Prediction and Evaluation of Cleanliness Levels Inside a Mini-Environment by Measuring Mean Air-Age and Effective Flow Rate

Numerical and experimental studies on the evaluation and prediction of the cleanliness level in a mini-environment system were carried out. The new direct method for estimating the cleanliness of a mini-environment, which involves determining the mean air-age and effective flow rate, is developed and its performance compared with that of the previous performance index based on airflow pattern characteristics. The result shows that the airflow pattern analysis is limited in terms of estimating the real performance of the mini-environment. This is because airflow pattern characteristics, like eddy size or turbulence intensity, can qualitatively show the path of contaminants and the degree of risk, but cannot predict the quantitative variation in the contamination level according to the variation in factors such as the location of the air supply and the exhaust and ventilation rate. In contrast, the mean air-age is capable of predicting quantitative variations in contaminant concentration as well as the effect of the turbulence intensity. Also, computational fluid dynamic (CFD) calculations and experiments show that mean air-age can help predict the relative cleanliness level variation at points in the modified mini-environment when the layout of the mini-environment is changed. It is concluded that mean air-age is a more exact and effective performance index than the previous airflow pattern characteristics like the turbulence intensity or eddy size.