A simplified methodology for the prediction of mean air velocity and particle concentration in isolation rooms with downward ventilation systems
Introduction
Isolation rooms are widely used in clean production workshops, hospital operating rooms and clean wards. For these places, the indoor air flow and particle distribution should be designed carefully to maintain a clean indoor environment. Previous studies suggest that engineering simulations using computational fluid dynamics (CFD) are a valid method for investigating air flow behavior, temperature distribution and particle dispersion in different types of isolation rooms [1], [2], [3], [4].
However, CFD is sometimes time-consuming and relatively expensive, which limits its application in actual engineering applications. Although the cost of CFD is acceptable when studying a limited number of cases for academic research nowadays, a more simplified method is always welcomed for those engineers or designers who are not CFD experts. Besides, in most cases the engineers or designers only concern the mean velocity and particle concentration in the key indoor areas, for example, the occupied zone in isolation rooms. For this purpose, the detailed air flow and particle distribution given by CFD seems unnecessary in terms of deciding the primary schemes for actual application. Furthermore, the indoor environment controlling or decision-making requests the real-time monitoring of air velocity and particle concentration indoor. The CFD calculation does not meet the request as it is hard to response the input parameters (e.g., air flow rate variation, source generating rate variation, and so on) in a short time or real time. A simple methodology that can real time response to the input parameters is necessary for this purpose.
This study therefore presents a simple way to predict the mean air velocity and particle concentration in the occupied zone (Occupied zone is defined as the central part of the room in this study, i.e. up to a height of 2 m and 0.5 m away from each wall [5].) in isolation rooms with downward ventilation systems, as downward ventilation system is regarded to be effective for controlling indoor particles and that current guidelines recommend use of downward ventilation systems for different kinds of isolation rooms [6], [7], [8]. The simple methodology is based on a similarity theory analysis, by which the key similarity criteria are deduced. The correlating equation to calculate the mean air velocity and particle concentration is established by multiple linear regressions (MLR), which is based on the numerical test results obtained by CFD method. The equation correlates the mean air velocity and particle concentration with air supply volume rate, indoor particle generating rate and other parameters. It can be used to aid the engineers or designers to calculate the mean air velocity and particle concentration in a quick and simple way and achieve results with similar precision of CFD method, so as to avoid the complicated computation by CFD software.
Section snippets
Methodology
The methodology consists of three key parts: similarity analysis; numerical test and equation regression.
Verification of the equation
Some cases are studied to verify the simple equations. If the results calculated by using above equations agree with the results calculated by CFD, the simple method is regarded to be working well. Besides, the measured data from the experiment performed in a real isolation room is also employed to compare with the results calculated by the simple equation.
Three new cases out of the 128 cases were again simulated by CFD. Then the results of mean air velocity and particle concentration, together
Discussion
The simple methodology is efficient for predicting the mean air velocity and particle concentration in the studied type of isolation rooms. Zonal model is another kind of simple approach to calculate the velocity, temperature and concentration with few costs. The present methodology would be simpler, as it is a simple algebraic equation which only requires simple input parameters. In this way, the real-time monitoring of mean air velocity and particle concentration in isolation rooms becomes
Conclusion
A similarity analysis is performed to get the key similarity criteria for mean air velocity and particle concentration in the occupied zone of isolation rooms with ceiling air supply and down-side return air flow pattern. Based on the numerical test of CFD, the correlating equation of the key similarity is established. The mean air velocity and particle concentration can be calculated with these correlating equations. The results have a maximal relative error of 27.5% compared with those
Acknowledgement
This work was sponsored by the National Natural Science Foundation of China (Grant No. 50908127).
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