Preliminary Study of Aerodynamic Characteristics of High Temperature Superconducting Maglev-Evacuated Tube Transport System

Transcription

1 7 nd International Conference on Industrial Aerodynamics (ICIA 7) ISBN: Preliminary Study of Aerodynamic Characteristics of High Temperature Superconducting Maglev-Evacuated Tube Transport System Shijie Bao, Bo Wang, Yong Zhang and Zigang Deng* ABSTRACT In the high temperature superconducting maglev-evacuated tube transport (HTS Maglev-ETT) system, the air drag is the most principal resistance to the speed improvement and energy conservation. Considering the aerodynamic performance of the HTS Maglev-ETT system has great significance in practical application. As the first step, we experimentally explored the influence of velocity, blockage ratio and airshaft on the air drag of a running maglev vehicle at atmospheric pressure (95.6 kpa in Chengdu), coupled with the simulation by using the ANSYS-FLUENT software. The results preliminarily confirmed the feasibility of the aerodynamic experiments on the HTS maglev-ett test system, as the drag is proportional to the blockage ratio and the square of velocity respectively. Moreover, airshafts are suggested to be added in the future design of the HTS Maglev-ETT system, aimed at its good relief on the air drag displayed in the experiment and simulation. KEYWORDS: HTS Maglev-ETT, Air drag, Velocity, Blockage ratio, Airshafts. *deng@swjtu.cn (Z. Deng) 6

2 INTRODUCTION The high temperature superconducting maglev-evacuated tube transport (HTS Maglev-ETT) system will be one candidate for the ideal rail transit in the future [,]. This new system consists of several distinct components, including the high temperature superconducting (HTS) maglev vehicle, the permanent magnet guideway (PMG) and the sealed tube [,]. Possessing both the self-stable levitation characteristic and low pressure property, the HTS Maglev-ETT system has many advantages, such as high-speed, low energy consumption and little noise [5]. Therefore, more and more researchers pay attention to this domain, the work of ET, Swissmetro, and Hyperloop are the typical representative [6-8]. Due to getting rid of mechanical friction, the initial design speed of the HTS Maglev-ETT system is more than 6 km/h. However, the air flow suffered by the HTS maglev train in a sealed space is extremely different from that the traditional high-speed train in open air. Particularly, when the vehicle is running in high speed, the aerodynamic drag of the train increases sharply, which affects the stability and efficiency, even endangers the safety of the whole system. Hence, aerodynamics of the HTS Maglev-ETT system is a new subject needed further research. Many groups have simulated aerodynamics of the vacuum tube train and analyzed the effect of the vehicle speed, blockage ratio, and air pressures [9, ]. Nonetheless, due to the lack of experimental facilities, there is little work on the related experiments. Based on the world's first HTS Maglev-ETT test system developed by TPL of SWJTU, China, in, shown in Fig. (a), this work primarily explores the aerodynamic characteristics of this HTS maglev-ett test system, involving different factors, such as the vehicle speed and the blockage ratio. Moreover, the function of airshafts was also studied in the experiments as well as the simulation performed by ANSYS-FLUENT software package. 7

3 EXPERIMENTAL SETUP AND PROCEDURE (a) (b) (c) (d) Figure. Photos of the experimental scene. (a) The 5 m-long HTS Maglev-ETT test system developed by TPL of SWJTU, China, in. (b) The monitoring and controlling system shown on the computer. (c) The full view of the vehicle. (d) The view of the chassis. The HTS Maglev-ETT test system mainly consists of four parts: the tube, the HTS maglev vehicle, the double-track PMG and the dynamic data acquisition system, and related parameters are listed as follow: a) The tube is 5 m in length, and its diameter is m. By employing water ring vacuum pumps, the pressure in tube can be reduced to. atm. Three doors on the tube are regarded as the airshafts, presented in Fig. (a). b) The HTS maglev vehicle [] is composed of four cryostats, two bogies, a vehicle body and a seat, revealed in Fig. (c). There are four cryostats in vehicle which were fixed on the four corners of the bogies, and each cryostat contains rectangular three-seeded melt-textured YBaCuO bulks with a dimension of 6 mm fabricated by ATZ GmbH, Germany []. Two bogies in the HTS maglev system are respectively on the front and latter part of the vehicle, on which the linear motor induction plate and the seat is installed. The vehicle body wraps the seat, cryostats and bogies. c) The double-track PMG is consisted of Nd-Fe-B permanent magnets and arranged in Halbach type with a cross section of 5 mm. And at the 8

