Fabrice Giuliani

Mapping the Density Fluctuations in a Pulsed Air-Methane Flame Using Laser-Vibrometry

Laser vibrometry (LV) is originally a laser-based, line-of-sight measurement technique dedicated to the analysis of surface vibrations. It was lately adapted at TU Graz for monitoring the stability of an air-methane flame (Giuliani, et al., 2006, ASME Turbo Expo, ASME Paper No. GT2006-90413). This paper reports on the mapping of density fluctuations measured with LV in a premixed air-methane flame (free jet; swirl stabilized) with a forced flow modulation (quarter-wave resonator; amplification with a siren). In order to correlate the density fluctuations with the jet aerodynamics and turbulent flame shape, stereoscopic particle image velocimetry and high-speed schlieren visualizations were used. This paper addresses issues regarding the estimate of density fluctuations, the transform from line-of-sight to local measurement with tomographic methods, and the potential of the method for detailed description of thermoacoustic couplings. One emphasized application of LV is its ability to perform precise and low-cost benchmark stability tests on a combustor during the design phase (time-resolved measurement, high frequency and phase resolution on the 5 Hz―20 kHz range with the present equipment and settings, near-constant spectral sensitivity over a large bandwidth, and no seeding required ; measurement possible over the whole combustion volume).

Frequency-resolved interferometric measurement of local density fluctuations for turbulent combustion analysis

A validation of a novel interferometric measurement technique for the frequency-resolved detection of local density fluctuation in turbulent combustion analysis was performed in this work. Two laser vibrometer systems together with a signal analyser were used to obtain frequency spectra of density fluctuations across a methane-jet flame. Since laser vibrometry is based on interferometric techniques, the derived signals are path-integrals along the measurement beam. To obtain local frequency spectra of density fluctuations, long-time-averaged measurements from each of the two systems were performed using correlation functions and cross spectra. Results were compared to data recorded by standard interferometric techniques for validation purposes. Additionally, Raman scattering and laser Doppler velocimetry were used for flame characterization.

STATISTICAL EVALUATION OF IGNITION PHENOMENA IN TURBOJET ENGINES

Turbojet engines must demonstrate their ability to be ignited on high altitude airfields or reignited in flight. These extreme conditions require the ignition of the engine in a large field of aerodynamic and thermodynamic conditions. Usually, a Mach-Altitude domain in which combustion can be established is used to represent the ignition performances of the aircraft engine. Nowadays, the prediction of this domain during the design process is mostly based on empirical rules. On the contrary, for the steady regimes, thanks to the use of computer aided design (CAD) and computational fluid dynamics (CFD) the combustor design is mainly done on the workstation. Finally the validation of the ignition configuration and the determination of the ignition domain demand an expensive experimental program. In an industrial context, it is therefore very important to include the ignition system design in the CAD process to reduce the costs of the experimental testing of the ability of a turbojet engine to be ignited. This requires the elaboration of a well tuned CFD model that is able to simulate the ignition phenomena properly. The aim of this study was therefore to gather experimental data on ignition to get a database for statistical evaluation of the ignition phenomena. A sample of 150 experiments at same operating conditions was performed. The experimental data were analyzed regarding the number of sparks needed to ignite and the ignition delay time. Further on, the probability that a certain spark will lead to ignition was computed. The paper discusses how the sample was statistically treated and the ignition probability was computed. Further the results on ignition probability are presented. The results of this study will be used to validate the ignition module of the AVBP LES CFD-Code at CERFACS in future works.Copyright

