Virtual Impactor Case Study

Although virtual impactors are commercially available, their performance depends strongly on design geometry, flow rate, and operating conditions. A generic design rarely achieves optimal efficiency for a specific manufacturing process. Factors such as particle size distribution, carrier-gas properties, and flow regime must all be tuned carefully to ensure effective separation. Traditional physical prototyping is often impractical: manufacturing and testing multiple prototypes to assess design sensitivity to flow parameters is costly, time-consuming, and limited in scope. Moreover, subtle issues such as particle clumping, deposition on walls, or unwanted chemical or thermal interactions can be difficult to predict experimentally. Jesmond Engineering sought to demonstrate how CFD can be used to overcome these limitations by allowing rapid, accurate virtual testing across a wide design space.

A detailed CFD model of a three-outlet virtual impactor was developed to analyse its particle separation efficiency. A Rossin–Rammler particle size distribution was used at the inlet to represent a realistic spread of particle diameters within the flow, and the simulation tracked the trajectories of these particles as they passed through regions of differing velocity to quantify separation patterns. The key governing parameter, the Stokes number, was used to characterise particle behaviour: large particles with high Stokes numbers possessed sufficient inertia to continue in a straight path towards the central outlet, while small particles with low Stokes numbers followed the diverging streamlines to the side outlets. By adjusting geometric parameters, flow rates, and outlet configurations, Jesmond Engineering determined how each design variable affected separation efficiency, with analytical validation performed alongside the CFD to ensure results corresponded with theoretical expectations.

The CFD simulations clearly demonstrated how design geometry and flow conditions influence the efficiency of virtual impactors. Small adjustments to the flow-split angle and outlet positioning were shown to significantly affect the proportion of correctly separated particles. The modelling confirmed that larger particles are efficiently captured by inertia in the central outlet, while smaller ones are diverted into the side channels by the local flow field. By comparing multiple design configurations, Jesmond Engineering successfully identified the parameters most critical to efficient and reliable separation performance.

This study highlights the power of CFD in accelerating the design and optimisation of complex particle separation devices. By replacing costly physical prototyping with computer-based simulation, Jesmond Engineering can evaluate numerous design variations in hours rather than weeks, enabling reduced development costs, shorter design cycles, and enhanced confidence in final product performance before fabrication. Beyond virtual impactors, the same simulation-based approach can be applied to a wide range of particle-laden flow systems, including filters, scrubbers, cyclones, and aerosol generators, delivering better-performing, more energy-efficient designs tailored precisely to each process.