Vn Viktor Kornilov

Accuracy assessment of thermoacoustic instability models using binary classification

We apply binary classification theory to assess the (in)stability prediction accuracy of thermoacoustic models. It is shown that by applying such methods to compare a large set of stability predictions and experiments it is possible to gain valuable qualitative insight in different aspects of prediction quality. The approach is illustrated with a 2-port model and a large experimental data set. The presented framework provides an unified and practical tool to answer questions such as (i) What is the chance that a stable prediction will be correct? and (ii) How conservative is the model? It is shown that the most suitable quality indicator is strongly dependent on the actual purpose of the model. The method provides a solid starting point for model comparison and optimization.

NOVEL BURNER CONCEPT FOR PREMIXED SURFACE-STABILIZED COMBUSTION

Surface-stabilized combustion is credited with high burning rates, extended lean flammability limits, wide modulation range and other advantages. This makes it an attractive technology for compact low-emission combustors. The experimental gas turbine surface burners reported to this date are produced from compressed and sintered Fe-Cr-Al fiber mats. The authors have developed a new concept of surface burner fabricated by braiding ceramic cords around a ceramic frame. This simple method produces a basket-type surface suitable for stabilizing lean premixed flames over a broad range of operating conditions. The use of ceramics extends possibilities for operation at very high inlet temperatures with reduced risks of material sintering and oxidation. This paper presents test results with an experimental burner on a pressurized combustion rig with optical access. The experiments were performed under the following conditions: inlet temperatures of 22-740 C, pressures of 1-3 bar, thermal power between 4 kWTh and 32 kWTh and equivalence ratios of 0.28-0.95. Measurements of flue gas composition and pressure drop are also reported in the paper. The operating window for low-NOx and low-CO combustion is analyzed. With the demonstrated performance, the burner could cover the operating envelope of a 3 kWe recuperated micro turbine [1]-[2] with no pilot and no staging. This would also limit NOx to <40 ppm @ 0% O 2 within the micro turbine load range of 100% to 50%. NOMENCLATURE

Intrinsic thermoacoustic modes and their interplay with acoustic modes in a Rijke burner

The interplays between acoustic and intrinsic modes in a model of a Rijke burner are revealed and their influence on the prediction of thermoacoustic instabilities is demonstrated. To this end, the system is examined for a range of time delays, temperature ratios and reflection coefficients as adjustable parameters. A linear acoustic network model is used and all modes with frequency below the cut-on frequency for non-planar acoustic waves are considered. The results show that when reflection coefficients are reduced, the presence of a pure ITA mode limits the reduction in the growth rate that usually results from a reduction of the reflection coefficients. In certain conditions, the growth rates can even increase by decreasing reflections. As the time delay of the flame and thus the ITA frequency decreases, the acoustic modes couple to and subsequently decouple from the pure ITA modes. These effects cause the maximum growth rate to alternate between the modes. This investigation draws a broad picture of acoustic and intrinsic modes, which is crucial to accurate prediction and interpretation of thermoacoustic instabilities.