CFD study of the sanitization of surfaces by means of hydrogen peroxide vapour

Chapter 1 – Introduction and Objectives Indoor confined environments such as rooms, offices, classrooms, meeting areas and commercial spaces represent complex microclimates where people spend a large portion of their time. In these spaces, microorganisms can persist on surfaces and, under certain conditions, remain suspended in the air. For this reason, effective room-scale sanitization strategies have become increasingly important not only in healthcare facilities but also in common indoor environments. Traditional surface disinfection methods are mainly manual and rely on liquid chemical agents applied locally. While effective at small scale, these approaches may lead to incomplete coverage, variability in contact time, and difficulty in reaching hidden or obstructed surfaces. Automated room decontamination systems based on vapor-phase disinfectants aim to overcome these limitations by distributing the active agent throughout the entire volume of the room. Among available technologies, vaporized hydrogen peroxide (VHP) is widely used because of its strong oxidizing properties and its ability to decompose into water and oxygen after treatment. This characteristic makes it suitable for confined spaces, where residue accumulation must be avoided. Hydrogen peroxide acts by generating reactive oxygen species capable of damaging cellular membranes, proteins and nucleic acids, leading to microbial inactivation. However, the real effectiveness of VHP at room scale is not determined only by its chemical properties. The disinfectant is transported by air, and therefore its distribution depends on airflow organization, ventilation rate, room geometry and the presence of obstacles. Poorly organized airflow may generate stagnation zones, recirculation areas or regions with insufficient contact time. As a consequence, a sanitization process must be understood as a coupled chemical and fluid-dynamic phenomenon. Computational Fluid Dynamics (CFD) provides a powerful tool to analyze airflow and disinfectant transport in confined environments. Through numerical modeling, it is possible to predict velocity fields, concentration distributions, and surface exposure patterns. This allows the optimization of ventilation configurations and disinfectant injection strategies before practical implementation. The objective of this doctoral work is to develop and analyze a CFD framework capable of describing hydrogen peroxide vapor transport in confined indoor environments. The study focuses on generic enclosed spaces such as rooms and offices, evaluating how ventilation design, air change rate and surface characteristics influence disinfectant distribution and overall sanitization effectiveness. Chapter 2 – Materials and Methods The numerical investigation was performed using a finite element CFD approach (COMSOL Multiphysics) applied to a simplified but representative confined room geometry. The computational domain includes walls, floor, ceiling and selected obstacles that mimic typical indoor furnishings such as desks or cabinets. Different ventilation configurations were considered by varying the number and position of inlet vents and by modifying the air change rate (ACH). Air was modeled as an incompressible Newtonian fluid. The airflow is governed by the conservation equations of mass and momentum. In compact form, the continuity equation for incompressible flow is written as: 𝛻 · 𝑢 = 0 The momentum conservation equation can be expressed in simplified form as: 𝜌 (𝑢 · 𝛻)𝑢 = −𝛻𝑝 + 𝜇 (𝛻^2)𝑢 The classification of the flow regime was based on the Reynolds number, defined rigorously as: 𝑅𝑒 =(𝜌𝑉𝐷)/𝜇 where 𝑉 is a characteristic inlet velocity and 𝐷 is a characteristic length (hydraulic diameter of the vent). For typical indoor ventilation conditions, Reynolds numbers indicate transitional or turbulent regimes, which justifies the use of turbulence modeling. The transport of hydrogen peroxide vapor was described by a convection–diffusion equation: 𝜕𝐶/𝜕𝑡 + 𝑢 · 𝛻𝐶 = 𝐷 (𝛻^2)𝐶 where 𝐶 represents disinfectant concentration and 𝐷 is the molecular diffusion coefficient. In confined environments with mechanical ventilation, transport is predominantly driven by convection, meaning that the disinfectant follows airflow streamlines. Boundary conditions included velocity inlets, pressure outlets and no-slip wall conditions. Both steady- state airflow simulations and time-dependent species transport simulations were carried out to evaluate the evolution of disinfectant concentration over time. A mesh independence study was performed to ensure numerical reliability. The quality of the computational grid was verified by monitoring mass conservation and comparing inlet and outlet flow rates. Once further refinement produced negligible changes in the solution, the mesh was considered adequate. Surface exposure was evaluated by analyzing near-wall concentration levels and estimating cumulative exposure over time. Although detailed surface reaction kinetics were not explicitly modeled, differences between non-porous and porous materials were qualitatively considered when interpreting results. Chapter 3 – Results The simulations show that airflow organization strongly influences disinfectant distribution. In confined rooms with limited inlet vents, airflow tends to form concentrated jets. These jets can create preferential paths from inlet to outlet, sometimes reducing mixing efficiency in peripheral zones. When the number of inlets is increased, airflow becomes more distributed and mixing improves. However, if inlet velocity becomes too low, penetration into remote corners may be reduced. This demonstrates that the number of vents and their positioning must be optimized rather than simply maximized. Air change rate significantly affects mixing dynamics. Higher ACH values increase overall air velocity and reduce the time required to achieve relatively uniform concentration. At the same time, excessive velocity may reduce local residence time near surfaces. Effective sanitization therefore requires a balance between mixing intensity and sufficient exposure duration. The concentration field of hydrogen peroxide vapor evolves over time. During the injection phase, the vapor propagates along airflow paths and gradually spreads to the entire volume. Recirculation regions delay the arrival of disinfectant in certain areas, particularly behind obstacles. In office-like environments with furniture and partitions, these localized effects are more pronounced than in empty rooms. Analysis of surface concentration indicates that uniform room-average concentration does not guarantee uniform surface exposure. Corners, floor–wall junctions and shielded regions tend to experience delayed or reduced disinfectant levels. These findings confirm the importance of evaluating local flow structures rather than relying solely on global ventilation parameters. Material characteristics also influence effective sanitization. Smooth, non-porous materials allow consistent exposure to vapor, while porous or absorbent materials may partially reduce local surface concentration. Although the present study does not model absorption explicitly, the results suggest that room composition must be considered when defining treatment duration. Chapter 4 – Conclusions This doctoral study demonstrates that hydrogen peroxide vapor sanitization in confined indoor environments is governed by coupled airflow and transport phenomena. The process cannot be evaluated solely on the basis of disinfectant chemistry; it must be analyzed through fluid dynamics. The Reynolds number, expressed as Re = (ρ V D) / μ, provides a fundamental parameter for classifying the ventilation regime and selecting appropriate modeling assumptions. In typical indoor scenarios, airflow conditions justify turbulence modeling and confirm the dominance of convective transport. CFD analysis reveals that ventilation configuration, air change rate and room geometry strongly influence disinfectant distribution. Increasing ACH enhances mixing but must be balanced against adequate exposure time. Multiple inlet configurations generally promote more homogeneous coverage, while poor vent placement may generate persistent dead zones. Furniture and obstacles introduce local recirculation regions that delay disinfectant arrival. Therefore, confined environments such as offices and commercial spaces require specific airflow evaluation rather than generic sanitization protocols. From a materials perspective, non-porous surfaces are more predictably exposed to vapor, whereas porous materials may require longer treatment durations. Future work may include more detailed modeling of surface interaction and deposition phenomena. In conclusion, CFD-based modeling provides a reliable engineering framework for optimizing hydrogen peroxide vapor sanitization in confined indoor spaces. By integrating airflow analysis, species transport modeling and surface considerations, it is possible to design safer, more efficient and more controllable indoor decontamination strategies suitable for a wide range of environments.