A theoretical model to calculate the distribution of air age in general ventilation system

Abstract

Air age is a widely used concept to evaluate indoor air quality. Available models of air age distribution apply only to simple ventilation systems with one room or one air handling unit (AHU). Furthermore, these models would be computationally intensive for complex ventilation systems since a large number of computational fluid dynamics (CFD) simulations or iterations would be required. This study presents a theoretical model which is capable of calculating the air age distribution in a wide variety of ventilation systems with recirculation. The model was validated by combining tracer gas experiments and CFD simulation and demonstrated by a case study using a recirculation ventilation system with multiple AHUs and rooms. A comparison between the model and the conventional iterative method indicated that for complex ventilation systems the model has a potential to significantly reduce the CPU-time requirements and allows additional new scenarios to be tested without further CFD simulations. Practical applications: The theoretical model presented can be used to obtain the distribution of air age in a wide variety of ventilation systems with recirculation and much more computationally efficient than the conventional iterative method. The model can be a useful tool to evaluate and improve a wide range of indoor environments, which are ventilated by a variety of practical ventilation systems that utilise fresh and return air mixing.