A new framework for understanding indoor chemical processes and dynamics using computational fluid dynamics (CFD) simulations
Open Access
- Author:
- Won, Youngbo
- Graduate Program:
- Architectural Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 26, 2021
- Committee Members:
- Donghyun Rim, Dissertation Advisor/Co-Advisor
Donghyun Rim, Committee Chair/Co-Chair
William P Bahnfleth, Committee Member
James Freihaut, Committee Member
Yuan Xuan, Outside Member
Somayeh Asadi, Program Head/Chair - Keywords:
- Indoor chemical reaction
Ozone
Chlorine
Hydroxyl radical - Abstract:
- Human exposure to gas-phase pollutants can lead to adverse health effects such as respiratory, cardiovascular, and neurological diseases. When some gases undergo chemical reactions in buildings, a number of reaction products are created and can influence the health and productivity of occupants. These air pollutants are heterogeneously distributed by air flow patterns, surface deposition, and chemical reactions so that the health effects of these species are different from one space to another in indoor environments. However, very little information is available in the literature on how the indoor environmental conditions affect concentrations and spatial distributions of reactants and reaction products. Based on this background, the objective of this study is to examine chemical processes and pollutant dynamics of gas-phase compounds in indoor environments under representative conditions of chemical reactions, ventilation, lighting, and indoor surfaces. This Ph.D. dissertation used three computational fluid dynamics (CFD) model frameworks designed to investigate spatial distributions of gas-phase species. The focus involved three critical reactive species (ozone, hydroxyl radical (OH), and chlorine atom (Cl)) in indoor environments as they can dominate indoor oxidation processes and reactive chemistry. Three different reaction scenarios were simulated: 1) ozone interaction with human surfaces; 2) indoor photolysis of nitrous acid (HONO); and 3) indoor surface cleaning by a chlorine bleach solution. The first modeling study showed that ozone was depleted on the human surface due to ozone reactions with skin oil and soiled clothing. Because of the ozone surface reaction, primary products were relatively concentrated near occupants, while secondary products were relatively well distributed throughout the room. Clean clothing with lower amounts of skin oil produced about 40% lower primary reaction products than the soiled clothing condition. Increasing air mixing near the human surface also enhanced ozone uptake to the human surface. With regard to indoor photolysis of HONO, production and spatial distribution of reaction products in indoor environments vary highly with light conditions. Photolysis of HONO generated OH that led to recycling reactions between OH and the hydroperoxy radical (HO2). Due to their high reactivities, such radicals (OH and HO2) are mainly concentrated where they are generated, while the oxidation products were produced in the lighting zone and dispersed to the ambient air. The increased volume of a daylight zone produced more oxidation products. Artificial lights also photolyzed HONO, but the impact was marginal compared to direct sunlight, due to the intensity decrease with increasing distance from the light source. The third CFD model simulated bleach cleaning experiments conducted in a test house where bleach solutions were applied to the living room floor. From the cleaning surface, hypochlorous acid (HOCl) and nitryl chloride (ClNO2) were emitted. Uptake of HOCl to aerosol surfaces produced chlorine (Cl2). These three gas-phase species (i.e., HOCl, ClNO2, and Cl2) were removed at approximately 80% due to surface deposition and 20% by ventilation. Photolysis of HOCl, ClNO2, and Cl2 were the key processes that generated radicals (OH and Cl). The radicals were confined in the sunlit zone and produced some toxic gas-phase species such as hydrogen chloride (HCl). Once oxidation products were generated, they were dispersed and recirculated to the ambient air by an indoor ventilation system. However, even with a high air mixing rate (8 h-1) and all indoor doors opened, the concentrations of the cleaning products in the bleach cleaning zone were 2-3 times higher than those in other rooms. In addition, regardless of ventilation conditions, the reactive species (OH and Cl) were concentrated near their sources, mainly due to the reaction time scale that was notably shorter than the transport time scale.