A Circumferential Slot Virtual Impactor

Abstract

A virtual impactor aerosol concentrator has been developed that uses circumferential slots for acceleration of aerosol particles and for collection of the coarse fraction. This allows for accurate and economical machining of small slot widths, which leads to lowpressure losses for the separation process. One important application of the device is in the concentration of bioaerosols, especially for military field applications where minimization of power consumption is necessary.A prototype configuration of the circumferential slot virtual impactor (CSVI), which was designed using numerical methods, was constructed and tested. The device has a curvilinear slit nozzle with a diameter of 150.3 mm (5.918 in), which provides a total slot length of 472 mm. Its slot width was 0.499 mm (0.0197 in). According to Loo and Cork, for circular-jet virtual impactors the misalignment between the axis of the acceleration jet and the receiver nozzle will cause an increase in wall losses of about 1.6% for each 1% of misalignment. Measurements were made of the nozzle dimensions in the critical region of the CSVI that showed 1.8% relative misalignment. When this prototype was operated at a flowrate of 122 l/min and a flow fraction (minor air flowrate/total air-flowrate) of 10%, the cutpoint was 2.2 µm aerodynamic diameter and the corresponding cutpoint Stokes number was 0.58. The collection efficiency was greater than 72% for particle sizes larger than twice the cutpoint, up to the largest particle size tested (≈10 µm aerodynamic diameter). The peak collection efficiency was greater than 95%. For virtual impactors, a critical performance parameter is the loss of particulate matter to the inner walls of the system. For the prototype system, where numerical methods had been used to generate designs that reduced wall losses, the losses at the cutpoint size of 2.2 µm aerodynamic diameter, are approximately 3%. For an operational condition of a total flowrate of 122 l/min and a coarse particle flow fraction of 10%, the pressure drop across the major flow stream (fine particle stream) was 63 Pa (0.25 in of water), with an ideal power consumption of 0.14 watts.

Measurements of the relation between aerosol properties and microphysics and chemistry of low level liquid water clouds in Northern Finland

“…Since this part of the study was focused on aerosol mass and since diffusion losses concern mainly ultrafine particles having a low mass concentration, the diffusion losses were neglected in our analysis. Particles with an aerodynamic diameter larger than 5 µm have wall looses in sharp angles (Haglund and McFarland, 2004). This effect increases with an increasing particle size (McFarland et al, 1997).…”

CFD Study on the Effects of the Large Particle Crossing Trajectory Phenomenon on Virtual Impactor Performance

“…Not only are small dimensions required, but also the fabrication must be precise (Loo and Cork 1988) and affordable, which generally leads to designs with acceleration nozzles that are chamfered, i.e., straight sides in the converging region. Haglund and McFarland (2004) reported results of tests conducted on linear and circumferential versions of a slot-type virtual impactor (denoted herein as LSVI and CSVI, respectively). The CSVI geometry is analogous to an LSVI with the two ends being joined together so the slot length forms a hoop.…”

“…However, some experimental results have indicated the presence of a third region in the transmission efficiency curve, where there is a steep decrease for larger particles, accompanied by increasing values of internal wall losses. Transmission efficiency curves, showing evidence of Region 3 behavior, have been given for slot virtual impactors (Ding et al 2000(Ding et al 2001Demokritou et al 2002;Haglund and McFarland 2004) and for circular nozzle virtual impactors (Ding et al 2000;Romay et al 2002). However, Region 3 does not appear on a transmission efficiency curve if the transmission efficiency is taken to be the ratio of the mass flow rate of aerosol particles in the minor flow stream to the sum of the mass flow rates in the minor and major flow streams.…”

A Circumferential Slot In-Line Virtual Impactor

“…By comparison, Haglund and McFarland (2004) presented minor flow transmission efficiency results for a slot virtual impactor, and the steepness of their curve, calculated from Equation (3), was 2.1. As a reference, the value of s g would be unity for an ideal fractionator with a step-type fractionation curve, with increasing values of the steepness value associated with poorer fractionation characteristics.…”

“…A circumferential-slot virtual impactor has certain inherent advantages relative to a circular jet virtual impactor in that a narrow slot can be easily fabricated and, provided the flow is stable, aerosol fractionation can be achieved with a low expenditure of pressure, Haglund and McFarland (2004). Herein, we present the design, development, and characterization of a dual-cone circumferential-slot In-Line Virtual Impactor (IVI).…”