The Influence of the History of Thermal Expansion on the Axial Turbine Characteristic

Abstract

The ability to predict the dynamic behaviour of a gas turbine is of great importance, even from the early design stages, as it impacts the designer’s ability to select higher performance operating regimes while maintaining the gas turbine within the required safety margins, inside stable operation envelope. Most of the models used for numerical simulations of the dynamic response of a gas turbine are based on the turbine characteristic curve, determined, in most cases, for the steady operating states of the gas turbine. From a manufacturing standpoint, the geometrical accuracy of the machined blade profiles is a major concern, as it strongly impacts on the actual gas turbine performance. Even relatively small deviations in the shape of the axial turbine blade profile affect the value of the area of the channel between two adjacent turbine blades minimum section, and, thus, impacts significantly upon of the actual gas turbine performance. With this in view, this paper aims at studying the influence of the thermal expansion history on the axial turbine characteristic curve. The thermal expansion that occurs between the temperature defining the idle operating regime and the temperature defining the maximum stable operating regime usually takes about 30 minutes, while the gas turbine acceleration process from idle to the maximum stable operating regime takes between four to eight seconds. Due to the thermal expansion, the minimum area of the channel between two adjacent turbine blades (a.k.a. the throttle area) increases by about 1.5% when the gas turbine moves from idle to the maximum stable operating regime, change that affects significantly the turbine characteristic curve. Hence, the deviation of a turbine characteristic curve due to the thermal expansion history must be taken into account in order to improve the gas turbine dynamic behaviour prediction. To demonstrate this improvement, this paper will present two numerical simulations. In the first case, the simulation reproduces the turbine characteristic for nominal regime. In the second case, the turbine characteristic at dilatation state at idle regime. The results of the numerical simulation are compared against each other in order to highlight the differences in the two turbine characteristics induced by the thermal expansion history.