Mixing Ventilation Research Articles

Effect of personalized air curtain combined with mixing ventilation on dispersion of aerosols released at different velocities from respiratory activities during close contact

Release velocities of aerosols vary among people and respiratory activities. To better promote the application of personalized air curtain (PAC) in practice, this work investigated the performance of PAC combined with mixing ventilation in terms of dispersion of aerosols released at different velocities using a simulation approach of computational fluid dynamics (CFD). The healthy person (HP) and infected person (IP) were in face-to-face and face-to-back scenarios during close contact. Results showed that the aerosol dispersion was highly affected by aerosol release velocities. When aerosols were released from 2.0 m/s to 8.0 m/s, the deposition rate was increased by more than 3 times and 10 times in the face-to-face and face-to-back scenarios, respectively. The intake fraction was increased by over 3 times in the face-to-face scenario. PAC could significantly reduce the intake fraction and deposition rate. When aerosols were released at 8.0 m/s, PAC could reduce the intake fraction and deposition rate by 76% and 66% in the face-to-face scenario, respectively. A higher PAC velocity did not necessarily lead to a better performance. The appropriate PAC velocity of 3.0–5.0 m/s was suggested in this work. The significantly reduced intake fraction and deposition rate suggest that PAC as an advanced ventilation system can be a promising control measure to prevent the spread of respiratory diseases in indoor environments. Additionally, different performances of PAC indicate the importance of investigating the appropriate PAC velocities in relation to aerosol release velocities. Otherwise, more energy is consumed by PAC, but the best performance is not achieved.

Spatial and temporal variation of cooking-emitted particles in distinct zones using scanning mobility particle sizer and a network of low-cost sensors

Exposure to ambient and household fine-particulate matter is identified as a substantial contributor to premature mortality in India, according to the Global Burden of Disease Studies. This study examines the impacts of typical Indian cooking practices on indoor air quality characteristics by monitoring the evolution of fine and ultrafine particle (UFP) concentration in the dining facility of a residential educational institute in India. The monitoring area was spread across the kitchen (zone 1) and the dining hall (zone 2). A combination of validated low-cost PM sensors (LCS), DustTrak8433, and Scanning Mobility Particle Sizer (SMPS) was utilized for real-time data acquisition while using Liquefied Petroleum Gas (LPG) as the cooking fuel. PM2.5 and UFP concentrations were monitored at 1.3 m and 1.8 m from the floor to assess the vertical variation of pollutants during cooking activities, including breakfast, lunch, and dinner, and processes such as preheating, reheating, stir-frying, and deep-frying. It was found that the prolonged cooking durations involved in high-heat cooking methods like stir-frying and deep-frying resulted in a rise in coarser UFP (300–550 nm) and PM2.5, causing a higher exposure to PM and UFP concentration. PM2.5 levels are higher at upper heights during typical cooking processes because of temperature-driven convection currents and hygroscopic growth of particles due to high humidity levels. Air exchange rates (AER) considerably varied by using chimneys and were low during the controlled (closed doors) compared to mixed ventilation (opened doors) conditions. The maximum AER was obtained during lunch (4.3–9.9 h−1) compared to breakfast (-7.8–6.8 h−1) and dinner (0.55–7.9 h−1). The decrement rate of PM2.5 inside zone 1 was highest during lunch (126 µgm−3 h−1), coinciding with the highest AER during mixed ventilation. It is recommended that improving ventilation and better design of the kitchen can reduce the exposure of PM and UFP in commercial and rural area kitchens.

