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Method to assess quality and quantity of outdoor ventilation airflow received by occupants.

Introduction

Indoor air is one kind of main exposure for human beings. There is mounting evidence that exposure to indoor air is the cause of excessive morbidity and mortality (Sundell 2004). Ventilation with outdoor air (or fresh air) is widely used to improve indoor air quality (IAQ), besides eliminating contamination sources and purifying the air (Fanger 2006; Yu and Hu 2008). Moreover, one important cause of sick-building syndrome and building-related illness is inadequate outdoor ventilation airflow (Wargocki et al. 2000; Takigawa et al. 2009).

However, improvement of IAQ by increasing outdoor ventilation airflow is not obvious in some practical applications (Bluyssen 2009). The main reason is merely paying attention to total outdoor airflow of a ventilation system but neglecting the effective quantity of outdoor ventilation air received by occupants. In addition, although pollutants concentration conforms to IAQ standards sometimes, the percentage of occupants' dissatisfaction (PD) is still very high (Chao et al. 2009). Therefore, by only giving concern to contamination sources and their contaminants, improvement of IAQ with ventilation cannot be gained completely.

Indoor ventilation process refers to the quantity of supplied outdoor air, airflow pattern, and emission intensities of contamination sources and their distribution. Meanwhile, the quality of indoor air at one position mainly depends on its component composition, including components of outdoor air and contaminants. The former reflects its positive quality and the latter indicates its negative quality. The outdoor air component of one position is the result of a joint effect of indoor airflow pattern and total outdoor airflow provided by the ventilation system. While the contaminants component of one position lies on contamination sources emission intensities and distribution, there is potential for outdoor air diluting and removing contaminants.

Therefore, in view of the above problems and causes, it is meaningful to build a relationship between local components composition and the combined effect of outdoor air stream and contamination sources for assessing ventilation performance.

Many indices, including local and global ones, have been proposed to assess ventilation performance from multiple aspects, which should conform to the criteria of using indices to evaluate ventilation performance as given by Sandberg (1981). Contaminants concentration, expressing the amount of contaminants contained in a unit volume of indoor air, is the most straightforward index to indicate influence of contamination source intensities, their distribution, and outdoor air dilution (Sandberg and Sjoberg 1983). The ventilation effectiveness factor, based on the work of Zhang et al. (2001), mainly reflects the potential of outdoor air to dilute and remove contaminants (Skaret and Mathisen 1982). Tommaso et al. (1999) defined the local mean age of air to depict air diffusion efficiency, which is derived from the opinion that the lower the local mean age of air is, the lower the degree of air being contaminated is, and the better the air quality is. The local mean age of air is not normalized and cannot be used to compare the air diffusion performance of different ventilation rooms, but it is allowable for air change efficiency and relative ventilation efficiency (Sandberg and Sjoberg 1983; Tommaso et al. 1999). Peng and Davidon (1997) suggested the local mean age of air was a poor index and gave a local specific contaminant accumulating index to show the joint effect of both a specific contamination source (through contaminant concentration) and indoor airflow (through local mean age of air). Meanwhile, they established another concept, the expected contaminant dispersion index, to portray mass transfer of contaminants emitted at a specific location for the situation when contamination source emission intensity is unknown. Chen et al. (1969) and Sandberg and Sjoberg (1983) introduced the concept of residence time distribution from chemical engineering (Nauman 1981) to indicate passive contaminants dispersion and air diffusing efficiency, which depends on indoor airflow characteristics (Robinson and Tester 1986). Zvirin and Shinnar (1976), Davidson and Olsson (1987), and Peng et al. (1997) proposed a definition of purging flow rate (also called regional purging flow rate) to reflect the effectiveness of outdoor air to dilute and remove contaminants and the influence of contamination sources emission intensities and distribution within a region. For the ventilation system with multiple inlets or outlets, Murakami (1992) and Kato et al. (1992) provided an index of contribution ratio to reflect the influence of inlets or outlets on one region. Moreover, the contribution ratio can be determined with the Markov chain model. It is worth pointing out that the contribution ratio becomes invalid for recirculating regions. On the other hand, Peng and Davidon (1997) established local purging effectiveness to demonstrate the potential of outdoor air diffusing to one position for diluting contaminants. In addition, for dynamic diffusion of outdoor air and contaminants in a limited period, Yang et al. (2004) purposed two indices--accessibility of supplied air and accessibility of contamination sources. When ventilation time is long enough, accessibility of supplied air equals the contribution ratio of inlet and accessibility of contamination sources equals the contribution ratio of outlet.