Laser Vibrometry for Real-Time Combustion Stability Diagnostic

Laser-based diagnostics for combustion monitoring are promising sensing techniques for the upcoming generation of build-in gas turbines measurement and control devices. Their principles are usually based on direct measurement of line-of-sight transmission, absorption, scattering or re-emission of laser light through the flame. We discuss here how a similar method based this time on interferometry can provide a refined analysis on dynamics of injected reactants and flame stability. Measurements are performed on a resonant premixed air/methane flame using Laser Vibrometry (LV). A method for detection of combustion instability within the primary zone is described, and dual LV measurements performed over the full flame cross section provide a refined analysis of the flow patterns. This technique, originally dedicated to structural dynamics, shows a high potential for low-cost and rapid flow characterisation during the benchmark tests of a gas turbine combustor. The discussion ends on feasibility for embarking Laser Vibrometry as a real-time combustion monitor.Copyright

Design and Validation of a Burner With Variable Geometry for Extended Combustion Range

A study on innovative gas turbine core concepts supported by the NEWAC project (NEW Aero-engine Core concepts, Integrated Project co-funded by the European Commission within the Sixth Framework Programme under contract No. AIP5-CT-2006-030876) focused on the ability of the combustor to maintain combustion during a drastic reduction of the main air, e.g. due to an active control on the core flow to improve the off-design efficiency. A feasibility study was performed at Graz University of Technology including dimensioning, design and validation of a test burner with variable geometry. A low power premixed methane / air burner with swirl-stabilised flame was chosen, on which the outlet surface and the ratio axial to tangential momentum on the mass flow rate could be controlled. During testing at atmospheric conditions, special attention was paid to the extension of the flammability domain and to the flame dynamics (transition attached-detached, flame stability, blowout limits). It was established for instance that based on this technology, a detached flame can be maintained when reducing the design mass flow rate by 40 per cent within a safe stability range. The paper discusses the background of the study, the burner’s design and technology, the measurement techniques and the results of the validation campaign. A discussion on possible advantages of using variable geometry in a combustion chamber versus conventional technologies closes the paper, taking into account the technical challenges to be met.Copyright

Air Flow Modulation for Refined Control of the Combustion Dynamics Using a Novel Actuator

A specific actuator able to modulate the air feed of a gas a burner at a given frequency and amplitude is presented. The Combustion Department at the Institute for Thermal Turbomachinery and Machine Dynamics at the Graz University of Technology has experience on the study of combustion instabilities in gas turbines using a flow excitor. The stability of an industrial burner is tested at elevated pressure and temperature conditions in the frame of the NEWAC project. For practical matters of operation among which the possibility to induce progressively a perturbation when the flame conditions are all set, the need was expressed to design, construct and validate a flexible actuator able to set an air flow modulation at a given frequency and at a desired amplitude level, with the possibility during operation to let these two factors vary in a given range independently from each other. This device should operate within the 0–1 kHz range and 0%–20% amplitude range at steady-state, during transients, or follow a specific time sequence. It should be robust and sustain elevated pressures. The objective is to bring a perturbation in the flow to which the combustor will respond, or not. For elevated levels of pulsation, it can simulate the presence of vortex-driven combustion instabilities. It can also act as a real-time actuator able to respond in frequency and in phase to actively damp a “natural” combustion instability. Other issues are a better and quicker mixing due to the enhanced turbulence level, and pushing forward the blow out limits at lean conditions with controlled injection dynamics. The basic construction is the one of a siren, with an elevated pressure side where the air is throttled, and a low pressure outlet where the resulting sonic jet is sheared by a rotating wheel. A mechanism allows to let vary the surface of interaction between the wheel and the jet. Two electromotors driven by Labview set both frequency and amplitude levels. This contribution describes the actuator’s principles, design, operation range and the results of the characterization campaign.

