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.