Resilience of Personalized Ventilation in Maintaining Acceptable Breathable Air Quality When Combined with Mixing Ventilation Subject to External Shocks

This work evaluates the ventilation resilience of the combined personalized ventilation (PV)-mixing ventilation (MV) system when implemented in a typical office space. This resilience is first evaluated by monitoring the ability of the PV devices when designed at different supply flow rates to maintain acceptable levels of CO2 at the occupant’s breathing zone when the MV system is subjected to a shock. The shock considers a malfunction of the MV system for periods of 3 h and 6 h, and at shutoff percentages of MV fan flow of 100% and 50%. This is followed by evaluating the resilience of the MV system when the PV air handling unit is shutoff for short periods. The following three aspects of resilience were calculated: the absorptivity, the recovery, and the resilience effectiveness. To monitor the CO2 temporal variation at the breathing zone, a computational fluid dynamic model was developed and validated experimentally. It was found that the resilience effectiveness varied between 0.61 (100% MV shutoff for 6 h and PV at 4 L/s) and 1 (50% MV shutoff for 3 h and PV at 13 L/s). Additionally, CO2 build-up and recovery took minutes during MV malfunctions and seconds during PV malfunctions.

Compatibility of integrated physical barriers and personal exhaust ventilation with air distribution systems to mitigate airborne infection risk

This study investigated the effectiveness of integrating desk partitions and personal exhaust ventilation to mitigate airborne infection risks in areas where people sit closely together. Experiments were conducted in a test chamber equipped with different air distribution systems, including two different mixing and one displacement ventilation. N2O tracer gas was utilized to study airborne transmission between occupants. Results showed effectiveness of preventive measures varied depending on air distributions. In the absence of preventive measures, mixing ventilation systems exhibited higher infection risk compared to displacement ventilation. After the introduction of physical barriers, N2O concentration increased at two measurement points in presence of one of the mixing ventilation systems. In contrast, the other mixing ventilation and displacement ventilation showed a reduction in N2O concentration, up to 63 % and 43 %, respectively, depending on measurement points. Combination of physical barriers and personal exhaust ventilation consistently reduced N2O concentration by 34 % to 83 %, depending on measurement point's location and type of air distribution. Furthermore, higher airflow rates of personal exhaust increased efficiency of proposed strategy in limiting infection risk. This study supports employing physical barriers and personal exhaust ventilation to reduce airborne infection risks. Tailoring preventive measures to specific air distribution system is crucial.

The airflow characteristics and thermal comfort evaluation in high-speed train cabin with mixing ventilation: An experimental and numerical study

The airflow characteristics and distribution in high-speed train (HST) cabins have been of great significance for the travel comfort of passengers, while the indoor dynamic airflow characteristics and overall thermal comfort are still unclear. This study first conducted a transient experiment in a real train cabin with the mixing ventilation mode. Measurements showed that the turbulence intensity of the whole airflow is between 20% and 60%. The center zone airflow has a higher turbulence kinetic energy, while the spectral characteristics are closer to the natural wind with the β value between 1.2 and 1.8. Moreover, considering the limitations of the experiment, a corresponding numerical study was carried out. To be closer to the actual situation, the numerical simulation adopted a partitioned heat transfer wall setting, the average deviation of the airflow velocity and temperature from the experimental comparison was 0.057 m/s and 0.25 °C, respectively. Numerical results revealed two vortices in the seating area, a significant longitudinal airflow and an increasing temperature near the carriage ends. Then, to further analyze the compartment's thermal environment and the acceptance of airflow, thermal comfort indicators based on the airflow parameters were used. The predicted mean vote (PMV) showed the thermal comfort of the cabin is slightly cold. Predicted percentage dissatisfaction (PPD) and draught rate (DR) in the center area are barely satisfactory, more than 25% and 20%, respectively. The results of this study can provide an essential reference for the optimization design of ventilation systems in public transport.

Comparison of Local Mean Age of Air between Displacement Ventilation System and Mixing Ventilation System in Office Heating Conditions during Winter

A novel displacement ventilation system (DVS) was designed using a four-way cassette fan coil unit (FCU) and air purifiers (APs) for supplying clean air. The proposed DVS in this study involved drawing indoor air through the FCU and diffusers installed in the ceiling, controlling air temperature using the FCU, and then discharging it back into the office through the APs placed on the floor. The comparative ventilation system considered was the typical mixing ventilation system (MVS) that intakes and exhausts indoor air using diffusers installed on the ceiling. The local mean age of air was used as an index to compare indoor air quality between DVS and MVS under winter heating conditions. It was found that the DVS was more effective in improving indoor air quality in winter than the MVS. Moreover, compared to the MVS, utilizing the DVS designed in this study resulted in the advantage of a much more uniform air temperature variation in the office space. Therefore, it is anticipated that modifying the structure of an indoor space with an FCU installed in the ceiling and APs on the floor to use the DVS designed in this study would greatly assist in enhancing indoor air quality.