Computer-Aided Dimensioning and Validation of a Versatile Test Facility for Combustion Chambers and Turbines

The continuous flow test facility at Graz University of Technology was originally designed for cold sub- and transonic experimental research on different turbine stages (2001-GT-0489). The operation range of the facility was recently extended to hot flows for investigations on the behavior of high-temperature resistant sensors embedded in gas-turbines and analysis of cooling systems of turbine blades or multiple-burner combustors, where each air supply is driven separately. Therefore, a 5 MW thermal air heater has been connected to the institute’s 3 MW compressor station. The dimensioning of the air system was done with IPSE-pro which is a commercial software package for simulation of basic thermodynamic processes. The standard modules of IPSE-pro were modified for calculating the mass flow distributions with respect to the prevailing pressure drops. As the air system is complex and relies on control valves to maintain specific mass flow rates, IPSEpro allows analysis of the behaviour of the test facility at several compressor station configurations. The main test facility dimensions and characteristics, as well as the most important equations describing the component models of IPSEpro are shown. Simulation results of several operation points are compared to measured data to validate the methodology. This work was done in the frame of the European research program New Aero Engine Core Concepts (NEWAC) at Graz University of Technology.© 2009 ASME

Validation of an Infrared Extinction Method for Fuel Vapor Concentration Measurements Towards the Systematic Comparison Between Alternative and Conventional Fuels for Aviation

Due to an increasing oil price and the obvious influence of the combustion of fossil fuel-derivatives on climate change on one hand and the steady growth of transportation needs on the other, it is necessary to develop alternatives to oil for aviation. For this purpose a specific research program on the investigation of adequate alternative fuels for aviation has been founded by the European Commission’s Framework Program. The project Alfa Bird (Alternative Fuels and Bio-fuels in Aircraft Development) focuses on an identification of possible alternative fuels to kerosene, the investigation of the adequacy of the selected ones, an evaluation of the environmental and economical impact of those and finally the creation of a future perspective for the industrial use of the “best” alternative. The main part of the investigation activities at TU Graz, in cooperation with ONERA Centre de Toulouse and Fauga-Mauzac on these specific topics consists of the analysis of the evaporation of the previously chosen fuel types in comparison to Fully Synthetic Jet Fuel (FSJF). Therefore qualitative measurements to obtain vapor concentration gradients will be done using the Infrared Extinction (IRE) measurement method. Based on a simplified Beer-Lambert-Law the integral vapor concentrations can be obtained. The main hypothesis is that if the line-of-sight extinction due to Mie-scattering is similar for both infrared and visible wavelengths because of the presence of the spray, only infrared light will be absorbed by the fuel vapor, being transparent to visible light. This contribution focuses on the validation of the infrared measurement technique on a well characterized spray. The tests are performed under controlled boundary conditions. Therefore an existing IRE test arrangement at ONERA Toulouse using an ultrasonic atomizer injecting n-octane at atmospheric conditions has been analyzed. Error sources related to misalignments in the hardware have been considered and an iterative alignment method of the laser beams followed by a beam diameter and diffraction analysis have been performed. Optimizing the setup to obtain a stable operation point has been successful. Improved experimental results at this operation point were compared with existing simulation results for the evaporation of the used ultrasonic atomizer. The achieved data has shown good accordance to the existing simulation results. This work has been supported by the Eccomet project (Efficient and Clean Combustion Experts Training) in the framework of Alfa Bird.© 2011 ASME

Measurement of Local Density Fluctuations for Combustion Diagnostics of Different Flames using Dual Laser Vibrometry

Experimental measurements of spatially- and frequency-resolved density fluctuations in two different types of flames, a non-premixed methane-jet flame and a premixed swirlstabilized methane-air flame were performed in this work. A novel interferometric measurement technique was applied for this purpose. Two laser vibrometers together with a signal analyser had been used to obtain frequency spectra of density fluctuations across both types of flames. Since laser vibrometry is based on interferometric techniques, the derived signals are path integrals along the measurement beam. To receive local frequency spectra of density fluctuations, long-time averaged time signals from each of the two systems were evaluated using correlation functions and cross spectra. Results obtained from measurements in the non-premixed jet flame and the premixed swirl-stabilized flame are compared and the application of the measurement technique for combustion diagnostics and detection and analysis of combustion instabilities is discussed.