A FRAMEWORK FOR INVESTIGATING THE REMOVAL EFFICIENCY OF BIOAEROSOLS IN IN-DUCT PHOTOCATALYTIC REACTORS
Creators: name: Anandan, Sudharshan
Publisher: name: Purdue University Graduate School
Year: 2022
Subjects: Mechanical engineering not elsewhere classified

description: ndoor air quality (IAQ) due to the presence of airborne microorganisms or bioaerosols (0.01-10 μm) in indoor spaces has been a concern for many years; however, it gained significant attention during the COVID-19 pandemic. Photocatalytic oxidation (PCO) has shown promising potential to kill microorganisms (removal/disinfection) and has already been in use within HVAC systems to treat volatile organic compounds (VOCs) (treatment). The main motivation of this work is to understand whether PCO devices can be used for bioaerosol removal in indoor spaces by integrating them with HVAC systems. Among the various factors that influence the adoption of PCO for large-scale bioaerosol removal, this work specifically tries to investigate two factors 1) whether the commercially available PCO reactors for treatment can be used for removal/disinfection or not, and 2) how to setup a standardized experimental setup for evaluating the removal efficiency of these systems. Generally, most of the commercial PCO devices use UV- based photocatalysis, so the removal efficiency is a combination of inactivation by UV and the reactive oxygen species produced by photocatalytic reactions (pure photocatalytic effect). In this work, the bioaerosol transport and the photon transport in a reactor is hypothesized as central to using the photocatalytic effect to inactivate microorganisms. This study uses analytical models to estimate the collection efficiency of the bioaerosols inside the honeycomb channels as a function of non-dimensional aspect ratios and velocity typical of HVAC systems. Subsequently, the collection efficiency results are overlaid with the prior literature results on photon transport inside such channels to present a limiting case for the removal efficiency of these systems. Another crucial factor for the performance of PCO systems is to investigate about the bioaerosol remediation on a photocatalyst substrate. Since there are many challenges associated with the numerical modeling of this phenomenon, this work developed a standardized experimental setup at the Herrick Laboratories, Purdue to investigate these interactions and further validate the previous hypothesis .The setup is constructed to systematically characterize the bioaerosol flowing through the airstream and measure data crucial to the PCO reactor performance, such as fluence rate field, number concentration (#/cm3), and viable concentration (CFU or PFU/m3) of the microorganisms upstream and downstream of the treatment sections. The collection efficiency (CE) of bioaerosols in honeycomb channels with velocities typical to HVAC systems were estimated using analytical models, and the results were presented in dimensionless aspect ratios (AR= Lch/ Dch). Based on the CE modeling results, the highest CE for aspect ratio 25 was less than 20% for the entire bioaerosol size range. From the prior literature results on photon transport, it was found that the intensity of the light reduced significantly for aspect ratios less than or equal to 6. Based on these results, it was found that the existing honeycomb geometries weren’t effective for PCO disinfection in operating conditions typical of HVAC systems. Since there aren’t any existing well-established methods to experimentally investigate these kinds of systems, this work will present the details about the development of the proposed methods inspired from prior literature for general air cleaning devices and small-scale PCO experiments. Furthermore, a detailed discussion about the important subsystems such as aerosol generation subsystem, sampling subsystem, and reactor subsystem which is crucial to investigating the hypotheses is presented in this thesis. Finally, some preliminary results on each of these characterization experiments to test the hypotheses has been presented in this thesis.