A pantograph receives aerodynamic force while a train is traveling. As the aerodynamic force increases in proportion to the square of the flow velocity relative to the pantograph, its influence on the pantograph becomes apparent, especially for high-speed trains. When a high-speed train runs in a tunnel, the flow velocity relative to the pantograph is faster than that in an open section. In this study, we measured the flow velocity around a train model running in a tunnel using a rake of total-pressure tubes mounted on the train model. The measured waveforms were distorted owing to the influence of the frequency characteristics of the measurement system. Therefore, we developed a restoration method and applied it to the measured waveforms to obtain the restored waveforms of the flow velocity. With the restored waveforms, we obtained several statistical values, such as the average flow velocity and standard deviation of the fluctuating flow velocity around the train model. Furthermore, we proposed a method for predicting the flow velocity around the train model, including fluctuating components in a frequency range of a pantograph contact performance. The proposed method can predict the flow velocity at the panhead of a pantograph by considering the average flow velocity profile and turbulence component profile in the tunnel cross section, which could not be taken into account so far.
This paper discusses that the unsteady aerodynamic forces caused by vehicle motion in road input have a large influence on the vehicle behaviors. In the present coupling analysis, unsteady aerodynamic forces were obtained by using the response functions for vehicle motion derived in our previous study. By attaching a protrusion on the roof which only measures about 3% of the vehicle height, pitch and heave motions are affected significantly by the change of unsteady aerodynamic effects. Due to the coupling of the unsteady aerodynamic forces and vehicle suspension responses, the vehicle behavior depends on the road input frequency. The pitch motion is enhanced in low-frequency road input (<1.6 Hz for a real vehicle) and is suppressed at high frequencies (>1.6 Hz). Meanwhile, the heave motion is suppressed over 1.2 Hz. Such vehicle behaviors were verified in actual running tests, and were consistent to the subjective evaluation of the drivers feeling. Comparing the unsteady aerodynamic coefficients to the equivalent vehicle specification factors, the aerodynamic inertia caused by pitch motion contributes to 39% of the rear vehicle mass inertia effect in pitch motion. This aerodynamic effect is important in designing suspension specifications of vehicles. These results show that the vehicle stability can be affected even by small detail in vehicle shape due to the unsteady aerodynamic forces.
The numerical scheme of the O-integral is formulated for precisely evaluating the stress intensity factor of an embedded crack with arbitrary shape in an infinite elastic body. In this study, we evaluate the O-integral using an efficient numerical integration method by introducing an iso-parametric element and the Gauss-Legendre formula, which is typically used in the finite element method. To verify the numerical procedure introduced herein, the mode-I stress intensity factors for cracks are evaluated based on a circle, an ellipse, and a perturbated circle cracks. Result shows that the K results obtained using the proposed method are consistent with the exact solution. Therefore, fatigue crack propagation is successfully simulated using the O-integral for the elliptical crack.
The void fraction distribution of a fuel rod bundle in a boiling water reactor is a critical parameter for accurately predicting the optimal thermal margin in the design of a reactor core. The rod bundle configuration, such as a part-length rod (PLR) and water rod, can affect void distribution. To clarify the influence of PLR on void fraction distribution, a boiling flow experiment was conducted using a 5 × 5 heated rod bundle that partially simulated a boiling water reactor (BWR) rod bundle, and three PLRs were arranged in the corner. The cross-sectional void fraction distribution was acquired using high-energy X-ray computed tomography at six height levels for wide flow conditions, system pressures of 0.1 − 7.2 MPa, inlet subcoolings of 20 - 90 kJ/kg, mass fluxes of 500 - 1250 kg/m2/s, and linear heat generation rates (LHGR) of 3.2 - 8.6 kW/m. In the PLR region, the local void fraction temporarily decreases because the PLRs disappear, and the flow channel rapidly expands. Together with the downstream PLRs, the voids propagate to the PLR region and concentrate in the center. The void fraction in the corner of the PLR region remains lower. A maximum 26% decrease in the subchannel void fraction was observed in the corner of the PLR region at the system pressure of 7.2 MPa, mass flux of 1.25 × 103 kg/m2/s, inlet subcooling of 50 kJ/kg, and LHGR of 8.6 kW/m.
