Abstract
Sound waves in a combustor are generated from fluctuations in the heat release rate (direct noise) or the acceleration of entropy, vorticity or compositional perturbations through nozzles or turbine guide vanes (indirect or entropy noise). These sound waves are transmitted downstream as well as reflected upstream of the acceleration point, contributing to the overall noise emissions, or triggering combustion instabilities. Previous experiments attempted to isolate indirect noise by generating thermoacoustic hot spots electrically and measuring the transmitted acoustic waves, yet there are no measurements on the backward propagating entropy and acoustic waves. This work presents the first measurements which clearly separate the direct and indirect noise contributions to pressure fluctuations upstream of the acceleration point. Synthetic entropy spots are produced by unsteady electrical heating of a grid of thin wires located in a tube. Compression waves (direct noise) are generated from this heating process. The hot spots are then advected with the mean flow and finally accelerated through an orifice plate located at the end of the tube, producing a strong acoustic signature which propagates upstream (indirect noise). The convective time is selected to be longer than the heating pulse length, in order to obtain a clear time separation between direct and indirect noise in the overall pressure trace. The contribution of indirect noise to the overall noise is shown to be non-negligible either in subsonic or sonic throat conditions. However, the absolute amplitude of direct noise is larger than the corresponding fraction of indirect noise, explaining the difficulty in clearly identifying the two contributions when they are merged. Further, the work shows the importance of using appropriate pressure transducer instrumentation and correcting for the respective transfer functions in order to account for low frequency effects in the determination of pressure fluctuations.
Highlights
Acoustic perturbations arising from the heat release in combustion devices are a topic of increasing concern due to stricter noise regulations
The response of the system to the generation and convection of synthetic hot spots was measured for four cases: (A) open tube with flow, the tube is terminated with an open end; (B) closed tube with no flow, the tube is terminated with a rigid cap; (C) accelerated flow, the tube is terminated with the 6.6 mm hole orifice plate; (D) accelerated flow, the tube is terminated with the 3.0 mm hole orifice plate, which is choked
As the flow rate increases, the time separation between the direct and the indirect noise peaks becomes shorter, due to the decrease in the convective time. These results suggest an important issue in the identification of indirect noise: increasing the flow velocity can increase the relative contribution of indirect noise through higher acceleration in the nozzle, yet the convective time of hot spots decreases
Summary
These entropy, vorticity and compositional waves are not directly associated with any pressure fluctuations in the linear regime As they convect through regions with mean flow gradients (such as through turbine vanes or exhaust nozzles) acoustic waves are created, generating indirect combustion noise. Due to the small temperature increase achieved (1 K) and the poor resolution of the data acquisition system available direct and indirect noise could not be separated This method of generating hot spots was applied more recently in the Entropy Wave Generator (EWG) rig developed at DLR Berlin, to study indirect combustion noise [17,18]. There have been no measurements of the upstream entropy noise generated by the acceleration of synthetic hot spots: the experimental data reported so far refers only to the transmitted acoustic waves (acquired downstream of the nozzle). In the ultra low frequency range, they behave as a high pass filter, leading to potentially erroneous outputs
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