On deposit sintering and detachment from gas turbines

Solid particles ingestion can deteriorate performance and stability of gas turbines through the build-up of deposits on the aerofoils. In some cases, the effect is sufficient to require an engine shut-down or to cause a failure. The fouling phenomenon can be characterized by two different phases: particle sticking and deposit evolution. The sticking process of an impinging particle has been deeply studied with several models available in the literature, both deterministic and stochastic. A less investigated phase is the the evolution of the deposit over time. A deposit is subjected to several forces that either tend to make it sticking to the surface or to detach from it. In the hot section of the gas turbine, the forces that act on the build-up tending to make it to adhere to the surface can be traced back to the van der Waals forces and, possibly capillary force if a certain amount of liquid phase is present. On the other hand, the detaching mechanism is related to the component investigated: if the particle is deposited on a vane, the drag and the shearing force are the only forces that tend to detach the particle, if a rotor blade is investigated, the centrifugal forces must be considered as well. On top of that, the deposit evolves over time in what is called sintering. During this process, the single particles deposited can melt together forming necks. If the temperature is sufficiently high, these necks increase in size until the former pores among particles are completely filled. This process is of paramount importance if the effect of the applied forces on the deposit needs to be investigated. The amount of material detached is indeed strongly dependent on the tension exchanged within various layers that constitute the build-up. This article focuses on the prediction of the build-up evolution on an HPT nozzle. The sintering process is modeled and related to the resisting strength of the deposit: an increasing sintering time reduces the deposit porosity and thus increase its strength. In order to monitor the stability of the deposit, the balance among detaching and attaching forces is carried out. The evolution of the vane shape is taken into account by using a moving mesh technique.