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Age of air as the criterion of ventilation efficiency in recirculating flows |
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Age of air as the criterion of ventilation efficiency in recirculating flows Author: M. Branny
Mining operations in copper mines in the Legnica-Głogów region employ mostly the room and pillar method. Workings up to 60 m long are ventilated by jet fans installed at the inlets. In the present study one of the ventilation efficiency parameters is considered: the mean, local age of air index 8 (Sandberg 1981). It is expressed as the time a particle travels from the inlet to the aired point. In the region where 0 reaches its highest values gas admixtures can hardly be rarefied. Two workings, differing in length and geometrical parameters (cross-sections) are considered in the study. The workings are ventilated with fans WOO-63. Fig. 2 and 3 show predicted age of air distributions for three considered cross-sections (x sub 1-X sub 2). In the chamber 27 m in length (Fig. 2) the mean age of air reaches its maximal value in the inlet section to the blind working, i.e. in the return stream, near the open cross-cat with the streamline fresh air. In the long chamber (Fig. 3) the most adverse conditions occur in the zone stretching from 35 to 55 m from the inlet (i.e. it takes longer to remove or rarefy gaseous admixtures). The maximal standardised value of 9 might be used as a nondimensional criterion of ventilation efficiency. Its maximal admissible value (expressing the efficiency of pollutants removal) is determined by comparing the time-space distribution of concentration of mine gas components with the mean, local age of air field. Accordingly, an assumption is made that at the initial moment the heading is filled with blast gas. Thus determined time-space distributions of gas concentration in the two chambers considered in the study are shown in Fig. 4 and 5. In qualitative terms the concentrations fields are the same as the age of air fields. This similarity is maintained at other time instants too. It appears that ventilation of blind workings aired with jet fans is sufficient as long as the maximal value of the standardised, local age of air is less than 5.
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2011