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

2016 ◽  
Author(s):  
Dominik Wassmer ◽  
Bruno Schuermans ◽  
Christian Oliver Paschereit ◽  
Jonas P. Moeck
Keyword(s):  
Temperature Measurement ◽  
Gas Turbine ◽  
Equivalence Ratio ◽  
Acoustic Pulse ◽  
Time Of Flight ◽  
Tunable Diode Laser ◽  
Phase Lag ◽  
Mean Flow ◽  
Unsteady Temperature ◽  
Flow Velocities

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.

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

2016 ◽  
Vol 139 (4) ◽  
Author(s):  
Dominik Wassmer ◽  
Bruno Schuermans ◽  
Christian Oliver Paschereit ◽  
Jonas P. Moeck
Keyword(s):  
Gas Turbine ◽  
Acoustic Waves ◽  
Fuel Injection ◽  
Acoustic Pulse ◽  
Time Of Flight ◽  
Phase Lag ◽  
Mean Flow ◽  
Unsteady Temperature ◽  
Arrival Times ◽  
Flow Velocities

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 measurement 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 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 measured 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 adhere well to a Strouhal number scaling for different mean flow velocities.

An Onion Peeling Reconstruction of the Spatial Characteristics of Entropy Waves in a Model Gas Turbine Combustor

2017 ◽  
Author(s):  
Dominik Wassmer ◽  
Felix Pause ◽  
Bruno Schuermans ◽  
Christian Oliver Paschereit ◽  
Jonas P. Moeck
Keyword(s):  
Gas Turbine ◽  
Reaction Zone ◽  
Equivalence Ratio ◽  
Acoustic Noise ◽  
Accurate Determination ◽  
Time Of Flight ◽  
Spark Discharge ◽  
Turbine Stage ◽  
Radial Temperature ◽  
Reconstructed Temperature

Entropy noise affects thermoacoustic stability in lean pre-mixed gas turbine combustion chambers. It is defined as acoustic noise that is emitted at the first turbine stage due to the acceleration of entropy waves that are advected from the reaction zone in the combustor to the turbine inlet. These non-isentropic temperature waves are caused by equivalence ratio fluctuations which are inherently present in a technically premixed combustion system. To experimentally study the generation and transport of entropy waves, an estimation of the spatial distribution of the entropy spots is highly valuable as it allows the accurate determination of the cross-section averaged entropy, which is the relevant quantity for the formation mechanism of entropy noise at the turbine stage. In this work, a time-of-flight based temperature measurement method is applied to a circular combustion test rig equipped with a premixed swirl-stabilized combustor. Downstream of the burner, an electric spark discharge is employed to generate a narrow acoustic pulse which is detected with a circumferentially arranged microphone array. The measured time of flight of the acoustic signal corresponds to the line-integrated inverse of the speed of sound between the acoustic source and each microphone. By modulating a share of the injected gaseous fuel, equivalence ratio fluctuations are generated upstream of the reaction zone and consequently entropy spots are advected through the axial measurement plane. The spark discharge is triggered at distinct phase angles of the entropy oscillation, thus allowing a time resolved-analysis of the thermo-acoustic phenomenon. Estimating the spatial temperature distribution from the measured line integrated inverse speed of sounds requires tomographic reconstruction. A Tikhonov regularized Onion Peeling is employed to deduce radial temperature profiles. To increase the number of independent data, the spark location is radially traversed, which enhances the resolution of the reconstructed temperature field. A phantom study is conducted, which allows the assessment of the capabilities of the reconstruction algorithm. By means of the reconstructed radial entropy field, spatially resolved entropy waves are measured and their amplitudes and phases are extracted. The characteristics of the entropy waves measured in this way correspond well to former studies.

Experimental Analysis of High-Amplitude Temporal Equivalence Ratio Oscillations in the Mixing Section of a Swirl-Stabilized Burner

2016 ◽  
Author(s):  
Richard Blümner ◽  
Christian Oliver Paschereit ◽  
Kilian Oberleithner ◽  
Bernhard Ćosić
Keyword(s):  
Equivalence Ratio ◽  
Near Infrared ◽  
Fuel Injection ◽  
High Amplitude ◽  
Operating Conditions ◽  
Tunable Diode Laser ◽  
Mean Flow ◽  
Injection Point ◽  
Driving Mechanisms ◽  
Acoustic Forcing

Unsteady temporal fluctuations of the equivalence ratio in lean premixed gas turbine combustors are one of the most important driving mechanisms for thermoacoustic instabilities. In this work, high-amplitude equivalence ratio fluctuations in the mixing section of a swirl-stabilized burner are assessed for the first time. The applied non-intrusive sensor is based on fixed-wavelength modulation spectroscopy of methane at 1653 nm using a near-infrared tunable diode laser. The measurements are performed at isothermal operating conditions without the presence of a flame at 25°C and at atmospheric pressure. The equivalence ratio fluctuations are generated by acoustic forcing of the air flow while the fuel injection flow rate is kept constant. Acoustic forcing amplitudes up to 220% of the mean flow velocity are assessed. Measurements are conducted at different axial distances from the fuel injection point to study the spatio-temporal evolution of the equivalence ratio fluctuations. The results show a frequency-dependent saturation of temporal equivalence ratio fluctuations with increasing forcing amplitude, which can not be described through the available model. These results are in good agreement with preceding studies and indicate the saturation of the flame response due to a saturation of equivalence ratio fluctuations. Furthermore, a decreased attenuation of temporal mixture inhomogeneities for small forcing amplitudes is found.

