Thomas Leitgeb

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).

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

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

Frequency resolved density fluctuations measurements of combustion oscillations in a model combustor

In this work, investigations of thermo-acoustic oscillations by means of density fluctuation measurements, are presented. For this, laser-vibrometers were used. Although these systems were initially developed for highly accurate detection of surface vibration, they are also capable of measuring the integral gas density fluctuation along the line of sight. In order to facilitate a laser based investigation of a thermo-acoustically unstable setup, a variable geometry burner – developed in a previous project was operated in an optically accessible gas turbine model combustion chamber at atmospheric conditions. Thermoacoustic coupling ocured at 165 Hz when the burner was operated at a power of 5 kW. Density fluctuations outside and inside the flame region were measured using vibrometers. Furthermore, the the eigenfrequencies of the combustion chamber were detected by measuring the surface vibration, as well as, the acoustic waves forming within the volume of the chamber without combustion, but with loudspeaker excitation. The spectrum of the cavity resonance was compared to the spectra obtained with combustion under different conditions. The variable geometry burner was helpful in order to easily change flow momentum or flow angular momentum. With a fixed vibrometer as a reference and a traversable laser-vibrometer moving along the flow axis, the phase relation of structures released by the flame, was measured. Consequently the velocity of these structures was estimated.