Experimental Investigation of the Stability Mechanism and Emissions of a Lifted Swirl Non-Premixed Flame

Abstract

We report on the experimental investigation of a confined lifted swirl non-premixed flame by applying a novel Airblast nozzle [1]. 3D-Laser Doppler Anemometry, a non-intrusive, laser-based measurements technique, is adapted for the measurement of all three mean velocity components and of the six Reynolds stress components. The determination of the temperature and mixture field occurs by employing in-flame measurement techniques. Valuable information concerning the mixing procedure, the temperature distribution, the turbulence level and the velocity field of the flame is provided. The results demonstrate that there is sufficient residence time in the pre-combustion area of the lifted flame in order to achieve spatial and temporal uniformity of the mixture, leading to a quasi premixed state. It was also found that hot reaction products, carried upstream by large vortices along the jet, could possibly react with fresh unburnt mixture in a reignition process. The determination of the flow pattern revealed the presence of an inner weak recirculation zone in the nozzle vicinity and a dominant external recirculation zone. The examination of the probability density function (PDF) of the velocity-measurements was also found to be a very useful tool in terms of the analysis of the turbulence structure of the flow. The bimodal distribution yield the existence of large scaled eddies in the shear layer between the downstream flow and the recirculated gases. Finally, the significant reduced NOx emissions in the lean area were also shown by means of emission measurements for elevated pressure conditions.

Experimental and Numerical Investigation of Swirl Induced Self-Excited Instabilities at the Vicinity of an Airblast Nozzle

“…The applied configuration was chosen at 30 • off-axis forward scattering, in order to ensure a high signal to noise ratio and support the performance of high data rate measurements. Detailed measurements of the velocity and turbulence field are provided in a previous work [16]. Power spectral density was assessed by means of Fast Fourier Transform technique (FFT).…”

Large Eddy Simulations of a Low-Swirl Gaseous Partially Premixed Lifted Flame in Presence of Wall Heat Losses

“…This kind of stabilization, as mentioned, is allowed by the upstream transport of hot vitiated gas from the principal reaction zones due to the ORZ: its relevance is such that the flame cannot be ignited without confinement walls [5]. Furthermore, the temperature of recirculating gas has a primary role in the ignition of incoming fresh mixture [7]. This fact also highlights the importance of the heat losses through confinement walls, which could cool the transported products and thus postpone the mixture ignition and elevate the lift-off height.…”

“…The test rig considered here employs a confined low-swirl lifted flame investigated by Fokaides et al at the Engler-Bunte Institut of the Karlsruhe Institute of Technology (KIT-EBI) [5,7]. The key feature of this burner is a radial double-swirl nozzle [14] with an overall theoretical swirl number S th,o below 0.4, where this number is defined as:…”

“…A lean global equivalence ratio φ equal to 0.65 is employed to investigate the flame stabilization and capability. Furthermore, the flame has been observed to also be stable for leaner operating conditions, when air pre-heating is increased [5,7,11].…”

“…The latter one is particularly interesting since it is similar to a usual aero-engine nozzle, but with better performance in terms of NO x reduction [5,6]. It has also demonstrated improved stability in terms of LBO occurrence [7,8], thus possibly allowing it to operate with even leaner equivalence ratios. Although many experimental and numerical investigations have been performed on lifted flames in recent years, a complete understanding of the flame-stabilization mechanism is still missing.…”