Impact of mechanical ventilation control strategies based on non-steady-state and steady-state Wells-Riley models on airborne transmission and building energy consumption

Ventilation is an effective solution for improving indoor air quality and reducing airborne transmission.Buildings need sufficient ventilation to maintain a low infection risk but also need to avoid an excessive ventilation rate,which may lead to high energy consumption.The Wells-Riley(WR)model is widely used to predict infection risk and control the ventilation rate.However,few studies compared the non-steady-state(NSS)and steady-state(SS)WR models that are used for ventilation control.To fill in this research gap,this study investigates the effects of the mechanical ventilation control strategies based on NSS/SS WR models on the required ventilation rates to prevent airborne transmission and related energy consumption.The modified NSS/SS WR models were proposed by considering many parameters that were ignored before,such as the initial quantum concentration.Based on the NSS/SS WR models,two new ventilation control strategies were proposed.A real building in Canada is used as the case study.The results indicate that under a high initial quantum concentration(e.g.,0.3 q/m^(3))and no protective measures,SS WR control underestimates the required ventilation rate.The ventilation energy consumption of NSS control is up to 2.5 times as high as that of the SS control.

Study on impact factors of tracer gas method in investigations of gaseous pollutant transport and building ventilation

Building ventilation is essential to discharge indoor pollutants and improve indoor air quality for occupant health.Tracer gas method is an efficient way in the field of building ventilation to measure ventilation rate and to evaluate the ventilation performance.Literature shows notable deviation of measured ventilation rate using different tracer gases.In the present study,CFD simulations are carried out to analyze He-,CO_(2)-and SF_(6)-based tracer gas methods.The effects of tracer gas density and release rate on the concentration distribution and ventilation effectiveness are studied.Various application scenarios of different ventilation rates and airflow distribution forms are compared.The results show that the deviation of ventilation effectiveness evaluated by different tracer gases can be above 2-4 times,and the error is introduced by non-passive dispersion.Whether tracer gas dispersion is passive or not depends on the relative importance of density difference driven mass transfer to forced convection mass transfer,which is due to the combined effects of density difference,release rate,and indoor airflow velocity,and can be judged by a dimensionless number θ.Under the geometry and ventilation settings in the present study,the critical value of θ is 1.0 for the error range of 5%,and 2.0 for the error range of 10%.When θ is below the critical value,the gas transport is passive and dominated by the indoor ventilation airflow.A release of tracer gas with smaller release rate and smaller density difference into a stronger indoor airflow behaves more passive.Heavier tracer gas tends to significantly overestimate the performance of upper supply and lower exhaust ventilation,and lighter tracer gas aggravates the overestimation of the performance for lower supply and upper exhaust ventilation.In mechanical ventilation rooms with air change rate of 3.0-6.0 h^(−1),a continuous release of tracer gas SF_(6),CO_(2) or He with release rate above 8 mg/s or source concentration above 8-75 ppm should not be considered as passive.This work clarified the passive and non-passive transport characteristics and mechanisms of various tracer gases,which is helpful for the engineering applications of tracer gas method in building ventilation studies.

The assessment of personal exposure in restaurants considering heat sources and ventilation strategies

Outbreaks of airborne infections during meal consumption in diverse restaurant settings have been extensively reported.It is widely recognized that effective ventilation strategies are essential to minimize the infection risk in indoor environments,and these strategies should be tailored to the heat sources.The purpose of this study is to compare the spatial distribution of risk in restaurant rooms that use mixing or displacement ventilation,specif-ically focusing on the heat sources used for different food types,namely hotpot,normal Chinese food,and iced food.Computational Fluid Dynamics(CFD)was employed to assess exposure risk.Our results indicate that the use of low-temperature heat sources can elevate the risk of infection by increasing the local vertical temperature gradient.In comparison to no heat source,the risk increased by 190.9%and 99.6%for displacement and mixing ventilation strategies,respectively.Under mixing ventilation,both low-temperature and no heat sources showed lower infection risks when compared to displacement ventilation.However,displacement ventilation is found to be highly effective in reducing the risk of infection when using a high-temperature heat source,with only 12.3%of the infection risk observed in mixing ventilation.Furthermore,the use of displacement ventilation resulted in a significant reduction in the odors emitted by hotpot,which were instead absorbed by clothes in the mixing ven-tilation scenario.Our findings provide crucial insights into the development of appropriate ventilation strategies for reducing personal exposure to airborne infections in diverse restaurant settings.Specifically,we recommend using displacement ventilation in restaurants that utilize high-temperature heat sources,as it can substantially reduce the risk of infection.

