Displacement ventilation and mixing ventilation are methods of air distribution that create different airflow patterns in rooms. Moreover, these two methods of air distribution involve diverse processes, such as turbulent flow, heat transfer and mass transfer. It is challenging to describe and compare the impact of these processes on the natural environment in a simple manner. Entropy generation, which represents energy degradation, is a direct and effective parameter for assessing irreversibility in multi-process systems. In this study, heat transfer irreversibility is found to be the primary source of entropy generation in displacement ventilation and mixing ventilation systems. In the tested scenarios, the entropy generation in a displacement ventilation system is approximately 30 % less than that in a mixing ventilation system. During space cooling, this difference in entropy generation is evidenced by the tendency of air to flow towards a uniform mixing state in mixing ventilation. When indoor air can be assumed to be completely mixed, a simplified model can effectively predict entropy generation. Considering entropy and exergy balance, the exergy efficiency for a displacement ventilation system is 25.2 %, whereas that for a mixing ventilation system is only 9.2 %.

Understanding the complex dynamics of indoor airflows is crucial for mitigating airborne infection risks in ventilated spaces. These airflows can be simplified into two populations: Recirculating air that spreads contaminants and outgoing air that evacuates them. Quantifying these populations involves analyzing mass transfer between zones in the room/building. This study builds on the newly proposed model that enhances the Wells-Riley model by incorporating indoor airflow interaction mechanisms. The study explores the transfer probability between zones and the recirculation and purging flowrate at the target location and its impact on the risk of infection in a ventilated room. Our contributions include: (i) Performance evaluation of the revised model that accounts for transfer probabilities between zones and purging flowrates; (ii) a novel tracer-gas measurement method to determine local purging flowrates; and (iii) an analysis of how different ventilation systems interact with internal room flow. We validated the proposed model through experimental measurements in a climate chamber, examining contaminant source locations under varying ventilation rates using mixing ventilation (MV) and displacement ventilation (DV). Results reveal significant spatial and temporal heterogeneities in contaminant distribution, with MV showing pronounced temporal variability and DV exhibiting significant spatial variations. Under MV, purging flowrates increase with higher ventilation rates, whereas DV shows no such change. Our findings underscore the importance of considering airflow dynamics in ventilation design to effectively reduce contaminant transfer and/or airborne infection transmission.

In this paper, the investigation focused on natural ventilation induced by wind and buoyancy effects through a wind tunnel experiment. A scaled-down model of a single-zone building featuring two opposing openings at varying heights was subjected to uniform heating from a source on the interior floor. Wind pressure coefficient differences were measured in a sealed model to assess the wind’s driving force. Indoor and outdoor temperatures were recorded to ascertain the buoyancy-driven driving force. The ventilation rate was assessed using the tracer gas constant emission method, while airflow characteristics were quantified through particle image velocimetry (PIV). The comparison between predicted and measured ventilation rates revealed generally good accuracy in cases where wind assists buoyancy. However, errors were evident in cases of opposing wind and buoyancy due to neglecting the effect of wind turbulence. The airflow in cases of opposing wind and buoyancy effects demonstrates the most dynamic changes, as illustrated through flow visualizations and transient temperature fluctuations at the openings.

In a ventilated room, the indoor airflows are complicated but can generally be defined by an internal recirculating airflow generated by flooding of ventilation air. This concept categorizes the internal room flow (air and contaminants) as consist of two populations: One leaving the room and the other recirculating. The one recirculating is spreading the contaminants while the one leaving is evacuating the contaminants, which are quantified by the transfer probability between the source and other locations in the room and by purging flow rate, respectively. This concept accounts for spatial and temporal aspects in risk of airborne infection transmission. The current paper proposes and discusses a revised risk infection model based on this concept and has demonstrated applicability of the model with a test measurement setup with both mixing and displacement ventilation systems. The results emphasize the importance of considering both spatial and temporal factors in assessing airborne infection risks. It underscores the need for dynamic models like the proposed revised Wells-Riley model to provide a more accurate representation of infection risks in various indoor environments. Additionally, it discusses the necessity for longer measurement periods to fully understand the evolving nature of these risks. 

As a preliminary investigation of the wind-driven purging process for densely built environments through the canopy layer, the ventilation efficiency of standalone semi-enclosed models incorporating a horizontal opening in the roof façade was investigated in the wind tunnel. For comparison, two models with different geometries were constructed, and each model was tested individually. Both models were equipped with replaceable roof covers, enabling the adjustment to the opening size. The ventilation efficiency was evaluated by continuous releasing and sampling of the tracer gas, from which the normalized purging velocity (PFVn) was derived. Additionally, the flow condition over the opening was monitored using the Laser Doppler Anemometer. It was found that separation flows from the frontal edge(s) of the model could introduce secondary circulations across large openings, resulting in dramatic increases in PFVn. Both the rectangular prism model and cylinder model possessed higher PFVn compared to prior studies on single-sided ventilation, while close values were observed with cylinder model mounted under the wind tunnel floor. Besides, the vertical distribution of integral length scales of streamwise velocity indicated the stratification feature of separation flows under low-turbulent incoming flow conditions. Measurement results provide validation data for further simulation studies including more detailed structures.

