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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.

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Fouling affects gas turbine operation, and airborne or fuel contaminants, under certain conditions, become very likely to adhere to surfaces if impact takes place. Particle sticking implies the change in shape in terms of roughness of the impinged surface. The consequences of these deposits could be dramatic: these effects can shut an aircraft engine down or derate a land-based power unit. This occurrence may happen due to the reduction of the compressor flow rate and the turbine capacity, caused by a variation in the HPT nozzle throat area (geometric blockage due to the thickness of the deposited layer and the aerodynamic blockage due to the increased roughness, and in turn boundary layer). Several methods to quantify particle sticking have been proposed in literature so far, and the experimental data used for their validation vary in a wide range of materials and conditions. The experimental analyzes have been supported by (and have given inspiration to) increasingly realistic mathematical models. Experimental tests have been carried out on (i) a full scale gas turbine unit, (ii) wind tunnel testing or hot gas facilities using stationary cascades, able to reproduce the same conditions of gas turbine nozzle operation and finally, (iii) wind tunnel testing or hot gas facilities using a coupon as the target. In this review, the whole variety of experimental tests performed are gathered and classified according to composition, size, temperature, and particle impact velocity. Using particle viscosity and sticking prediction models, over seventy (70) tests are compared with each other and with the model previsions providing a useful starting point for a comprehensive critical analysis. Due to the variety of test conditions, the related results are difficult to be pieced together due to differences in particle material and properties. The historical data of particle deposition obtained over thirty (30) years are classified using particle kinetic energy and the ratio between particle temperature and its softening temperature. Qualitative thresholds for the distinction between particle deposition, surface erosion, and particle break-up, based on particle properties and impact conditions, are identified. The outcome of this paper can be used for further development of sticking models or as a starting point for new insight into the problem.

Fouling in gas turbines is caused by airborne contaminants which, under certain conditions, adhere to aerodynamic surfaces upon impact. The growth of solid deposits causes geometric modifications of the blades in terms of both mean shape and roughness level. The consequences of particle deposition range from performance deterioration to life reduction to complete loss of power. Due to the importance of the phenomenon, several methods to model particle sticking have been proposed in literature. Most models are based on the idea of a sticking probability, defined as the likelihood a particle has to stick to a surface upon impact. Other models investigate the phenomenon from a deterministic point of view by calculating the energy available before and after the impact. The nature of the materials encountered within this environment does not lend itself to a very precise characterization, consequently, it is difficult to establish the limits of validity of sticking models based on field data or even laboratory scale experiments. As a result, predicting the growth of solid deposits in gas turbines is still a task fraught with difficulty. In this work, two nondimensional parameters are defined to describe the interaction between incident particles and a substrate, with particular reference to sticking behavior in a gas turbine. In the first part of the work, historical experimental data on particle adhesion under gas turbine-like conditions are analyzed by means of relevant dimensional quantities (e.g. particle viscosity, surface tension, and kinetic energy). After a dimensional analysis, the data then are classified using non-dimensional groups and a universal threshold for the transition from erosion to deposition and from fragmentation to splashing based on particle properties and impact conditions is identified. The relation between particle kinetic energy/surface energy and the particle temperature normalized by the softening temperature represents the original non-dimensional groups able to represent a basis of a promising adhesion criterion.