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Abstract: The temperature increase problem of city road tunnel appears as the increase of the amount of motor vehicles. This paper analyzed how the problem comes about, the method to control temperature using high pressure water mist system and the probably defects. It provides reference for project design.
Keywords: temperature increase, high pressure water mist system, blocking condition
Urban highway tunnels are one of the important means to alleviate congestion and strengthen urban traffic capacity. Large traffic flow is a characteristic of urban highway tunnels. The accumulation of waste heat discharged by motor vehicles causes the temperature of the tunnel to rise. When the temperature in the tunnel is too high (>40°C), it may cause the failure of the automobile oil circuit and the increase of the temperature of the water tank, which may cause the car to break down and cause a traffic accident. At the same time, the high temperature in the tunnel will also cause serious discomfort for passengers and tunnel staff. On July 20, 2009, the Wuhan Yangtze River Tunnel, the outdoor temperature was 34C. When the ventilation system ventilation system duct making machine was working normally, the temperature rose to 43.4C after 500m in the tunnel, and the maximum temperature reached 619. Traffic accidents occurred frequently. As the tunnel design only considers the pollutant concentration control ventilation design under normal and blocked conditions and the smoke exhaust design under fire conditions, there is a lack of targeted control measures for the temperature in the tunnel. Moreover, there are no codes or standards applicable to the calculation of temperature and heat dissipation of urban highway tunnels. Based on the temperature problems encountered in the design of Shanghai Hongmei South Road-Jinhai Road Passage Project (cross-river section) (hereinafter referred to as "this tunnel"), a preliminary analysis of the temperature control of urban highway tunnels is carried out.
1 Tunnel temperature standard
"Preliminary Discussion on Temperature Rise of Long and Large Highway Tunnels" comprehensively "comfort of passengers and suitable working environment for maintenance personnel in tunnels under normal and blocked operating conditions. In addition, engineering investment and operating costs also need to be considered, as well as other high temperatures. According to the temperature design requirements of the site, according to the human body's tolerance to temperature," the air temperature standard for highway tunnels is proposed. When formulating temperature standards for this tunnel, the following aspects should be considered:
1) Normal operating temperature of car air conditioner;
2) Passenger comfort requirements when there is no on-board air conditioner;
3) Working environment temperature requirements for tunnel equipment maintenance personnel.
First of all, the temperature in the tunnel to ensure the normal operation of the car air conditioner should not be higher than 45C; second, if there is no car air conditioner, taking into account the design speed of this tunnel 60km/h, the car travel time in the tunnel is about 5 minutes, and in summer When the outside temperature is high, the passengers need to endure the high temperature environment of the tunnel for a long time, so the temperature in the tunnel should be ≤40C; third, starting from the working environment requirements of the maintenance staff in the tunnel, the temperature in the tunnel should be <40C. Refer to the temperature control standards of existing tunnel projects, including the Chongming River Crossing Yangtze River Tunnel and the Snow Mountain Tunnel in Taiwan, as well as the requirements of the "Metro Design Specification" for the installation of air-conditioning in the train compartments and the tunnel temperature of not higher than 40C when the station is equipped with screen doors. The tunnel uses 40C as the temperature control standard.
2 Tunnel temperature rise analysis
The temperature changes in urban highway tunnels depend on factors such as outdoor temperature, traffic flow, tunnel ventilation methods, and heat transfer on the tunnel wall. Among them, the tunnel ventilation method has a great difference in the temperature effect. It is mentioned in reference : "... Among the various ventilation methods, the temperature of the transverse ventilation and the air supply type semi-transverse ventilation GI air duct fabrication machine is relatively even in the depth direction of the tunnel. ...Longitudinal ventilation. As there is no air entering along the way and the motor vehicles continuously emit heat, the temperature in the depth direction of the tunnel continues to rise.....'. This tunnel crosses the Huangpu River, the underground section is 4746m, and the inner diameter of the circular tunnel 13.3m, belongs to the extra-long tunnel. Under normal working conditions, the peak hour traffic on the west route (north to south) is 2760pcu/h, and the peak hour traffic on the east route (south and north) is 3050pcu/h. The traffic is heavy, and gasoline and diesel burn when vehicles pass. After that, a large amount of waste heat is discharged. The tunnel ventilation system uses jet fans to induce, and a centralized exhaust outlet is set near the exit of the tunnel to discharge the exhaust gas. In the case of outdoor ventilation calculation temperature of 31.2C in summer, the temperature distribution in the tunnel is calculated according to the normal working condition and the 10km/h blocking working condition of the whole line. The temperature distribution curve is shown in Figure 2.
