NEW DEVELOPMENT OF A STRAIGHTFORWARD METHOD OF ESTIMATING AGE-OF-AIR USING CFD

Ventilation efficiency is important in agricultural facilities such as greenhouse and livestock house to keep internally optimum condition because it relates to the controlling of proper environmental conditions. Age-of-air concept, such as LMA and LMR, allows assessing the ventilation efficiency of an agricultural facility and most of these methods use a tracer gas method. The tracer gas experimental approaches have some difficulties such as control of the unstable gas, many experiments for verification at many points. Therefore, the aim of this study is to develop a straightforward method to calculate LMA and LMR values with CFD steady-state simulation which has the advantage of saving computational time and resources.
This study suggested the simple and economic method of solving transport equation of time scalar to calculate LMA and LMR. The main idea of LMA computa¬tion is to solve the passive scalar transport equation with the assumption that the production of the time scalar throughout the room is uniform (Sandberg and Sjoberg, 1983). Diffusivity of the time scalar was controlled by user-defined function (UDF). In case of LMR calculation, the transport of the time scalar was reversed compulsively by using UDF.
The methodology to validate the results of this study was established by comparing with previous research; Kwon (2011) that had performed a computing LMA and LMR value by laboratory experiments and CFD simulations using tracer gas. As a result the presented error was almost 1.1%. And in terms of R-square, the LMA and LMR values showed good agreements with results of Kwon et al. (2011) while the computational time was reduced to almost one fourth of that by Kwon et al. (2011).