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Experimental study of the cross-infection risk due to the cross-flow of exhaled airflows and a plane jet with the protected occupied zone ventilation.

INTRODUCTION

In modern society, people spend more than 90% of daily time indoors, where cross infection may occur due to exposure to various infectious sources in a ventilated space. Some studies show that the performance of different ventilation methods to reduce the exposure risk of airborne cross-infections depends on various factors in ventilated room, such as internal heat loads of the room, the distance between occupants and even the types of air distributions systems and the location of air inlet and exhaust outlet (Bjorn and Nielsen 2002, Nielsen 2009, Olmedo et al. 2012). The interaction between occupants and the indoor airflow is rather complex with the presence of thermal plume around the human body and the breathing processes. For example, one study found the exhalation flow from a person is able to penetrate the natural convection boundary layer and the breathing zone of another person standing nearby. This penetration is a function of the distance between the person and it exists up to a distance of 1.2 m (3 feet 11.24 in) (Nielsen 2008). As matter as a fact, the boundary layer flow around the body may protect the occupant from the pollution source to some extent. But the exhalation jet of one person is able to penetrate the breathing zone of another person standing nearby.

Air curtains are used to eliminate airflows through doorways (doorway tightness), which are affected by jet discharge momentum flux and nozzle design parameters. The tightness of an upwards blowing air curtain is defined in Valkeapaa & Siren, 2011. One recent study shows that a room may be separated by a plane jet, which prevent the transmission of pollutants from one zone to the other (Cao et al. 2014). This study found that the protection efficiency increases as the increasing of supply air velocity. When the jet velocity is very low and the jet discharge momentum is too weak, a breakthrough phenomenon occurs. There is still a lack of knowledge to predict the critical supply air velocity.

The main objective of this study is to quantify the interaction of breathing airflow with downward plane jet in a room with protected occupied zone ventilation (POV). The finding in this study will help designers in the design phase of a POV system to reduce the risk of airborne cross-infection between people in a room. Experimental measurements are carried out in a full scale office climate with two breathing thermal manikins (BTM).

EXPERIMENT SET UP

Climate chamber

The climate chamber used in this study is an experimental room with surfaces that can change thermal conditions according to the ambient conditions. The test room was inside the chamber has a size of height, width, length equal to 5.20 m (17 feet 0.72 in) (length) x 2.00 m (6 feet 6.74 in) (width) x 2.50 (8 feet 2.42 in) m (height) (as shown in Figure 1). Figure 2 shows the sketch of the test room and the locations of the breathing thermal manikin and air diffusers.

Measurement conditions

The measurement of air velocity was taken at a height of 1.5 m (4 feet 11.05 in) between two breathing manikin. Velocity is measured at 10 Hz with a hot sphere anemometer to observe the fluctuation of breathing phenomenon, which normally has a value of 16 times per minutes. The maximum velocity occurs about every 4 seconds. The supply air temperature is set 20.5 [+ or -] 0.5[degrees]C (68.9 [+ or -] 0.9[degrees]F). The room temperature is set as 24 [+ or -] 0.5[degrees]C (75.2 [+ or -] 0.9[degrees]F), which is recommended by ISO 7730. Heat source power is always 142 W inside the room, which generated by the two breathing thermal manikins. The distribution of measurement point is shown in Figure 2. The points marked with "O" represent the positions of the air velocity sensors. The maximum velocity of the plane jet was measured in the room with one or two breathing thermal manikin.

Breathing thermal manikins

Two breathing thermal manikins (BTM) are used in the experiment to simulate the source person and target person. Manikin is a model of a 168 cm (5 feet 6.14 in) tall female person. It is constructed of aluminum and has bendable knees and waist, so it can be set to various positions (sitting, standing, etc.). Its photograph and dimensions is shown in Figure 3.