It can be seen that the existing ventilation indices may depict the characteristic of indoor airflow, potential of outdoor air diluting and removing contaminants, influence of contamination sources emission intensities, and distribution. But attempts are needed to quantify the relationship between components composition of one position and the combined effect of outdoor air stream and contamination sources, which determines quality and quantity of outdoor ventilation airflow received by occupants. In order to achieve this purpose, a new scale was established in this study, and the application of this scale was shown with the aid of numerical simulations.

New scale development

An outdoor air stream of ventilation relates to indoor airflow pattern and flow rate of supplied outdoor air; contamination sources involve their emission intensities and distribution. Meanwhile, the former two factors determine the outdoor air component and its transfer time; adding the latter two factors collectively determines contaminants components and their transfer time. Therefore, in order to quantify the relationship between components composition of one position and the combined effect of outdoor air stream and contamination sources, the amount of each kind component and its transfer time should be considered simultaneously, although time is the ruler and component composition is the real cause for air quality. In a certain ventilation time range, the transfer time of the outdoor air component affects its effective quantity received by occupants and that of contaminants component impacts occupants' contamination exposure level.

Since the local mean age of air represents the time of outdoor air component transferring from inlet to indoor specific position, it is significative to quantify the effective quantity of outdoor air reaching to one position for a time equal to its local mean age of air. Moreover, outdoor air may be seen as one kind of hypothetical special gas ([C.sub.oA]); the effective quantity of outdoor air at one position can be obtained as the following:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where t is the ventilation time, and [[tau].sub.A] is the local mean age of air.

The local mean age of air is governed by its transport equation (Etheridge and Sandberg 1996),

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where [u.sub.j] is the velocity vector, and [GAMMA] is the effective diffusion coefficient.

Theoretically, the time of contaminants components transferring from contamination sources to an indoor specific position can be seen as the local mean age of contaminants. However, the local mean age of contaminants is indeterminate at the ventilation inlet. The accumulation of contaminants components for a time equal to the local mean age of air at one position can be regarded as a feasible choice, which is given as the following:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

where [C.sub.P] is the concentration of contaminant produced by the human body, and [C.sub.B] is the concentration of contaminant produced by building material.

Local air freshness (LAF) is proposed to depict the ratio of the effective quantity of the outdoor air component and the cumulant of contaminants components at one position. According to Equations 1 and 3, the LAF of one indoor position can be obtained as the following:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

where [alpha] is the correction factor.

The correction factor indicates the influence of indoor air speed frequency on air freshness; e.g., natural wind may increase freshness of air (Song and Xu 2008).

When the effective quantity of the outdoor air component at the inlet and the removed amount of contaminants components at the outlet are seen as a reference, the first factor of outdoor air effect (FFOAE) can be given as the following:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

where

C[O.sub.A,s] is the outdoor air component at inlet,

[[tau].sub.n] is the nominal time constant,

[C.sub.P,e] is the outlet concentration of contaminant produced by the human body,

[C.sub.B,e] is the outlet concentration of contaminant produced by building material,

[E.sub.OA] is the transfer efficiency of outdoor air component,

[E.sub.[tau]A] is the time efficiency of indoor airflow, and

[E.sub.P,B] is the transfer efficiency of contaminants components.

In addition, the FFOAE at the initial moment and the steady state of the ventilation process can be acquired from Equation 5:

FFOAE(x,y,z,0) = 0, (6)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

Equation 6 means that the FFOAE is zero at the initial moment of the ventilation process, since the outdoor air has not begun to transfer and there is no outdoor air component at arbitrary position. Equation 7 illustrates that the FFOAE is independent on the ventilation time at the steady state of ventilation.

The FFOAE is normalized and can be comparable in a different building room. It quantifies ventilation performance by building one relationship between components composition of one position and the combined effect of outdoor air stream and contamination sources. The effective quantity of outdoor ventilation airflow received by occupants can be assessed with this scale, which indicates the influence of indoor airflow pattern, potential of outdoor air diluting and removing indoor contaminants, and contamination sources emission intensities and their distribution in ventilation process.

