MODELLING OF THE AIRFLOW IN THE PASSENGER COACH

Every day people spend a lot of time in public transport, for example buses, trams, trains or subways. To make their journey comfortable, appropriate thermal conditions must be provided by HVAC (Heating, Ventilation, Air Conditioning) systems. Thermal comfort is achieved when passengers perceive the air temperature, humidity, air movement, and heat radiation of their surroundings as ideal and would not prefer warmer or colder air or a different humidity level [3].

Passengers often decide to travel by railway vehicles. For example, during the 2017 year over 7.5 million passengers travelled by train in Europe and went almost 600 000 million kilometres [4]. Thus, the correctness of design and work of the HVAC system is one of the most important issues in passenger coaches. More and more modern trains have been produced for a few years, however, the work of ventilation and air-conditioning systems is dissatisfactory. In summer passengers often complain about the method of air distribution. In older passenger coaches, too high indoor temperature occurs. Moreover, in some zones of the compartment the air velocity distributions are uneven, as a result, draught and discomfort zones are created. In modern, air-conditioned railway vehicles passengers are also often dissatisfied, due to too low temperature of the supplied air, especially at the head level. Therefore, HVAC systems should be improved and optimized to achieve better thermal comfort for their users.

Computational Fluid Dynamic (CFD) software is used to predict the air distribution in different ventilated rooms, for example in offices, sports halls, swimming pools, ice rinks and even in vehicles. For the railway industry, thermal comfort in passenger vehicles was described in few studies, for example [5, 6, 7, 8, 9, 10]. Zhang [5] simulated and compared three different air distribution systems for sleeping spaces in a transport vehicle. They were displacement, personalized and mixing ventilation systems. After the research, it was found that the best air quality for sleeping passengers was personalized ventilation. Aliahmadipour [6] studied the airflow in a compartment of a passenger coach in summer conditions. The research was conducted for two cases in presence of seated/slept manikins and without. The first results showed the non-symmetrical air distribution, so the thermal comfort was unfulfilled in the part of the compartment. Due to performing some simple modifications of the HVAC system, the thermal conditions improved. Thermal comfort was also investigated in double-decker train cabins with passengers in summer and winter conditions [7]. Schmeling and Bosbach [10] studied the influence of heat release on ventilation efficiency and thermal comfort with the use of thermal manikins. Different ventilation systems in passengers’ rail vehicles were observed in the literature. Aliahmadipour [6] used a mixing system, where the fresh air was delivered by the main air duct located under the floor of the coach. The displacement ventilation was used by Schmeling and Bosbach [10] and Konstantinov [7]. The air inlet was located in the ceiling. Numerical simulations were made for different vehicles by researchers, for example, Suárez [8] studied air distribution in trams and Bosbach [9] compared six different ventilation concepts (mixing ventilation, cabin displacement ventilation, ceiling based displacement as well as combined ceiling and floor-based displacement ventilation for the aircraft cabin. Due to the application of CFD, it was possible to simulate the distribution of air parameters in vehicles and also better optimization of the ventilation for subway side-platforms in Tianjin Metro. It would help to build a more dependable, effective and ventilation system [11]. Moreover, simulations CFD can be used to predict coach fire. Chen [12] simulated and analyzed the impact of the ventilation factor on the burning characteristic of coach fire.

Generally, the design of HVAC systems in the passenger coach may be a source of difficulties and doubts. This issue is a great challenge for the constructors. The aim of this paper was to prepare a numerical model of passenger coach and carry out the numerical calculation of the airflow. The simulations were made for two cases of boundary conditions: 1) obtained by using design calculations common for ordinary buildings, 2) obtained by using information included in standards [1, 2]. This was to answer the question of many constructors, which approach in design will be better to obtain the required parameters of thermal comfort. Prior to this work, the boundary conditions have been determined based on the results of measurement [11],[6],[10] or assumed values in computational simulations [8]. The information included in standards was not taken into account.

Current requirements and guidelines for the design HVAC systems in railway vehicles are found in European standards PN-EN 13129 [1] and UIC leaflet 553 [3]. They determine thermal comfort parameters by extension, the capacity of the air conditioning systems under defined conditions and describe the testing program and the measurement procedures for estimating the HVAC system [3]. The air distribution systems issues were not included in these documents.

