Measurement and Analysis of Flame Transfer Function in a Sector Combustor Under High Pressure Conditions

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. A flame transfer function describes the change in the rate of heat release in response to perturbations in the inlet flow as a function of frequency. It is a quantitative assessment of the susceptibility of combustion to disturbances. The resulting fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result. Flame transfer functions for LPP combustion are poorly understood at present but are crucial for predicting combustion oscillations. This paper describes an experiment designed to measure the flame transfer function of a simple combustor incorporating realistic components. Tests were conducted initially on this combustor at atmospheric pressure (1.2 bar and 550 K) to make an early demonstration of the combustion system. The test rig consisted of a plenum chamber with an inline siren, followed by a single LPP premixer/duct and a combustion chamber with a silencer to prevent natural instabilities. The siren was used to induce variable frequency pressure/acoustic signals into the air approaching the combustor. Both unsteady pressure and heat release measurements were undertaken. There was good coherence between the pressure and heat release signals. At each test frequency, two unsteady pressure measurements in the plenum were used to calculate the acoustic waves in this chamber and hence estimate the mass-flow perturbation at the fuel injection point inside the LPP duct. The flame transfer function relating the heat release perturbation to this mass flow was found as a function of frequency. The same combustor hardware and associated instrumentation were then used for the high pressure (15 bar and 800 K) tests. Flame transfer function measurements were taken at three combustion conditions that simulated the staging point conditions (Idle, Approach and Take-off) of a large turbofan gas turbine. There was good coherence between pressure and heat release signals at Idle, indicating a close relationship between acoustic and heat release processes. Problems were encountered at high frequencies for the Approach and Take-off conditions, but the flame transfer function for the Idle case had very good qualitative agreement with the atmospheric-pressure tests. The flame transfer functions calculated here could be used directly for predicting combustion oscillations in gas turbine using the same LPP duct at the same operating conditions. More importantly they can guide work to produce a general analytical model.

Thermodynamic Performance of Wet Compression and Regenerative (WCR) Gas Turbine

Thermodynamic performance of wet compression and regenerative (WCR) gas turbine are investigated in this paper. The regenerative process can be achieved by a gas/air (and steam) heat exchanger, a regenerator, or by a heat recovery steam generator and then the steam injected into the gas turbine. Several schemes of the above wet compression and regenerative cycles are computed and analyzed. The calculated results indicate that not only a significant specific power can be obtained, but also is the WCR gas turbine an economic competitive option of efficient gas turbines.

Numerical Prediction of Partially Premixed Flames Based on Extended BML Model Coupled With Mixing Transport and ILDM Chemical Model

To improve the numerical prediction of partially premixed flames occurring in gas turbine combustors the extension of the well-known Bray-Moss-Libby model for premixed combustion is presented. The model modification based on the algebraic closure for a mean chemical source term is coupled to the mixing transport model providing variable equivalence ratio distinguishing partially premixed flames. Finite rate chemistry is incorporated by means of ILDM model solving transport equations for two reaction progress variables conditioned on the flame front. Multivariate presumed PDF model is used for the turbulence chemistry interaction treatment. Turbulence models of two levels of complexity are applied in order to investigate the influence of non-gradient turbulent transport phenomenon. Redistribution terms in second moment transport equations are extended to take into account strongly variable density effects. Model combinations considered are assessed simulating piloted partially premixed flame. The obtained results agree well with experimental data.

Statistical Methodologies for Reliability Assessment of Gas Turbine Measurements

All measurements, although taken as accurately as possible, are subjected to uncertainty. So the analysis of errors and uncertainty is crucial in all applications since such errors need to be estimated and, when possible, reduced. In particular, when gas turbine mathematical models based on the processing of field measurements (such as the Gas Path Analysis models) are used, the evaluation of measurement reliability is a key point. In fact, it has been demonstrated that these kinds of techniques are sensitive to measurement errors: thus, tools for field data processing to evaluate the presence of the so-called outliers are advisable. In this paper, some statistical methodologies for the assessment of the reliability of the measurements taken on a gas turbine are presented. The methodologies, taken from literature and used for historical measurements, are discussed. Moreover, a new methodology, based on a modified t-Student distribution, is proposed.

