The Hyperloop system is a new concept that allows a train to travel through a near-vacuum tunnel at transonic speeds. Aerodynamic drag is one of the most important factors in analyzing such systems. The blockage ratio (BR), pod speed/length, tube pressure, and temperature affect the aerodynamic drag, but the specific relationships between the drag and these parameters have not yet been comprehensively examined. In this study, we investigated the flow phenomena of a Hyperloop system, focusing on the effects of changes in the above parameters. Two-dimensional axisymmetric simulations were performed in a large parameter space covering various BR values (0.25, 0.36), pod lengths (10.75–86 m), pod speeds (50–350 m/s), tube pressures (~100–1000 Pa), and tube temperatures (275–325 K). As BR increased, the pressure drag was significantly affected. This is because of the smaller critical Mach number for a larger BR. As the pod length increased, the total drag and pressure drag did not change significantly, but there was a considerable influence on the friction drag. As the pod speed increased, strong shock waves occurred near the end of the pod. At this point, the flows around the pod were severely choked at both BR values, and the ratio of the pressure drag to the total drag converged to its saturation level. At tube pressures above 500 Pa, the friction drag increased significantly under the rapidly increased turbulence intensity near the pod surface. High tube temperatures increase the speed of sound, and this reduces the Mach number for the same pod speed, consequently delaying the onset of choking and reducing the aerodynamic drag. The results presented in this study are applicable to the fundamental design of the proposed Hyperloop system.

In this study, the critical velocity was investigated with various aspect ratios (0.5, 0.667, 1.0, 1.5, 2.0) of the tunnel cross-section in tunnel fires. Reduced-scale experiments were carried out under the Froude scaling using ethanol pool fire. Temperatures were measured under the ceiling and vertical direction along the center of the tunnel. The effects of aspect ratio on critical velocity were analyzed from smoke movement. It has been found that the critical velocity increases with aspect ratio, where the aspect ratio is height/ width for a tunnel of rectangular cross-section. This study used new dimensionless velocity and dimensionless heat release rate parameters considering aspect ratio of the tunnel. The critical velocity varied with the one-third power of the heat release rate for under-medium fires. The results of this study are compared with other small-scale and full-scale experiments. A good agreement has been obtained.

The present article deals with the experimental study to investigate the effects of tunnel slope on the critical velocity in the tunnel fires. The 1/20 reduced-scale model tunnel is adopted on the basis of Froude scaling law. The methanol, acetone, and n-heptane pool fires are used as fire sources with the heat release rate ranging from 1.11 to 1.85 kW, 3.13 to 5.21 kW, and 9.16 to 15.6 kW, respectively. The angle of tunnel slope is varied as five different degrees 0, 2, 4, 6, and 8°. A load cell is used to measure the mass loss rate of the burning fuel. The location of smoke layer is determined on the basis of temperature variation measured at the tunnel ceiling by K-type thermocouples. The present experiments observe the effects of the tunnel slope, fire size, and ventilation velocity on the back-layering distance. The results showed that critical velocity increases with tunnel slope due to the stack effect. The correlation between the critical velocity and the angle of tunnel slope was obtained for the pool fires whose heat release rate was varied with the ventilation velocity.

In this study, the 1/20 reduced-scale experiments using Froude scaling are conducted to investigate the effect of longitudinal ventilation velocity on the burning rate in tunnel fires. The methanol pool fires, acetone pool fires, and n-heptane pool fires with a heat release rate ranging from 2.02 to ~6.15 kW, 2.76 to ~11.04 kW, and 2.23 to ~15.6 kW, respectively, are used. A load cell is used to measure the mass loss rate of the burning fuel and the temperature distributions are measured by K-type thermocouples in order to investigate smoke movement. The ventilation velocity in the reduced-scale tunnel is controlled by the wind tunnel through an inverter. In the case of a methanol pool fire, the increase in ventilation velocity reduces the burning rate. On the contrary, for acetone and n-heptane pool fires, the increase in ventilation velocity leads to the enhanced burning rate of fuels. The reason is that, for acetone and n-heptane pool fires, the oxygen supply effect prevails rather than the cooling effects as the ventilation velocity increases. However, for a methanol pool fire the cooling effect outweighs the effects of oxygen supply. The relationship between the critical velocity (V_c, m/s) and the heat release rate (Q, MW), for a full tunnel, has been found to be:[ILLEGIBLE].