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Fouling and erosion are two problems that severely affect gas turbines. The shape of the blade, its roughness, and its structural stability can vary as a consequence of these phenomena. The outcomes of this occurrence can span from the efficiency reduction to the engine shut down according to the nature of the material ingested, to the concentration of contaminants in the air, the cleanliness of fuel and to the particular design of the machine. In this work, an axial turbine airfoil is modified according to the requirement of less sensibility to the phenomena above mentioned, utilizing an automatic optimization algorithm. An artificial neural network surrogate approach is used for searching the optimal shape, minimizing the computational cost of the entire process. The optimum design of the blade is therefore achieved, in order to reduce the effects of deposition on the performance. The methodology here proposed is fully general and it is applied to an HPT nozzle in the present analysis.

The consequences of particle deposition on gas turbine blade surface are studied since the first gas turbine application. The effects generated by particle adhesion range from performance deterioration to life reduction to complete loss of power. Even if, the effects generated by fouled blade surface are well known, the mechanisms responsible for the particle adhesion are still less clear. The variability related to the nature of materials, the impact conditions and the presence of promoting substances (such as water, oil, glue agents, etc.) imply several difficulties for comparing the results and for extracting general trends and rules useful for generating up-to-date predictive models. In the present work, an attempt to realize a general comparison among several different particle deposition tests is carried out. Starting from the previous review work, which has collected experimental tests carried out over thirty years, in the present study, an original elaboration data is proposed. Over seventy adhesion tests realized with particle composition, size, velocity, and temperature similar to those characterize gas turbine fouling are collected and post-processed. After a dimensional analysis, the data are then classified using non-dimensional groups such as Reynolds, Weber, and Ohnesorge numbers. In this way, general threshold values for the transitions between erosion, deposition, and splashing are identified according to the literature data. This general tool allows the a priori identification of the driving phenomena (such as inertia, viscous/capillary forces), based on the knowledge of basic inputs (such as impact and particle characteristics). The general approach adopted in this work gives the opportunity to increase the gas turbine fouling knowledge based on an interdisciplinary approach.

Compressor fouling is a severe problem for both heavy-duty and aero-propulsion gas turbines. Particles can impinge on the blade and annulus surfaces, sticking there. The consequences of particle deposition are the increase of the roughness and an uncontrolled variation in the surface shape. These problems have a major effect on the performance of the compressor over time. Variations in the flow field can make the flow quantities close to the deposit to change, and it may happen that the conditions for the sticking do not hold any longer. If this is the case, the build-up detachment may happen. This occurrence can mitigate the fouling effects and may be exploited for keeping the performance of the compressor as high as possible over the operating period. In this work, an innovative model is proposed in order to evaluate the adhesion forces and the possible detachment. Particularly, the same forces that keep a gecko stuck to a surface are considered: the van der Waals forces (due to the proximity of the two bodies) and the Laplace force (due to the curvature of the liquid film related to the humidity). The so formulated model, named gecko-like for such a reason, is used for the numerical analyses of a deposition problem. Both the sticking and possible build-up detachment are considered. The outcome of this work can be regarded as an a-priori estimate of the forces to be kept into account when dealing with compressor fouling problems.

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.