Indoor Air 2014 wileyonlinelibrary.com/journal/ina Printed in Singapore. All rights reserved
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd INDOOR AIR doi:10.1111/ina.12120
Human convective boundary layer and its interaction with room ventilation flow Abstract This study investigates the interaction between the human convective boundary layer (CBL) and uniform airflow with different velocity and from different directions. Human body is resembled by a thermal manikin with complex body shape and surface temperature distribution as the skin temperature of an average person. Particle image velocimetry (PIV) and pseudocolor visualization (PCV) are applied to identify the flow around the manikin’s body. The findings show that the direction and magnitude of the surrounding airflows considerably influence the airflow distribution around the human body. Downward flow with velocity of 0.175 m/s does not influence the convective flow in the breathing zone, while flow at 0.30 m/s collides with the CBL at the nose level reducing the peak velocity from 0.185 to 0.10 m/s. Transverse horizontal flow disturbs the CBL at the breathing zone even at 0.175 m/s. A sitting manikin exposed to airflow from below with velocity of 0.30 and 0.425 m/s assisting the CBL reduces the peak velocity in the breathing zone and changes the flow pattern around the body, compared to the assisting flow of 0.175 m/s or quiescent conditions. In this case, the airflow interaction is strongly affected by the presence of the chair.
D. Licina1,2, A. Melikov1, C. Sekhar2, K. W. Tham2 1 School of Design and Environment, National University of Singapore, Singapore, 2Civil Engineering, Technical University of Denmark, International Centre for Indoor Environment and Energy, Lyngby, Denmark
Key words: Convective boundary layer; Thermal manikin; Particle image velocimetry; Pseudocolor visualization; Ventilation flow; Airflow interaction.
D. Licina School of Design and Environment National University of Singapore – Building 4, Architecture Drive Singapore City 117566, Singapore Tel.: +45 5029 7735 Fax: +45 4593 2166 e-mail: [email protected] Received for review 12 February 2014. Accepted for publication 10 April 2014.
Practical Implications
Interaction of the human convective boundary layer flow with the surrounding flows modifies the airflow around the human body, affects spread of pollution, personal exposure and convection heat transfer from the body. Understanding the nature of this interaction can be used for better distribution of ventilation air and improving occupants’ thermal comfort.
Introduction
Understanding the air movement in the indoor environment is important as people spend 80–90% of their time indoors (Spengler and Sexton, 1983). One of the most common ways to reduce human exposure to airborne pollutants is to use mechanical ventilation. In current room air distribution design practice, airflows induced by the building occupants are not taken into account resulting in inaccurate prediction of the personal exposure. Several recent studies have challenged existing ventilation standards by showing that increase in the air change rate can lead to the exposure increase (Bolashikov et al., 2012; Pantelic and Tham, 2013; Popioek et al., 2012). These studies emphasize the
importance of understanding the complex interactions between airflows generated by the ventilation system and buoyant airflows induced by human body heat. A healthy human body dissipates substantial part of metabolic heat by means of convection (Murakami et al., 2000; Zukowska et al., 2012). The convective heat loss, caused by the temperature gradient between a human body surface and cooler surrounding air, induces upward natural flow of the surrounding air, creating a convective boundary layer (CBL) around the human body. The CBL further rises above the head developing into a human thermal plume. Understanding the physics of the human CBL is essential to both indoor air quality and thermal comfort, because it has the ability to transport potentially contaminated air to 1
Licina et al. the breathing zone and influences the body heat release, respectively. Several studies in the past investigated the CBL in a quiescent indoor environment. Lewis et al. (1969) found that a nude standing man induces a buoyant flow which remains laminar at the lower leg region and develops into a fully turbulent flow at about 1.5 m from the floor. Homma and Yakiyama (1988) revealed that the CBL has a thickness of 150 mm at the head level with a mean velocity up to 0.25 m/s. These studies as well as several other studies performed in a quiescent environment (Clark and Toy, 1975; Craven and Settles, 2006; Licina et al., 2014) provide a basic knowledge of the airflow behavior in the vicinity of a human body. Air motion in mechanically ventilated spaces is highly unpredictable and influenced by momentum flux at the supply terminal, type and location of supply and exhaust terminals, room