Surface-Stabilized Fuel Injectors With Sub-Three ppm NOx Emissions for a 5.5 MW Gas Turbine Engine

ALZETA Corporation has developed surface-stabilized fuel injectors for use with lean premixed combustors which provide extended turndown and ultra-low NOx emission performance. These injectors use a patented technique to form interacting radiant and blue-flame zones immediately above a selectively-perforated porous metal surface. This allows stable operation at low reaction temperatures. A previous ASME paper (IJPGC2002-26088) described the development of this technology from the proof-of-concept stage to prototype testing. In 2002 development of these fuel injectors for the 5.5 MW turbine accelerated. Additional single-injector rig tests were performed which also demonstrated ultra-low emissions of NOX and CO at pressures up to 1.68 MPa (16.6 atm) and inlet temperatures up to 670 °K (750 °F). A pressurized multi injector ‘sector rig’ test was conducted in which two injectors were operated simultaneously in the same geometric configuration as that expected in the engine combustor liner. The multi-injector package was operated with various combinations of fired and unfired injectors, which resulted in low emissions performance and no adverse affects due to injector proximity. To date sub-3 ppm NOx emissions with sub-10 ppm CO emissions have been obtained over an operating range of 0.18 to 1.68 MPa (1.8 to 16.6 atm), inlet temperatures from 340 to 670 °K (186 to 750 °F), and adiabatic flame temperatures from 1740 to 1840 °K (2670 to 2850 °F). A full scale multi-injector engine simulation is scheduled for the beginning of 2003, with engine tests beginning later that year.

Suppression of Instabilities in Gaseous Fuel High-Pressure Combustor Using Non-Coherent Oscillatory Fuel Injection

This paper describes the application of active, open loop, control in effective damping of severe combustion instabilities in a high pressure (i.e., around 520 psi) gas turbine combustor simulator. Active control was applied by harmonic modulation of the fuel injection rate into the combustor. The open-loop active control system consisted of a pressure sensor and a fast response actuating valve. To determine the dependence of the performance of the active control system upon the frequency, the fuel injection modulation frequency was varied between 300 and 420 Hz while the frequency of instability was around 375 Hz. These tests showed that the amplitude of the combustor pressure oscillations strongly depended upon the frequency of the open loop control. In fact, the amplitude of the combustor pressure oscillations varied ten fold over the range of investigated frequencies, indicating that applying the investigated open loop control approach at the appropriate frequency could effectively damp detrimental combustion instabilities. This was confirmed in subsequent tests in which initiation of open loop modulation of the fuel injection rate at a non resonant frequency of 300Hz during unstable operation with peak to peak instability amplitude of 114 psi and a frequency of 375Hz suppressed the instability to a level of 12 psi within approximately 0.2 sec (i.e., 75 periods). Analysis of the time dependence of the spectra of the pressure oscillations during suppression of the instability strongly suggested that the open loop fuel injection rate modulation effectively damped the instability by “breaking up” (or preventing the establishment of) the feedback loop between the reaction rate and combustor oscillations that drove the instability.

Adaptive Closed-Loop Control on an Atmospheric Gaseous Lean-Premixed Combustor

Active control of pressure oscillations has been successfully applied to a lean premixed prevapourised (LPP) combustion rig operating at atmospheric conditions. The design of the rig is based on the primary stage of the Rolls-Royce RB211-DLE industrial gas turbine. Control was achieved by modulating the fuel flow rate in response to a measured pressure signal. The feedback control is an adaptive, model-based self-tuning regulator (STR), which only requires the total time delay between actuation and response to achieve control. The STR algorithm achieves a reduction of up to 30 dB on the primary instability frequency. This performance was an improvement of 5–15 dB over an empirical control strategy (simple time-delay controller) specifically tuned to the same operating point. Initial robustness studies have shown that the STR retains control for a 20% change in frequency and a 23% change in air mass flow rate.

