The mechanism of deposit formation on the blade surfaces of a cooled turbine vane is investigated numerically. The prediction of dispersed particles trajectories is affected by temperature, by the mechanics of impact on a solid surface, and by the interaction between particles and film cooling jets and all these aspects must be accounted for. The model here proposed is obtained as a high temperature extension of the well-known Thornton and Ning (1998) approach in a temperature interval ranging between 500 K (where basic model — based on an elastic-plastic impact mechanism assumption — holds) and 1500 K (where the critical viscosity model of Walsh et al., 1990 is usually employed). The transition between the two extreme conditions is modelled through a temperature-driven modification of the mechanical properties of both particles and target surfaces. Our computations demonstrate that the updated model is able to return credible predictions of deposit formation when compared with the baseline models of Thornton and Ning and of Walsh and co-authors. Moreover in the region where particles bounce off, the model predict the coefficient of restitution according to the actual mechanical properties of particles, thus providing a better particle dynamics description than in both the critical viscosity and original Thornton and Ning models.

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In turbomachinery applications blade fouling represents a main cause of performance degradation. Among the different techniques currently available, online water washing is one of the most effective in removing deposit from the blades. Since this kind of washing is applied when the machine is close to design conditions, injected droplets are strongly accelerated when they reach the rotor blades and the understanding of their interaction with the blades is not straightforward. Moreover, undesirable phenomena like blades erosion or liquid film formation can occur. The present study aims at assessing droplets dragging from the injection system placed at the compressor inlet till the first stage rotor blades, with a focus on droplets impact locations, on the washing process and the associated risk of erosion. 3D numerical simulations of the whole compressor geometry (up to the first rotor stage) are performed by using Ansys Fluent to account for the asymmetric distribution of the sprays around of the machine struts, IGV and rotor blades. The simulations are carried out by adopting the k-ε realizable turbulence model with standard wall functions, coupled with the discretephase model to track injected droplets motion. Droplets-wall interaction is also accounted for by adopting the Stanton-Rutland model which define a droplet impact outcome depending on the impact conditions. The induced erosion is evaluated by adopting an erosion model previously developed by some of the authors and implemented in Fluent through the use of a User Defined Function (UDF). Two sets of simulations are performed, by considering the rotor still and rotating, representative of off-line and on-line water washing conditions, respectively. In the rotating simulation, the Multiple Reference Frame Model is used. The obtained results demonstrate that the washing process differs substantially between the fixed and the rotating case. Moreover, to quantify the water washing effectiveness and the erosion risk, new indices were introduced and computed for the main components of the machine. These indices can be considered as useful prescriptions in the optimization process of water washing systems.

Jet impingement against surfaces is uses in several industrial applications, including the internal cooling of turbine blades. Coolant used for blade cooling is air bleed from a compressor and is laden with particles. A numerical simulation of a particle-laden impinging jet is here proposed, aiming at studying and analysing the effect of surface curvature on flow field, particle motion and deposit formation. To this aim, an impinging jet on a flat and a curved walls is considered. Flow motion is be solved using Direct Numerical Simulation, thus no additional model is needed for turbulence effect on particle motion. Results show that the deposit patterns follow some secondary flow structures, showing a series of peaks forming according to those structures. The peaks are present in both the main orthogonal direction of the jet, but they are not symmetrical due to surface curvature.

Recent year witnessed an increasing interest in online water washing technique since it allows to minimize compressor performance losses in the time interval between two off-line washing. However, the washing capability and the related erosion risk depend on several parameters such as the injection duration, the droplet size, the spray angle, the water mass flow rate and the injector positions. The influence of such parameters on the washing capability and erosion rate is analysed. Results are discussed with reference to number of impacts, wetted surface, capture efficiency, accumulated energy and erosion. The numerical simulation is performed with ANSYS Fluent in which a new water droplet erosion model, introduced in previous papers, is here included as a User Defined Function. The discussion provides useful information for prescribing the injector characteristics and the water washing procedure with the aim of minimizing the erosion risk.