In the present thesis it is introduced a miniaturized, single-stage inertial impactor that has been designed, numerically modelled, prototyped and tested, for the collection of microparticles in ambient air. The target particles are PMs since in recent years, air pollution has increased extensively and clinic studies have largely demonstrated that these microparticles are responsible for causing respiratory and cardiovascular diseases to the human being . The traditional sysems used to assess PMs concentration in air are stationary and cumbersome, using optical and gravimetric methods. However, increasing attention has raised towards micro- and miniaturized technologies able to assess particles concentrations. Inertial impactors are a common method used to sample coarse and fine particles. Their working principle exploits the inertia of a particle suspended in a flowing fluid: a particle laden fluid, flows in a vertical jet, passing through a nozzle and impinging on the surface of a plate placed at a fixed distance from the outlet of the nozzle. This confined jet, is forced to change direction due to an abrupt T-junction, forming a stagnation point on the impaction plate. Therefore, particles with great inertia are induced to deviate from the streamlines of the flow, while particles with small inertia tend to follow the streamline, exiting in the side channels. Inertial impactors have been widely studied and employed, due to their simple design and functioning. The foundation of modern study on inertial impactors have been performed by V. A. Marple (1970), who has studied numerically and experimentally the performance of round and rectangular inertial impactors, giving the guidelines for a good design of such devices. Therefore, we decided to realize an single-stage miniaturized inertial impactor with the intent to assemble it with a micro-sensor able to count single particles by perceiving very small capacitance differences. We have followed the design procedure presented by Marple and Willeke, 1976, to assess the dimensions and the operating points of our inertial impactor. The operating points chosen have been numerically simulated with a Viscous laminar model using a CFD program Fluent v. 16.2. They show two completely different fluid dynamics performances: as a matter of fact, when operating at Re = 1000, a laminar jet flow is observed inside the fluid domain. However, at higher Reynolds number (Re = 3000), the fluid dynamics is completely unsteady, because at the corner of the T-junction a vortex shedding starts to form, keeping the flow unstable. The particles trajectories have been simulated in steady and unsteady conditions, using the the Discrete Phase Model availble in Fluent v. 16.2. Moreover, we numerically investigated the flow field in the domain, at Re=3000, with a jet-to-plate distance doubled with respect to the one of our domain. As a matter of fact, the solution showed that the unsteadiness of the flow increases with higher jet-to-plate distance, inducing an oscillation of the stagnation point. To validate the numerical results, a prototype have been created, with the aid of 3D printing, and it has been tested at Re = 1000 and at Re = 3000, for the original configuration, and at Re = 3000, for the configuration with the doubled jet-to-plate distance. The results have demonstrated that particles are deposited in a line, focussing at the stagnation point. By qualitatively comparing the results of the experiments, the particles have shown a behaviour in agreement with the numerical results.
Il presente lavoro di tesi riguarda la progettazione, la modellizzazione numerica, la prototipazione e il test di un filtro a impatto inerziale a un singolo stadio, miniaturizzato, per la detezione di microparticolato atmosferico. Negli ultimi anni, l'inquinamento aereo e' aumentato drasticamente e numerosi studi hanno dimostrato che le microparticelle PM, relative alle polveri sottili, sono dannose per la salute della popolazione e responsabili di procurare malattie respiratorie e cardiovascolari. I sistemi tradizionali utilizzati per valutare la concentrazione aerea di PM sono fissi e ingombranti, tipicamente funzionano sfruttando tecniche ottiche o gravimetriche. Tuttavia, micro- e mini- dispositivi per la detezione di microparticelle hanno acquistato grande interesse nel mondo scientifico. I filtri a impatto inerziale sono un metodo comunemente utilizzato per separare particelle grandi e piccole. Il loro principio di funzionamento si basa sull'inerzia di una particella sospesa in un fluido in movimento: un flusso carico di particelle entra in un ugello verticale e impatta su di una superficie che lo confina e forza a cambiare direzione bruscamente, grazie a una giunzione a T. Questo fenomeno genera un punto di stagnazione a velocita nulla sulla superficie di impatto. Di conseguenza, particelle aventi una grande inerzia deviano dalle linee di flusso e impattano sulla superficie che confina il getto, mentre le particelle con bassa inerzia proseguono sulle linee di flusso del fluido. I filtri a impatto inerziale sono ampiamente studiati e utilizzati, grazie alla loro semplicita di funzionamento e facile progettazione. V. A. Marple ha eseguito nel 1970 una profonda, numerica e sperimentale, sui filtri a impatto, ponendo le basi per i successivi studi e fornendo dei validi strumenti per la progettazione di filtri a impatto. Pertanto si e' deciso di realizzare un filtro a impatto a singolo stadio miniaturizzato con l'intento di accoppiarlo con un microsensore in grado di contare singolarmente le particelle