The main requirements are described in this chapter. The climatic zone, which is taken into account in the selection of ventilation, heating or air conditioning devices for railway vehicles, is different than the one commonly known in building engineering. First of all, one of three zones, separately for summer and winter, is assigned for each country, in which vehicle is homologated. Central European countries, including Poland, are placed in II zone for both summer and winter conditions. Parameters of outdoor air are assigned for each zone, for example. for II zone, in summer, outdoor air temperature should not exceed t_e =35°C, RH=50% of relative humidity and a solar load equals to 700 W/m². Other information was shown in Tab. 1 and Tab. 2. The heat transfer coefficient is also determined on the basis of zones and its values are different for single and double deck vehicles. For the single deck it equals to 2.0 W/(m²·K), 1.6 W/(m²·K), and 1.2 W/(m²·K), for I, II and III climatic zone, respectively. For double deck the coefficient is 2.5 W/(m²·K), 2.2 W/(m²·K) and 2.0 W/(m²·K) for I, II and III climatic zone.

The minimal amount of fresh air per one seating or bed should be in the range of 10-20 m³/h, depending on the outdoor air temperature (Tab. 3). According to UIC 553 leaflet [2], it is recommended to increase the airflow up to 5 m³/h for each place for a smoker. In order to obtain thermal comfort conditions, the adequate parameters should be provided. The indoor air temperature is determined on the basis of the adjustment curve, depending on the outdoor air temperature. Velocity and relative air humidity are read on the basis of limiting curves as a function of the indoor air temperature. In air-conditioned passenger coach velocity should be in the range of 0.07–0.60 m/s, air temperature 22–27°C and relative humidity about 52–60%. Excluding comfort zones, airspeed must not be lower than 0.05 m/s. More information about comfort parameters can be found in standards and UIC leaflet [1, 2].

The obtained results of the simulations were studied using the ANSYS CFX-Post module. The results of the calculations were performed in a graphical form (Fig. 5–10). The planes, in which values of air parameters (velocity and temperature) changed in a wide range, were analyzed. The XY, Z=1.804 m vertical plane was a symmetry plane (Fig. 5, 8). The vertical planes YZ were located in different places. The first plane YZ, X=-11.63 m passed through the area of air inlets (Fig. 6, 10), the second plane YZ, X=-9.55 m was placed between windows (Fig. 7, 9). According to the Polish standards [14] the boundaries of the thermal comfort were between 0.1÷1.7 m of the height for a standing position and 0.1÷1.1 for a sitting position. Therefore, distributions of air velocity and temperature out of this range were not taken into account in the result description. They were shown for only illustrative purposes. The range of values and colours were selected adequately so that the air distributions were shown legibly. In order to make the accurate comparison of values of air velocity and air temperature from two cases monitoring points were introduced. They were located in the vertical axis of symmetry at the height of 1.95 m (points 1–6) and in the comfort zone at the height of 0.85 m and near the seats (point 7–12).

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

The comparison of the air velocity distribution for the two cases was shown in Fig. 5–7. The values of them were similar, average air velocity for both variants was 0.79 m/s. In monitoring points (Fig. 11) the values also were similar (without point 6). The airstream flowing out from inlets mixed with the indoor air in a large degree. Due to this the dead zones (w_max <0.7 m/s for t_i = 27°C, w_min > 0.05 m/s for t_i = 18°C) were located only near floor between seats and near the ceiling where people seldom stay.

Figure 11.

For case 1 the average indoor air temperature was 25.07°C. It was higher by about 1.54°C than for case 2. The value of temperature obtained near the floor was (Fig. 8, 9) too low (23–24°C). The stratification of temperature was unnoticeable, the bigger part of the coach temperature was similar (24.7–26°C). For case 2 the average indoor temperature (Fig. 9) was significantly too low t_i = 23.53°C, especially in the area from 0.1 to 0.6 m. It was due to cold air outflow from diffusers. According to the design assumption, the indoor temperature should be 27°C (+-1°C), and the range of mean temperature measured 1.1m above the floor should not exceed 2K. The stratification of temperature was at significant level ranging between 22÷26°C. Therefore, passengers could have felt locally uncomfortable. In monitoring points values of air temperature were much higher (without point 5) in case 1 than case 2 (Fig. 12).

Figure 12.

The comparison of the numerical distribution of air velocity of simulations for two cases was shown in Fig. 10. It was noted that the air distribution of case 2 was more similar to the theoretical scheme. The streamlines were more symmetrical and regular. However, in both cases, there was a bigger amount of air on the left side of the coach.

By means of CFD, it was possible to take into account most of the phenomena associated with the flow of the air and heat in the passenger coach. It was found that: