Thermal boundary effects on a GT liner structure

GT combustor liners are subjected to mechanical and thermal loads that damage the structure and reduce their operational life. Among those, the thermo-acoustic instabilities develop, generating pressure oscillations because of the interaction between heat release, acoustic waves and structure vibrations. The vibratory behaviour of the structure is the result of these phenomena and undergoes repeated reversals of the main deformation mechanisms as a function of the operating load of the engine. Monitoring and evaluating the operational load history and the life consumption rate of combustor components is essential to sustain a reliable risk-based maintenance in the GT combustion hardware. The non-linear material behaviour can activate possible interactions causing coupled damage mechanisms and become a life threatening mode of failure. A methodology for modelling both the dynamic and static behaviour of a GT cannular combustion chamber by utilizing a combined fluid-structure approach is presented in this study. Together with the calculation of the heat fluxes through the liner, the effects of the modifications at the thermal boundary conditions were used to investigate the modifications in the liner structural properties and the stresses development at different GT loads. The monitored pressure oscillations during operations has been investigated by performing both acoustic and structural dynamics. A correlation with the observed failure has been proposed by investigating stress relaxation phenomena’s, creep and plastic effects for base load and part load operations

An investigation on the impact fatigue characteristics of valve leaves for small hermetic reciprocating compressors in a new automated test system

This paper presents an investigation on the impact fatigue characteristics of valve leaves that are prevalently used in hermetic reciprocating compressors especially for the household type refrigerators. A unique automated impact fatigue test system has been designed and produced, which enables to carry out impact fatigue tests of the compressor valve leaves under the desired impact velocities. The test system serves investigations on the impact fatigue characteristics with the ability of crack detection and as the subsequent step of automatically terminating the test. The crack detection technique incorporates a non-contact actuation, a data acquisition system and a microphone. The investigation relates the impact fatigue lifetime of the valve leaves with the impact velocity, asymmetrical impact, operation temperature, material type (carbon strip steel, stainless strip steel and new stainless strip steel grade) and tumbling operation duration. Microscopic and metallographic observations were performed on the specimens. It was observed that the crack initiated at the edge of the valve leaves on the contact surface of valve leaf and vale plate and a particle is torn away from the edge before propagation. As the crack propagates, branching along the crack path is caused by the geometrical shape and stress waves on the valve leaves. The investigation and introduced test system guide the design optimization of valve leaves in terms of compressor performance due to energy consumption and lifetime of the valve leaf

Sensitivity of combustion driven structural dynamics and damage to thermo-acoustic instability: Combustion-acoustics-vibration

The dynamic combustion process generates high amplitude pressure oscillations due to the thermo-acoustic instabilities, which are excited within the gas turbine. The combustion instabilities have a significant destructive impact on the life of the liner material due to the high cyclic vibration amplitudes at elevated temperatures. This paper presents a methodology developed for mechanical integrity analysis relevant to gas turbine combustors and the results of an investigation of combustion-acoustics-vibration interaction by means of structural dynamics. In this investigation, the combustion dynamics was found to be very sensitive to the thermal power of the system and the air-fuel ratio of the mixture that feed into the combustor. The unstable combustion caused a dominant pressure peak at a characteristic frequency, which is the first acoustic eigenfrequency of the system. Besides, the higher-harmonics of this peak were generated over a wide frequency-band. The frequencies of the higher-harmonics were observed to be close to the structural eigenfrequencies of the system. The structural integrity of both the intact and damaged test specimens mounted to the combustor were monitored by vibration-based and thermal-based techniques during the combustion operation. The flexibility method was found to be accurate to detect, localize and identify the damage. Furthermore, a temperature increase was observed around the damage due to the hot gas leakage from the combustor that can induce detrimental thermal stresses to consume the lifetime

Fluid-structure interaction on the combustion instability

The multi-domain problem, the limit cycle behaviour of unstable oscillations in the LIMOUSINE model combustor has been investigated by numerical and experimental studies. A strong interaction between the aerodynamics-combustion-acoustic oscillations has been observed during the operation. In this regime, the unsteady heat release by the flame is the acoustic source inducing pressure waves and subsequently the acoustic field acts as a pressure load on the structure. The vibration of the liner walls generates a displacement of the flue gas near the wall inside the combustor which generates an acoustic field proportional to the liner wall acceleration. The two-way interaction between the oscillating pressure load in the fluid and the motion of the structure under the limit cycle oscillation can bring up elevated vibration levels, which accelerates the degradation of liner material at high temperatures. Therefore, fatigue and/or creep lead the failure mechanism. In this paper the time dependent pressures on the liner and corresponding structural velocity amplitudes are calculated by using ANSYS workbench V13.1 software, in which pressure and displacement values have been exchanged between CFD and structural domains transiently creating two-way fluid-structure coupling. The flow of information is sustained between the fluid dynamics and structural dynamics. A validation check has been performed between the numerical pressure and liner velocity results and experimental results. The excitation frequency of the structure in the combustor has been assessed by numerical, analytical and experimental modal analysis in order to distinct the acoustic and structural contribution

The analysis of mechanical integrity in gas turbine engines subjected to combustion instabilities

Stringent regulations have been introduced towards reducing pollutant emissions and preserving our environment. Lowering NOx emissions is one of the main targets of industrial gas turbine engines for power generation. The combustion zone temperature is one of the critical parameters, which is directly proportional to NOx emission levels. Premixing an excessive amount of air with fuel before delivering to the combustor can reduce the temperature, at which combustion takes place, by burning a leaner mixture. Therefore, new generation combustion systems for modern gas turbines have been introduced, which are\ud named lean, premixed (LP) combustion systems. However, LP combustion systems are prone to thermo-acoustically induced combustion instabilities, which are excited by a feedback mechanism between heat release, pressure and flow-mixture oscillations. Consequently, high amplitude oscillations of pressure are generated and heat transfer is generated, which results in mechanical vibrations at elevated temperatures, and hence degradation of mechanical\ud integrity of combustor components due to fatigue and creep damage.\ud \ud The present work in this thesis is focused on the development of efficient analysis tools to investigate the sensitivity of mechanical integrity and to assess the lifetime of structures at combustion instabilities

Accelerated life consumption due to thermo-acoustic oscillations in gas turbines: XFEM & Crack

The combustion instability phenomenon in the gas turbine engines brings out elevated vibrations under high temperature levels. The present work addresses the projection of a life assessment methodology applied in a laboratory-scaled generic combustor onto the typical gas turbine engine combustor section in terms of crack and its evaluation. A temperature-structural analysis based on the combustion experiments was utilized to obtain the crack initiation region. Sequentially coupled extended finite element method (XFEM) based fracture mechanics analysis was performed to characterize the crack-tip near fields in a typical combustor nickel-based superalloy