Abstract
In an open air system where outside air is supplied and room air is exhausted, methods to measure the age of air distribution using tracer gas have been established. However, in air recirculating systems where exhausted air is re-supplied back into the room, no tracer gas technique for evaluating fresh air distribution effectiveness and contaminated air removal efficiency has been developed. Here, we carried out a theoretical study on the tracer gas experimental method for measuring the age of air distribution in an air recirculating system by applying the concept of dynamic steady-state concentration. Furthermore, in order to determine key issues faced when applying this theoretical method to perform accurate measurements of the ventilation efficiency of air recirculating systems, case studies were carried out using CFD. In this paper, we theoretically considered the relationship between the concept of dynamic steady-state concentration in the air recirculation system and the age of air distribution and studied the effect of leakage when measuring the age of air distribution in an actual space. When the source was located in contact with the supply outlet, the concentration increase rate at each point in the air recirculation system without air leakage was constant, which was consistent with the increase rate of the average concentration. On the other hand, the ratio of the concentration increase rate at each point to the average concentration increase rate when there was air leakage became constant after 2 to 3 times the nominal time constant of the recirculating ventilation rate to the room volume. The results confirmed that the smaller the leakage is, the closer the value of the rate at each point is to 1.