4 location of the Door, the primary of a linear induction motor with a length of m is installed in the middle of the two PMGs. The vehicle can be accelerated or decelerated by the linear induction motor, and its speed is controlled by changing frequency of the motor. d) The dynamic data acquisition system, including photoelectric speed sensors, laser displacement sensors, acceleration sensors, a data collection box and a computer, is to acquire and display the vehicle running signals, as shown in Fig. (b). This experiment was conducted in atmospheric pressure (95.6 kpa) to realize a constant pressure environment. Firstly, four cryostats were cooled by liquid nitrogen at a field-cooling height (FCH) of mm for 6 min to ensure the stable levitation. Then the working frequency of the linear motor is set up to Hz per lap and the HTS maglev vehicle starts to speed up. Until the average velocity is above 6 km/h (the max velocity of the chassis is about km/h), the motor no longer actuated the vehicle forward. At this moment, the HTS maglev vehicle started on inertial motion, and the computer began to record data. When the average velocity was below km/h, the vehicle braked and the data collection system was stopped. The whole vehicle and the chassis were set as different blockage ratios, the weight of them are respectively 9.7 kg and 8. kg, correspondingly the cross-sectional area are.9 m and.9 m, as displayed in Fig. (c) and (d). Besides, the doors are assumed as airshafts in the ETT system. Five experimental conditions by the combination of diverse opening doors are arranged as Table I, where the number represents the amount of opening doors and the single quote indicates using different doors. TABLE I. THE EXPERIMENTAL GROUPS WITH DIFFERENT NUMBERS AND LOCATIONS OF DOORS. Groups The opening doors None Door Door and Door Door and Door Door, Door and Door 9

5 Average velocity (km/h) Average velocity (km/h) EXPERIMENTAL RESULTS AND DISCUSSION. The Initial Data and Theory ' 5 Distance (m) (a) ' Distance (m) Figure. The average velocity per lap at atmospheric pressure (95.6 kpa) during the inertial motion: (a) the speed curve of the vehicle and (b) the speed curve of the chassis. The numbers - stand for the experimental groups with different number and location of doors open, which are listed in Table I. (b) Fig. presents the relationship between the average velocity and the running distance, directly obtained from the data acquisition system. The velocity decreases clearly under the action of aerodynamic drag. In order to more accurately reflect the airflow characteristics on the HTS maglev vehicle, the aerodynamic drag calculated by the equation of kinetic energy theorem, described as: mvb mva Fs () where m is weight of the vehicle; v a and v b are the average velocities of any two adjacent circles, v b is the latter; F is the external force; and s is the distance. From Eq. (), the average aerodynamic drag was obtained as: mvb mva F () s The values of the drag per circle are revealed in Fig.. It appears that the aerodynamic drag is decreasing, as same as the trend of the velocity, both for the vehicle and the chassis conditions.

6 Aerodynamic drag (N) Aerodynamic drag (N) Aerodynamic drag (N) Aerodynamic drag (N) Aerodynamic drag (N) Aerodynamic drag (N) 8 6 ' 5 Distance (m) 5 ' 6 8 Distance (m) (a) (b) Figure. The average aerodynamic drag per lap of the maglev vehicle in FCH mm during the experimental process: (a) the drag of the vehicle and (b) the drag of the chassis. The numbers - stand for the experimental group with different number and location of doors open, which are listed in Table I.. The Feasibility Analysis of the Experiment ' Average velocity (m/s) (a) Model Polyno Adj. R-S.9996 Value Standard T Intercept T B T B ' Average velocity (m/s) The chassis Polynomial Fit Average velocity (m/s) Average velocity (m/s) (c) (d) Figure. The average aerodynamic drag against average velocity of the maglev vehicle per lap in FCH mm under the ambient pressure of 95.6 kpa: (a) the drag of the vehicle, (b) the drag of the chassis, (c) the polynomial fitting curve for the drag-velocity relation of the chassis in the case of the Group, and (d) the drag with different blockage ratios in the case of the Group, as listed in Table I. 8 6 (b) The vehicle The chassis