It can be seen from Figure 2 that due to wall heat dissipation under normal working conditions, the temperature in the front section of the tunnel (before the entrance of 2500m) basically maintains the outdoor temperature. As the length increases, the accumulation of heat exceeds the wall heat dissipation capacity, and the back section of the tunnel ( After 2500m from the entrance to the tunnel, the temperature rises significantly, and the temperature at the exit to the tunnel reaches 37.89C; the temperature of the 10km/h blockage condition of the whole line is approximately proportional to the length of the tunnel. When the increase in ventilation and other cooling measures are not considered, the tunnel is at 2150m The temperature is 40.05C, and the temperature at 4746m from the exit reaches 50.74C. If the outdoor temperature in summer is above 38C or extreme traffic conditions, the calculated temperature in the tunnel can be as high as 55C. The temperature rise problem cannot be ignored. Appropriate cooling measures should be taken to ensure the safety of personnel and vehicles.
3 Tunnel temperature control measures
3.1 Tunnel temperature control method
At present, there are two ways to control the temperature of domestic tunnels: use the tunnel's own ventilation system Rectangular duct forming machine to cool down and use the high-pressure water mist system to cool down. Before the Chongming Cross-River Passage Yangtze River Tunnel, domestic tunnels mainly used ventilation to achieve the purpose of cooling. However, for this tunnel, if the temperature is cooled by pure ventilation, there will be the following main problems:
1) The cross-sectional wind speed in the tunnel will be as high as 10.3m/s, which will affect driving safety;
2) A set of jet fans needs to be installed every 40m in the tunnel, which does not meet the requirement of 80m minimum installation spacing of jet fans in the specification;
3) Power consumption will increase by 3000kW.
It can be seen from the above three points that the use of pure ventilation to cool down is not feasible for engineering. This tunnel considers the use of high-pressure water mist as a special auxiliary cooling system for the tunnel while maintaining normal ventilation.
3.2 The principle of high-pressure water mist cooling
The high-pressure water mist has the characteristics of extremely small particle diameter, the adopted particle size is only 60um, the distribution is dense and uniform, and the residence time in the air is long. Table 2 lists the relationship between droplet diameter, surface area per liter of water, vaporization time, and free fall speed. It can be seen from Table 2 that when the total amount of water is constant, the smaller the droplet diameter, the larger the surface area, the shorter the time required for vaporization, and the higher the heat absorption and efficiency. For the same amount of water, the surface area formed by the high-pressure fine water mist droplets is at least 100 to 1000 times larger than that of the water droplets sprayed by the traditional water mist. Table 2 The relationship between droplet diameter, surface area, vaporization time and free fall speed On the other hand, the high-pressure water mist droplets are light in weight and will not easily fall to the ground. Instead, they follow the air flow air flow ductwork pipe forming machine , which increases the heat exchange time with the air, which is conducive to the heat absorption and cooling process of the droplets.
3.3 High-pressure water mist cooling system application scheme of this tunnel
According to the calculation results of the temperature rise of the blocking conditions, the total water demand of the single-tube tunnel of the high-pressure water mist cooling system is about 400L/min (6.67kg/s), and the air temperature at the outlet can be controlled to not exceed 40°C. The spray particle size of the high-pressure water mist nozzle is 60um, and the spray volume of a single nozzle is about 1.75L/min. The design temperature at the entrance of the tunnel is 31.2C, and the calculated temperature at the exit is 50.749C. The protection starts at the section where the calculated temperature is 359C. The temperature rise curve is approximately proportional to the length of the tunnel, that is, it starts at 900m after entering the tunnel. Set the temperature control section of high-pressure water mist. Figure 3 shows the working principle diagram of high-pressure water mist. Each tunnel needs to be equipped with 45 spray sections, divided into 3 spray sections, each section contains 15 spray sections, each spray section is a group, each group has 5 Only the nozzle. One section is close to Minhang working shaft, one section is located in the middle of the tunnel, and the third section is close to Fengxian working shaft. The distance between each group of nozzles is 75m, and the length of each spray section is about 1125m, and spray nozzles are required for the whole 3375m. Combining temperature control and energy saving requirements, three sections are used as a group of control units, and each group of control units has 3 measuring points, which are located at the top and two sides of the tunnel. The spray section in a single tunnel is divided into 15 groups of control units. Each group of control units determines whether to spray or not according to the temperature at the location and the temperature at the exit. The control conditions are as follows: each spray section is independently controlled by 5 control units. When the outlet temperature is higher than 40C and the average temperature of the control unit is higher than 409C, the 3 section sprays contained in the control unit are turned on; when the control point When the average temperature is lower than 409C, the spray is turned off.