Development of self-label method to distinguish supply air from room air without tracer in numerical simulations

Traditionally, the tracer method has been used to distinguish the supply air and room air in buildings by labeling the target air with a tracer. However, due to difference in the physical properties of the target air and the tracer, use of the tracer method inevitably produces tracer error. Furthermore, due to the lack of an exact comparative reference, it is difficult to determine which tracer is the best to represent the target air distribution. In this study, a method (i.e. self-label method) which can distinguish supply air from ambient air without tracer in numerical simulations has been developed. In this paper, the principle, governing equations, accuracy, and applications of the self-label method are presented. Further, due to its simple and typical mixing characteristics of supply air and ambient air, a jet has been used to validate the numerical model by comparing the numerical results with the classical theoretical and experimental results in the earlier studies. The validity of the self-label method has been verified by comparing the flow rate and the velocity field of the jet with those by the single air approach. The tracer gas error has been compared with the exact reference obtained by self-label method. The results show that the tracer gas error increases with increasing tracer concentration or decreasing air velocity under isothermal conditions. Finally, the dynamic mechanism of the tracer gas error has been analyzed. It shows that an additional concentration buoyancy force can occur, which can cause tracer gas error under isothermal conditions.