geometry, movement of the occupants, obstacles and furniture, and thermal loads in the room (Etheridge and Sandberg, 1996). These suggest that mechanical ventilation will generate different airflow patterns in every room, hence any generalization is challenging. In practice, human CBL interacts with the airflow in the occupied spaces. To understand the phenomenon, a simplified approach is needed which is based on the physics of the interaction between the CBL and the surrounding airflows. In such an approach, the interacting flows can be assisting, opposing, or transverse to each other. So far, little is known about the nature of the interaction between these flows. The study that used a combination of two personalized ventilation systems coupled with mixing or displacement ventilation highlighted the importance of airflow interaction around the human body on inhaled air quality (Melikov et al., 2003). Interaction between the CBL and surrounding airflows is largely influenced by the characteristics of the invading flow generated by the air delivery systems, such as velocity, direction, and turbulence intensity. These factors will determine the extent of disturbance that invading flow exerts over the human natural convection flow. Interaction between the CBL and locally supplied personalized ventilation airflow from the front (Bolashikov et al., 2011a), assisting from below (Bolashikov et al., 2011b), and opposing from above (Yang et al., 2009) has been studied and reported. An airflow of 6 l/s supplied from the front was able to penetrate the CBL, while personalized jet at the lower flow rate was deflected upwards by the CBL without reaching the breathing zone. In the study with two assisting confluent jets, increased airflow from below led to an increase in the velocity in the breathing zone. The velocity increase from 0.17 to 0.34 m/s was recorded across the mouth, when the airflow of 4 l/s increased to 10 l/s. These studies investigated only the local airflow interaction in the breathing zone and did not focus on how airflow patterns in the 2
occupied spaces globally affect the development of the CBL. Melikov and Zhou (1996) studied airflow interaction between the human free convection flow and uniform horizontal airflow from behind. Results indicated that the convection flow with a maximum velocity of 0.13 m/s measured at the neck of a seated manikin was penetrated by invading flow with the velocity of 0.1 m/s, resulting in 4°C lower air temperature near the skin. Similar study performed in a low-speed uniform environment revealed that heated and non-heated manikin creates very distinctive airflow patterns with respect to the free air stream (Johnson et al., 1996). Both studies focused only on the interaction between the human convection flow and the airflow approaching from behind the manikin. In another, low-speed wind tunnel study, it was found that the human metabolic heat considerably alters the airflow patterns on the downwind side of the child-sized manikin and that increased wind velocity reduces the importance of the human convection flow (Heist et al., 2003). As seen, most of the measurements on the interaction between airflows induced by the human body heat and the surrounding airflows have been carried out in the wind tunnel that gives limited body orientation with respect to the free stream or localized invading flows with relatively small cross-section area. Neither of the previous studies considered interaction between the entire CBL and assisting flow from below nor opposing flow from above. Previous experiments have also shown that increase in the free stream velocity decreases the relative importance of the buoyancy effect produced by the human body. The widely adopted velocity at which invading flow starts to disturb the CBL is 0.1 m/s or above (Bjørn and Nielsen, 2002; Melikov and Zhou, 1996). However, these findings apply only to disturbance due to horizontal airflow or due to a walking person. None of the studies in the past considered the velocity magnitude at which opposing and assisting airflows create disturbance to the CBL. Finally, most of the aforementioned studies had been performed with point-wise measurement technique, which may not be sufficient to fully understand the nature of air movement near the human body. This paper presents findings from an experimental investigation of the CBL and its interaction with opposing flow from above, transverse flow from front, and assisting flow from below. Measuring techniques employed for this study consist of particle image velocimetry (PIV) complemented with pseudocolor visualization (PCV) technique, which have been shown to provide a good synergy between quantitative and qualitative airflow characteristics and can be adequately employed for the CBL investigation (Licina et al., 2014). Findings of this study contribute to the fundamental knowledge of how the human CBL interacts with the airflow in the occupied spaces of the room.
