Numerical models for rotating Lagrangian particles in turbulent flows

Abstract

The primary motivation for this dissertation is to address the problem of aircraft engine degradation due to ingestion of non-aqueous particulates such as sand, dust, and ash. This dissertation introduces high fidelity Lagrangian particle models to account for the rotational effects of particles in complex turbulent flows. Part of the focus of the research is to provide novel techniques to model particle-surface collision induced rotation as well as develop correlations for forces and torques on non-spherical particles. Particulates in nature have a tendency to damage turbomachinery components through a number of mechanisms which include erosion and adhesion and will lead to engine failure. The trajectory, size, velocity, chemical composition and shape of the particles play an important role in predicting the damage occurring in the engine. An aircraft engine consists of a cold section, comprising of an inlet and compressor, and a hot section consisting of the combustion chamber and turbine. The particulates enter through the inlet and impact against the components within the compressor eroding away the surfaces as well as causing the particles to fracture into smaller sizes. On entering the hot section, the particles encounter drastic phase changes leading to change in their intrinsic properties. Some of these particles may also adhere to the blade surfaces blocking cooling ports. In this report we focus on particle interaction with the cold sections of aviation gas turbine engines, and so we do not study any of the phenomena that occur due to the heating of particles. To predict how particles will interact with the engine components it is first important to understand the trajectory of particles. For flow into the inlets and early stages of the compressor, particle trajectory is dominated by a two factors: particle aerodynamics and rebounds from surface collisions. Near-wall aerodynamics also play an important role in particle impact and surface erosion. The particle trajectories show the adverse effects of the near wall aerodynamic effects just before collision. The particle velocities are influenced significantly by these effects, and in order to predict particle rebound properties and erosion accurately, these effects have to be taken into account. One important, but often neglected aspect of particle trajectory is accurate prediction of particle rotation. The primary source of the angular velocity of the particles is through particle collisions with walls. However, few models of particle-wall impact introduce rotation. Several studies indicate that the particle angular velocity plays a significant role on their trajectories even in simple geometries such as curved pipes. The research in this dissertation introduces a collision-induced particle rotation model that improves the prediction of particle trajectories after rebound. The improved collision model compares well to experimental data provided in literature and its importance is demonstrated in a simple pipe bend. There have been several experimental studies from past literature, that have developed lift and drag correlations for rotating particles. However, from the literature we also see that these correlations are very specific to the particle shape, Reynolds Number and their orientation relative to the oncoming flow. Real particles are non-spherical, and almost all existing models for particles consider only spheres. Non-spherical particles have different values of aerodynamic lift, drag, and torque compared to their spherical counterparts. As a means to explore and quantify the effect of particle shape, and orientation on the aerodynamic forces and torques on non-spherical particles, we developed a CFD framework that has the ability to measure the lift and drag on arbitrarily shaped non-spherical particles by rotating a single particle in space in an airstream. On changing parameters such as the air velocity, rotation rate, orientation we can tabulate the lift and drag forces and torques on the particle. These correlations can be implemented into Lagrangian particle models to improve the predictions of particle trajectories due to rotation induced lift and drag. The importance of particle rotation is demonstrated by injecting particles into a high pressure compressor section of a gas turbine engine and comparing erosion profiles and impact locations between particles with and without the rotation models. The research presented in this dissertation aims to improve the prediction of particle trajectories by considering non-ideal parameters such as the aerodynamic effects on non-spherical particles and the influence of rotation on particle motion. Particle-surface collisions play a significant role in particle trajectories and so the first step in improving these predictions is to gain a better understanding of particle rebound phenomena.