7 For further research, it is important to investigate the relationship between the drag and the speed. The aerodynamic drag for the high-speed train is generally expressed: Fx CxS xvt () where F x is the aerodynamic drag, μ the air density, the general value in atmospheric environment is.9 kg/m, C x the air drag coefficient, S x the maximum cross-section area of the vehicle, V t the velocity []. From Fig. (a) and (b), it can be seen that the growth of aerodynamic drag is non-linear with increasing velocity. In order to get clear function between the force and velocity, the polynomial fitting curve for the drag to velocity is revealed in Fig. (c) and the polynomial fitting expression is: F.56V.58V.8 () x t The correlation coefficient is up to.9996, which strongly confirms that F x is linear to V t. Whereas, there is not only the quadratic term in the Eq. (), which is maybe explained by the Reynolds number (Re). Re is the significant parameter in air motion and proportional to speed, as: t vl Re (5) where l is the characteristic length, μ the viscosity coefficient. When R e is below 5, the air pressure distribution on the train surface is unstable, and the shifty speed even influences the value of the air drag coefficient, resulting in the Eq. () containing the constant term and the first item. So in the subsequent experiment, the data should be chosen under the high speed condition for matching Eq. (). On the other hand, the air drag of the vehicle is much higher than that of the chassis in the Fig. (d), which indicates that the blockage ratio is a crucial factor in the air drag. For example, at the speed about.6 m/s, in the condition of none opening door, the drag and the blockage ratio of the vehicle are. N and. respectively, while those of the chassis are. N and.9. The drag ratio between the vehicle and the chassis is.9, in accordance with the ratio of their cross-sectional area of.. The data shows a very good agreement with Eq. (), that is, the drag is linear with the blockage ratio. Compared with Fig. (a) and Fig. (b), the fluctuation in the drag-velocity curve of the vehicle is more unapparent than that of the chassis. With the vehicle body and the seat installed, the maximum cross-section area of the vehicle is larger, prompting the air drag to be the major resistance. Compared to the air drag, other

8 forces have less interference on the whole inertial motion, such as the electromagnetic resistance caused by the irregularity magnetic field of the PMG and frictional force among components of the vehicle []. As mentioned above, after the direct data being disposed by Eq. (), the relationships between the air drag and velocity are obtained and displayed in Fig.. These outcomes demonstrate that the drag is proportional to the blockage ratio and square of the velocity respectively, which are satisfactory to Eq. () in a great extent. Furthermore, other forces have little impact on the accuracy of experimental results. So this experimental method proves good feasibility and reliability.. The Influence of Airshafts Fig. (a) and (b) explicitly illustrate that the drag-velocity curves with different opening doors are nearly coincident at lower speed, both for the maglev vehicle and the chassis conditions. This is mainly because that, the door is only m-wide. When three doors are all open, the total area is even small, which only occupies at most.59% of the 5 m-long test line. The external airflow just exchanges air around the door, rather than immediately acting on the internal air in the tube. But at higher speeds, the value of the force in different conditions is evidently unequal, particularly above m/s. The reason of this phenomenon is that: when the vehicle is running rapidly, the large mass air in tube is forced to flow quickly, generating the turbulent flow. Then the external airflow at doors aggravates and impacts the drag greater, with the enhancement on the exchange interaction at doors. This is the reason why passengers feel an air flow, when the HTS maglev vehicle is passing by the door at higher speeds. To highlight the comparative results, the influence of doors was enlarged by calculating the average drag during the whole process in accordance with Eq. (), and the results are listed in Table II and III. TABLE II. THE AIR DRAG OF THE HTS MAGLEV VEHICLE. The grouping of doors The initial velocity (km/h) The final velocity (km/h) The air drag (N) NO NO NO NO NO (The total distance is all 95 m, namely running rounds.)