che impattano sulla sua superficie, percependo differenze di capacita molto piccole. E' stata seguita la procedura di design proposta da Marple e Willeke nel 1976, per dimensionare il dispositivo e scegliere i punti di lavoro. I punti di lavoro scelti, a Re = 1000 e Re = 3000, sono stati modellizzati numericamente utilizzando Fluent v. 16.2. Si e' dimostrato che la fluidodinamica dei due punti di lavoro e' completamente differente: infatti per Re = 1000, si e' osservato, all'interno del dominio fluido, un flusso laminare. Tuttavia a Re = 3000 la fluidodinamica e' completamente non stazionaria, perche lo spigolo vivo della giunzione a T genera un distacco di vortici che mantiene il flusso instabile. La traiettoria delle particelle e' stata simulata in regime stazionario e tempo-dipendente, usando il Discrete Phase Model disponibile in Fluent v. 16.2. Inoltre, e' stato studiato numericamente il campo di flusso a Re = 3000 nel dominio fluido con la distanza dall'uscita del getto al piatto di impatto pari a 2 volte quella del dominio originale. Infatti, la soluzione ha mostrato che le instabilita' del flusso aumentano all'aumentare di questa distanza, inducendo il punto di stagnazione ad oscillare. Per validare i risultati numerici, e' stato realizzato un prototipo, per mezzo di stampanti 3D. Successivamente e' stato testato a Re = 1000 e Re = 3000, nella configurazione originale, e a Re = 3000 nella configurazione con la distanza tra l'uscita del getto e il piatto di impatto piu' grande. I risultati hanno dimostrato che le particelle si depositano in fila lungo la centerline del dispositivo, nel punto di stagnazione. Comparando qualitativamente i risultati degli esperimenti realizzati, si e' dimostrato che le particelle si comportano coerentemente con i risultati delle simulazioni numeriche.
Numerical modelling, design, prototyping and testing of a miniaturized detector for airborne micrometric particulate
PASINI, SILVIA
2015/2016
Abstract
In the present thesis it is introduced a miniaturized, single-stage inertial impactor that has been designed, numerically modelled, prototyped and tested, for the collection of microparticles in ambient air. The target particles are PMs since in recent years, air pollution has increased extensively and clinic studies have largely demonstrated that these microparticles are responsible for causing respiratory and cardiovascular diseases to the human being . The traditional sysems used to assess PMs concentration in air are stationary and cumbersome, using optical and gravimetric methods. However, increasing attention has raised towards micro- and miniaturized technologies able to assess particles concentrations. Inertial impactors are a common method used to sample coarse and fine particles. Their working principle exploits the inertia of a particle suspended in a flowing fluid: a particle laden fluid, flows in a vertical jet, passing through a nozzle and impinging on the surface of a plate placed at a fixed distance from the outlet of the nozzle. This confined jet, is forced to change direction due to an abrupt T-junction, forming a stagnation point on the impaction plate. Therefore, particles with great inertia are induced to deviate from the streamlines of the flow, while particles with small inertia tend to follow the streamline, exiting in the side channels. Inertial impactors have been widely studied and employed, due to their simple design and functioning. The foundation of modern study on inertial impactors have been performed by V. A. Marple (1970), who has studied numerically and experimentally the performance of round and rectangular inertial impactors, giving the guidelines for a good design of such devices. Therefore, we decided to realize an single-stage miniaturized inertial impactor with the intent to assemble it with a micro-sensor able to count single particles by perceiving very small capacitance differences. We have followed the design procedure presented by Marple and Willeke, 1976, to assess the dimensions and the operating points of our inertial impactor. The operating points chosen have been numerically simulated with a Viscous laminar model using a CFD program Fluent v. 16.2. They show two completely different fluid dynamics performances: as a matter of fact, when operating at Re = 1000, a laminar jet flow is observed inside the fluid domain. However, at higher Reynolds number (Re = 3000), the fluid dynamics is completely unsteady, because at the corner of the T-junction a vortex shedding starts to form, keeping the flow unstable. The particles trajectories have been simulated in steady and unsteady conditions, using the the Discrete Phase Model availble in Fluent v. 16.2. Moreover, we numerically investigated the flow field in the domain, at Re=3000, with a jet-to-plate distance doubled with respect to the one of our domain. As a matter of fact, the solution showed that the unsteadiness of the flow increases with higher jet-to-plate distance, inducing an oscillation of the stagnation point. To validate the numerical results, a prototype have been created, with the aid of 3D printing, and it has been tested at Re = 1000 and at Re = 3000, for the original configuration, and at Re = 3000, for the configuration with the doubled jet-to-plate distance. The results have demonstrated that particles are deposited in a line, focussing at the stagnation point. By qualitatively comparing the results of the experiments, the particles have shown a behaviour in agreement with the numerical results.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/133456