Modeling particle fate in ventilation system—Part II: Case study

In this paper the particle filter group model, which was presented in the first part of this series of study, is employed to predict particle fate in a typical ventilation system. The model simultaneously takes into account the interactions between particle transport in ventilation ducts and rooms and particle spatial distribution. It has been proven that an entire ventilation system, including filters, ducts and rooms, can be regarded as a serial of filters in steady-state cases, hence the name “particle filter group model”. With this model, the particle concentration and quantity of deposited particles in each part of the ventilation system can be easily calculated.     

Modeling particle fate in ventilation system—Part I: Model development

Aerosol particles are regarded as one of the most significant pollutants in the indoor environment. These particles can travel through ventilation systems, thus impacting indoor air pollution. Accurate modeling and prediction of aerosol particle transport in ventilation systems is therefore a key component of indoor particle pollution. Since recycled air takes particles into the ventilation system again, it can be challenging to calculate the particle concentration in each part of the system. This study theoretically and experimentally proves that the entire ventilation system, including filters, ventilation ducts and ventilation rooms, can be regarded as a serial of filters in steady-state cases. We call this the particle filter group model. Equations are presented to calculate the particle concentration in each component of normal kinds of mechanical ventilation systems. With this model, the particle concentration and deposited particle quantity in each part of the ventilation system can be easily calculated. This model may help to control indoor particle pollution and guide the cleaning of ventilation ducts and filters.     

Numerical analysis of particle deposition in ventilation duct

This paper adopts computational fluid dynamics (CFD) to numerically analyze particle deposition in the ventilation duct. A three-dimensional drift-flux model combined with particle deposition boundary conditions for wall surfaces is presented. The numerical method is used to analyze the particle deposition velocity and deposited particle mass flux in the ventilation duct after validation. Twelve groups of particle size, two average air speeds in ducts are investigated to understand the particle deposition in the straight ventilation duct, which ensures a fully developed turbulent duct flow. And then, the particle accumulation by deposition in the ventilation duct is analyzed according to the cleaning code for air duct system in heating, ventilation and air conditioning (HVAC) systems of China. The cases with or without air filter installed are studied by assuming that the duct inlet particle concentration is that of outdoor air in Beijing city, China.     

Experimental investigation of reduced-mixing personal ventilation jets

This paper presents an investigation of the design and performance characteristics of personalized ventilation (PV) systems that, in combination with general ventilation, deliver high quality air to the breathing zone (BZ) with no more clean air supply than indicated by ANSI/ASHRAE 62.1-2004, while satisfying acceptable ergonomic and aesthetic considerations. Under these conditions, the energy used for conditioning the clean air will not exceed that of a conventional ventilation system. We introduce a novel PV nozzle that achieves high BZ air quality with a small fraction of the clean air indicated by the ANSI/ASHRAE Standard. Tracer gas experimental results presented in this paper demonstrate the advantages of the novel nozzle relative to conventional PV nozzles. The results show that, at a PV clean air supply of only 2.4 l/s, the new nozzle achieves a BZ ventilation effectiveness close to 7 versus less than 2 for a conventional nozzle delivering the same amount of clean air. A companion paper presents a computational analysis of the same concept, validated against the experimental results of the present paper.     

Effect of door opening on the performance of displacement ventilation in a typical office building

The purpose of this paper is to investigate, using a validated computational fluid dynamics simulation, the effect of the position of doors on performance of the displacement ventilation system. The results are reported in terms of thermal comfort and indoor air quality. The study focuses on a typical Hong Kong office under local thermal and boundary conditions. It was found that the presence of large heat sources such as from a window can cause the lateral movement of airflow, disrupting the convection effect which the displacement ventilation system relies on. Doors can create this situation when they are opened by changing the thermal boundary conditions of indoor spaces. The designer should be made aware of this possibility and make appropriate design decisions to accommodate for this fact.     