Inlet Fogging for a 655 MW Combined Cycle Power Plant: Design, Implementation and Operating Experience

The design, installation, commissioning and operation of a fogging system for a large 655 MW combined cycle power plant is described. Technical details and practical installation issues are discussed. Special considerations as to how the fogging system could help in the augmentation of power during high temperature and low frequency operation of the gas turbine is discussed. Finally a discussion is made regarding the importance of inlet filtration and the proper selection of blade coatings.

Development of Detailed Chemical Kinetic Mechanisms for Ignition/Oxidation of JP-8/Jet-A/JP-7 Fuels

Progress on development and validation of detailed chemical kinetic mechanisms for the U.S. Air Force JP-8 and JP-7 fuels [1] is reported in this article. Two JP-8 surrogate fuel blends were considered. The first JP-8 surrogate blend contained 12 pure hydrocarbon components, which were 15% n-C10H22, 20% n-C12H26, 15% n-C14H30, 10% n-C16H34, 5% i-C8H18, 5% C7H14, 5% C8H16, 5% C8H10, 5% C10H14, 5% C9H12, 5% C10H12 and 5% C11H10 by weight. The second JP-8 surrogate blend contained 4 components, which were 45% n-C12H26, 20% n-C10H22, 25% C10H14, and 10% C7H14 by weight. A five-component surrogate blend for JP-7 was also considered. The JP-7 surrogate blend components were 30% n-C10H22, 30% n-C12H26, 30% C10H20, 5% i-C8H18, and 5% C7H8 by weight. The current status of the JP-8 and JP-7 mechanisms is that they consist of 221 species and 1483 reactions and 205 species and 1438 reactions respectively. Both JP-8 and JP-7 mechanisms were evaluated using a lean fuel-air mixture, over a temperature range of 900–1050 K and for atmospheric pressure conditions by predicting autoignition delay times and comparing them to the available experimental data for Jet-A fuel. The comparisons demonstrated the ability of the 12-component JP-8 surrogate fuel blend to predict the autoignition delay times over a wider range of temperatures than the 4-component JP-8 surrogate fuel blend. The 5-component JP-7 surrogate blend predicted autoignition delay times lower than those of JP-8 blends and Jet-A fuel. The JP-8 and JP-7 mechanisms predictions, however, showed less agreement with the measurements towards the lower end of the temperature range (i.e., less than 900 K). Therefore, low temperature oxidation reactions and the sensitivities of the autoignition delays to reaction rate constants are still needed.

Experimental and Computational Study of Hybrid Diffusers for Gas Turbine Combustors

The increasing radial depth of modern combustors poses a particularly difficult aerodynamic challenge for the prediffuser. Conventional diffuser systems have a finite limit to the diffusion that can be achieved in a given length and it is, therefore, necessary for designers to consider more radical and unconventional diffuser configurations. This paper will report on one such unconventional diffuser; the hybrid diffuser which, under the action of bleed, has been shown to achieve high rates of diffusion in relatively short lengths. However, previous studies have not been conducted under representative conditions and have failed to provide a complete description of the relevant flow mechanisms making optimisation difficult. Utilising an isothermal representation of a modern gas turbine combustor an experimental investigation was undertaken to study the performance of a hybrid diffuser compared to that of a conventional, single passage, dump diffuser system. The hybrid diffuser achieved a 53% increase in area ratio within the same axial length generating a 13% increase in the pre-diffuser static pressure recovery coefficient which, in turn, produced a 25% reduction in the combustor feed annulus total pressure loss coefficient. A computational investigation was also undertaken in order to investigate the governing flow mechanisms. A detailed examination of the flow field, including an analysis of the terms within the momentum equation, demonstrated that the controlling flow mechanisms were not simply a boundary layer bleed but involve a more complex interaction between the accelerating bleed flow and the diffusing mainstream flow. A greater understanding of these mechanisms enabled a more practical design of hybrid diffuser to be developed that not only simplified the geometry but also improved the quality of the bleed air making it more attractive for use in component cooling.