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Lean fuel burning is considered as one of the most prominent methods to achieve the low temperature combustion reduces nitrogen oxides (NOx) emissions. However, as a result lean burning suffers from strong flame instabilities and a very lean operating condition near to the flame quenching limit is associated with the high risk of lean blowout (LBO) which has to be avoided. Therefore, the capability to sense the origin of LBO will provide substantial settlements to improve combustor reliability in order to achieve optimum performance, to extend engine life and to reduce maintenance cost. From the last few decades, many engine manufacturers and research institutions are working on the novel approaches based on the lean combustion concepts. For achieving desired objects, several passive control technologies were studied to improve the flame stabilization by modifying the injector geometry or flame holders in a permanent way. Adding to this, Non thermal plasma assisted combustion (NTPAC) technology is the most encouraging techniques to actively control the control the flow and combustion dynamics in consonance with real-time operating conditions without altering the combustor design. NTPAC improves the efficiency of combustors, increases flame stabilization, enhances low temperature oxidation, and extends the LBO limits, in the meantime it also reduces the significant amount of nitrogen oxides (NOx). From literature review it has been concluded that the effects of NTPAC is broadly classified into three ways: thermal effects, kinetic effects, and transport effects. Among all the NTPAC technologies, the dielectric barrier discharge (DBD) is widely considered and credit goes to the simplicity of its design and the wide range of operating modes.However, the effects of NTPAC on flame stability and LBO limits are not clearly understood yet, due to the complexity of each specific process involves in the plasma combustion. Therefore, for the better understanding of NTPAC processes, an experimental study was performed which investigates the behavior of premixed methane/air flames close to LBO limits at different operating conditions using plasma assisted lifted swirl combustor. The combustor was equipped with ring-needle shaped plasma actuator operated by sinusoidal generator at the maximum frequency of 20 kHz directly coupled with flame near to the nozzle. The test system was covered by faraday cage to avoid interference of electromagnetic fields produced through plasma discharge. The plasma actuator configuration involves a tungsten needle having a diameter of 1 mm and length equal to 46 mm. The needle was placed at the center of nozzle which referred as a high voltage (HV) electrode. However, the nozzle was acted as a grounded (GND) electrode. Electrical characterization of combustion characteristics was performed at different plasma actuation conditions and air/fuel mass flow rates. Flame behavior was also captured by high resolution CCD camera by predicting the recirculation zone produced by plasma, have the benefits of generating compact flames and allowing the high reaction rates. Acoustic measurements were performed using high sensitivity microphone. Furthermore, post processing of the microphone data involved application of Fast Fourier Transform (FFT). |