Artificial lungs are connected to the manikin to simulate breathing functions. The volume of breathing air is kept 8.8 L/minutes (2.32 gal/min) for each manikin. The exhalation is simulated by one piston, which pumps air out via the mouth. The volume of exhaled air can be regulated by changing travel of the piston. An engine moves the piston at certain frequency, that can be controlled as well. Before entering the manikin, exhalation air is heated. This is done with the use of simple electric heater. Power can be regulated on a transformer. There are three heat sources in the manikin. One is the inside heater, which can be regulated by a transformer and the heat load can be set up to 145 W (495 BTU/hr). In this study, the power is set 30 W (102 BTU/hr). Another source is consisting two circulating fans, by which to distribute air around the manikin, thus creating an uniform distribution of temperature on manikin's surface. However, the fans also create a heat load, which in this case equals to 30 W (102 BTU/hr). The last heat source is the breathing air, which is heated up before the exhalation. In fact, the exhalation air is heated before entering the manikin.

RESULTS

Visualization of the cross-infection risk between two persons

As the cross-infection risk will be very high when the distance between two manikins gets as close as 0.35 m (1 feet 1.78 in), this section presents the visualization results of the cross-infection risk between two persons with POV. Figure 4 shows that when there is no downward plane jet between the two breathing thermal manikins, the exhaled air from the source manikin can easily approach the breathing zone of the target manikin. The exposure risk of the target manikin to the source manikin will be substantially high. When the velocity of the downward plane jet is 2.2 m/s (7.22 fps), the exhaled the airflow can only penetrate partly the downward plane jet. The exhaled airflow bends to the lower part of the target manikin, which lowers the risk of the cross infection between source manikin and target manikin. The risk of indirect cross infection might rise due to thermal boundary layer of the target manikin, which can entrain the polluted air from the lower part of the room. When the supply velocity is increased up to 3.0 m/s (9.84 fps), the exhaled airflow cannot penetrate the download plane jet anymore. So, two supply air velocities, 3.0 (9.84 fps) and 4.0 m/s (13.12 fps) were used in the measurement to avoid the possible penetration.

Instantaneous velocity

Instantaneous velocity at point Ps. Figure 5 shows the instantaneous velocity at point Ps for different cases. Without the downward jet, the maximum velocity of the exhaled airflow of source manikin is 2.8+0.2 m/s (9.19 [+ or -] 0.65 fps) (see Figure 5 a). With downward plane jet, the maximum velocity of the exhaled airflow of source manikin varies depending on the jet velocity. With a jet velocity of 3.0 m/s (9.84 fps), the maximum exhalation velocity decreases to 2.5+0.1 m/s (8.2 [+ or -] 0.33 fps) (see Figure 5 b). While with a jet velocity of 4.0 m/s (13.12 fps), the maximum exhalation velocity increases to 3.3 [+ or -] 0.2 m/s (10.83 [+ or -] 0.65 fps) (see Figure 5 c). The maximum velocity does not change so much when two breathing thermal manikins are employed.

Instantaneous velocity at point Pc. Figure 6 shows the instantaneous velocity at point Pc in different cases. Without downward jet, the maximum velocity of the exhaled airflow of source manikin is 0.6+0.2 m/s (1.97 [+ or -] 0.65 fps) (see Figure 6 a). With downward plane jet, the maximum velocity of the exhaled airflow of source manikin varies depending on the jet velocity. With a jet velocity of 3.0 m/s (9.84 fps), the maximum exhalation velocity decreases to 0.7 [+ or -] 0.2 m/s (2.30 [+ or -] 0.65 fps) (see Figure 6 b). While with a jet velocity of 4.0 m/s (13.12 fps), the maximum exhalation velocity increases to 1.0 [+ or -] 0.2 m/s (3.28 [+ or -] 0.65 fps) (see Figure 6 c). The maximum velocity does not change so much when two breathing thermal manikins are employed.