Human CBL and its interaction with ventilation flow Methods Experimental facility
Measurements were performed in an environmental chamber: 11.1 9 8 9 2.6 m (L 9 W 9 H) equipped with displacement ventilation. A thermal manikin was positioned in the center of the chamber. Air from a dedicated air-handling unit was supplied via six lowmomentum floor standing diffusers and exhausted through six ceiling mounted grills. All supply and exhaust devices were located at far enough distance from the thermal manikin to eliminate interference between the supply airflow and the CBL, as shown in Figure 1 (left). Furthermore, supply diffusers were sealed on the side facing the manikin to prevent any such interference. To minimize radiant heat exchange with the surroundings, one external wall was insulated and heat sources from the lighting fixtures were eliminated. The other three walls and the floor were adiabatic, while the ceiling was suspended below an insulated roof of the building. Experimental equipment
Airflow generator. A specially designed rectangular box with dimensions 1.8 9 1 9 0.2 m (L 9 W 9 H) was used to generate a uniform forced convection flow around the thermal manikin. The top plate of the box was perforated with 18 000 holes of 5 mm diameter. The box contained 66 DC fans that were controlled with a fine-tuning frequency regulator capable of providing a velocity range from 0 to 1 m/s. The amount of heat added to the airstream by the fans was negligible. The uniformity test was performed at 0.7 and 1 m distance above the perforated plate to assure that the velocity remains constant at both distances. Test was performed in accordance with the standard (ASHRAE, 2012) that uses the coefficient of variation as an
indicator of the flow uniformity. Results obtained with omnidirectional thermal anemometers (Dantec Dynamics, Copenhagen, Denmark) with 0.02 m/s and 0.5 K accuracy showed that at both 0.7 and 1 m distances, the velocity remained constant with a low discrepancy (
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd INDOOR AIR doi:10.1111/ina.12120
Human convective boundary layer and its interaction with room ventilation flow Abstract This study investigates the interaction between the human convective boundary layer (CBL) and uniform airflow with different velocity and from different directions. Human body is resembled by a thermal manikin with complex body shape and surface temperature distribution as the skin temperature of an average person. Particle image velocimetry (PIV) and pseudocolor visualization (PCV) are applied to identify the flow around the manikin’s body. The findings show that the direction and magnitude of the surrounding airflows considerably influence the airflow distribution around the human body. Downward flow with velocity of 0.175 m/s does not influence the convective flow in the breathing zone, while flow at 0.30 m/s collides with the CBL at the nose level reducing the peak velocity from 0.185 to 0.10 m/s. Transverse horizontal flow disturbs the CBL at the breathing zone even at 0.175 m/s. A sitting manikin exposed to airflow from below with velocity of 0.30 and 0.425 m/s assisting the CBL reduces the peak velocity in the breathing zone and changes the flow pattern around the body, compared to the assisting flow of 0.175 m/s or quiescent conditions. In this case, the airflow interaction is strongly affected by the presence of the chair.
D. Licina1,2, A. Melikov1, C. Sekhar2, K. W. Tham2 1 School of Design and Environment, National University of Singapore, Singapore, 2Civil Engineering, Technical University of Denmark, International Centre for Indoor Environment and Energy, Lyngby, Denmark
Key words: Convective boundary layer; Thermal manikin; Particle image velocimetry; Pseudocolor visualization; Ventilation flow; Airflow interaction.
D. Licina School of Design and Environment National University of Singapore – Building 4, Architecture Drive Singapore City 117566, Singapore Tel.: +45 5029 7735 Fax: +45 4593 2166 e-mail: [email protected] Received for review 12 February 2014. Accepted for publication 10 April 2014.
