The effects of temperature distortion on aerodynamics and low engine order forced response in axial turbines
The flow entering a high-pressure turbine in a gas turbine engine is characterised by a loss of symmetry due to temperature distortions in both radial and circumferential directions, known as hot streaks. In industrial simulations it is common practice to assume uniform inlet temperature conditions to simplify the aerodynamic analysis. However, hot streaks may have significant impact on the turbine aerodynamics with the redistribution of the hot fluid affecting the development of secondary flows with consequent effects on enhanced local heat transfer and aerodynamic losses. The loss of symmetry has also been linked to the excitation of low-order nodal diameter assembly modes of the downstream rotor blades leading to potential blade failure and thus, should be taken into account during the design process. In today’s carbon-constraint environment additional parameters arise as gas turbines are challenged to adapt to variations of the fuel composition driven by the need of efficient and lowCO2 power generation. Introducing syngas, a synthesis gas fuel that is used to power integrated gasification combined cycle (IGCC) power plants, is likely to affect the operating conditions of existing gas turbines leading to the requirement of re-design of components. With particular focus on the turbine hot flow path, the propagation mechanism of hot streaks throughout the turbine will be affected with consequent impact on the turbine aerodynamics and forced response excitation levels originating from the different hot flow patterns. Motivated by the lack of relevant studies, the current work provides a first step towards the evaluation of the effects of syngas on hot streaks aerodynamics and the induced forced response excitation levels. Using full annulus multi-bladerow unsteady 3D CFD simulations and applying combustor representative hot streak profiles in two different gas turbines, a complete analysis of the hot streaks migration is achieved, with respect to a number of geometric parameters such as the hot streaks shape and injection location in both spanwise and circumferential directions, the coolant configurations as well as the combined effects on the secondary flow development. The aerodynamic analysis indicated the propagation of the hot streaks up to the exit of the turbines under investigation with differences in characteristics depending on design parameters. With respect to the effect of fuel composition variations on the blades temperature levels and the flow pattern is observed between the natural gas and syngas turbine with the syngas showing a more concentrated wake shape. In effect of the syngas different flow pattern, differences are observed in the secondary flows with consequent interaction with the hot streaks. Contrast to initial expectations, the forced response analysis iii resulted slightly lower amplitude unsteady force of lower harmonics for syngas compared to natural gas; however, both fuels showed significant levels of the hot streak induced low engine order excitation compared to the burners and stator related blade passing frequency vibration.
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