A Sobol' Sequence Parametric Analysis of Rotor Thermal Bow in Gas Turbines

Abstract

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.

The impact of gas turbine compressor rotor bow on aircraft operations

“…A comprehensive study of the relative contribution of relevant design and operational parameters by Smith (42) , as well as other preliminary studies by the authors, identified three noteworthy trends. These three trends fall roughly into three areas of responsibility and influence: engine-airframe integration is the realm of airframe-engine OEM; initial rotor temperature is the parameter of most interest to, and most able to be influenced by, operators; and engine geometry is strictly the realm of the engine OEM.…”

“…Ongoing research into the rotor bow phenomenon by Smith and Neely (37-41) has investigated a range of potential driving parameters, including rotor length, rotor thermal mass, internal geometry, initial rotor temperature, ambient conditions and installation design. Smith and Neely (40) have developed an approach for investigating rotor thermal bow using 3D unsteady CHT CFD simulations coupled one-way with FEA of fundamental geometries/analogues, as shown in Fig.…”

“…While these preliminary studies (37-41) did not yet represent a comprehensive breakdown of the phenomenon and its driving parameters, they did provide a useful basis for later work. These results were subsequently used by the authors to develop a robust Plackett-Burman (43) screening test, followed by a more targeted quasi-random sampling study (43) .…”

Prediction of the thermal bow of rotor based on the measured displacement and temperature

“…The numerical simulation is widely applied to unveil the mechanism of the rotor thermal bow of aeroengine. With the commercial packages, it is convenient to investigate the effects of the rotor geometry, 6,7 the case and the integration design, 6,8 the blades and vanes, 9 and the operation of the aeroengine before shutdown, 10 on the rotor thermal bow by numerical simulation. Numerical simulation is an effective method to study the thermal bow of aeroengine qualitatively, while the quantitative research on this problem still requires experimental test.…”

A new computational method for predicting the thermal bow of a rotor

“…Smith and Neely 8 turned to conjugate heat transfer (CHT) computational fluid dynamics (CFD) to calculate the transient thermal loads. Smith 9 further devoted his efforts to investigate the effects of various factors on thermal bow of rotor during natural cooling conditions, include the effect of blades, 10 aircraft integration design 11 and shaft geometry. 12 Later, they carried out experiments to validate their previous numerical studies.…”