A CFD study on the effect of portable air cleaner placement on airborne infection control in a classroom.

The utilization of portable air cleaners (PACs) is a recommended supplemental approach to help remove airborne pathogens and mitigate disease transmission in learning environments. To improve PAC effectiveness, science-based information is needed to optimize their implementation strategies such as the deployment location, height, and number of PACs. In this study, we developed a Computational Fluid Dynamics (CFD) model to assess how PACs perform in occupied classrooms equipped with displacement and mixing ventilation systems. The results show that PACs with a flow rate of 2.6 h-1 reduce the mean aerosol intake of all students by up to 66%. A key benefit of using PACs is to facilitate air mixing and movement in indoor environments with inadequate ventilation, thereby effectively reducing high aerosol concentrations near the infector. Furthermore, our results highlight the impact of PAC location on its performance. PACs achieve the best effectiveness when placed closed to the infector (within a distance <3 m). In the absence of knowing who is infected, deploying a PAC at the center of the room is recommended. Moreover, adjusting PAC flow discharge height to the breathing height of occupants (e.g., 0.9-1.2 m for seated people) can enhance their effectiveness in spaces with poor air mixing.

Analysis of viral aerosol distribution characteristics in typical body positions of patients under local exhaust air

In this paper, CFD numerical simulation and experiment were used to simulate the diffusion process of human exhaled aerosol particles, and the influence of human lying down, sitting and standing posture on the diffusion of aerosol particles under local exhaust air conditions was studied. The results show that different body positions have significant effects on local exhaust air effect. Only 13.2% of the aerosol particles could be removed by local exhaust air in the lying state. The exclusion rate in sitting posture was 25.4%; The exclusion rate for standing posture was 20.6%. Local exhaust air alone is not enough to prevent infection. It is recommended to adopt the combination of mixed ventilation and local exhaust air, which may have a better effect on preventing cross infection, indoor particulate matter transmission and controlling pollution.

Comparison of Ventilation Performance between Displacement Ventilation and Mixing Ventilation Installed in Factories with High Floor Heights

Comparison of Ventilation Performance between Displacement Ventilation and Mixing Ventilation Installed in Factories with High Floor Heights

Static and dynamic airflow and contaminant transport in a full-scale elevator-lobby mockup

Elevators facilitate people's movement in daily life. The confined space with high population density in elevators, however, assists the transmission of infectious diseases. To study disease transmission under various transit and boarding-period scenarios, this study used a full-scale elevator mockup with a connected lobby area. This investigation measured both static and dynamic distributions of air velocity/temperature and contaminant concentration. The static experiment involved the closed elevator cabin with mixed ventilation, while the dynamic measurements were conducted in the elevator with the door open to the adjacent lobby area when a passenger exited the elevator. The dynamic experiment showed that high-frequency instruments and better background airflow design could increase the data quality. The experimental data was then used to validate a computational fluid dynamics (CFD) model. This study compared the average and individual experimental data to minimize the uncertainties caused by the initial conditions and instrument limitations. The simulation results revealed that an in-transit elevator cabin is characterized by relatively uniform distributions of air temperature and contaminant concentration. The airflow in the dynamic case was greatly affected by the wake generated by passenger movement. This wake effect was short in duration, but it drew contaminants farther along the direction of movement.

