An Acoustic Time-of-Flight Approach for Unsteady Temperature Measurements: Characterization of Entropy Waves in a Model Gas Turbine Combustor

Abstract

Lean premixed combustion promotes the occurrence of thermoacoustic phenomena in gas turbine combustors. One mechanism that contributes to the flame-acoustic interaction is entropy noise. Fluctuations of the equivalence ratio in the mixing section cause the generation of hot spots in the flame. These so called entropy waves are convectively transported to the first stage of the turbine and generate acoustic waves that travel back to the flame; a thermoacoustic loop is closed. However, due to the lack of experimental tools, a detailed investigation of entropy waves in gas turbine combustion systems has not been possible up to now. This work presents an acoustic time-of-flight based temperature measurement method which allows the detection of temperature fluctuations in the relevant frequency range. A narrow acoustic pulse is generated with an electric spark discharge close to the combustor wall. The acoustic response is measured at the same axial location with an array of microphones circumferentially distributed around the combustion chamber. The delay in the pulse arrival times corresponds to the line-integrated inverse speed of sound. For validation of this new method an experimental setup was developed capable of generating well defined entropy waves. As a reference temperature measurement technique a hot-wire anemometer is employed. For the measurement of entropy waves in an atmospheric combustion test rig, fuel is periodically injected into the mixing tube of a premixed combustor. The subsequently generated entropy waves are detected for different forcing frequencies of the fuel injection and for different mean flow velocities in the combustor. The amplitude decay and phase lag of the entropy waves adheres well to a Strouhal number scaling for different mean flow velocities. In addition, simultaneously to the entropy wave measurement, the equivalence ratio fluctuations in the mixing tube are detected using the Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique.

Flow Inhomogeneities in a Realistic Aeronautical Gas-Turbine Combustor: Formation, Evolution, and Indirect Noise

“…The flow in a gas-turbine combustor is usually characterized by a high level of unsteadiness to ensure rapid mixing in a limited space. Although rapid mixing and formation of a homogeneous mixture allows for a better control of the temperature and emissions, the highly unsteady nature of the reacting flow, the cooling flows and the requirement of compactness could generate combustion inhomogeneities that reach the combustor's exit [1,2]. Typical examples of flow inhomogeneities are temperature inhomogeneities (also known as entropy spots) [3] and fluctuations of the mixture composition [4,5] (also known as compositional blobs).…”

“…Non-uniform velocity profiles and turbulent mixing may decrease the strength of flow inhomogeneities as they are transported towards the combustor's exit. The dispersion and diffusion of entropy spots have been analyzed in simple geometries [7,8] and configurations close to realistic combustors [2,9] using both experiments and numerical simulations. As discussed in [9,10], the role of turbulent mixing on the attenuation of entropy spots is expected to be significant in real geometries and strongly dependent on the configuration.…”

Flow Inhomogeneities in a Realistic Aeronautical Gas-Turbine Combustor: Formation, Evolution and Indirect Noise

“…Non-uniform velocity profiles and turbulent mixing may decrease the strength of flow inhomogeneities as they are transported towards the combustor's exit. The dispersion and di↵usion of entropy spots have been analyzed in simple geometries [7,8] and configurations close to realistic combustors [2,9] using both experiments and numerical simulations. As discussed in [9,10], the role of turbulent mixing on the attenuation of entropy spots is expected to be significant in real geometries and strongly dependent on the configuration.…”

Non-intrusive acoustic measurement of flow velocity and temperature in a high subsonic Mach number jet

“…In recent years, the use of acoustic measurements for nonintrusive gas turbine engine flow monitoring has been proposed and several studies have been conducted to assess its feasibility in practical applications [1][2][3][4][5]. An acoustic approach is of par ticular interest because it offers several unique advantages over optical methods (such as particle image velocimetry) in gas turbine engine environments.…”