactions, and Zeldovich mechanism for NOx emission. Especially a careful consideration on the turbulence modeling was possible by modulating the parameters concerned with the LEM. Initial and boundary conditions for the code were extracted from the experimental results.
With the carefully calibrated input variables for the simulation, the measured temperature and NOx emission were successfully predicted.
Numerical experiments were conducted to reveal the effects of AE on the reduction of NOx emission. The simulation was focused on revealing the effects of turbulence modulation by the AE, which is incorporated in the changes of integral length, inlet velocity and length of the shear layer for the LEM. The cold fluid entrainment suppresses excessive temperature increase while the increased reaction rate and shortened shear layer enhances the consumption of fuel more rapidly and thus reduces the volume of high temperature region. We concluded that NOx reduction mechanism in a acoustically driven dump combustor is the decrease of residence time in high temperature region.
