Experimental Observations of Thermal Bow due to Natural Convection on a Gas Turbine Compressor Rotor Shaft Analogue

Abstract

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.

The impact of gas turbine compressor rotor bow on aircraft operations

“…Ongoing research into the rotor bow phenomenon by Smith and Neely (37)(38)(39)(40)(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.…”

“…4), can restrict convective heat transfer within the rotor and thus dampen the thermal gradient, reducing the thermal bow. While these preliminary studies (37)(38)(39)(40)(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

“…In order to measure the natural convective heat transfer coefficient in the thermal bow process, Fahy et al 11 put emphasis on the transient temperature of the model rotor, and the displacement was not recorded in the experiment. Smith et al 12,13 conducted a progressive experimental research on the thermal bow of rotor. At the initial stage, the thermocouple and the linear variable displacement transducer were applied to record the temperature and the displacement of the rotor, respectively.…”

“…At the initial stage, the thermocouple and the linear variable displacement transducer were applied to record the temperature and the displacement of the rotor, respectively. 12 In the improved experiment, the thermographic infrared camera was used to measure the temperature, while the laser sensor was adopted to record the deflection of the rotor. The temperature measurements generally matched well with the simulations, while the thermal deformation of the rotor support and the uniformity of the rotor initial temperature will cause obvious difference of the displacement measurement with the numerical simulations.…”