An investigation concerning optimal design of confluent jets ventilation with variable air volume

This parametric study aims to predict the performance of confluent jets ventilation (CJV) with variable air volume (VAV) from four CJV design parameters. A combination of computational fluid dynamics (CFD), and response surface method (RSM) has been used to predict the energy efficiency, thermal comfort and IAQ for the four expected vital design variables, i.e., heat load (XH), number of nozzles (XN), airflow rate (XQ) and supply temperature (XTS). The RSM was used to generate a quadratic equation for the response variables exhaust temperature (TE), supply temperature (TP), PMV, DR, εT and ACE. The RSM shows that the TE, TP and PMV were independent of the number of nozzles. The proposed equations were used to generate setpoints optimized for thermal comfort (PMV) for summer, spring and winter cases with different CLO factors and different TS under a scenario where the heat load varied between 10-30 W/m2. TE was used as setpoint to regulate the airflow rate to keep the PMV values close to zero. The results show that by adapting the TS to the CLO factor both thermal comfort and the energy efficiency can be improved. Further energy reduction can be gained by downregulating the airflow rate to keep the TP at a fixed setpoint when the heat load is decreased. This means that a CJV can effectively be combined with VAV to improve environmental performance with good thermal comfort (-0.5 < PMV < 0.5, DR < 20%), above average IAQ (ACE ≈ 106%) and with a higher heat removal efficiency (ε T ≈110%) than conventional mixing ventilation.

19th-century thermosiphon ventilation and its potential for heat recovery in buildings today

A forgotten thermosiphon scheme is found in Montreal's former Royal Victoria Hospital and traced back to the original Center Block of Canada's Parliament Hill. This discovery inspires an investigation into the fluid mechanics of heat recovery with buoyancy ventilation, where interior spaces are arranged in an open thermal loop with heat exchange through partition walls. Flow visualizations with physical models are used to corroborate the archival evidence and show how the historical scheme worked. The scheme is then generalized, defining a criterion for steady unidirectional flow and a heat recovery limit when room temperatures upstream and downstream reach equilibrium This mathematical model is validated experimentally, demonstrating steady flow close to the efficiency limit with a balanced thermal design . Further analysis shows significant heating savings are possible in mildly cold seasons compared to natural displacement (74%) and natural mixing (60%) ventilation.

Airborne Pollutant Removal Effectiveness and Hidden Pollutant Source Identification of Bionic Ventilation Systems: Direct and Inverse CFD Demonstrations

A healthy and efficient ventilation system is essential for establishing a comfortable indoor environment and significantly reducing a building’s energy demand simultaneously. This paper proposed a novel ventilation system and applied it to the IEA Annex 20 mixing ventilation enclosure to verify its feasibility through mathematical modeling and CFD simulations. First, two bionic ventilation systems, single-side and dual-side ventilations, were compared to a conventional constant-volume supply system using CFD simulations, with the results demonstrating that the bionic ventilation system could provide higher ventilation efficiency and more effective pollutant removal from stagnant regions. Furthermore, the present work exercised these two bionic ventilation systems with different temporal periods of sine and rectangular wave functions, identifying a turning point at a period of 0.06 τ n , which could contribute to further enhancement of these bionic ventilation systems. Finally, a methodology depending on the Bayesian inference algorithm was developed for identifying pollution sources in the bionic ventilation system with unstable flow fields, and factors influencing source identification accuracy were discussed. The results show that the peaks of the KDE distributions and the sampling average values of both the source location and intensity are all consistent with the actual source parameters. The potential of the proposed bionic ventilation systems has been well demonstrated by direct and inverse CFD models, paving the way for further engineering applications.

Enhancing Indoor Environmental Quality and Sustainability in Post-Pandemic Office Settings: A Study on Displacement Ventilation Feasibility