The time constant of ventilation of rooms in buildings is between 15 minutes (in office spaces) to 2 hours (in residential buildings). Currently, most of the tracer gas system analyzers on the market have a minute-based time constant and depending on the channels a cycle of sampling and analysis may take up to 6 minutes, E.g., 6 channel system. Essentially, only mean values are recorded with most present tracer gas analyzers. This is a hindrance for detailed temporal analysis of conditions in the room and consequently is does not capture the resolution of the influence of the internal flow on air and contaminant distribution. The current paper presents work on the development and testing of a fast response CO2 tracer-gas system with a time constant of 1 second. In contrast to the present analyzers, not only the mean values but also the whole statistical distribution of variables can be recorded, and pulse responses can be analyzed. This makes the system viable for measurement and analysis of not only spatial but also temporal distribution of contaminants. For example, recirculating airflow in the room generated by flooding of ventilation air is possible to be measured and thus making it easy to extend the analyses of the process of ventilation far beyond the possibilities with current systems.

The primary goal of ventilation is to provide dilution capacity for lowering the concentration of contaminant by diluting contaminants generated within a room. First is discussed the properties of the dilution curve which is the concentration as a function of the ventilation flow rate and the varying efficiency of dilution at different levels of concentration. The concept purging flow rate which is the flow rate that ventilates is introduced. In situations where the supply air is laden with the contaminant that is going to be removed it is important to distinguish between the purging flow rate and the flow rate supplied by the ventilation system. The trade-off between control of contaminant levels by ventilation and source control is addressed. Factors that may diminish the dilution capacity as short circuiting and direct loss of air are dealt with. A sensation of draught may be counteracted by the occupants’ manipulation of the ventilation system which may lead to reduced dilution capacity. Situations where there is a risk of draught occurring are identified and design parameters are introduced whose use in design can help in avoiding systems that cause draught. Communication about how ventilation works is important for avoiding mistakes and wrong expectations about what can be achieved with ventilation. 

This study adapted the mean age of air, a time scale widely utilized in evaluating indoor ventilation, to assess the impact of building layouts on urban ventilation capacity. To distinguish it from its applications in enclosed indoor environments, the adapted index was termed the effective mean age of air (T_E). Based on an experimentally validated method, computational fluid dynamic (CFD) simulations were performed for parametric studies on four generic parameters that describe urban morphologies, including building height, building density, and variations in the heights or frontal areas of adjacent buildings. At the breathing level (z = 1.7 m), the results indicated three distinct distribution patterns of insufficiently ventilated areas: within recirculation zones behind buildings, in the downstream sections of the main road, or within recirculation zones near lateral facades. The spatial heterogeneity of ventilation capacity was emphasized through the statistical distributions of T_E. In most cases, convective transport dominates the purging process for the whole canopy zone, while turbulent transport prevails for the pedestrian zone. Additionally, comparisons with a reference case simulating an open area highlighted the dual effects of buildings on urban ventilation, notably through the enhanced dilution promoted by the helical flows between buildings. This study also serves as a preliminary CFD practice utilizing T_E with the homogenous emission method, and demonstrates its capability for assessing urban ventilation potential in urban planning.

Reduced-scale experiments and simulations are important approaches in natural ventilation research, and the similarity requirement is fundamental for generalising the flow characteristics obtained from reduced-to full-scale conditions. However, the similarity requirement of a nonisothermal natural ventilation flow in a reduced-scale model poses additional challenges because of the reduced approaching flow, which can potentially result in Reynolds dependence issues. This study investigated the Reynolds number (Re) independence of indoor airflow in natural ventilation under isothermal and nonisothermal conditions using computational fluid dynamics (CFD) with Reynolds-averaged Navier–Stokes. A wind tunnel experiment was first conducted to validate the accuracy of the CFD using a reduced-scale model. Indoor airflow fields characterised by the same Archimedes number (Ar) but with varying approaching wind velocities and temperatures were compared between the full-scale and 1/10 reduced-scale simulations. The dimensionless ventilation rate showed the least dependence on the Re number, while the temperature field was very sensitive to the Re number, especially in the near-wall region. However, the temperature field on the ventilation pathway is much less dependent on the Re number, the deviation of which is less than 10 % compared to the full-scale simulation. The temperature distribution in the reduced-scale simulation exhibits a thermal stratification pattern similar to that in the full-scale simulation.

Wind-induced single-sided ventilation is a prevalent form of natural ventilation extensively used in buildings. Nevertheless, prior experimental investigations predominantly focused on single-zone buildings, neglecting the multizone buildings with internal partitions which is representative of more common scenarios. This study addresses this gap by investigating the impact of internal partitions on single-sided ventilation, employing a combination of wind tunnel experiment and numerical analysis. Airflow rate (AFR) was measured with a split-fibre probe and purging flow rate (PFR) was assessed by the tracer gas methodology. The PFR exhibits greater sensitivity to internal partitions in unidirectional airflow compared to bidirectional flow. Large Eddy Simulation (LES) was conducted to elucidate the intricate airflow characteristics in single-sided ventilation. The ventilation efficiency (ratio of PFR and AFR) derived from LES ranges between 0.74 and 0.79, which means that <80% of the AFR actively contributes to the removal of contaminants. Notably, the investigation discerned that the AFR of a single room approximates that of the entire room, whereas the PFR of a single room is smaller than that of the whole room. The disparities in AFR and PFR were caused by the recirculating flow, which was elaborated by the theoretical analysis.