Instantaneous velocity at point Pt. Figure 7 shows the instantaneous velocity at point Pc in different cases. Without downward jet, the maximum velocity of the exhaled airflow of source manikin is 0.5 [+ or -] 0.2 m/s (1.64 [+ or -] 0.65 fps) (see Figure 7 a). With downward plane jet, the maximum velocity of the exhaled airflow of source manikin varies depending on the jet velocity. With a jet velocity of 3.0 m/s (9.84 fps), the maximum exhalation velocity decreases to 0.3 [+ or -] 0.2 m/s (0.98 [+ or -] 0.65 fps) (see Figure 7 b). While with a supply velocity of 4.0 m/s (13.12 fps), the maximum exhalation velocity increases to 0.4 [+ or -] 0.1 m/s (1.31 [+ or -] 0.65 fps) (see Figure 7 c). The maximum velocity increases up to 1.0 [+ or -] 0.2 m/s (3.28 [+ or -] 0.65 fps) when two breathing thermal manikins are employed with a supply velocity of 3.0 m/s (9.84 fps). When increasing the supply velocity to 4.0 m/s (13.12 fps), the maximum velocity at [P.sub.t] reaches even 2.0 m/s (6.56 fps).

DISCUSSION

The impingement of the exhalation jet and the downward plane jet is similar to a cross-flow to two constant jets. Penetration of the exhalation airflow through the plane jet might be modelled as the constant flow field of a round jet in a cross-flow Muppidi and Mahesh(2005):

r = [{[[rho].sub.j][u.sup.2.sub.j]/[[rho].sub.cf][u.sup.2.sub.cf]}.sup.1/2] (1)

which simplifies to r = [u.sub.j]/[u.sub.cf] for constant-density flows. Here, [u.sub.j] is the jet velocity, [u.sub.cf] is the velocity of the crossflow, [[??].sub.j] is the density of the jet fluid and [[??].sub.cf] is the crossflow fluid density.

The flow field of a jet in crossflow is believed to be influenced primarily by the effective velocity ratio. Broadwell & Breidenthal (1984) use a similarity theory to treat the jet exit as a point source of momentum. They conclude that the global length scale in the flow is rd in the region away from the jet exit. This length scale is used to scale the trajectory as

y/rd = A[(x/rd).sup.B] (2)

where A and B are constants. Pratte and Baines (1967) obtain A=2.05 and B =0.28.

Assuming the cross-flow of the exhalation jet and downward jet has a steady state velocity field at one moment. The maximum velocity of the two jets may be superimposed. Figure 8 shows the simplified schematic velocity field of the exhalation jet and downward plane jet.

Assume the velocity at the center point of the cross-flow of the exhalation airflow and the downward plane jet define is the tangential velocity of the trajectory of the bending exhalation jet flow, then the velocity may be expressed as:

[u.sub.p] = [square root of [u.sub.x.sup.2] + [u.sub.y.sup.2]] (3)

Furthermore the concentration in the exhaled jet will be deluded due to the mixing process in the two jets.

CONCLUSION

The micro-environment of the breathing thermal manikin will be determined by the interaction of room airflow distribution and the exhalation airflow. Likewise, the transmission of exhaled gaseous infectious pollutants from one person to the other is also affected by the interaction. The risk of cross infection between two persons becomes very high when the distance between two persons is as close as 0.35 m (1 feet 1.78 in) in a room without a downward plane jet. The exhaled airflow from a source manikin can directly reach the breathing zone of the target manikin. In this study, the promising results show that the downward plane jet of POV system can break the exhalation airflow from the source manikin. With the downward plane jet, the personal exposure risk from the exhalation airflow is significantly reduced. The supply air velocity of the plane jet plays a critical role in reducing the personal exposure. The supply air velocity higher than 3.0 m/s (9.84 fps) is able to break the exhaled airflow.