The COVID-19 pandemic has catalyzed global efforts toward transitioning to a sustainable society, driving rapid innovation in building technologies, working practices, building design, and whole life cycle environmental impact consideration. In this pursuit, this study explores the enduring impact of these on an alternative ventilation approach for both existing building renovations and new building implementations. Comparing displacement ventilation to mixed-mode ventilation in a Finnish office building with varying occupancy densities, this study examines indoor air quality (IAQ), thermal comfort, total building energy performance, and embodied carbon. The findings reveal that the basic case of mixed ventilation has a specified system primary energy value of 38.83 kWh/m2 (with 28 occupants) and 39.00 kWh/m2 (with 24 occupants), respectively. With the displacement ventilation alternative, it reduces this by 0.3% and 0.1%, enhancing thermal comfort and decreasing turbulence as well as having a marginal decrease in embodied carbon. In general, the study offers three-fold contributions: insights into post-pandemic office mechanical ventilation design with an emphasis on sustainability and ecological footprint considerations, a concrete case study addressing climate action and human-centric IAQ design, and a multifaceted analysis using the Environmental, Social, and Governance (ESG) paradigm, contributing to the groundwork for associated future research and policy progress.

Aspiration efficiency and respiratory tract deposition of indoor suspended micro-particles during steady and transient breathings

The assessment of potential health risks from exposure to airborne contaminants and the inhalability of microparticles through human nasal breathing is crucial for the design of ventilation systems. In this study, using a standing computer-simulated person (CSP) directly linked to a numerical model of the respiratory tract, we investigated the aspiration efficiency (AE) of microparticles ranging between 1 and 80 μm under steady and transient breathing conditions. Two ventilation scenarios with displacement and mixed ventilation systems (DV and MV) were assumed for AE calculations. To determine the appropriate particle injection location, we investigated the position of highly inhaled particles corresponding to the breathing zone using a reverse particle-tracking simulation with reversed time progression. Additionally, the total and regional deposition fractions (TDF and RDF) of the inhaled microparticles on the respiratory wall surfaces were evaluated using the Lagrangian approach. The results indicate that the transient breathing cycle showed relatively higher AE and TDF rates compared to the assumed steady inhalation conditions, particularly for the size range 1–10 μm. An insignificant variation was observed in the AE and RDF results between different ventilation systems. However, microparticles in the MV room showed slightly higher AE than the DV system. Moreover, AE and penetration ratio were highly sensitive to the initially assumed particle density. For a comprehensive and accurate assessment of inhalation exposure to microparticles, we should predict the heterogeneous flow field formed around the human body and then analyze the AE and TDF in the respiratory tract during realistic transient breathing.

Portable Air Purifiers’ Predicted Efficacy in Mitigating Airborne Pathogen Transmission in an Office Room Featuring Mixing Ventilation

Portable air purifiers have been extensively used to improve indoor air quality and mitigate the transmission of airborne diseases. However, the efficacy of mitigation is strongly affected by the interactions between jet flows of processed air from the air purifiers and the background airflows driven by the ventilation system. Critical factors in this context include the position and capacity of air purifiers and the ventilation rate of the heating ventilation and air-conditioning (HVAC) system. These factors are investigated in this study via computational fluid dynamics (CFD) simulations and the infection probability for different scenarios is quantified using the latest airborne infection predictive model incorporating recent pathological and clinical data for SARS-CoV-2. The results show that the use of air purifiers can significantly reduce the concentration of particulate matter, thus contributing to a generally lower risk of airborne transmission. However, the position of air purifiers affects their overall efficacy remarkably. Comparatively, a central HVAC system is more efficient at removing airborne particles under an equivalent ventilation rate assuming it uses a mixing ventilation scheme.

Optimizing indoor air quality: CFD simulation and novel air cleaning methods for effective aerosol particle inhibition in public spaces.

In contemporary building ventilation, displacement and mixing ventilation demand high air volumes for rapid virus elimination, resulting in elevated energy consumption. To minimize the spread of viruses and decrease energy consumption for ventilation, this study employed CFD to explore the efficacy of a downward uniform flow field in impeding the transmission of aerosol particles in a high-traffic public facility, like a supermarket. The findings indicate that the downward uniform flow field proves insufficient when individuals remain static for extended periods. A wind speed of 0.1 m/s or higher becomes essential to overpower the stationary thermal plume, which disrupts this flow field. In areas with human presence, however, this technique is found to be particularly efficient since mobile heat sources do not generate a fixed thermal plume. A 0.05 m/s downward uniform flow field can settle 90% of particles within just 22 s. This flow pattern contributes to the swift settling of aerosol particles and effectively diminishes their dispersion. Employing this flow pattern in public places with increased foot traffic, like supermarkets, can lower the risk of contracting novel coronavirus without augmenting energy consumption. In order to implement the flow field in a part of the domain, a new air purification device is proposed in this study. The device combined with shelves can optimize the flow field uniformity through the MLA (PSO-SVR) algorithm and alteration of the air distribution structure. The uniformity of the final flow field increased to 0.925. The combination of data-driven MLA with CFD showed good performance in predicting the flow field uniformity. These findings offer valuable insights and practical applications for the prevention and control of respiratory diseases, particularly in post-epidemic scenarios.

