1990 Volume 15 Issue 44 Pages 69-78
In casting and welding workshops, a large amount of fumes is generated from various heat sources. As a countermeasure against such fumes, general ventilation systems are usually used jointly with local ventilation. A lot of reports has been given on the general ventilation in workshops over a long time. However, various problems have been left unsolved in the actual factories adopting general ventilation. For example, a venting condition is dependent on weather or season, namely, a good venting condition occurs when it is fine weather, but a phenomenon that fumes stay stratiformly at a certain height is observed frequently in rainy weather. In this report, venting flows in a simple model with a heat source on the floor are simmulated numerically in order to investigate the appearance of the natural venting in workshops generating fumes. The P-V method is used as a calculation procedure and also the k-ε model is used for turbulent model. As the results of numerical calculation are well coincide with experiments, it becomes clear that the natural venting characteristics can be well estimated by numerical calculations using the k-ε turbulent model. Some series of numerical calculations were performed to investigate the influence of roof temperature and opening sizes on the venting characteristics. The followings have become evident from the calculation results. 1) The larger the opening size becomes and the higher the roof temperature is, the more air change rate increases. 2) The thermal stratification is formed when the roof is under an adiabatic condition. In this case, it is observed that the plume rising from heat source falls along the roof and the walls and then is entrained again in the plume at the height of thermal stratification. 3) When the roof is cooled, the thermal stratification is not formed any longer. In this case, the downflow containing a lot of fumes reaches almost to the floor, and the results that the working spaces are contaminated by fumes are supposed.
