A single-pump parallel micro virtual impactor integrating choked flow regulation and orifice-induced particle beam focusing for enhanced SAW detection

Abstract

In modern industrial and urban environments, the health impacts arising from ultrafine particle exposure have become increasingly significant, creating a demand for compact, low-power classification technologies capable of accurately separating and measuring such particles in real time. Conventional micro virtual impactors (μVIs) often rely on multiple pumps or mass flow controllers, which increases system complexity and power consumption. In this study, a single-pump parallel micro virtual impactor (SPP-μVI) is proposed and its flow control and particle detection performance are numerically investigated under a low flow rate of 50 mL/min. A choked-flow nozzle was introduced to passively maintain a stable major-to-minor flow-rate ratio of 9:1 for each μVI under inlet pressures down to 5% below atmospheric pressure. The predicted cutoff sizes of 2.45, 0.95, and 0.31 μm for the designed targets of 2.5, 1.0, and 0.3 μm, respectively, showed errors within 2–5%. In addition, an aerosol collimation orifice designed to satisfy the condition of Stokes number ≈ 1 was incorporated to enhance aerosol beam focusing and thermophoretic deposition on a sensor surface. The average velocity was minimized approximately 10,000 μm downstream of the orifice, where extended particle residence time and strengthened thermophoretic effects above the SAW sensor led to maximum deposition. These results demonstrate that the proposed SPP-μVI enables passive flow balancing among parallel μVIs under single-pump operation while integrating particle focusing, thermophoretic deposition enhancement, and SAW-sensor-based real-time detection, highlighting its potential as a core low-flow-rate classification technology for future portable and low-power fine-particle measurement platforms.