# Energy-Saving Network Ventilation Technology of Extra-Long Tunnel in Climate Separation Zone
## 1 Introduction  
Climate separation zone usually means the transition region between two different climate zones. Miles of towering mountains stand and interdict the extending of one climate from one side of the mountain to the other side. Rich wind resources in these regions are available for further exploitation but never fully used. Moreover, natural wind, inside the extra-long tunnels located in climatic separation zone, shows more obvious effect on ventilation than in other regions [1,2] and its velocity in the tunnels can be 4-7 m/s [3]. However, in the design phase, natural wind is assumed as resistance in the tunnel ventilation modes [4]. Actually, the defect of the assumption is that natural wind sometimes can be assistance. If the traditional ventilation method is applied in the design of extra-long tunnel, the number of fans will be unreasonable. Consequently, traditional ventilation design is not suitable for tunnels in climatic separation zone. In these extra-long tunnels, the control strategy combining natural ventilation and mechanical ventilation together is of great significance. According to our literature search, different from the abundant research of natural ventilation in building, the application of natural ventilation in long tunnels is rarely reported. [PERSON] et al. reported that the airflow caused by the natural ventilation pressure is large enough (up to 29.26% of mechanical ventilation) to increase the efficiency of the ventilation system in tunnels with shaft [5]. [PERSON] et al. analyzed the naturally ventilated urban vehicular tunnels with multiple roof openings and held the view that piston effect exists in such tunnels but is weak [6]. Under traffic jam, thermal pressure is a main factor which influences inside flow field [7]. [PERSON] et al. simulated the piston effects in a subway system. He found that a larger sectional area will result in smaller air velocity in shaft and less effective air exchange between tunnel and outside ambient [8]. Mechanical ventilation usually takes up huge space in tunnel for ventilation equipment and costs much for maintenance and electrical energy consumption in operation [9]. In semi-transverse ventilation, the driving space is treated as an air delivery duct or air exhaust duct. If setting air delivery ducts and air exhaust ducts in full-transverse ventilation, no longitudinal wind flow happens inside the tunnel. Good ventilation results are obtained in extra-long tunnels after applying this ventilation mode, but the drawback is high construction costs due to large occupied space in the tunnel [10]. Shafts or inclined shafts serve as air inlets or outlets in segmented longitudinal ventilation, the extra-long tunnel is divided into several segments [11]. Construction expense will obviously decline compared to semi-transverse ventilation [12]. How can sufficient airflow in tunnels be provided with minimum energy consumption? With the development of sensor technology, control strategies of intelligent ventilation were put forward [13; 14]. Tunnel ventilation is optimized by controlling jet fans and dust collectors installed inside the tunnel [15]. Jet fans blow polluted air outside the tunnel and sensors measure the contents of pollutants in the tunnel. By means of the approach, it was possible to reduce energy consumption while keeping the degree of pollution within allowable range. Modern control methods in tunnel ventilation include fuzzy control, neural networks, expert systems, etc. Several studies about the intelligent control on tunnel ventilation were reported [16; 17; 18; 19; 20; 21; 22; 23; 24]. The development tendency of tunnel ventilation will be comprehensive, systematic, intelligent and energy-saving [25]. The Nibashan tunnel is located in climatic separation zone of China. The law of natural wind velocity in the tunnel was studied in this paper. According to the meteorological parameters on both sides of the tunnel and the meteorological principles, the natural wind velocity inside the tunnel was calculated.