scope:

INTRODUCTION

The aim of personalized ventilation (PV) is to supply clean air to the breathing zone of each room occupant. Together with total volume ventilation, PV can provide superior air quality and can greatly reduce the risk of cross-infection for occupants who spend a relatively long time at their workplace (Cermak and Melikov 2007). Individual control of the flow rate, temperature and direction of the supplied personalized air makes it possible to achieve a preferred microenvironment for each occupant. It has been documented that PV can significantly improve occupants' inhaled air quality and thermal comfort and can significantly decrease SBS symptoms (Kaczmarczyk et al. 2004, 2006).

The performance of PV with regard to occupants' thermal comfort and inhaled air quality depends on the interaction of the flows in the vicinity of the human body, in most cases the personalized airflow, the free convection flow around the human body, the airflow generated by the background total volume ventilation and the flow of exhaled air.

The personalized flow is typically a free jet issued from a circular or rectangular opening or a nozzle. The first region of the jet, known as the potential core region, contains a core with constant velocity, low turbulence intensity and supply air unmixed with the polluted room air. A non-uniform velocity field at the air supply and a high initial turbulence intensity that generates velocity fluctuations increase the mixing of the supplied clean air with the polluted surrounding room air and decrease the length of the potential core (Melikov 2004).

The free convection flow is generated by the difference between the room air temperature and the surface temperature of the human body. The greater the temperature difference, the stronger the free convection flow. The free convection flow develops from laminar, with low velocity at the lower legs, to turbulent, with relatively high velocity at the upper chest and the head region (Clark and Toy 1975). Body shape and posture, room air temperature, clothing insulation, etc. define the mean velocity in the free convection flow, which may be as high as 0.25 m/s (49.21 fpm) at the head region, and the thickness of the boundary layer, which may measure 0.2 m (0.66 ft) or more (Homma and Yakiyama 1988). This flow induces and transports air (as well as pollutants if present) from lower heights in the room to the breathing zone. Therefore the greater portion of the air that is inhaled by sedentary and standing persons in rooms is from the free convection flow (Brohus and Nielsen 1994).

The background airflow is influenced by the location and type of air supply devices, supply airflow rate and temperature, type and location of heat sources, etc. The flow of exhalation depends on the breathing mode (nose/mouth, mouth/ nose, etc.), respiration flow rate (which depends on activity level, body weight), nose and mouth shape (different from person to person), body and head posture, etc.

The interaction of the background flow with the free convection flow is important for the heat loss from the human body. In order to avoid draught discomfort, present indoor climate standards (ISO 7730 2004, ASHRAE 55 2004) recommend low velocity (less than 0.2 m/s (39.37 fpm)) in the occupied zone at the low range of comfortable room air temperature (20 °C (68 oF) - 24 °C (75.2 oF)). Under these conditions, the strength of the free convection flow may be equal to or even stronger than the strength of the background flow. The thermal plume generated above the human body by the free convection flow affects the room air distribution (Homma and Yakiyama 1988, Zukowska et al. 2008). The interaction of the personalized flow with the flow of exhalation determines the spread of bioeffluents and exhaled air between room occupants (Cermak and Melikov 2007).

This study focuses on the performance of PV with regard to inhaled air quality. In this respect the interaction between the free convection flow and the personalized flow is of major importance. The interaction depends on many factors: strength of free convection flow and thickness of its boundary layer, velocity, turbulence intensity, direction and temperature of PV flow, body posture, shape and clothing design, etc. It has been documented that personalized flow directed against the face with a mean velocity higher than 0.3-0.35 m/s (59.06- 68.89 fpm) is able to penetrate the free convection flow and provide 100% clean air. This however, may pose draught discomfort, especially at a relatively low room air temperature (Melikov 2004). The risk of draught will decrease when the velocity of the personalized flow decreases, i.e decrease of the personalized flow rate when the air supply terminal device is not changed. This strategy will require a decrease of the strength and the thickness of the free convection flow at the breathing zone to enable its penetration by the personalized flow and to supply clean air for inhalation. However, methods for control of the free convection flow have not yet been developed or studied.

Two methods, passive and active, for controlling the free convection flow at the breathing zone were developed. The effect of these methods on the interaction of the personalized flow with the free convection flow and the resulting improvement of inhaled air quality was studied. The results are presented and discussed in this paper.