Abstract:In order to evaluate the direct and indirect contributions to the total combustion noise emission, a combustion chamber consisting of a swirl burner and an exit nozzle of Laval-shape, representing a gas turbine combustor, is investigated by means of experiments and large eddy simulation. Focused on the isothermal flow case first and encouraged by a good overall agreement between the LES and the experimental data for the flow field, a first characterisation of the flow with respect to noise sources is performed… Show more
“…For validation purpose and to map the flow field topology, the flow velocity field was measured using a quartz glass combustion chamber with square cross section and a threecomponent Laser-Doppler-System. Results were reported in [13], [14], and [15].…”
Strong evidence is presented that entropy noise is the major source of external noise in aero-engine combustion. Entropy noise is generated in the outlet nozzles of combustors. Low frequency entropy noise — which was predicted earlier in theory and numerical simulations — was successfully detected in a generic aero-engine combustion chamber. It is shown that entropy noise dominates even in the case of thermo-acoustic resonances. In addition to this, a different noise generating mechanism was discovered that is presumably of even higher relevance to jet engines: There is strong evidence of broad band entropy noise at higher frequencies (1 kHz to 3 kHz in the reported tests). This unexpected effect can be explained by the interaction of small scale entropy perturbations (hot spots) with the strong pressure gradient in the outlet nozzle. The direct combustion noise of the flame zone seems to be of minor importance for the noise emission to the ambiance. The combustion experiments were supplemented by experiments with electrical heating. Two different methods for generating entropy waves were used, a pulse excitation and a sinusoidal excitation. In addition, high-frequency entropy noise was generated by steady electrical heating.
“…For validation purpose and to map the flow field topology, the flow velocity field was measured using a quartz glass combustion chamber with square cross section and a threecomponent Laser-Doppler-System. Results were reported in [13], [14], and [15].…”
Strong evidence is presented that entropy noise is the major source of external noise in aero-engine combustion. Entropy noise is generated in the outlet nozzles of combustors. Low frequency entropy noise — which was predicted earlier in theory and numerical simulations — was successfully detected in a generic aero-engine combustion chamber. It is shown that entropy noise dominates even in the case of thermo-acoustic resonances. In addition to this, a different noise generating mechanism was discovered that is presumably of even higher relevance to jet engines: There is strong evidence of broad band entropy noise at higher frequencies (1 kHz to 3 kHz in the reported tests). This unexpected effect can be explained by the interaction of small scale entropy perturbations (hot spots) with the strong pressure gradient in the outlet nozzle. The direct combustion noise of the flame zone seems to be of minor importance for the noise emission to the ambiance. The combustion experiments were supplemented by experiments with electrical heating. Two different methods for generating entropy waves were used, a pulse excitation and a sinusoidal excitation. In addition, high-frequency entropy noise was generated by steady electrical heating.
“…At smaller fluid dynamic timescales relative to the chemical timescales (Da= f / c ~ 0.3), reaction zones can be stretched to include (localised) extinction and elongation of the flame [95]. In non-reacting studies where a Dual Swirl Burner type nozzle has been longitudinally coupled with a combustion chamber followed by a narrowing nozzle (in the downstream), the upstream PVC motion can also trigger higher frequency instabilities at the downstream nozzle exit [96].…”
“…Many experimental and computational studies have sought to explain the mechanisms of combustion instability. 7–17 In a swirl-stabilized combustion system, the unsteady vortical structures formed on either side of the flame lead to unsteady heat release rates that can drive the instability. 18 Figure 1.Illustration of the flow field and heat release zones within (a) swirl-stabilized combustor and (b) swirl-stabilized combustor with porous insert.…”
In this study, a porous insert is placed at the dump plane of a swirl-stabilized lean premixed combustor to passively suppress thermoacoustic instabilities. The diffuser-shaped annular ring of porous inert material influences the turbulent flow field directly, including recirculation zones and vortical and/or shear layer structures to passively control the acoustic performance of the combustor. The porous inert material is made of silicon carbide-hafnium carbide coated, high-strength, high-temperature-resistant open-cell foam materials. In this study, the porous insert concept is investigated at above-ambient operating pressures to demonstrate its suitability for practical combustion applications. Experiments are conducted in quartz and metal combustors, without and with the porous insert while varying operating pressure, equivalence ratio, and reactant flow rate. Measurements show that the porous insert, and consequent changes in the combustor flow field, decrease the sound pressure levels at the frequency of combustion instability at all operating conditions investigated in this study. The porous insert also decreases the broadband combustion noise, i.e. the measured sound pressure levels over a wide frequency range.
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