Practical Implications
Interaction of the human convective boundary layer flow with the surrounding flows modifies the airflow around the human body, affects spread of pollution, personal exposure and convection heat transfer from the body. Understanding the nature of this interaction can be used for better distribution of ventilation air and improving occupants’ thermal comfort.
Introduction
Understanding the air movement in the indoor environment is important as people spend 80–90% of their time indoors (Spengler and Sexton, 1983). One of the most common ways to reduce human exposure to airborne pollutants is to use mechanical ventilation. In current room air distribution design practice, airflows induced by the building occupants are not taken into account resulting in inaccurate prediction of the personal exposure. Several recent studies have challenged existing ventilation standards by showing that increase in the air change rate can lead to the exposure increase (Bolashikov et al., 2012; Pantelic and Tham, 2013; Popioek et al., 2012). These studies emphasize the
importance of understanding the complex interactions between airflows generated by the ventilation system and buoyant airflows induced by human body heat. A healthy human body dissipates substantial part of metabolic heat by means of convection (Murakami et al., 2000; Zukowska et al., 2012). The convective heat loss, caused by the temperature gradient between a human body surface and cooler surrounding air, induces upward natural flow of the surrounding air, creating a convective boundary layer (CBL) around the human body. The CBL further rises above the head developing into a human thermal plume. Understanding the physics of the human CBL is essential to both indoor air quality and thermal comfort, because it has the ability to transport potentially contaminated air to 1
Licina et al. the breathing zone and influences the body heat release, respectively. Several studies in the past investigated the CBL in a quiescent indoor environment. Lewis et al. (1969) found that a nude standing man induces a buoyant flow which remains laminar at the lower leg region and develops into a fully turbulent flow at about 1.5 m from the floor. Homma and Yakiyama (1988) revealed that the CBL has a thickness of 150 mm at the head level with a mean velocity up to 0.25 m/s. These studies as well as several other studies performed in a quiescent environment (Clark and Toy, 1975; Craven and Settles, 2006; Licina et al., 2014) provide a basic knowledge of the airflow behavior in the vicinity of a human body. Air motion in mechanically ventilated spaces is highly unpredictable and influenced by momentum flux at the supply terminal, type and location of supply and exhaust terminals, room geometry, movement of the occupants, obstacles and furniture, and thermal loads in the room (Etheridge and Sandberg, 1996). These suggest that mechanical ventilation will generate different airflow patterns in every room, hence any generalization is challenging. In practice, human CBL interacts with the airflow in the occupied spaces. To understand the phenomenon, a simplified approach is needed which is based on the physics of the interaction between the CBL and the surrounding airflows. In such an approach, the interacting flows can be assisting, opposing, or transverse to each other. So far, little is known about the nature of the interaction between these flows. The study that used a combination of two personalized ventilation systems coupled with mixing or displacement ventilation highlighted the importance of airflow interaction around the human body on inhaled air quality (Melikov et al., 2003). Interaction between the CBL and surrounding airflows is largely influenced by the characteristics of the invading flow generated by the air delivery systems, such as velocity, direction, and turbulence intensity. These factors will determine the extent of disturbance that invading flow exerts over the human natural convection flow. Interaction between the CBL and locally supplied personalized ventilation airflow from the front (Bolashikov et al., 2011a), assisting from below (Bolashikov et al., 2011b), and opposing from above (Yang et al., 2009) has been studied and reported. An airflow of 6 l/s supplied from the front was able to penetrate the CBL, while personalized jet at the lower flow rate was deflected upwards by the CBL without reaching the breathing zone. In the study with two assisting confluent jets, increased airflow from below led to an increase in the velocity in the breathing zone. The velocity increase from 0.17 to 0.34 m/s was recorded across the mouth, when the airflow of 4 l/s increased to 10 l/s. These studies investigated only the local airflow interaction in the breathing zone and did not focus on how airflow patterns in the 2
