This study aims to understand a nuanced intra-urban PM2.5 distribution pattern and provide insights into the environmental consequences of urban development. Inspired by the impact that the urban-rural divide has had on nation-wide urbanization progress, the study approaches urban areas as a series of hotspots and non-hotspots, applying Zipf's law-ruled spatial clustering method on nighttime light data. We found that PM2.5 concentrations and exposures in China decreased notably from 2005 to 2020, affirming the efficacy of that country's air pollution control policies. However, the difference in exposure between hotspot and non-hotspot regions indicates that outside-hotspot PM2.5 exposures across most cities consistently exceeded within-hotspot ones, despite the observed declining trend in exposure disparity during the 15-year time span. The diminishment in exposure disparity primarily stems from the reduction in PM2.5 levels rather than targeted equity measures. All of these outcomes point to the importance of integrating air quality management with broader city planning, economic, and health policies.

Previous studies have rarely addressed single-sided ventilation driven by the external flow over the roof, which exhibits considerable potential owing to its highly turbulent nature and strong suction associated with leading-edge flow separation. In this study, wind tunnel experiments on single-sided ventilation through a roof opening were conducted using two isolated generic models: a cylinder and a rectangular prism, each with a set of replaceable openings. Both models were tested either flush- or floor-mounted. Two inflow conditions, each with three free-stream velocities, were considered. For both models mounted beneath the floor, the nondimensional ventilation rates (Q*) are comparable to values reported in the literature; for the prism, a slight increase in Q* with orientation suggests the development of a mixing layer along the streamwise extent of the floor-level opening. In the floor-mounted configuration, body-induced flow disturbances tend to enhance ventilation. Three primary governing rooftop flow regimes are identified—recirculation zone, flow reattachment, and conical vortex—whose relative dominance over the opening depends on inflow turbulence, wind incidence angle, and model configuration. When the opening lies entirely within the recirculation zone, Q* is proportional to the normalized local fluctuation intensity, with a coefficient of about 0.16. For certain yaw angles, the marked increase in Q* strongly correlates with the presence of a conical vortex over the prism model roof, which features strong suction and intense fluctuations. Direct advection through the opening could occur with a favorable opening size and location, allowing deep penetration of the reattaching shear layer.

To quantitatively assess the effectiveness of ventilation in controlling contamination, ventilation efficiency indices such as air exchange efficiency and contaminant removal efficiency are commonly used. Since indoor airflow plays a significant role in the dispersion of contaminants or scalars within a space, it is necessary to assess contaminant dispersion more comprehensively. In this context, tracer gas measurements were conducted using a device capable of determining the volume-averaged scalar concentration within a local domain. By applying this device within the breathing zone, we quantified the exposure risk from the source/infective agent to the receptor/susceptible individuals in the experimental setup. The measurements were performed in a climate chamber with thermal manikins under two types of ventilation systems: displacement and mixing ventilation (DV and MV). The results revealed that contaminant dispersion within indoor spaces varied significantly depending on the airflow patterns formed by the ventilation systems, with DV systems more effectively suppressing contaminant dispersion compared to MV systems. In conclusion, when designing an HVAC (Heating, Ventilating, Air Conditioning) system, it is important to account for this heterogeneity to ensure effective implementation.

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. 

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 %.

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.

An air distribution system has two ends. To the inlet the capacity for the dilution of contaminants generated within the room is delivered. To the outlet the capacity for removal of the contaminants is delivered. The flow in a room is controlled by a driving flow. A driving flow does not need to be the supplied ventilation flow rate. A driving flow entrains ambient air, and the flow rate increases with the distance from the source. When the flow is driven by a jet the initial flow rate is the ventilation flow rate but when it is driven by a plume the initial flow rare is zero. Due to entrainment the flow rate increases with the distance from the source After a certain distance the flow rate in the driving flow is larger than the evacuation capacity. Thus, a surplus flow larger than the evacuation capacity is generated. The surplus flow cannot be evacuated immediately, and it returns to the room. This is flooding of ventilation air. It has several consequences for the air quality and air flow pattern in the room. The purpose of the paper is to present he mechanism for generation of surplus air, its consequences and the concepts related to this. 

The importance of natural ventilation has become more recognized for energy saving. Given the contemporary architectural designs, the utilization of wind-induced single-sided ventilation has become a prevailing format. This paper presents a wind tunnel experiment to assess the influence of a partition wall on the single-sided wind-induced ventilation rate considering the effect of wind pressure fluctuation. A rectangular building model whose shape was 1:1:2 (Height: Width: Length) was studied. The internal space is separated by an internal partition with an opening, and the position of this opening was varied as a parameter. The external wind direction was also changed from 0 to 180.In order to assess the actual ventilation performance including ventilation efficiency, the purging flow rates (PFR) were also measured by the continuous dose method of the tracer gas technique. Here, two types of PFR were measured based on the target domain, i.e., dosing/sampling at only one room or two room. To evaluate the driving pressure, the instantaneous wind pressure coefficient and its fluctuation were measured at the positions of the assumed openings using the sealed model with a sampling frequency of 1.0 kHz.

This study investigates entropy generation minimization in indoor air distribution to enhance exergy efficiency of ventilation systems while ensuring thermal comfort. Using computational fluid dynamics (CFD) simulations and a gradient descent-based optimization framework, two 2D mixing convection models were analyzed. Model 1 validated the optimization algorithm, achieving an average Predicted Mean Vote (PMV) of 0 within the design region, with maximum and minimum PMV values within 0.1 and-0.1, respectively. Model 2 minimized entropy generation by 73% (0.0054 W/K) compared to the baseline, maintaining PMV between-0.2 and 0.2 and Draught Rate (DR) below 15%. The framework, integrating Kriging metamodeling and Nonlinear Programming by Quadratic Lagrangian (NLPQL), optimized design parameters efficiently. Results demonstrate that entropy-based optimization reduces irreversible losses in HVAC systems without compromising comfort, offering a practical approach for sustainable indoor environment design.