Research Of The Effect Of Short Shafts’ And Jet Fan’s Position On Urban Tunnel’s Ventilation

The elimination of air pollutant that is diffused whencars are runningalong theurban tunnel is limited inside due to the relative confined space,which causes the airpollution in the urban tunnel and has a risk to damage the healthy of drivers and thesafety of the vehicles driving. Meanwhile rigidrestrictions also have to be imposedonthe total pollution emission oftunnel exit, because in most cases urban tunnel issurrounded by residential quarters, business district and culture area, all of which havestrict protection requirement. Longitudinal ventilation system with shafts is a newtunnel ventilation model which can ues piston effect effectively to reduce the cost ontunnel construction, and can save electric energy and also can help to exhaustpollutantsto alleviate the pollution of tunnel exit, so the ventilation mode is morecompetitive among numerousventilation designs. But the designing parameters are notincluded in the current design code, so essential study on its feasibility is verynecessary.In this paper, the mathematical model of longitudinal ventilation system withshafts is built according to the structural characteristics of the urban tunnel anddiffusion condition inside the ruban tunnel. And the calculation result was comparedand verified by that measured from a model experiment. Rechearch onhow theregularities of the airflowandthedistributionof air pollutant in and out the tunnel arechanging when we alter the relative position of jet fans and shafts was conducted.Theresultshows that in the same investment conditions, the pollutant concentration oftunnel exit is the minium when the shafts are centralized layoutin the front of thetunnel, and the value is respectively smaller17.5%and15.5%than that shaftscentralized in the middle and rear of the tunnel; And when the shafts are set up in themiddle of the tunnel, the mean pollutant concentration in the tunnel is respectivelysmaller35.5%and19.9%than that when the shafts are built in the front and rear ofthe tunnel. We can also get the conclusion that when the position of air-intake shaftsare fixed, the position of exhaust shaft affects the pollutant concentration of tunnelexit little. And the total pollutant emission of tunnel exit trends to decrease first andthen increase with the distance between exhaust shaft and tunnel exit increasing.There is a certain placement of exhaust shaft to make the tunnel let the least air pollutant out. It can also be found that air-intake shafts being set up far from thetunnel portal helps reducethepollutant concentration and the total pollutant emissionof tunnel exit.At last, by analyzing and comparing the technical level, safety, economicalefficiency, operation and service level of severalventilation designs,a mark is giventoeach one. Base on the score, an evaluation system for tunnel ventilation designs wasbuilt.