Performance Prediction of Gas Turbine Under Different Strategies Using Low Heating Value Fuel

2013 ◽  
Author(s):  
Edson Batista da Silva ◽  
Marcelo Assato ◽  
Rosiane Cristina de Lima
Keyword(s):  
Natural Gas ◽  
Gas Turbine ◽  
Equivalence Ratio ◽  
Safe Operation ◽  
Engine Operation ◽  
Heating Value ◽  
Gasification Process ◽  
Surge Margin ◽  
And Performance ◽  
Industrial Gas Turbine

Usually, the turbogenerators are designed to fire a specific fuel, depending on the project of these engines may be allowed the operation with other kinds of fuel compositions. However, it is necessary a careful evaluation of the operational behavior and performance of them due to conversion, for example, from natural gas to different low heating value fuels. Thus, this work describes strategies used to simulate the performance of a single shaft industrial gas turbine designed to operate with natural gas when firing low heating value fuel, such as biomass fuel from gasification process or blast furnace gas (BFG). Air bled from the compressor and variable compressor geometry have been used as key strategies by this paper. Off-design performance simulations at a variety of ambient temperature conditions are described. It was observed the necessity for recovering the surge margin; both techniques showed good solutions to achieve the same level of safe operation in relation to the original engine. Finally, a flammability limit analysis in terms of the equivalence ratio was done. This analysis has the objective of verifying if the combustor will operate using the low heating value fuel. For the most engine operation cases investigated, the values were inside from minimum and maximum equivalence ratio range.

The Effect of Transient Fuel Staging on Self-Excited Instabilities in a Multi-Nozzle Model Gas Turbine Combustor

2017 ◽  
Author(s):  
Wyatt Culler ◽  
Janith Samarasinghe ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca ◽  
Jacqueline O’Connor
Keyword(s):  
Gas Turbine ◽  
Gas Turbines ◽  
High Speed ◽  
Equivalence Ratio ◽  
Growth Time ◽  
Stable Operation ◽  
Time Constants ◽  
Decay Times ◽  
Fuel Staging ◽  
Passive Techniques

Combustion instability in gas turbines can be mitigated using active techniques or passive techniques, but passive techniques are almost exclusively used in industrial settings. While fuel staging, a common passive technique, is effective in reducing the amplitude of self-excited instabilities in gas turbine combustors at steady-state conditions, the effect of transients in fuel staging on self-excited instabilities is not well understood. This paper examines the effect of fuel staging transients on a laboratory-scale five-nozzle can combustor undergoing self-excited instabilities. The five nozzles are arranged in a four-around-one configuration and fuel staging is accomplished by increasing the center nozzle equivalence ratio. When the global equivalence ratio is φ = 0.70 and all nozzles are fueled equally, the combustor undergoes self-excited oscillations. These oscillations are suppressed when the center nozzle equivalence ratio is increased to φ = 0.80 or φ = 0.85. Two transient staging schedules are used, resulting in transitions from unstable to stable operation, and vice-versa. It is found that the characteristic instability decay times are dependent on the amount of fuel staging in the center nozzle. It is also found that the decay time constants differ from the growth time constants, indicating hysteresis in stability transition points. High speed CH* chemiluminescence images in combination with dynamic pressure measurements are used to determine the instantaneous phase difference between the heat release rate fluctuation and the combustor pressure fluctuation throughout the combustor. This analysis shows that the instability onset process is different from the instability decay process.

scholarly journals Smoke Characteristics of Distillate and Residual Fuel Burning in Gas Turbine Combustors

1980 ◽  
Author(s):  
T. M. Liu ◽  
R. M. Washam
Keyword(s):  
Trace Metals ◽  
Chemical Analysis ◽  
Gas Turbine ◽  
Equivalence Ratio ◽  
Appreciable Amount ◽  
High Concentration ◽  
Gas Turbine Combustors ◽  
Fuel Burning ◽  
Distillate Fuel

During the development of a rich-lean staged dry low NOx combustor, the conventional trend of increasing smoke with increasing operating equivalence ratio was found when tests were run with distillate fuel (%H = 13.0). However, when tests were run with residual fuel (%H = 11.4), the trend was reversed. In addition, when the same combustor was run with blends of distillate fuel and residual fuel, a drastic improvement of smoke was observed when only 6 percent of residual fuel was mixed with distillate fuel, and for any blending of more than 10 percent of residual fuel the combustor was practically smoke free. A chemical analysis of fuel samples revealed an appreciable amount of trace metals in the residual fuel, giving rise to the suspicion that the smoke reduction may have been due in part to these trace metals. Of these elements found, vanadium is believed to be the most likely to cause smoke reduction because of its relatively high concentration.

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