Local experimental methodology for the study of microparticles resuspension in ventilated duct during fan acceleration

Abstract

The purpose of this study is to develop an experimental methodology with relevant space and time resolutions to track the velocity properties responsible for the resuspension of microparticles during the acceleration stage of a fan start. Microparticles release is investigated over a time period of several seconds, i.e. at short time. This methodology involves velocity signal measurements thanks to Hot Wire Anemometry, and an optical counting method to build resuspension kinetics curves. During the fan acceleration the velocity evolution is characterized by two stages: a first increase without fluctuations, and then the acceleration with fluctuations. The same behavior is observed whatever the distance to the wall at which velocity is considered. The resuspension phenomenon seems to be initiated by a threshold turbulent kinetic energy, i.e. by turbulent events powerful enough to release microparticles having the lowest adhesion forces. For the studied particles properties/wall properties/aeraulic conditions, a significant fraction of particles remains on the duct wall at the end of experiments, despite the fact that the remaining fraction is stabilized. This may reveal that the highest energy levels of flow events seen by microparticles were not powerful enough to release particles having the highest adhesion forces.

Influence of the transient airflow pattern on the temporal evolution of microparticle resuspension: Application to ventilated duct during fan acceleration

“…The results reported in Table 3 show that rather significant deviations were observed between F eq values obtained for the different trials carried out for each temporal airflow pattern. As mentioned by Theron, Debba, and Le Coq (2020) this is not explained by temperature and relative humidity variations, which are quite low (see Table 1), apart from experiments conducted at U mean ¼ 9.0 m.s À1 and a mean ¼ 2.1 m.s À2 . For these experiments, the RH range indicated in Table 1 is rather high: 31-57%, but among the seven trials five were performed for RH comprised between 31 and 35%, and only two at 57%.…”

“…The experimental facilities, materials and methods used to conduct the experiments are the same as those presented in Theron, Debba, and Le Coq (2020). The experimental methodology was detailed for one temporal airflow pattern (mean duct velocity at steady state of 7.6 m.s À1 and mean acceleration of 2.1 m.s À2 ).…”

“…For each temporal airflow pattern the curve representing the temporal evolution of the remaining fraction F was obtained thanks to the image processing methodology (using the Image J software) detailed in Theron, Debba, and Le Coq (2020). In this procedure particles of diameter lower than 9 mm are not counted.…”

“…In Figure 6 the temporal evolution of the remaining fraction is superimposed to the evolution of the velocity in the viscous sublayer for cases for which resuspension occurred during every trials, except for the case U mean ¼ 7.6 m.s À1 and a mean ¼ 2.1 m.s À2 described in Theron, Debba, and Le Coq (2020). For the three temporal airflow patterns presented in this figure the graphs show that the resuspension phenomenon starts during the fan acceleration and more precisely during the acceleration with fluctuations stage.…”

“…In parallel to these modeling works, many experimental studies were dedicated to the resuspension phenomenon for cases of monolayer deposits. Theron, Debba, and Le Coq (2020) summarized those conducted in wind tunnels, by emphasizing the experimental methodologies that were implemented; especially in terms of temporal airflow patterns to which particles were exposed; and the type of results that were reported. Most of these studies aimed at determining detachment fractions versus mean air velocity, free stream velocity or friction velocity, or threshold velocities responsible for the detachment of 50% of particles (Barth et al 2014;Brach 2003, 2004;Ibrahim, Dunn, and Qazi 2008;Ibrahim and Dunn 2006;Jiang et al 2008).…”