[FIGURE 1 OMITTED]

Application and discussion

Modeling

One two-dimensional ventilation space is chosen for application of this new scale. Of course, the new scale can also be used in three-dimensional ventilation space. The size of the ventilation space is 10.5 ft (3.2 m) wide x 21 ft (6.4 m) high. The sizes of inlet and outlet are both 1.3 ft (0.4 m), and their locations include four situations: upper inlet at left side and upper outlet at right side (ULUR), upper inlet at left side and lower outlet at right side (ULLR), lower inlet at left side and lower outlet at right side (LLLR), and lower inlet at left side and upper outlet at right side (LLUR).

The occupant-related contamination source (P-source) and building material-related source (B-source) lie in the bottom of this ventilation space. The configuration of this space is shown in Figure 1. For simplification, there is no heat source inside the space; all walls are well insulated and indoor air temperature keeps isothermal. The supplied outdoor air does not contain contaminants components. The influence of indoor air speed frequency is neglected, and [alpha] equals 1. In all conditions, the ventilation process is assumed to be steady.

The renormalization-group (RNG) K-[epsilon] turbulence model combined with standard wall functions is adopted to model turbulence, owing to its applicability for this case (Chen 1995). For discretization, the standard scheme for pressure; the QuICK scheme for momentum, turbulence, energy, and species; and the SIMPLE algorithm for pressure-velocity coupling are chosen (Peric et al. 1988; Leonard and Mokhtari 1990). A grid-independent solution is tested using different grid sizes, and the amount of the grid is 2048.

[FIGURE 2 OMITTED]

Influence of indoor airflow pattern

For analyzing the influence of indoor airflow pattern, the supplied outdoor airflow rate of ventilation is 25.6 cfm (12.1 L/s). Emission intensities of occupant-related contamination source and building material-related contamination source are 0.19 grain/s (12.63 mg/s) and 0.0003 grain/s (0.02 mg/s), respectively, the interval of which is 2.6 ft (0.8 m) at the bottom of ventilation space. Indoor airflow pattern can be obtained based on the above conditions (see Figure 2).

[FIGURE 3 OMITTED]

Figures 3 and 4 show the concentration (mass fraction) distribution of contaminants emitted by the P-source and B-source for the four kinds of airflow patterns. The maximum contaminant concentration emerges at the lower-left region, and the minimum contaminant concentration locates in the top region for ULUR and ULLR. The maximum and minimum contaminant concentrations of LLLR and LLuR appear in lower-right region and left region, respectively. Meanwhile, compared with the contaminant concentration of uLuR, that of ULLR significantly decreases in the lower-right region owing to its outlet being on the lower-right side. Due to the contaminant being removed rapidly, the contaminant concentration of LLLR is less than that of other airflow patterns in the whole region. In addition, there exist longitudinal concentration stratifications for ULUR and ULLR and transverse concentration stratifications for LLLR and LLUR.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The local mean age of air for different airflow patterns is given in Figure 5. Due to the lack of air direct transport, the local mean age of air in recirculating regions is much higher than that of other regions, though contaminant concentration is rather low in the former. Meanwhile, contaminant concentrations of ULUR and ULLR in the lower-left region, and that of LLLR and LLUR in the lower-right region, are highest, but their local mean age of air is not so in these regions. Therefore, a position with a high local mean age of air may be a position with low contaminant concentration, or vice versa.

Figure 6 illustrates the FFOAE for different airflow patterns. Due to the large quantity of outdoor air component and low contaminant concentration, the FFOAE becomes higher in the region closer to the inlet. Meanwhile, under the effect of mixing and dilution between the outdoor air component and contaminant component, the FFOAE gradually decreases along the path of airflow. The minimum FFOAE appears in the lower-left region of ULUR and ULLR and in the lower-right region of LLLR and LLUR, owing to the high contaminant concentration. In addition, the FFOAE increases from top to bottom for ULUR and ULLR and decreases from left to right for LLLR and LLUR in the recirculating region, although the distribution of contaminant concentration and local mean age of air produce the opposite effect on it. This result manifests that the influence of outdoor air diluting and removing contaminants on ventilation performance is more obvious for ULUR and ULLR, and that of the airflow pattern on ventilation performance is more remarkable for LLLR and LLUR in the recirculating region. It is worth pointing out that the FFOAE is comparatively high for LLLR and LLUR in the human respiration zone, which indicates the improvement of IAQ is preferable in that zone.

