Particle Transport and Losses when Sampling Aircraft Gas Turbine Combustion Engine nvPM Emissions

Abstract

Incomplete combustion of hydrocarbon fuels in aircraft engines leads to the production of a complex mix of gaseous, volatile, and non-volatile Particulate Matter (nvPM) emissions. nvPM emissions have been linked to several health and environmental implications, where it is estimated to cause 16,000 premature death and act as nucleation sites for contrail formation. To combat these aforementioned implications, international agencies have introduced reporting standards to regulate the global nvPM emitted by aircraft engines. However, due to harsh environments experienced at an aircraft engine exhaust, and the complex nature of measuring ultrafine particles, an intricate sampling system is required with long sampling tubing that causes up to 90% of the particles to be lost. Consequentially, a correction methodology has to be employed to account for the particle losses and produce representative reporting. The current correction methodology accounts for five distinct particle losses mechanisms, although only diffusion and thermophoresis are usually included in the final calculations as the other mechanisms are assumed to have minimal impacts. The aim of this thesis was to explore potential unaccounted for effects from particle loss mechanisms that are not currently included in the aircraft engine nvPM corrections, specifically electrostatic and surface roughness induced particle losses. This included testing the correction methodology assumptions and sampling system design parameters, ultimately to recommend improvements to provide more representative nvPM measurements. Particle loss induced by surface roughness was initially investigated, which was found to be the explained by three regimes when in turbulent flow. The particle loss was observed to depend on the relative roughness, flowrate, and tube diameter. The investigation found that no additional losses occurred for 1/4" ID commercially available tubes (average roughness between 0.25-1.75 µm). However, as the roughness increased to that of an Additive Layer Manufactured 3D printed tube (with a roughness of 10 µm), the particle loss increased to above 30%. Therefore, it is recommended that any commercially available 1/4" tube can be used, but 3D printed tubes should be avoided in turbulent flow. In addition to surface roughness, electrostatic particle losses were also investigated. It was found that both naturally charged nebulised particles and unipolar particles were lost in cPTFE tubes by up to an additional 50% compared to stainless steel, depending on the tube diameter, flowrate, and charge state. It is therefore recommended that cPTFE tubes should be avoided in the sampling systems, which has subsequently been adopted on combustion test by Rolls-Royce in Germany. Furthermore, this study predicted that particle losses could occur in the initial part of the sampling system due to electrostatic dispersion when the particle number concentration is above 1e6 particles/cm3. To quantify the impact of electrostatic losses for real aircraft nvPM sampling, this thesis was the first experimental study to measure the charge of nvPM emitted from an aircraft engine. It was observed that nvPM carried an asymmetrical charge distribution, where up to 60% of the particles were charged and the distribution tended towards positive polarity as engine power increased, confirming previous theoretical predictions. Furthermore, using the measured values, it was predicted that electrostatic loss could cause an additional 35% nvPM loss for sub-20 nm particles. The measurements conducted in this thesis provide recommendations for feature sampling system designs to reduce nvPM losses that will improve the representativeness of reported aircraft nvPM. In addition, the nvPM charge quantification not only has implications on the particle loss, but could impact contrail formation, engine efficiency, and health studies that could lead to a better understanding of the global impacts of aircraft nvPM.

Date of Award	1 Aug 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Paul Williams (Supervisor) & Amanda Lea-Langton (Supervisor)