Thermal Design Optimization of a Water-Based Condensate Particle Counter

ABSTRACT

Condensate Particle Counters (CPCs) are used to measure particle number densities to ascertain nano-scale (30nm-100nm) particulate exposure to adults and children. An air sample is introduced in a supersaturated vapor environment maintained in the condensation chamber of the CPC to facilitate vapor condensation on the particle surface. The size of the condensate-immersed particle grows as the particle moves through the condensation chamber and eventually becomes sufficiently large to detect by optical methods. In this paper we have computationally modeled the particulate-humid air two-phase flow in a water-based micro-scale condensate particulate counter. The numerical model is based on a finite volume method where the mass, momentum, energy and species conservation equations are solved simultaneously with appropriate interface and boundary conditions. A parametric study is performed by considering various combinations of wall temperature, ambient temperature, air sampling rate, and length of the CPC. The design intent was to create a large supersaturated region of water vapor-air mixture inside the condensation chamber which would result in required rate of growth at different ambient conditions for a range of particle sizes. Effects of different flow parameters, ambient conditions, and CPC geometry are delineated in the paper. The study showed that a low flow rate or low wall temperature would result in less supersaturation than required, whereas, a very high flow rate condition would restrict the supersaturated region close to the walls.

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