Actually, influence of outdoor air stream, contamination sources, and their interaction with each other on ventilation performance may be embodied by the relationship between those three efficiencies ([E.sub.OA], [E.sub.[tau]A], and [E.sub.P,B]) and the FFOAE. Distributions of the three efficiencies and FFOAE at the vertical centerline are given in Figure 7 for different airflow patterns. Increasing [E.sub.[tau]A] and [E.sub.P,B] is the main reason for the rise of the FFOAE in the top region of ULUR and ULLR. Owing to high [E.sub.[tau]A], the FFOAE of ULLR is more than that of ULUR in the bottom region. Meanwhile, the FFOAE of LLLR and LLUR first rise due to the increase of [E.sub.P,B], then descend due to decrease of [E.sub.[tau]A] and [E.sub.P,B] in the bottom region. Furthermore, owing to low [E.sub.P,B,] the FFOAE of LLUR is less than that of LLLR in the top region. In addition, the [E.sub.OA] is relatively stable along the vertical centerline.

[FIGURE 9 OMITTED]

Influence of supplied outdoor airflow rate

Three kinds of supplied outdoor airflow rates are chosen, including 17.6 cfm (8.3 L/s), 20.3 cfm (9.6 L/s), and 25.6 cfm (12.1 L/s). The emission intensities of the P-source and B-source are 0.19 grain/s (12.63 mg/s) and 0.0003 grain/s (0.02 mg/s), respectively, the interval of which is 2.6 ft (0.8 m). The mean FFOAE of the human respiration zone for different outdoor airflow rates is shown in Figure 8. Increasing of outdoor airflow rate promotes mean the FFOAE of the human respiration zone to descend for ULUR and ULLR and to rise for LLLR and LLUR. The reason is that the increase of outdoor airflow rate strengthens the mixing effect in the recirculating region for ULUR and ULLR, but makes contaminants exhaust to the outside more efficiently for LLLR and LLUR.

Influence of contamination source emission intensity

Three levels of contamination sources emission intensities are taken into account, including (G1) 0.119 grain/s (7.91 mg/s) for the intensity of the P-source and 0.0001 grain/s (0.008 mg/s) for the intensity of the B-source, (G2) 0.19 grain/s (12.63 mg/s) for the intensity of the P-source and 0.0003 grain/s (0.02 mg/s) for the intensity of B-source, and (G3) 0.337 grain/s (22.5 mg/s) for the intensity of the P-source and 0.0005 grain/s (0.032 mg/s) for the intensity of the B-source. Supplied outdoor airflow rate is 25.6 cfm (12.1 L/s), and the interval of these two contamination sources is 2.6 ft (0.8 m). The mean FFOAE of the human respiration zone for different contamination sources emission intensities is given in Figure 9. Increasing contamination sources emission intensities leads the mean FFOAE to rise first and then to decline for ULUR, LLLR and LLUR, but it is to the contrary for ULLR.

[FIGURE 10 OMITTED]

Influence of contamination sources distribution

Three kinds of interval between the P-source and B-source are chosen, including 0.65 ft (0.2 m), 1.3 ft (0.4 m), and 2.6 ft (0.8 m). Intensities of the P- and B-sources are 0.19 grain/s (12.63 mg/s) and 0.0003 grain/s (0.02 mg/s), respectively. Supplied outdoor airflow rate is25.6cfm(12.1 L/s). The mean FFOAE of the human respiration zone for different intervals between the P-source and B-source is shown in Figure 10. When the interval increases, the mean FFOAE descends first and then rises for ULUR, varies for LLLR, declines for ULLR, and rises for LLUR. Actually, the change of contaminant concentration distribution is the reason for above results.

Conclusion

The new scale established in this study is capable of reflecting the combined effect of outdoor air stream, contamination sources, and their interaction with each other on ventilation performance, quality, and quantity of outdoor ventilation airflow received by occupants. Meanwhile, it provides some guides for the determination of needed outdoor airflow rate in building room, ventilation system design, and optimization.

DOI: 10.1080/10789669.2011.585419

Acknowledgments

This research has been supported by a project on residential building energy conservation system for the Yangtze Delta region (grant 2006BAJ01A05) and the State Key Laboratory of Pollution Control and Resource Reuse Foundation of China (grant PCRRF09010)

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Received November 22, 2010; accepted April 14, 2011

Jun Wang, PhD, is doctoral student. Xu Zhang, PhD, is Professor.

Jun Wang * and Xu Zhang

Institute of HVAC & Gas Engineering, School of Mechanical Engineering, Tongji University, 4800, Caoan Road, Shanghai 201804, China

* Corresponding author e-mail: wangjunhvac@163.com
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Date:Jul 1, 2011
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