9 TABLE III. THE AIR DRAG OF THE CHASSIS. The grouping of doors The initial velocity (km/h) The final velocity (km/h) The air drag (N) NO NO NO NO NO (The total distance is all 5 m, namely running 7 rounds.) The results in Table II and III demonstrate that the value of air drag declines during adding the number of opening doors, but the effect of the different location is inconspicuous, for example, the value of the Group is approximate to the Group. For accounting the function of airshafts, it is necessary to understand the aerodynamic characteristics of the HTS maglev in the sealed tube, which is analogue with the situation that the train goes through a van. While the vehicle is running in open air, a series of compression and expansion waves spread in all directions at approximately the speed of sound ( m/s). Nevertheless, in the sealed tube, the wave propagation is obstructed and restrained by walls, bringing about greater pressure [8]. Specifically, the head of the vehicle forms a compression wave which is going ahead along the tube, meanwhile, the tail produces an expansion wave which is spreading backward, then the waves superpose or counteract constantly during the vehicle running on the ring test line. As the radius of tube is small, the waves are reflected from the tube wall, and then act on the vehicle surface again. Consequently, the airflow on the vehicle surface is more irregular and the air drag is larger, which leads to the external disturbance and even impacts on the stability of the maglev vehicle. As discussed previously, the sealed tube makes the air drag reinforced. After adding some airshafts, the air changes rapidly from the exhaust flow to the suction flow through doors, similar to the entry and exit of the tunnel. Except a part of the compression (expansion) wave still spread along the tube, the partial remaining is reflected as an expansion (compression) and exchanges many times at doors, making the energy weakening, and the another part is directly exhausted to the outside. These air waves all lower the pressure in whole tube, accordingly, the air drag suffered by the vehicle is reduced. Moreover, while the vehicle is passing by the door, the pressure on the vehicle surface also gets in flux. Firstly, as the vehicle upon arrival at the door, the part of the compression waves diffuse into external air, diminishing the positive pressure on head; secondly, during the vehicle body

10 passing the door, the high-density air flows out, reducing the aerodynamic damping force on the vehicle body; finally, as the tail departures, the external air is sucked into the tube, decreasing the absolute value of the negative pressure. Eventually, the whole pressure on the vehicle surface is lessened. In short, the overall value of the air drag is decreasing with the addition of the opening doors, as shown in the Table II and III. By comparing Table II and III, there is another discovery that the air drag difference between the none-opening-doors condition and the three-opening-doors condition of the vehicle is higher than that of the chassis. This is related to the lateral-area ratio between the vehicle and the door. It has long been known that the air drag is actually received by the areal integral of the pressure, as described by Eq. (6). F ( p ) ds (6) where p is the pressure on surface, is the viscous shearing stress, and S is superficial area []. As the airflow at the door primarily acts on the lateral windward of the vehicle, the bigger area enlarges the great altering of the pressure and shearing stress, as a result, the drag of the vehicle lowers obviously. THE SIMULATION VERIFICATION AND PRELIMINARY TUBE OPTIMIZATION. The Calculation Model (a) (b) Figure 5. The simulation of the HTS maglev vehicle in ETT tube with three opening doors: (a) the D model grid meshing and (b) the velocity vectors. 5

