Design and modelling of a low-cost aerosol particle counting device

The measurement of atmospheric airborne particles is important for contributing to human health and assessing a number of manufacturing processes. Inexpensive sensors exist for measuring larger particles (> 1000 nm), but those for ultrafine particles (<300 nm) remain too expensive (>£10,000) for use in sensor networks, yet these represent about 90% of the number of typical airborne particles in urban settings. These include vehicle emissions within 10-100 nm and viruses around 100 nm.

This project will work with Cambridge university startup Aetosense who are looking to bridge this gap with their low-cost ultrafine sensor. A prototype sensor for quantitative nanoparticle measurements has been built and tested. The sensor is based on using water vapour to grow individual particles into larger droplets, and using a beam of laser light to scatter off the droplets along a flow column. Water vapour is delivered using a saturated wick at a controlled heating rate, followed by condensation on the particles in a colder section. Some preliminary model has been undertaken, but further studies are required to optimise the overall approach and help invert the signal to provide particle number concentrations.

Objectives:

• Produce a quantitative model of how particles grow by water condensation in the system
• Couple the model to modelling of the optical extinction of the signal
• Compare the model results and experimental data
• Indicate directions for improving the overall power and water consumption rate of the device.

Expected Tasks

1. Basic CFD modelling: Produce a CFD model of the unit based on previous modelling using COMSOL, by adding a porous flow model for the wicking medium, and produce a coupled heat and mass transfer analysis of the droplet growth, which will be initiated by a point-based Lagrangian model. An extension of the model would be to add particle diffusion to the Lagrangian (or Eulerian) model of particle growth. Validation will be produced against a state-of-the art condensation particle counting (CPC) device
2. Optical modelling: The output of the CFD simulations of particle extinction along the line of sight of the laser beam. Validation will be provided against the signal obtained in the device prototype.
3. Optimisation: if time allows, recommendations from the model simulations will be made to reduce water and power consumption.

Requirements:

• interest in CFD simulations
• familiarity and interest with programming languages (python, matlab) sufficient to extract data for visualisation
• interest in heat and mass transfer (ok to learn on the job by looking at 3A6 notes)