Aerosol Deposition on Surface Microstructures in Turbulent Flow

Speaker :
Mr. Haolun XU
Department of Mechanical and Aerospace Engineering, HKUST
Date : 23 Jun 2020 (Tue)
Time : 2:30 - 4:30 pm
Venue :
Organizer : Meeting ID: 999 3856 9418


Deposition of aerosol from turbulent airflow to a surface has received considerable attention in the past few decades because of its significance in numerous applications. One of the interests that emerged previously applies to protect the microelectronic devices from micro-contamination. Due to the fact that accumulation of dust may cause localized hot spots, and therefore resulting a reduction of the heat transfer coefficient of electronic components. The aerosol deposition on micrometer-scale obstructions can be applied to decrease the accumulation efficiency of dust around tiny electronic components. Another application field that has emerged in recent years is the disease infection through aerosol transmission. The relationship between the airborne particle exposure and human morbidity has received significant attention in the COVID-19 (2019 novel coronavirus). Coronaviruses are believed to be able to transmit from person-to-person through large respiratory droplets. Other routes during the aerosol-generating procedures can also lead to the inhalation of aerosols. There is sufficient evidence to demonstrate that these infectious diseases can spread through aerosols by ventilation, air movements in buildings. Building ventilation ducting systems play a core role in controlling indoor air quality by recirculating the indoor air and mixing with ambient air. The ventilation ducting systems can serve as an air cleaning system itself either through the filtration system or integrating other means, while at the same time, attention to energy consumption is needed. The high-efficiency fibrous filters in a conventional filtration system not only cause high-pressure drops that consume fan energy but also add to the high operation cost. This research proposed an air cleaning technique aimed at submicron particles by means of installing patterned surfaces on the ventilation ducting systems, which can be easily cleaned by water and reused.

The research consisted of experimental, numerical simulation and theoretical work. In the numerical methodology, the discrete phase model and Lagrangian particle tracking model were applied and the numerical results were validated with benchmark cases and experimental results. Then different numerical results on aerosol deposition mechanisms were discussed. Different forms of patterned surfaces inspired by nature were designed, and parametric studies were performed to enhance deposition efficiency. In the particle deposition experiment, a fully developed wind tunnel experiment was carried out to quantify the particle deposition on the semi-circular micropatterns for a wide range of heights. Different flow velocity was characterized by using the particle image velocimetry experiment. The experimental deposition velocity was calculated based on the deposition velocity at different locations multiplied by the area-weighted percentage at the upstream, center and downstream regions. A feasibility study of applying semi-circular patterned surfaces as an air cleaning technique in a ventilation ducting system was conducted in comparison with the high-efficiency particulate air (HEPA) filter. A semi-mathematical deposition velocity model was developed to calculate deposition velocity in different dimensions of ventilation ducting systems. This semi-mathematical model included the particle deposition mechanisms of Brownian motion, eddy diffusion-impaction effect, gravitational sedimentation and the inertia-interception effect caused by flow structures. In the end, particle deposition in different degree bend ducts was studied numerically, and the overall efficiency of installing patterned surface before and within the bend area was compared. The results indicated that the core effect of the deposition enhancement at different bend degree is due to the bend enhanced inertia impaction. After installed the semi-circular patterned surface, the particle deposition for different bend degree was enhanced for submicron particles. This study can contribute to controlling the contaminant aerosol in ventilation ducting systems.

(Supervisor: Prof. Christopher Yu-Hang Chao and Prof. Huihe Qiu)