An innovative methodology for the analysis of microparticle deposits in transonic and subsonic blades for the assessment of compressor degradation

Abstract

Solid particle ingestion is one of the principal degradation mechanisms in the compressor section of heavy-duty gas turbines. Usually, foulants in the ppm range not captured by the air filtration system cause deposits on blading and result in a severe performance drop of the compressor. It is of great interest to the manufacturer and industry to determine which areas of the compressor airfoils are affected by these contaminants as a function of the location of the power unit. The aim of this work is the estimation of the actual deposits on the blade surface in terms of location and quantity. Particle trajectory simulations use a stochastic Lagrangian tracking method which solves the equations of motion separately from the continuous phase. Then, a transonic rotor and subsonic rotor are considered as a case study for the numerical investigation. The compressor rotor numerical model and the discrete phase treatment have been validated against the experimental and numerical data available in literature. The size of the particles, their concentrations and the filtration efficiency are specified in order to perform a realistic quantitative analysis of the fouling phenomena in an axial compressor. This study combines the impact/adhesion characteristic of the particles obtained through a Computational Fluid Dynamics (CFD) numerical simulation and the real size distribution of the contaminants in the air swallowed by the compressor. The kinematic characteristics (velocity and angle) of the impact of micrometric and sub-micrometric particles with the blade surface of an axial transonic and subsonic rotor are shown. The blade zones affected by particle impact are extensively analyzed. This work has the goal of combining the kinematic characteristics of particle impact on the blade with fouling phenomenon through the use of a quantity called ‘sticking probability’ adopted from literature. The analysis shows that particular fluid-dynamic phenomena such as separation, shock waves and tip leakage vortex strongly influence pattern deposition. The combination of the smaller particles (0.15 ― 0.25) μm and the larger ones (1.00 ― 1.50) μm determines the highest amounts of deposits on the leading edge of the compressor airfoil. The blade zones affected by deposits are clearly reported by using easy-to-use contaminant maps realized on the blade surface in terms of contaminant mass per unit of time. From these analyses, some guidelines for proper installation and management of the power plant (in terms of filtration systems and washing strategies) can be drawn.