Exploring the potential to mitigate airborne transmission risks with convective and radiant cooling systems in an office

Based on the recognized airborne infection risk, there is a raised demand to develop innovative ventilation systems to mitigate the airborne transmission risk indoors. This paper focused on two micro-environment ventilation systems, namely personalized ventilation combined with radiant panel system (PVRP) and a local low velocity unit combined with radiant panel system (LVRP) and studied the potential to minimize the airborne infection risk. The performance was compared with a typical mixing ventilation system, where supply air is released from a perforated duct. The droplet nuclei of an infected person were simulated with tracer gas (SF6) released by a thermal breathing manikin. The effect of the heat gain (38 W/m2 and 73 W/m2), breathing pattern of the infector (exhaled via the nose or mouth), desk partition wall, and air distribution methods on the infection risk were studied. The results show the infection risk of the exposed person is around 0.5 % with micro-environment systems (42 l/s) and 0.7% (61 l/s) with the perforated duct system when the occupants remain for 102 min in the space. The higher heat gain slightly increased the infection risk (from 0.71 % to 0.81 %) with the LVRP system, but it did not have an effect with the PVRP system. The desk partition wall could reduce the infection risk only to an extent. The breathing patterns of the infector do not have any influence on the infection risk for the three studied air distribution methods.

Comparative evaluation of air distribution systems for controlling the airborne infection risk in indoor environments

Air distribution systems are essential for controlling indoor airborne cross-infection risks. However, there is no consensus on the most effective system. This study compares the effectiveness of common air distribution methods, including mixing ventilation (MV), displacement ventilation (DV), and stratum ventilation (SV), while considering specific influencing factors for each system. Factors such as inlet type for MV, airflow direction for SV, and relative body posture for DV were evaluated. Computational thermal manikins with actual breathing functions were simulated, and tracer gas (N2O) and particles of various sizes were used to simulate pathogen-laden droplet nuclei from an infected individual. The modified Wells-Riley model was employed to assess infection risk. Results indicate that no single air distribution system consistently outperforms others, with effectiveness depending on different conditions. Under the DV system, the highest infection risk (20.86 %) occurred when the infected person was seated, and the exposed person was standing, decreasing to 1.2 % when their positions were reversed. In SV system, the highest risk (17.58 %) was observed when the infected person with the same direction as the airflow, dropping to 1.01 % in the opposite direction. The study suggests using MV systems with adequate air volume when people's locations and relative positions are unclear.

Validation of simulated velocity distribution in isothermal mixing ventilation cases with particle streak tracking

This is a validation study for the velocity distribution in mixing ventilation. Two different supply air diffusers a slot and a swirl diffuser form two different room airflows. For the swirl diffuser two different and for the slot diffuser five different exhaust positioning are tested numerically and experimentally. A comparison of the flow structure shows good agreement between simulation and experiment for six air changes per hour, but not for the lower air change rate of 1.5 per hour. The velocity deviations between experiment and simulation are higher for the swirl diffuser. These exist partly due to the experimental methodology, but also due to an overestimation of the supply air momentum. Thus, further sensitivity investigations are carried out for the swirl diffuser. The overestimation of the supply air momentum depends mainly on a geometric model error in the computational fluid dynamics simulation. A comparison of different turbulence models confirms the advantages of the RST elliptic blending turbulence model over the k-epsilon realizable model for the swirl diffuser case.