Determination of particle concentration in the breathing zone for four different types of office ventilation systems

Many factors affect the airflow patterns, thermal comfort, contaminant removal efficiency and indoor air quality at individual workstations in office buildings. In this study, four ventilation systems were used in a test chamber designed to represent an area of a typical office building floor and reproduce the real characteristics of a modern office space. Measurements of particle concentration and thermal parameters (temperature and velocity) were carried out for each of the following types of ventilation systems: (a) conventional air distribution system with ceiling supply and return; (b) conventional air distribution system with ceiling supply and return near the floor; (c) underfloor air distribution system; and (d) split system. The measurements aimed to analyse the particle removal efficiency in the breathing zone and the impact of particle concentration on an individual at the workstation. The efficiency of the ventilation system was analysed by measuring particle size and concentration, ventilation effectiveness and the indoor/outdoor ratio. Each ventilation system showed different airflow patterns and the efficiency of each ventilation system in the removal of the particles in the breathing zone showed no correlation with particle size and the various methods of analyses used.     

Effect of internal partitions on the performance of under floor air supply ventilation in a typical office environment

This paper presents a case to investigate the effect of partitions in an office on the performance of under floor air supply ventilation system via computational fluid dynamics. The assessment is in terms of thermal comfort and indoor air quality with the use of a validated computer model. The results indicate that the partitions may significantly affect airflow and performance of a under floor air supply ventilation system. In particular, the presence of a gap above the partition wall is able to improve air distribution owing to less air re-circulation in the upper zone. Its effect on thermal comfort and indoor air quality indicators are evaluated.     

A review of ventilation and the quality of ventilation air

Ventilation is pivotal in terms of securing optimum indoor air quality. In addition, it also has a major impact on energy use in buildings. It is important, therefore, that the role and impact of ventilation is fully understood and that ventilation is employed efficiently. The purpose of this paper is to review these aspects with particular reference to recent research and developments. Key aspects are concerned with identifying the role of ventilation and reviewing this role in the context of the other measures that must be taken to secure a healthy indoor environment. References are particularly made to the development of standards and recent related research. Although good progress is being made, areas that still need to be addressed include maintaining good outdoor air quality and preventing contaminated outdoor air from entering buildings. The outcome of recent research must also be disseminated in practical ways to policy makers, building occupiers and practitioners. Good indoor climate can be achieved, not so much by introducing expensive concepts, but by developing a rationale approach to identifying needs and applying the necessary tools to deal with each need.     

Sorption of VOCs on material surfaces as the deciding factor when choosing a ventilation strategy

The interaction between different ventilation strategies, and the adsorption and desorption of volatile organic compounds on material surfaces in small test chambers, is investigated. In test chamber experiments, nylon carpet was exposed to a mixture of toluene and α-pinene at two different dosing rates. The ventilation strategies were chosen to mimic the conditions in real buildings, i.e. with an air exchange rate of 2 h⁻¹ during the working day (8–17) and a rate of 0.67 h⁻¹ during the remainder of the 24 h. The results show that the sorption behavior has to be included when estimating the concentration variations in a room based on source characteristics and ventilation rates. The software application “EnviSim” was used to model the concentrations in a model room based on the experimental conditions. As the ventilation strategy influences the resulting concentrations, it is recommended that the ventilation system be “turned on” a couple of hours before the start of the working day and “turned down” again soon after the occupants have left the building.     

Effects of ventilation strategies and source locations on indoor particle deposition

A computer model for predicting aerosol dispersion in indoor spaces was validated with experimental data found in the literature. The validated model was used to explore the effect of the area or point source locations on aerosol particle transportation in ventilation rooms with rough surfaces. Two different ventilation strategies were studied: mixing ventilation (MV) and underfloor air distribution (UFAD) system. The simulation results show that in MV, the particle concentration and removal effectiveness are little dependent on the position of the pollutant sources. In UFAD, the source location has a strong impact on the distribution and removing of the contaminants. The particle removal performance strongly depends on the ventilation efficiency and the particle deposition loss in the room with rough surfaces. The important consideration in both the relative ventilation efficiency and the deposition rate is the relative position of the sources to the main airflow and the occupied zone in an UFAD room.     

Modeling of cooking-emitted particle dispersion and deposition in a residential flat: A real room application

Maintenance of good indoor air quality for residences could be very challenging. Episodic event such as cooking emits a large amount of ultrafine and supermicron particles. A numerical model is used to simulate a 10-min cooking process in a real room, followed by a few more minutes to allow the decay and removal of particles. Particle dispersion and deposition in the kitchen and the living room are simulated by a new drift-flux model. Strong buoyancy flow is observed and particle concentration is significantly affected by the thermal plume. Results show that for supermicron particles strong non-uniformity of concentration is observed in the kitchen but the non-uniformity is less obvious in the living room. Exposure analysis must take into account the influence of the particle sizes.     

