Shutdown Modelling to Extend Operation to Extreme Ambient Conditions

This paper presents a discussion of heat soak-back for various gas turbine engine shutdown scenarios, compares with experimental data and discusses how the temperatures at the shutdown point can be used to approximately determine a suitable shutdown procedure for ambient and hot ambient conditions in order to control the bearing soak back peak. The investigation was performed by running the test vehicle at various engine conditions and shutdown procedures while measuring the temperatures at the bearing outer tracks. The data acquisition continued after the engine shutdown until the temperature data showed that the soak-back peak temperature had passed. The tested engine conditions were replicated in a FEM model of the turbine, from which knowledge was gained of the turbine discs temperatures during the shutdown and subsequent cooling down. It was shown that the bearing soak-back and cooling down after shutdown is determined by the cooling down of the turbine discs. The discs are directly affected by the blade temperature reduction, which in turn is depending on how the air trapped in the main gas annulus after the engine has stopped rotating is cooling down. The cooling down of the main annulus air is due to free convection and conduction to the surrounding metal, and cannot be readily predicted with current tools (the air is not moving). Therefore, an approximation of the temperature behaviour is needed. It was decided for the present study to investigate the correlation between the disc temperature at the shutdown point with the bearing soak-back peak temperature and time. It was found that the disc rim temperature at the shutdown point could be used to determine an approximate soak-back peak and thereby the need for motor-overs (MO). The conclusions are that various shutdown procedures and motor over cooling can be used in order to extend the engine operability without expensive bearing redesign.