Theoretical and experimental study of subway ventilation systems to determine safe modes

Subway is the main transport of Tbilisi. It serves the major share of the passenger flow. Safety of mentioned transport mean is provided by all the existing systems and technical means designed for such transport. Technical means involves rolling stocks created on the bases of railway transport base and the necessary infrastructure for their movement. Subway infrastructure involves general and accessory buildings which might be located both in the over ground and underground also. In most cities of the world general buildings of subways are located under grounds and in this point of view Tbilisi subway is not an exception. For proper performance of the said transport including all related systems, a reliable performance of ventilating system is of vital importance. The purpose of it is a provision and maintenance of normal microclimate in subways in everyday basis, herewith polluted air to be returned in atmosphere and in emergency situations the system shall answer all arisen challenges. In subways, with their infrastructures located over grounds, reasons of emergency situations are mainly technogeneous and there is a quite a big risk of getting a fire emergency situation. In such situations choosing of right working regime of ventilation system has a major importance, because saving of people’s lives depends on it. Also, prevention of toxic gases and pathogenic microbes in the events of aerosol terrorist acts are very important.  Mentioned danger became even actual after Tokyo (Japan) subway accident, when the group of terrorists managed to spread a poisonous gas Sarin in subway space. As a result, several people died and thousands had been poisoned. the mentioned accident the question came up quite thoroughly: Is penetration of toxic gases and pathogenic microbes in subway possible by relevant attempts? There is no qualified answer to this question for today, because there have been conducted quite a few studies and works on the issue.  The abovementioned work is one of the first attempts for regulation of the issue, in specific: based on analyses of underground working systems, we established the weakest circle from where it is theoretical possibility for unplanned air to penetrate into subway. The said weakest circle is ventilation system, because it is the only direction by which underground space of subway is connected to over ground. Specifically, the most insecure points are air shafts which return polluted air from undergrounds to atmosphere, especially in cases if mentioned air shafts are not equipped with exhaust ventilation equipment. It should be noted, that none of Tbilisi (Georgia) subways are equipped with air shafts equipped with exhaust ventilation systems for polluted air.  Modern computer technologies give possibility to describe created air flows resulted from piston effect of moving wagons in tunnel with high accuracy. Consequently, we made a computer modelling of the situation in Pyro Sim 2016 program. The technical characteristics of piston effect of moving wagons in the tunnel, also, speed character of transmitted and rushing air flow in gaps, in front and back sides of wagons had been revealed. There are provided specified tables and charts of their variations, by means of which and according to the geography of cross over tunnels ventilation characteristics can be established. It may be noted, that established parameters according to the abovementioned charts are in a good coincidence with figures received by theoretical counting. A new regularity has been established: a non-stationary character of air flows arisen from piston effect during the wagon movement in channel caused by length of cross over tunnel. According to the given geometry and relevant ventilation characteristics we have determined numerical indicators of excessive pressure and rarefication in front and behind of wagons correspondingly. Based on determined indicators tab-les and charts have been created. Analyses of modelling results showed the impact of air gap between perimeters of wagon and tunnel on created numismatic indicators of excessive pressure and rarefication in front and behind of wagon. Based on the abovementioned we corrected the chart describing rarefication dynamic created behind wagon for the given speed of wagon. It should be noted, that the said knot of ventilation system is really problematic. Behind wagon there is created enough rarefication which may result in getting of unplanned air to the subway area.  Thus, a technological gap in ventilation system performance has been revealed, which is not a subject of insufficient quantity of ventilation air or improper quality of air, the problem is that existing ventilation systems can not perceive toxic gases or pathogenic microbes and prevent from possibilities of their penetrating and spreading in underground space. The abovementioned problematic creates a serious threat to security of metropolitan. In order to overcome the problematic, in the present work there is formulated requirements to ventilation systems for new threats and possibilities of using appropriate technical means for proper and secure performance of the problematic knot.