| In recent years,the interest for fire safety issues in tunnels has increased dramatically due to a significant increase in number of tunnels worldwide and due to numerous catastrophic tunnel fires.Due to the relatively limited cross-sectional area in tunnels,hot smoke can spread rapidly,e.g.,downstream with the traffic flow or due to longitudinal ventilation.Consequently,people downstream of the fire may be exposed to high temperatures and toxic gases,especially in urban tunnels that are likely to clog during rush hours.Inspired by fire compartmentation in buildings,it is worth investigating whether a tunnel can be partitioned by a water mist system into a fire zone and safety zones.If so,people can move from the fire zone into a safe zone through the water mist system.Obviously,an essential question is to examine to what extent the fire-induced heat and smoke can be blocked by the water mist system.So far,there is still a lack of investigation of using water mist systems as a curtain to prevent smoke and heat spreading in tunnels,and there are no clear design specifications.Therefore,the main purpose of present study is to investigate the effectiveness of water mist systems with respect to blocking smoke and heat in tunnels.Small-scale experiments and a numerical study were conducted for abovementioned purpose.A small-scale tunnel platform was built to investigate the effect of smoke blocking and temperature reduction by a water mist system.Two different levels of water working pressures(0.3 MPa and 0.5 MPa)and ventilation conditions(without ventilation and with 0.8 m/s forced longitudinal ventilation)were considered.Different nozzle arrangements(2+2,2 x 3 or 2+3 nozzles)were also taken into account.Experimental data concerns temperature fields,extracted from thermocouple trees at different longitudinal positions in the vertical center plane of the tunnel,with measurements at 3 heights per tree.The experimental results show that in the naturally ventilated tunnel,the installed water mist system effectively prevents smoke spreading.The water also has a strong cooling effect on the smoke in the tunnel.These observations were confirmed,regardless of the nozzle arrangement(2+2,2 x 3 or 2+3 nozzles)or the water pressure imposed(0.3 MPa or 0.5 MPa).With longitudinal ventilation in place,with velocity 0.8 m/s in the reduced-scale tunnel,the smoke is no longer blocked.The smoke is still cooled down by the water,but the effect is smaller than for the naturally ventilated tunnel,because residence times are shorter and the forced ventilation also already has a cooling effect on the smoke.With longitudinal ventilation in place,the impact of the nozzle arrangement is small,for a given level of water pressure.For all configurations,a higher water pressure in the water mist system leads to stronger cooling effects,due to the production of more and smaller water droplets.A numerical study was conducted to analyze the flow field in more detail to know the mechanisms leading to the smoke blocking effect of water mist system and the interaction between a water mist system and longitudinal flow.Before we conduct numerical study of small-scale experiments,the impact of the longitudinal ventilation condition and the impact of the sidewalls on flow fields induced by one single nozzle are discussed,by means of CFD simulations(FDS 6.0.1).The simulation results of single nozzle show that 1).an upward flow occurs near the sidewall,regardless of the distance between nozzle and sidewall,but its intensity will be higher when the nozzle is placed closer to the sidewall.2).The strong upward flow could impinge onto the ceiling and deflect sideward.3).The downward flow induced by water spray and upward flow near sidewalls tilted backward because of the longitudinal ventilation flow.4).Higher water working pressure makes the entrainment and upward flow more pronounced,regardless of the ventilation condition.Numerical studies of small-scale experiments(without longitudinal ventilation system)show the presence of a clear upward motion in the region in between the sprays and near sidewalls.Such motion is caused by spray-induced impinging jets onto the floor.These jets,induced by entrainment into the sprays,merge and create a zone of upward motion that acts as a 'barrier',preventing smoke spread downstream the water sprays.Different nozzle arrangements affect the temperature field,more nozzles indicates stronger entrainment.But as the presence of upward flow in-between water sprays and near sidewalls are not strongly affected by the nozzle arrangements,the smoke and heat blocking effect were found in each case.This is in line with the experimental observations.Besides,analysis of the flow field near water mist system shows that more nozzles and higher water working pressure induce stronger entrainment and upward motion in-between sprays and near sidewalls.For the cases with activation of longitudinal ventilation system,the impact of longitudinal mechanical ventilation has been discussed on the basis of mean flow and temperature fields.In contrast to the situation without longitudinal ventilation,the smoke is not blocked by the water mist.The upward flow in between the water sprays,which caused the smoke blocking by the water mist in the absence of longitudinal ventilation,is still observed,but it does not impinge onto the ceiling anymore:the horizontal momentum from the mechanical ventilation weakens the 'pushing' force of the spray-induced downward flows impinging onto the floor and moves them further downstream.Moreover,the downward momentum induced by the water mist destroyed smoke stratification.Higher water working pressure leads to a stronger cooling effect and spray-induced velocities.Different nozzles combinations have a slight impact on the temperatures downstream.However,as spray-induced momentum destroyed smoke stratification,activating fewer nozzles is better from this point of view.In the last part,we mainly focus on applying water mist in a full-scale tunnel to block fireinduced smoke.A full-scale tunnel model is built corresponding to the small-scale model based on Froude scaling laws.Comparison of full-scale to small-scale simulation results reveals that the temperatures in the full-scale tunnel are higher than in the small-scale tunnel simulations.However,better agreement is observed for the upward flow,as well as for the entrainment by the water mist system.As scaling up water flow rates from the small-scale model to full scale leads to unrealistically high water flow rates,a new tunnel model was simulated to investigate the application of water mist systems in a full-scale tunnel in more realistic conditions.The characteristics of the water droplets under a working pressure of 5 MPa were determined beforehand.Investigation of impact of the nozzle combinations,with the same total water flow rate,on temperature and flow fields shows that the combination of nozzles has a slight impact on temperatures downstream,but the upward flow and the entrainment are more significant with more nozzles installed in one row(higher coverage of water mist).The study of the distance between two nozzle rows also indicates that the temperatures downstream the water mist system are not affected by this distance.For a certain level of heat release rate,activating more nozzles means lower temperature and longitudinal velocity downstream water mist system.The entrainment caused by the water mist system plays the main role on blocking fireinduced smoke.The momentum of the water mist system was taken as the sum of all the momentums per nozzle,while the momentum of the ceiling jet flow was calculated as the product of the mass flow rate of the ceiling jet flow and its maximum velocity.The total water flow rate(number of nozzles)is proportional to the 2/3 power of the heat release rate(supposing that the thickness of the smoke layer changes only slightly from 1 MW to 3 MW),as illustrated by simulation results for 5 different values of HRR. |
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