Analysis of stack ventilation system effectiveness in an average kindergarten in north-eastern Poland

The present research is concerned with effectiveness of stack ventilation system in an average Polish kindergarten. The research was carried out in June (2010), before the cleaning of the ventilation ducts, and in December (2009), after they had been cleaned. Two aspects of possible improvement of effectiveness of stack ventilation were considered, namely the influence of duct cleaning, and the supportive role of window unsealing. It seems that duct cleaning considerably improves effectiveness of ventilation: minimum growth in effectiveness of stack ventilation equalled 10%, whereas maximum was up to 100%. With unsealed windows, the air change rate grew up to 30%.     

PROBE-PM: A new way to simulate particle transport in ventilation systems

A ventilation system usually runs on a certain schedule. The boundary conditions, such as the time-dependent outdoor particle concentrations and indoor particle generating sources, vary dynamically. Ventilated rooms are connected to ventilation ducts and filters, and indoor particle concentration and particle deposition on duct surfaces are interdependent. Thus it is important to study particle transport in the entire ventilation system and take the dynamic characteristics into account to assess particle pollution in the entire system more accurately. A generalized model is proposed in this study to estimate particle concentration throughout an entire ventilation system as well as mass loading of particles on ventilation components. Model equations describe particle movement in different ventilation components, including filters, ducts, and rooms. Penetration factors are adopted for filters and ducts, and particle concentrations in rooms are calculated by a lumped parameter method. This generalized model can be applied to any ventilation system, and a new software, PROBE-PM, was developed based on the presented model. Four case studies are carried out using this new software to demonstrate the application of the model.     

Experimental investigation of thermal and ventilation performances of stratum ventilation

Stratum ventilation has been proposed to cope for elevated indoor temperatures. Air speed, temperature and CO₂ concentration of a stratum ventilated office are investigated experimentally. The data obtained under well defined conditions and therefore can be used for validating numerical models. Thermal comfort conditions and ventilation efficiency are studied based on the experimental results of four experimental cases. Thermal comfort indices, i.e. PMV, PPD and PD are calculated from measured data. The values of these indices are found to satisfy the requirements of ISO 7730, CR 1752-1998 and ASHRAE 55-2010. In terms of thermal comfort, the two cases with supply air temperature of 21 °C are found to perform better compared with the two cases with supply air temperature of 19 °C. For all the cases, the ventilation effectiveness is close to 1.5. This ventilation method could therefore be expected to provide indoor air quality in an efficient way.     

Comparing the effectiveness of interventions to improve ventilation behavior in primary schools

Poor air quality in schools has been associated with adverse health effects. Indoor air quality can be improved by increasing ventilation. The objective of this study was to compare the effectiveness of different interventions to improve ventilation behavior in primary schools. We used indoor CO(2) concentrations as an indicator. In 81 classes of 20 Dutch primary schools, we applied three different interventions: (i) a class-specific ventilation advice; (ii) the advice combined with a CO(2) warning device and (iii) the advice combined with a teaching package. The effectiveness of the interventions was tested directly after intervention and 6 weeks after intervention by measuring the CO(2) concentrations and comparison with a control group (iv). Before intervention, the CO(2) concentration exceeded 1000 ppm for 64% of the school day. The class-specific ventilation advice without further support appeared an ineffective tool to improve ventilation behavior. The advice in combination with a CO(2) warning device or the teaching package proved effective tools and resulted in lower indoor CO(2) concentrations when compared with the control group. Ventilation was significantly improved, but CO(2) concentrations still exceeded 1000 ppm for more than 40% of the school day. Hence, until ventilation facilities are upgraded, the CO(2) warning device and the teaching package are useful low-cost tools. PRACTICAL IMPLICATIONS: To improve ventilation behavior and indoor air quality in schools, CO(2) warning device and teaching package combined with a class-specific ventilation advice, are effective tools, while giving the ventilation advice solely, is not effective. Although ventilation is significantly improved through behavioral change, the ventilation rate is still insufficient to maintain good air quality during the full school day. Therefore, the improvement of the ventilation facilities is recommended. Hence, until ventilation facilities in schools are upgraded, the CO(2) warning device and the teaching package are useful low-cost tools to improve current indoor air quality.     