The impingement of the exhalation jet and the downward plane jet is similar to a cross-flow to two constant jets. However, the exhalation jet is a sinusoid kind of fluctuating airflow, which interacts with the downward plane jet in a different way. The bending trajectory of the exhalation jet can be modelled by the semi-empirical model, which can be used to calculate the location of the maximum velocity of the center line of the exhalation jet. A more comprehensive study is needed to extend the understanding and knowledge of the simultaneous airflow pattern during the penetration processes of the exhaled airflow and the downward plane jet in POV ventilation system.

NOMENCLATURE

u = velocity
r = radius in the cross-flow

Subscripts

c = cross/center
f = flow
j = jet
s = source
t = target
x = downward vertical direction of the plane jet
y = horizontal direction of the exhalation airflow


ACKNOWLEDGMENTS

The authors wish to thank the financial support of the Academy of Finland and VTT Technical Research Centre of Finland in the POWER-PAD project (NO. 252708).

REFERENCES

Bjorn, E., and P.V. Nielsen. 2002. Dispersal of exhaled air and personal exposure in displacement ventilated rooms. Indoor Air, 12(2): 147-164.

Broadwell, J.E., and R.E. Breidenthal. 1984. Structure and mixing of a transverse jet in incompressible flow. Journal of Fluid Mechenics, 148: 405-412.

Cao, G.Y., Siren, K., and S. Kilpelainen. 2014. Modelling and experimental study of performance of the protected occupied zone ventilation, Energy and Buildings, 68: 515-531.

Gupta, J.K., Lin, C.H., and Q. Chen. 2010. Characterizing exhaled airflow from breathing and talking. Indoor Air, 20: 31-39.

Muppidi, S., and K. Mahesh. 2005. Study of trajectories of jets in cross flow using direct numerical simulations. Journal of Fluid Mech. 530: 81-100.

Nielsen, P., Buus, M., Winther, F.V., and M. Thilageswaran. 2008. Contaminant flow in the microenvironment between people under different ventilation conditions. ASHRAE Transactions, SL -08-064: 632-638

Nielsen P.V., Olmedo, I., de Adana, M.R., Grzelecki, P., and R.L., Jensen. 2012. Airborne cross-infection risk between two people standing in surroundings with a vertical temperature gradient. HVAC&R Research, 18(4):1-10.

Nielsen, P.V. 2009. Control of airborne infectious diseases in ventilated spaces, J.R. Soc. Interface, 6: 747-755.

Olmedo, I., Nielsen, P.V., de Adana, M.R., Jensen, R.L., and P. Grzelecki. 2012. Distribution of exhaled contaminants and personal exposure in a room using three different air distribution strategies. Indoor Air, 22: 64-76.

Pratte, B.D. and W.D. Baines. 1967. Profiles of the round turbulent jet in a cross flow. Journal of Hydraulics Division, ASCE 92 (HY6), 53-64.

Valkeapaa, A., and K. Siren. 2010. The influence of air circulation, jet discharge momentum flux and nozzle design parameters on the tightness of an upwards blowing air curtain. International Journal of Ventilation, 8(4): 337-346.

Guangyu Cao, PhD

Member ASHRAE

Peter V. Nielsen, PhD

Fellow ASHRAE

Chunwen Xu

Rasmus Lund Jensen, PhD

Guangyu Cao is a Senior Scientist at VTT Technical Research Centre of Finland, Espoo, Finland. Peter V. Nielsen is a professor in the Department of Civil Engineering, Aalborg University, Aalborg, Denmark. Chunwen Xu is a PhD research in College of Civil Engineering, Hunan University, Changsha, China. Rasmus Lund Jensen is an associate professor in the Department of Civil Engineering, Aalborg University, Aalborg, Denmark.
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Author:Cao, Guangyu; Nielsen, Peter V.; Xu, Chunwen; Jensen, Rasmus Lund
Publication:ASHRAE Transactions
Article Type:Report
Date:Jul 1, 2014
Words:2752
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