occupied spaces globally affect the development of the CBL. Melikov and Zhou (1996) studied airflow interaction between the human free convection flow and uniform horizontal airflow from behind. Results indicated that the convection flow with a maximum velocity of 0.13 m/s measured at the neck of a seated manikin was penetrated by invading flow with the velocity of 0.1 m/s, resulting in 4°C lower air temperature near the skin. Similar study performed in a low-speed uniform environment revealed that heated and non-heated manikin creates very distinctive airflow patterns with respect to the free air stream (Johnson et al., 1996). Both studies focused only on the interaction between the human convection flow and the airflow approaching from behind the manikin. In another, low-speed wind tunnel study, it was found that the human metabolic heat considerably alters the airflow patterns on the downwind side of the child-sized manikin and that increased wind velocity reduces the importance of the human convection flow (Heist et al., 2003). As seen, most of the measurements on the interaction between airflows induced by the human body heat and the surrounding airflows have been carried out in the wind tunnel that gives limited body orientation with respect to the free stream or localized invading flows with relatively small cross-section area. Neither of the previous studies considered interaction between the entire CBL and assisting flow from below nor opposing flow from above. Previous experiments have also shown that increase in the free stream velocity decreases the relative importance of the buoyancy effect produced by the human body. The widely adopted velocity at which invading flow starts to disturb the CBL is 0.1 m/s or above (Bjørn and Nielsen, 2002; Melikov and Zhou, 1996). However, these findings apply only to disturbance due to horizontal airflow or due to a walking person. None of the studies in the past considered the velocity magnitude at which opposing and assisting airflows create disturbance to the CBL. Finally, most of the aforementioned studies had been performed with point-wise measurement technique, which may not be sufficient to fully understand the nature of air movement near the human body. This paper presents findings from an experimental investigation of the CBL and its interaction with opposing flow from above, transverse flow from front, and assisting flow from below. Measuring techniques employed for this study consist of particle image velocimetry (PIV) complemented with pseudocolor visualization (PCV) technique, which have been shown to provide a good synergy between quantitative and qualitative airflow characteristics and can be adequately employed for the CBL investigation (Licina et al., 2014). Findings of this study contribute to the fundamental knowledge of how the human CBL interacts with the airflow in the occupied spaces of the room.
Human CBL and its interaction with ventilation flow Methods Experimental facility
Measurements were performed in an environmental chamber: 11.1 9 8 9 2.6 m (L 9 W 9 H) equipped with displacement ventilation. A thermal manikin was positioned in the center of the chamber. Air from a dedicated air-handling unit was supplied via six lowmomentum floor standing diffusers and exhausted through six ceiling mounted grills. All supply and exhaust devices were located at far enough distance from the thermal manikin to eliminate interference between the supply airflow and the CBL, as shown in Figure 1 (left). Furthermore, supply diffusers were sealed on the side facing the manikin to prevent any such interference. To minimize radiant heat exchange with the surroundings, one external wall was insulated and heat sources from the lighting fixtures were eliminated. The other three walls and the floor were adiabatic, while the ceiling was suspended below an insulated roof of the building. Experimental equipment
Airflow generator. A specially designed rectangular box with dimensions 1.8 9 1 9 0.2 m (L 9 W 9 H) was used to generate a uniform forced convection flow around the thermal manikin. The top plate of the box was perforated with 18 000 holes of 5 mm diameter. The box contained 66 DC fans that were controlled with a fine-tuning frequency regulator capable of providing a velocity range from 0 to 1 m/s. The amount of heat added to the airstream by the fans was negligible. The uniformity test was performed at 0.7 and 1 m distance above the perforated plate to assure that the velocity remains constant at both distances. Test was performed in accordance with the standard (ASHRAE, 2012) that uses the coefficient of variation as an
indicator of the flow uniformity. Results obtained with omnidirectional thermal anemometers (Dantec Dynamics, Copenhagen, Denmark) with 0.02 m/s and 0.5 K accuracy showed that at both 0.7 and 1 m distances, the velocity remained constant with a low discrepancy (
