Over-speed turbine aerodynamics

The event of a HP shaft breakage will foster the rapid acceleration of the turbine. The current research has shown that an over-speed scenario (turbine rotating over design speed) in a contra-rotating turbine layout increases incidence angles in the IPT NGVs. The e↵ect of the over-speed in the IPT performance parameters has been evaluated by means of the capacity and efficiency based on the steady state assumption. The observed trend shows an incidence angle augmenting with the shaft rotational speed causing the capacity and efficiency to diminish for high levels of over-speed. The comparison of the steady and unsteady results reveals an interesting fact. At nominal conditions, the large distance separating the HPT rotor blades and IPT NGVs allows the flow to mix out before entering the IPT stage. Therefore, the transport of the unsteadiness downstream the turbine does not justify the added computational cost to be assumed in an unsteady simulation. The e↵ect of the axial displacement of the HPT rotor as a consequence of the over-speed event has been also assessed. The purpose of this part was to study the influence that the increase of massflow through the tip clearance exerted on the downstream component.