Numerical analysis on the condenser inlet air temperature of train-mounted air conditioner when a train stops in subway station tunnel
Introduction
Due to the expansion of cities and societies, there is an ever-growing urge for traffic relief and pollution control. Therefore, urban rail transit has become the primary mode of transportation for a large number of citizens (Ahn, Cho, & Chung, 2016; Luan, Cheng, Song, & Zhao, 2020). Among the different types of urban rail transit, urban subway takes the largest part. According to statistics, by the end of 2019, 40 cities have established 208 subway lines and 3982 stations, the total length is 5180.6 km in China (China Association of Metros, 2020).
Most subway trains are equipped with air-conditioning system to provide a comfortable environment for passengers. The condenser of the air-conditioning system is installed on the top of the train. Therefore, the heat generated by the air-conditioning system is released directly into the tunnel area. Previous studies have shown the fact that if the condenser inlet air temperature is above 45℃, the air-conditioner is likely to face high-pressure failure (Bogdanovská, Molnár, & Fedorko, 2019). And the failure of the air-conditioning in trains leads to heatstroke and syncope in passengers during summer (Ampofo, Maidment, & Missenden, 2004), a problem which should be effectively avoided. On this account, good control of the thermal environment in subway tunnels is of great importance and has drawn the attention of researchers.
According to the study in UK (Ampofo, Maidment, & Missenden, 2004), subways can generate enough heat from their operations to raise the tunnel temperature substantially. After investigating the heat load in a generic underground railway network, researchers pointed out that additional cooling to the train should be provided by cooling the tunnels within which they operate. Zhang et al. (2016) pointed out the significant effect of braking energy on tunnel temperature and proposed the way of utilizing this energy to improve the thermal environment in cold areas. Wang, Zou, Tao, Song, and Zheng (2017) studied the heat reservoir effect of the surrounding rock in subway tunnels by scale model experimental test, and found out that tunnel temperature can reach up to 41 °C during summer, which is much higher than the setting value in the designing handbook. Based on a Green's function method and finite element method, Zhang & Li (Zhang & Li, 2018) proposed a two-dimensional unsteady heat transfer formalism to study thermal environment in the underground tunnels. Using Subway Thermal Environment Simulation Software (STESS) (Wang & Li, 2018), the formalism-based simulation tool, they also studied the interval tunnel temperature by multi-factor-analysis and put forward a response-surface model to quickly predict interval tunnel temperature (Zhang & Li, 2019).
These studies provided insights into the tunnel thermal environment. However, they only concentrated on the study of interval tunnel, which is the main drawback of previous studies. In fact, the underground tunnel is composed of interval tunnel and station tunnel; the interval tunnel is the underground space where train moves, and the section of tunnel where the train stops is called the station tunnel. Hence, the condition of air temperature can be extremely different in these two tunnels.
On one hand, station tunnel is where trains stop, and the total heat release of the air-conditioner is much greater than any place of the interval tunnel. On the other hand, natural ventilation is mostly caused by piston effect (Cross, Hughes, Ingham, & Ma, 2017; Izadi, Mehrabian, Abouali, & Ahmadi, 2019; Li et al., 2020; Lin, Chuah, & Liu, 2008; Liu et al., 2019), and its rate is closely related to the train motion (Li & Wang, 2018); therefore, the piston effect is not as prominent in the station tunnel as in the interval tunnel. To sum up, the air temperature in station tunnel is higher than interval tunnel, due to the higher heat release rate and weaker piston wind. This will cause a harsh condition for the Air Conditioner (AC) on the train. Therefore, the air temperature in subway station tunnel is the critical issue for train’s operation safety.
Analytical method, experimental method, and numerical method are three main approaches to study the thermal environment in the underground areas. Krarti et al. (Krarti & Kreider, 1996) applied analytical method to study the heat transfer in the underground air tunnel by proposing a 1-D model to predict the hourly air temperature variation along the tunnel. However, it cannot show the temperature distribution on the cross-section. Although, experiments of tunnel ventilation and tunnel temperature can present real situations (Ninikas, Hytiris, Emmanuel, Aaen, & Younger, 2016), it is not appropriate for system optimization, since the experiment is both time-consuming and difficult to repeat.
Compared to analytical and experimental methods, numerical simulation has the merits of few simplified assumptions, repeatability, and less time requirement. Basically, numerical simulation method is categorized as 1-D network simulation and 3-D Computational Fluid Dynamics (CFD) simulation. Some researchers applied network simulation to reflect the network structure in underground tunnels (Lin et al., 2008; Wang & Li, 2018), and they were able to use 1-D simulation to acquire the average temperature of each branch. Others used CFD simulation in a typical station or tunnel. Izadi (Izadi et al., 2019) studied the transient airflow field around the train in underground tunnel by 3-D numerical analysis. Khayrullina et al. (Khayrullina, Blocken, Janssen, & Straathof, 2015) evaluated the effect of trains on the wind conditions induced on a passenger platform inside the underground tunnel by CFD simulation. Based on the above literature reviews, CFD simulation method is most suitable to tackle the topic of this study, since it is essential to investigate the air distribution and temperature field in the station tunnel.
In order to explore whether the train-mounted air conditioner can operate safely in the subway station tunnel, the numerical method was adopted to show the air temperature distribution and AC condenser inlet air temperature (). With a number of surveys in subway lines in China, the typical simulation model was summarized to maximize the applicability of simulation. The CFD method was verified by field measurements in both mechanical ventilation scenario and non-mechanical ventilation scenario. Moreover, main factors including the form of airflow organization, Over Tract Exhaust (OTE) ventilation volume and heat release rate of the AC condensers were analyzed. This study can provide guidance to the tunnel mechanical ventilation system design and operation.
Section snippets
Simulation method
This section describes the method used in this numerical simulation study, including the introduction of simulation model, the tool and meshing method, as well as the boundary conditions and initial conditions.
Validation
In order to verify the accuracy and reliability of the simulation method, several field measurements were conducted.
Guangzhou metro line 5, station 07 (Xiaobei) was selected as the validation station, as it shown in Fig. 11. The selected subway station is located in Guangzhou, a city in southern China. Based on the field investigation, the station 5−07 is an island station with standard dimension. The subway train used in Line 5 is the standard B-type train, which is the same as the simulation
Airflow and temperature field
Using the above-mentioned numerical method, the airflow and temperature distribution can be presented in the simulation domain. Fig. 15 illustrates the selected left view and top view sections to present the airflow and temperature field. The left view section is chosen at the first AC condenser on carriage 3, the section represents the temperature in middle carriages, which is considered to be the hottest area in the tunnel area. The top view section is selected at the height of train’s top,
Conclusion
This research focuses on the condenser inlet air temperature of train-mounted air-conditioner when train stops in the subway station tunnel.
Based on the observation on dynamic process and temperature distribution, it is found that the application of mechanical ventilation leads to the even distribution of temperature among the different carriages during the 30-second stop. When there is no mechanical ventilation, the temperature shows dynamic rise and the situation in the 6 carriages is quite
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key R&D Program of China (No. 2018YFC0705000) and the National Natural Science Foundation of China (No. 51521005).
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