Nanofluid, a liquid containing choroidal dispersion of nanometer-sized solid particles, enables high-temperature bodies to be cooled more rapidly during quenching than in pure liquid. Drastic rise of the minimum heat flux temperature (TMHF) caused by the layer of nanoparticles formed on the heat transfer surface is the key phenomenon of heat transfer enhancement. In the present work, using alumina, silica, and titanium dioxide as the nanoparticle materials, quenching experiments were carried out to explore the mechanisms of the rise of TMHF in nanofluids; stainless steel 304 and Inconel 718 were used as the materials of the specimen and distilled water was used as the base liquid. In the experiments, TMHF increased in all the nanofluids but the increasing rate was dependent significantly on the nanoparticle material and the nanoparticle layer thickness. To elucidate the mechanisms of the heat transfer enhancement, the relations of TMHF with the three basic surface parameters of roughness, wettability, and wickability were examined but no clear relationship was found. When the metal specimen of higher thermal conductivity is covered with the nanoparticle layer of lower thermal conductivity, the contact temperature during quenching should decrease and the contact duration would be dependent on the thermal properties and thickness of the nanoparticle layer. Assuming that TMHF rises with an increase in the contact duration, a new model describing the rise of TMHF in the nanofluid was proposed.
Pellet-Cladding Mechanical Interaction (PCMI) failure is one of the failure mode which must be evaluated in nuclear fuel safety. PCMI is caused by mechanical load to the cladding due to fuel pellet expansion. Under high fuel burnup condition, the fuel cladding may become degraded by embrittlement under neutron-induced irradiation and hydrogen accumulation due to the waterside corrosion. In order to consider the further deterioration of the material with higher burnup, evaluation using mechanical indicators, e.g. strain and stress, might be required. As fuel burnup proceeds, cracks occur in the pellet due to an internal temperature gradient, which induces the fuel pellet relocation, and fission gas may be accumulated in the pellet and the gap between pellet and cladding. Cracks and fission gas may cause more deformation of the pellet under the power excursion than in low burnup condition. In this study, a transient model is developed, which can mechanistically evaluate the PCMI behavior, in particular, for fuel rods under higher burnup condition. The model is incorporated in a fuel behavior analysis code and verified by benchmarks with other similar codes. The PCMI predictability of this code is validated using the experimental test data.
Fission product removal by pool scrubbing is known to be largely affected by the gas-liquid two-phase flow regime. The pool scrubbing performance of various carrier gases was evaluated in two-phase flow experiments by injecting helium, nitrogen, or argon through a pool of stagnant water in a column with an inner diameter of 0.5 m and a height of 8 m. The gases were supplied through a cylindrical nozzle with an inner diameter 69 mm at superficial gas velocities ranging from 0.013 to 0.053 m s-1. Measurements were conducted using a camera and two sets of a 128 × 128 wire-mesh sensors, separated by 40 mm. The visually observed gas hold-up and wire-mesh sensor-measured average void fraction decreased with increasing fraction of lighter gas in the supplied gas. Detailed analysis of the flow regime using the obtained wire-mesh sensor signals revealed that lighter gases led to a greater fraction of relatively large bubbles in the flow compared to heavier gases, causing the gas phase with the lighter gases to have a higher average rise velocity in the flow. This leads to a hypothesis that, compared to heavier gases, lighter gases break up less or coalesce more in the flow, resulting in distinct two-phase flow characteristics depending on the inlet gas composition.
A sodium-cooled fast reactor has been designed to attain a high burn-up core in commercialized fast reactor cycle systems. The sodium-cooled fast reactor adopts a wire spacer between fuel pins. The wire spacer performs functions of securing the coolant channel and mixing between subchannels. In high burn-up fuel subassemblies, the fuel pin deformation due to swelling and thermal bowing may decrease the local flow velocity in the subassembly and influence the heat removal capability. Therefore, understanding the flow field in a wire-wrapped pin bundle is important. This study performed particle image velocimetry (PIV) measurements using a wire-wrapped three-pin bundle water model to grasp the flow field in the subchannel under conditions, including the laminar to turbulent regions. The PIV results confirmed that the normalized flow velocity near the wrapping wire in the low Re number condition was relatively decreased compared to that in the high Re number condition. Meanwhile, in the region away from the wrapping wire, the maximum flow velocity was increased by decreasing the Re number. Accordingly, the PIV measurements using the three-pin bundle geometry without the wrapping wire were also conducted to understand the effect of the wrapping wires on the flow field in the subchannel. The results confirmed that the mixing due to the wrapping wire occurred, even in the laminar condition. These experimental results are useful not only for understanding the pin bundle thermal hydraulics, but also for the code validation.