Natural Ventilation Design:Predicted and Measured Performance of a Hostel Building in Composite Climate of India

Natural ventilation is recognized for improving the thermal comfort of the built environment and indoor air quality.It provides comfortable conditions for building occupants and reduces energy consumption for air-conditioning.Therefore,it is important to study and explore effective means of ventilation to improve the building designs.This study investigates the thermal comfort of a naturally ventilated hostel operational building in the composite climate of Jaipur,India using Computational Fluid Dynamics(CFD)simulation tool‘Cradle scSTREAM’.A 3D building model has been developed to analyze the thermal comfort for different natural ventilation strategies with advanced mesh algorithms which generate fewer mesh elements and maintain good mesh quality.A field study was carried out to collect the actual data and to validate the model which was further used to evaluate the thermal comfort range based on the ASHRAE-55 standard.Several design strategies have been applied to enhance thermal comfort.It was found that an increase in air velocity up to 0.5 m/s was achieved by Cross Ventilation while a drop of 2.0-2.5℃in the air temperature was found using Night Ventilation.It can be stated that cross ventilation increases the air movement while night ventilation gives comparatively higher comfort regarding air temperature and relative humidity.

Adaptive thermal comfort in naturally ventilated hostels of warm and humid climatic region,Tiruchirappalli,India

Thermal comfort is an important factor in hostel buildings when the aim is to maximize the productivity of the students.Due to the extreme weather conditions,achieving thermal comfort in a hostel building in a hot and humid climate is even more difficult.Studies conducted in naturally ventilated hostel buildings in warm-humid climates involved the influence of outdoor air temperature only up to 34.4℃ and have been conducted in a specific season.In contrast,the Tiruchirappalli climate is characterized by a higher range of environmental variables.Therefore,to understand the thermal comfort conditions and usage of the environmental controls in naturally ventilated hostel buildings at the higher range of the environmental variables,a thermal comfort field study spread over one year was carried out at the National Institute of Technology,Tiruchirappalli,India,in twenty-seven hostel buildings.This study relies on field observation and thermal comfort responses from 2028 questionnaires collected from the students between September 2019 to August 2020.The analysis revealed a neutral temperature of 29.5℃ and a comfort range from 26.1℃ to 32.8℃,indicating a wide range of ther-mal adaptation than suggested by the National Building Code of India and ASHRAE standard 55.The preferred temperature was 27.8℃,indicating that students preferred a cooler environment.Acceptability with sweating conditions extended the upper limit of thermal acceptability from 31.8℃ to 32.4℃.The use of a mosquito net can increase the probability of opening a window.Results indicated that overall behavioral adjustment could extend the comfort limits.The study results would be helpful to develop guidelines and designs for naturally ventilated hostel buildings in warm and humid climates that will contribute to reducing energy demand.

Pressure and flowrate distribution in central exhaust shaft with multiple randomly operating range hoods

Central flues are now commonly adopted in high-rise residential buildings in China for cooking oil fumes(COF)exhaust.Range hoods of all floors are connected to the central shaft,where oil fumes were gathered and exhausted through the outlet at the building roof.As households may cook and use their range hood at random periods,there is great uncertainty of the amount of COF being exhausted.In addition,users can often adjust the exhaust rate of the range hood according to their needs.As a result,thousands of possible operating conditions consisting of distinct combinations of on/off conditions and fan speed occur randomly in the central COF exhaust system,causing the exhaust performance to vary considerably from condition to condition.This work developed a mathematical model for characterizing the operation of the central COF exhaust system in a high-rise residential building as well as its iterative solving method.Full-scale tests coupled with CFD simulation referring to a real 30-floor building were conducted to validate the proposed model.The results show that the model agreed well with the CFD and experimental data under various system operating conditions.Moreover,the Monte-Carlo method was introduced to simulate the random operating characteristics of the system,and a hundred thousand cases corresponding to distinct system operating conditions were sampled and statistically analyzed.