Investigation of Adjacent Lifted Flames Interaction in an Inline and Inclined Multi-Burner Arrangement

The main objective of this research is to assess an innovative, low nitrogen oxides emission combustor concept, which has the potential to achieve the long term European emissions goals for aircraft engines. Lean lifted spray flames and their very low nitrogen oxides emissions are combined with an inclination of burners in annular combustor leading to a more compact combustor with superior stability range. The presented combustor concept was developed in the frame of the European research project CHAIRLIFT (Compact Helical Arranged combustoRs with lean LIFTed flames). CHAIRLIFT combustor concept is based on “low swirl” lean lifted spray flames, which features a high degree of premixing and consequently significantly reduced nitrogen oxides emissions and flashback risk compared to conventional swirl stabilized flames. In the CHAIRLIFT combustor concept, the lifted flames are combined with Short Helical Combustors arrangement to attain stable combustion by tilting the axis of the flames relative to the axis of the turbine to enhance the interaction of adjacent flames in a circumferential direction. A series of experimental tests were conducted at a multi-burner array test rig consisting of up to five modular burners at different burner inclination angles (0° and 45°), equivalence ratios, and relative air pressure drop at ambient conditions. For all investigated configurations, a remarkable high lean blow out for non-piloted burners (ϕLBO = 0.29–0.37), was measured. The multi-burner configurations were observed having a superior stability range in contrast to the typical decrease in stability from single to high swirl multi-burner. The unwanted flow deflection of highly swirled flames in Short Helical Combustors arrangement, could be avoided with the investigated low swirl lifted flames. Moreover, the flame chemiluminescence (OH*) measurements were used to provide a qualitative characterization of the flame topology. Complementary numerical investigations were carried out using different numbers of burners to evaluate the effect of boundary conditions.

Numerical and Experimental Flame Stabilization Analysis in the New Spinning Combustion Technology Framework

Global warming, climate change and pollution are burning environmental issues. To reduce the carbon footprint of the aviation sector, aeronautical companies have been striving to lower engine emissions via the development of reliable lean combustors. In this context, effort has been devoted to the better understanding of various flame dynamics with emphasis on thermoacoustic instabilities, lean blow-off and extinctions. In line with this effort, Safran Helicopter Engines has recently developed and patented the revolutionary spinning combustion technology (SCT) for its next generation of combustors. This technology has indeed great flexibility when it comes to ignition and blow-off capabilities. To better understand the various physical mechanisms occurring in a SCT combustor, a joint numerical and experimental analysis of the flame stabilization in this spinning combustion technology framework has been devised. On the experimental side, the NTNU atmospheric annular combustor has been modified to introduce a relevant azimuthal component of velocity while operating under premixed fuel conditions, following the SCT concept. Note that to reduce temperature at the backplane of the chamber, film cooling is incorporated to avoid fuel injector damage. On the numerical side, high fidelity Large Eddy Simulations of the test bench have been carried out with the AVBP code developed at CERFACS, providing insights on the flame stabilization in this unique SCT geometry. In particular, it is noted that there is a strong interaction between the cooling film and the highly swirled flames exiting from the fuel injector bend. In that respect, changing the injector or global equivalence ratios while operating the SCT is shown to affect the combustion of this design.

A Comparison of the Transfer Functions and Flow Fields of Flames With Increasing Swirl Number

The frequency response of premixed swirled flames is investigated by comparing their Transfer Function (FTF) between velocity and heat release rate fluctuations. The equivalence ratio and flow velocity are kept constant and four different swirling injectors are tested with increasing swirl numbers. The first injector features a vanishing low swirl number S = 0.20 and produces a flame anchored by the recirculating flow in the wake of a central bluff body. The three other swirling injectors produce highly swirled flows (S > 0.6) leading to a much larger internal recirculation region, which size increases with the swirl level. When operating the burner at S = 0.20, the FTF gain curve smoothly increases to reach a maximum and then smoothly decreases towards zero. For the highly swirled flames (S > 0.6), the FTF gain curve shows a succession of valleys and peaks attributed to interferences between axial and azimuthal velocity fluctuations at the injector outlet. The FTF phase-lag curves from the vanishing low and highly swirled flames are the same at low frequencies despite their large differences in flame length and flame aspect ratio. Deviations between the FTF phase lag curves of the different swirled flames start above the frequency corresponding to the first valley in the FTF gain of the highly swirled flames. Phase averaged images of the axial flow fields and of the flame chemiluminescence are used to interpret these features. At forcing frequencies corresponding to peak FTF gain values, the cold flow response of all flames investigated is dominated by large coherent vortical structures shed from the injector lip. At forcing frequencies corresponding to a valley in the FTF gain curve of the highly swirled flames, the formation of large coherent structures is strongly hindered in the cold flow response. These observations contrast with previous interpretations of the mechanisms associated to the low FTF response of swirled flames. It is finally found that for flames stabilized with a large swirl number, heat release rate fluctuations result both from large flame luminosity oscillations and large flame volume oscillations. For conditions leading to a small FTF gain value, both the flame luminosity and flame volume fluctuations are suppressed confirming the absence of strong perturbations within the flow at these frequencies. The experiments made in this work reveal a purely hydrodynamic mechanism at the origin of the low response of swirling flames at certain specific frequencies.

Numerical Simulation of Biomass Derived Syngas Combustion in a Swirl Flame Combustor

In this work a numerical investigation is carried out on a model combustor characterized by swirl flow conditions, fed with a biomass derived syngas fuel (which incorporates CH4, CO and H2) and operated in laboratory at atmospheric pressure. The combustor internal aerodynamics and heat release in case of syngas combustion have been simulated in the framework of CFD-RANS techniques, by means of different available models and by adopting different levels of kinetic mechanism complexity. In particular, the applicability of reduced mechanisms involving CO and H2 species and also of detailed kinetic mechanisms are assessed. The results obtained by means of the CFD simulations on the model combustor and a comparison with available experimental data on flow field and thermal field are presented in the paper. In the test-case of syngas-air swirled flames, the turbulent non premixed combustion “flamelet” model with detailed non-equilibrium chemistry, originally developed for methane-air combustion, provides encouraging results in terms of temperature distribution. Nevertheless, a simpler chemical path including the main fuel species integrated in a general purpose, widely used in industry, turbulent combustion model still provides acceptable results.