Experimental validation of thermophoretic and bend nanoparticle loss for a regulatory prescribed aircraft nvPM sampling system

Abstract

Aircraft gas turbine engines produce ultrafine PM which has been linked to local-air-quality and environmental concerns. Regulatory sampling and measurement standards were recently introduced by ICAO to mitigate these emission of nonvolatile PM (nvPM). Currently, reported nvPM emissions can significantly under-represent engine exit concentrations due to particle loss. A System-Loss-Tool (SLT) has been proposed to correct for particle loss in the standard sampling and measurement system permitting an estimation of engine exit concentrations for airport environment inventories. Thermophoretic and bend particle loss mechanisms are predicted in the SLT using expressions derived from the literature, which are not in all cases empirically validated to conditions representative of aircraft nvPM exhaust sampling methodologies. In this study, thermophoretic (Tgas≤910 °C) and coiling-induced (≤3960°) particle loss were measured using sampling variables relevant to aerospace certification. Experiments were performed using laboratory generated solid particles (fractal graphite, cubical salt and spherical silica) bounding the upper and lower limits of aircraft soot morphology (i.e., particle effective density, mass-mobility exponent, primary-particle-size). These were aerodynamically classified using a Cambustion Aerodynamic-Aerosol-Classifier (AAC) at electrical-mobility diameters ranging from 30 to 140 nm. The AAC was shown to efficiently classify salt and silica particles, producing monomodal distributions ≥25 nm electrical-mobility GMD, whilst classifying fractal graphite >40 nm electrical-mobility GMD (calculated as da≥20 nm) albeit generally displaying larger GSD’s. Thermophoretic loss at ΔTgas of 0–880 K correlated well with the SLT for non-fractal particles with losses ≤39.2% measured, with higher depositions observed for graphite (4.1%) considered insignificant compared to overall measurement uncertainty. Coiling a 25 m sample line in compliance with ICAO standards induced negligible additional particle loss at flowrates relevant of aircraft exhaust sampling, in agreement with SLT-predicted bend losses. However, additional losses were witnessed at lower flowrates (≤13% at 30 nm), attributed to secondary flow diffusion loss induced by the coiling.