CFD analysis of ventilation effectiveness in a public transport interchange

A study was conducted into the ventilation effectiveness of a ventilation system within a public transport interchange (PTI) in Hong Kong. A computational fluid dynamics (CFD), steady state computational model of the PTI was used to investigate and predict the typical pollutant emission pattern for buses. In Hong Kong, the displacement ventilation (DV) scheme is often employed for the PTI. The numerical simulation investigates the effectiveness of the DV system in removing pollutants from the occupied zone. An alternative model is proposed where the supply is located at the ceiling and the exhausts are located at the lower part of the columns. It was found that both systems could adequately ventilate the PTI; however, the ceiling based air supply system is able to provide improved thermal comfort and indoor air quality (IAQ).     

A preliminary study on the association between ventilation rates in classrooms and student performance

Poor conditions leading to substandard indoor air quality (IAQ) in classrooms have been frequently cited in the literature over the past two decades. However, there is limited data linking poor IAQ in the classrooms to student performance. Whereas, it is assumed that poor IAQ results in reduced attendance and learning potential, and subsequent poor student performance, validating this hypothesis presents a challenge in today's school environment. This study explores the association between student performance on standardized aptitude tests that are administered to students on a yearly basis, to classroom carbon dioxide (CO2) concentrations, which provide a surrogate of ventilation being provided to each room. Data on classroom CO2 concentrations (over a 4-5 h time span within a typical school day) were recorded in fifth grade classrooms in 54 elementary schools within a school district in the USA. Results from this preliminary study yield a significant (P < 0.10) association between classroom-level ventilation rate and test results in math. They also indicate that non-linear effects may need to be considered for better representation of the association. A larger sample size is required in order to draw more definitive conclusions. Practical Implications Future studies could focus on (1) gathering more evidence on the possible association between classroom ventilation rates and students' academic performance; (2) the linear/non-linear nature of the association; and (3) whether it is possible to detect 'no observed adverse effect level' for adequate ventilation with respect to academic performance in schools. All of this information could be used to improve guidance and take regulatory actions to ensure adequate ventilation in schools. The high prevalence of low ventilation rates, combined with the growing evidence of the positive impact that sufficient ventilation has on human performance, suggests an opportunity for improving design and management of school facilities.     

Particle dispersion and deposition in ventilated rooms: Testing and evaluation of different Eulerian and Lagrangian models

Indoor particle dispersion in a three-dimensional ventilated room is simulated by a Lagrangian discrete random walk (DRW) model and two Eulerian models: drift flux model and mixture model. The simulated results are compared with the published measured data to check the performance of the three models for indoor particle dispersion simulation. The deposition velocity of the particles is also computed and compared with published data. The turbulent airflow is modeled with the renormalization group (RNG) k−ε and a zero equation turbulence model. Comparison of the calculated air velocities with measurement shows that both the two turbulence models can simulate the airflow well for the presented case. For the Lagrangian DRW model, a post-process program is used to state the particle trajectories and transfer the results to particle concentration distribution. For Eulerian models, the effect of particle deposition towards wall surfaces is incorporated with a semi-empirical particle deposition model. The comparison shows that both the Lagrangian DRW model and drift flux model yield satisfactory predictions, while the predicted results by the mixture model are not satisfied. The deposition velocity obtained by the three models match the experimental data well.     

Natural ventilation of indoor spaces while minimizing heat losses: Theoretical and experimental investigation

Limiting heat losses during ventilation of indoor building spaces has become a basic aim for architects, particularly since the oil crisis of 1970. Much experience has been gained in terms of ventilation of indoor spaces. Nevertheless, due to the complex applications, attempts to create a theoretical base for solving the problems related to the issue are limited, especially determining the minimum ventilation period required within a designated space. In this paper we have approached this matter, both theoretically and experimentally. The conclusion we reached was that controlled ventilation of spaces through vent holes that successively open and close at regular time intervals can limit the excessive circulation of air masses, which in turn limits heat losses.