11 For simulating the aerodynamic drag imposed on the HTS maglev vehicle running in the tube with opening doors, the Navier-Stokes equations coupled with k-ε turbulent models were set up by using the commercial software ANSYS-FLUENT. Because the speed in the experiment is below 5 m/s, the airflow is assumed as a steady three dimensional, incompressible viscous flow. The operating condition is atmospheric pressure and boundary conditions are shown in Fig. 5(a), as follows: Inlet: velocity-inlet, the velocity magnitude is. m/s. Outlet: Pressure-outlet Door: Pressure-outlet Vehicle: Wall, the wall motion is stationary wall and the shear is no slip. Tube: Wall, the wall motion is moving wall, speed is. m/s, and the shear is no slip. The corresponding meshing and velocity vectors are shown in Fig.. The velocity vectors clearly portray the velocity variation in the domain, and the velocity magnitude of the airflow at doors is above 5 m/s, which is a little greater than the value of internal airflow in Fig. (b).. The Simulation Results and Preliminary Tube Optimization Table IV and V show the comparison of data between the experiment and the simulation, by separating into the vehicle and the chassis situations respectively. And it is apparent that the air drag in the simulation is decreasing with the addition of doors, which is consistent with the experimental phenomenon. Therefore, the results strongly confirm that the airshafts could relax the air pressure on the HTS maglev vehicle. TABLE IV. THE AIR DRAG COMPARISON BETWEEN THE EXPERIMENT AND THE SIMULATION OF THE HTS MAGLEV CHASSIS. The experiment The simulation The grouping of doors S x =.9 m S x =. m The velocity The air drag The velocity The air drag (m/s) (N) (m/s) (N)

12 TABLE V. THE AIR DRAG COMPARISON BETWEEN THE EXPERIMENT AND THE SIMULATION OF THE HTS MAGLEV VEHICLE The experiment The simulation The grouping of doors S x =.8 m S x =.9 m The velocity The air drag The velocity The air drag (m/s) (N) (m/s) (N) Some other findings should been noted in the simulation. Adding the door, the reversed flow emerges during the iteration, and the phenomenon is similar to the previously discussed in the experiment, that is, the airflow at doors changes very quickly from the exhaust to the suction. In addition, the distinction of drag with respect to the location was emulated. When the door is near to the inlet, the distribution of doors is scattered along the tube, or the cross-area of the door is enlarged, the drag would get lower, because the atmospheric diffusion is enhanced. But it is difficult to provide definite answers concerning the position of airshafts at the same cross-section with the vehicle. When the door position is elevated, the air drag of the chassis is reduced, on the contrary, the drag of the vehicle increases. The evidence suggests that the function of airshafts is likely limited to the vehicle model. If the airshaft position is closed to the high pressure area of the vehicle surface, the airshafts are more effective to reduce the drag. Summarizing the findings mentioned above, some proposals about the prospective design of the HTS Maglev-ETT system could be given, as exhibited in Fig 6. The symmetric branches or airshafts are more adaptive to the whole system, as the drag could be comforted without impacts on the air moment and lift. The widespread symmetric branches should be fixed at the accelerating and decelerating periods to reduce the energy consumption of the driving motor. At the inertial drift section of the route, airshafts should be placed on the top of the tube, avoiding the vehicle vibration caused by the airflow interaction. What s more, the design of the escape passage, the adapter module and else installations could incorporate the efficacy of symmetric branches, for cutting down the cost of the whole system. 7

13 Figure 6. The preliminary design sketch of the ETT tube with airshafts and branches. CONCLUSION Based on the first HTS Maglev-ETT test system in the world, the aerodynamic characteristics of the HTS Maglev vehicle running in the tube were preliminary studied. By calculated from the experimental data with the theorem of kinetic energy, the air drag is evidently proportional to the blockage ratio and square of the velocity respectively. These results prove the feasibility of the method and principle in our experiment, and also propose that this method is more appropriate to measure the aerodynamic characteristics of the maglev vehicle, which lay foundation for following experiments. Moreover, another finding in this work is that the air drag could be relieved effectively by airshafts, which is verified by simulations. Therefore, the reasonable design of airshafts should be recommended in the HTS Maglev-ETT application to improve the stability and energy conservation. It is very important to explore the effect of the blockade ratio and the structure of evacuated tube on air resistance, because the results can provide a guide for the design of evacuated tube. However, it is equally important to explore the magnitude of the air resistance of a maglev train under different low-pressure environments. Therefore, the next experiment will study the aerodynamic characteristics of the maglev train operating in a low pressure. 8

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