During the cooling process after shutdown, gas turbines can suffer from differential thermal expansion due to buoyant convection. This process can result in asymmetric cooling of the shaft, which can in turn lead to differential thermal expansion, causing deformation of the shaft, known as thermal bow. Attempts to start a gas turbine in this bowed condition can lead to rotor-to-stator contact, triggering further heating, and subsequently further bow. This phenomenon, known as the Newkirk Effect, can result in severe damage to the engine, representing a risk to both airworthiness and logistics. This study utilises a technique previously developed by the authors for modelling shaft thermal bow in gas turbines using a combination of 3D conjugate heat transfer (CHT) computational fluid dynamics (CFD) and finite element analysis (FEA). A baseline model comprises a simple hollow shaft supported at each end, enveloped inside a simple case. Body temperatures obtained through 3D CHT CFD at set time intervals are transferred to FEA, where the physical distortion associated with the application of an asymmetric thermal load is measured. The baseline model was allowed to cool down from representative operational temperatures, with the shaft thermal bow measured for 90 minutes of flow time. Simple modifications were then made to the baseline model including the addition of representative helicopter and fast jet inlet and exhaust analogues, and the use of porosity to simulate the presence of blades, to analyse their influence on the onset time, duration, and severity of the shaft deformation. While the geometries used in this initial study are basic, the results indicate that these aspects of gas turbine design do have an appreciable effect on the onset time, severity, and duration, as well as the axial distribution of the shaft thermal bow. This also indicates the importance of further work in this area using more realistic geometries.
After a jet engine is shut down, hot air rising inside the compressor disc cavities, secondary air systems, and the gas path annulus will result in a vertical temperature gradient. As the compressor rotor cools and contracts in the presence of this thermal gradient, it will bend, in a phenomenon known as thermal rotor bow. Starting an engine under bowed conditions can result in rubbing of the rotor and stator seals, adding heat to the rotor, exacerbating the rotor bow. This causal sequence of rubbing and bending is called the Newkirk Effect. In this study, 30 simulations of simplified compressor geometries have been run in three-dimensional unsteady conjugate heat transfer computational fluid dynamics coupled with finite element modelling, using a Sobol’ quasi-random sequence coupled with kriging interpolation to study the effects of three important geometric parameters on the thermal bow response of an engine. The three parameters, rotor length, rotor diameter, and compressor case wall thickness, were selected based on a similar screening test analysis performed by authors in a previous study. The results include response maps of each parameter with respect to rotor bow and clearance reduction onset time, duration, and severity, and show that length and case wall thickness exhibit linear responses due to their effect on stiffness, whereas diameter exhibits a non-linear response, due to the conflicting and competing effects on stiffness and vertical temperature difference.
This study builds upon previous work by the authors, using a combination of 3D conjugate heat transfer (CHT) computational fluid dynamics (CFD) and finite element analysis (FEA) to characterise the thermal bow behaviour of a simple compressor shaft and case model under natural cooling. As with previous studies by the authors, body temperatures obtained from 3D CHT CFD solutions at set time intervals are transferred to FEA, where the physical distortion associated with the asymmetric thermal load is measured. The current study examines the influence of a range of shaft design parameters on the severity and duration of the shaft deformation. The parameters of interest include shaft length, annulus geometry, degree of shaft ventilation, and shaft internal cavity geometry. Each time the baseline model is modified to analyse the contribution of a parameter, the model is allowed to cool down from representative operational temperatures for a period of 180 minutes, over which time the shaft thermal bow, and shaft-to-case clearance, are measured. The results of this study indicate that the shaft’s thermal bow response and shaft-to-case clearance over time are highly sensitive to changes to its geometry, whereas the change in 180-degree out-of-phase shaft-to-case clearance is more sensitive to the case geometry, rather than the shaft. These results indicate that increasing the length of the shaft, reducing its wall thickness, or introducing a rising-line annulus, will increase the severity of the shaft thermal bow phenomenon; whereas introducing disc geometry inside the shaft will reduce the severity of the bow.
This study describes the development of an experimental apparatus designed to provide initial validation of numerical simulations of a gas turbine compressor rotor shaft under thermal bow due to natural convection. The experimental analogue represents a simplified model developed by the authors in previous works, designed to represent the basic elements of a compressor rotor shaft, as part of an ongoing parametric study into the influence of compressor design and integration on the onset time, duration, and severity of thermal bow, and the propensity of an engine to suffer from the Newkirk Effect. The semi-enclosed steel shaft, mounted on a support frame, was heated to approximately 600K in a convective oven before being removed and allowed to cool under ambient conditions. Observations of the shaft under natural cooling were made using several experimental techniques, including temperature profiling using thermographic imaging and a thermocouple array, and physical distortion measurement using linear variable displacement transducer (LVDT) probes. The behaviour of the buoyant plume was also observed using background oriented schlieren (BOS). Using these techniques, the natural convection-driven thermal gradient and resultant physical deformation were measured and recorded over a period of 60 minutes. The observed thermal gradients and resultant thermal bow distortions were then compared to a one-way fluid thermal structural interaction (FTSI) 3D conjugate heat transfer (CHT) computational fluid dynamics (CFD) and finite element analysis (FEA) model developed by the authors, in order to validate the numerical model. The behaviour of the thermal plume from the BOS imagery was also used as a qualitative validation method. The temperature measurements and overall cooling rate measured on the experimental model showed good agreement with the numerical predictions (within 1%); however, uncertainties in the initial phase of the experiment led to error in the numerical prediction of the thermal gradient and resultant thermal bow measurements (error of up to 63%). Noting the uncertainties in the experiment, the agreement between numerical and experimental results with respect to the overall cooling rate indicates that the numerical approach being employed as part of the larger parametric study into gas turbine compressor rotor shaft thermal bow is appropriate and valid.
When a gas turbine engine is shut down it will develop a circumferential thermal gradient vertically across the compressor due to hot air rising from the cooling metal components and pooling at the top. As the hot compressor rotor drum and casing cool and contract in the presence of this thermal gradient, they do so non-uniformly and therefore will bend slightly, in a phenomenon known as rotor bow. Starting an engine under bowed conditions can result in damage, representing a risk to both airworthiness and operational capability. This study consolidates some preliminary findings by the authors relating to the drivers for rotor bow, such as engine geometry, aircraft-engine integration and rotor temperature on shutdown. The commercial and military operational considerations associated with rotor bow are also discussed, including limitations which may result from a bowed rotor; the influence of operations including the final flight and descent profiles, taxi procedures and rapid turnaround requirements; as well as some practical solutions which may be implemented to reduce the impact of rotor bow.
One of the few acoustic travelling wave systems (TWS) in the world has been developed by Defence Science and Technology Group Australia (DST). This system uses acoustic excitation of blisks by means of a high-powered speaker cluster. The response to this acoustic excitation is measured by a laser doppler vibrometer which shows the natural frequencies and mode shapes of the blisk. To date, this system has been verified and validated on a simple, twelve-bladed research blisk with a maximum error of 3.78% from FEM modal and harmonic analysis. Furthermore, the system has been refined and tested on a current in-service blisk with an accuracy of approximately 1%. This study outlines how the TWS has been used to investigate the effects of mistuning on blisks. Promising results have been achieved with the system able to accurately identify changes in resonant behaviour due to mistuning with correlation to finite element modelling predictions. The limitations are also outlined, along with suggested areas of research and improvement. This may ultimately result in the ability to verify manufacturer blending limits and assess the effect of foreign object damage and repairs on the dynamic response characteristics of a blisk without a complex and labour intensive mechanical spin rig investigation.
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