Co-flow jets application for occupant targeted ventilation: Focus on thermal comfort and air quality

Co-flow air jets can be used in occupant-targeted ventilation, such as personalized ventilation, stratum ventilation, aiming at improving air quality in the breathing zone of occupants. However, these have not been studied sufficiently. The velocity field and cleanness of co-flow jets comprising inner jet of clean air encircled by outer jet of room air were studied under isothermal conditions by physical measurements and CFD simulations at numerous combinations of initial diameter and velocity of the inner and outer jets appropriate for practical application. The co-flow jet had a lower axial velocity decay along the jet direction than that of the single jet. The magnitude (turbulence intensity) and frequency of the velocity fluctuations in co-flow jets were lower compared to those in a single jet. The results revealed that with regard to thermal comfort co-flow jets may provide more cooling compared to single jets, especially when supplied toward to the user. The level of cleanness depended on the initial velocity of the inner and outer jets as well as their diameters. The air on the jet axis of the co-flow was up to 40 % cleaner compared to the single jet. The increased area ratio of outer nozzle to inner nozzle from 1.0 to 3.0 improved the cleanness of the air by at least 10% to the distance from 8 to 14 initial diameters compared to a single jet. The co-flow jet with velocity ratio of 1.5 had longest concentration core length, Lu, (up to 6 initial diameters to the inner jet) with clean air. The results can be used for design of occupant targeted ventilation.