Numerical Study on Different Series Modes of Jet Fan in a Longitudinal Tunnel Ventilation System

Hindawi Publishing Corporation Mathematical Problems in Engineering Volume 2014, Article ID 194125, 9 pages http://dx.doi.org/10.1155/2014/194125 Research Article Numerical Study on Different Series Modes of Jet Fan in a Longitudinal Tunnel Ventilation System Guihong Pei and Jie Pan School of Civil Engineering and Architecture, Southwest Petroleum University, Chengdu 610500, China Correspondence should be addressed to Jie Pan; Received 26 May 2014; Revised 21 August 2014; Accepted 4 September 2014; Published 29 September 2014 Academic Editor: Jaromir Horacek Copyright © 2014 G. Pei and J. Pan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Efficient ventilation systems will contribute to maintaining air quality in the tunnel. In order to improve ventilation performance in normal traffic condition, the 3D tunnel models were established according to the original design for the tunnel located in central plains of China. Based on the commercial CFD software Fluent 6.3 and 𝑘-𝜀 turbulence model, numerical simulations were carried out to study the patterns of jet flow and the optimization of fan combinations. It is found that the axial velocity profile obtained from numerical simulation agrees quite well with turbulent free jet theory although there is a little difference on the magnitude. The comparison of four combination modes under the condition of operating four fans indicates that the ventilation effectiveness is affected mainly by both the interval of adjacent groups of fans and the combination modes of operational fans. According to the simulation results, a novel combination mode which consists of a group double paralleled fans and two groups single fan is designed. The novel combination mode is regarded as the optimum combination mode with respect to maximizing air velocity in the tunnel. Compared to the traditional combination modes, it will increase the air velocity by 5.7%. 1. Introduction The air quality inside tunnel is gradually taken seriously because of the increasing of tunnel length. Owing to the special structure of tunnels, when a fire occurs, the smoke spreads fast and widely, which prohibits the safe evacuation of occupants and the firefighters from extinguishing the fire, while, in most tunnels, the ventilation system plays a crucial role in controlling the smoke and maintaining acceptable conditions within the tunnel during evacuation, rescue, and firefighting procedures [1]. Statistics have shown that smoke is the most fatal factor in fire and about 85% of victims in building fire were killed by hot and toxic smoke [2]. Therefore, efficient ventilation in a road tunnel is extremely important to prevent the smoke and the harmful substances from affecting tunnel users. The longitudinal ventilation mode with jet fans is highly anticipated due to its flexibility, reliability, lower cost, and easy control. Therefore, it not only gets the favor of engineers to be used in most of the tunnel but also causes the attention of scholars to do deeper research. The four main methods of studying tunnel ventilation at home and abroad are theoretical calculations, model tests, field measurements, and numerical simulations. For example, Ohashi et al. used numerical method to study the vertical concentration distribution of pollutants in Kan-Etsu highway tunnel of Japan which is 10.885 km long. The results had provided the basis for design of ventilation and air pollution control in highway tunnel [3]. Rohne carried out an experimental research in which the fans were located near a flat plate to study the resistance and jet thrust. The influence of different installation separation of fans on the ratio of resistance and thrust had been investigated; at the same time, the influence of various distances from the plate on the ratio of resistance and thrust also had been analyzed [4]. Gao of Lanzhou Jiaotong University had analyzed the characteristics of jet flow and the pressure-rise of jet fan. Not only the relationship between the pressure-rise and the induced length but also the relationship between the pressure-rise and the separation of jet fans had been investigated by him [5]. Zhong of South China University of Technology had conducted field measurements in Dabaoshan tunnel, and the test results provided valuable data for the installation location of jet fans in longitudinal ventilation system [6]. According to the ideas of the composite structure method, Wang and Xia of 2 Mathematical Problems in Engineering Chang’an University had carried out numerical simulations on longitudinal ventilation, and the optimization ventilation method was picked out [7]. Colella et al. [1, 8, 9] had conducted a large number of researches in the field of tunnel ventilation, and they had obtained plenty of important results. They applied multiscale modeling approach to study the ventilation flows and fire in a tunnel domain. The multiscale modeling approach uses different levels of details when describing the fluid flow in different tunnel regions. The far field region is modeled by using a 1D “network” model and the near field region is modeled by using a 3D CFD model. The “network” and the CFD models exchange information at the 1D–3D interfaces [8]. The adoption of multiscale modeling approach allows for a more rational use of the computational resources without loss of accuracy [9]. The research results obtained by scholars mentioned above had laid the foundation for the study of tunnel ventilation system. All the scholars had put forward some valuable opinions. The longitudinal ventilation systems with jet fans have been continuously updated. On the one hand, as the width of the tunnel increases, 2 or 3 jet fans are paralleled and suspended in the tunnel vault. On the other hand, the number of jet fans in series increases with the increasing of the tunnel length. In general, only the total number of operational jet fans will be given in design plan, while the combination mode of jet fans in normal traffic condition is not stipulated. The wind speed is different in various combination modes, and so it is the air pressure. Therefore, there is clear need to analyze the difference of pressure distribution and velocity distribution in various combination modes. Numerical simulations were carried out to study the air flow and the optimization of fan combinations inside tunnel based on CFD software Fluent 6.3. According to the simulation results, the best combination mode which is conductive in improving the ventilation performance in the tunnel is selected. where 𝑢𝑖 is turbulent velocity, 𝑢𝑖 is time-averaged velocity, 𝑢𝑖󸀠 is velocity fluctuation, and 𝑓𝑖 is mass force (𝑖, 𝑗 = 1, 2, 3). Turbulent free jet is a common form of jet flow, and the important features are shown as follows [12]. 2. The Mathematical Mode (...truncated)