Shin Ichi Tsuda

Limits of classical molecular simulation on the estimation of thermodynamic properties of cryogenic hydrogen

In this study, we investigated the limits of classical molecular simulation on the estimation of thermodynamic properties of cryogenic hydrogen. Three empirical potentials, the Lennard-Jones (LJ) potential, two-centre LJ (2CLJ) potential, and modified Buckingham (exp-6) potential and an ab initio potential model derived by the molecular orbital calculation were applied. Molecular dynamics (MD) simulations were performed across a wide density–temperature range. Using these data, the equation of state (EOS) was obtained by Kataokas method, and they were compared with National Institute of Standards and Technology (NIST) data using the principle of corresponding states. As a result, it was confirmed that the potential model has a large effect on the estimated thermodynamic properties of cryogenic hydrogen. On the other hand, from the viewpoint of the principle of corresponding states, we obtained the same results from the empirical potential models as from the ab initio potential showing that the potential model has only a small effect on the reduced EOS: the classical MD results could not reproduce the NIST data in the high-density region. This difference is thought to arise from the quantum effect in actual liquid hydrogen.

Validation of Bubble Dynamics Equation for a Nano-scale Bubble via Molecular Dynamics Simulation

For a validation of the application of conventional bubble dynamics to a nano-scale bubble behaviour, we simulated a nano-scale bubble collapsing or vibration by Molecular Dynamics (MD) method and compared the result with the solution of Rayleigh-Plesset (RP) equation and that of Confined RP (CRP) equation, whose boundary condition was corrected to be consistent with that of MD simulation. As a result, a good coincidence was obtained between MD, RP, and CRP in the case of one-component fluid. In addition, also a good correspondence was obtained particularly in the comparison between MD and CRP in the case of two-component fluid containing non-condensable gas. The present results indicate that conventional bubble dynamics equation can be applied even to a nano-scale tiny bubble.

An analysis of quantum effects on the thermodynamic properties of cryogenic hydrogen using the path integral method

In this paper, we describe the analysis of the thermodynamic properties of cryogenic hydrogen using classical molecular dynamics (MD) and path integral MD (PIMD) method to understand the effects of the quantum nature of hydrogen molecules. We performed constant NVE MD simulations across a wide density-temperature region to establish an equation of state (EOS). Moreover, the quantum effect on the difference of molecular mechanism of pressure-volume-temperature relationship was addressed. The EOS was derived based on the classical mechanism idea only using the MD simulation results. Simulation results were compared with each MD method and experimental data. As a result, it was confirmed that although the EOS on the basis of classical MD cannot reproduce the experimental data of saturation property of hydrogen in the high-density region, the EOS on the basis of PIMD well reproduces those thermodynamic properties of hydrogen. Moreover, it was clarified that taking quantum effects into account makes the repulsion force larger and the potential well shallower. Because of this mechanism, the intermolecular interaction of hydrogen molecules diminishes and the virial pressure increases.

An analysis of the quantum effect on the thermodynamic and transport properties of cryogenic hydrogen using molecular dynamics method

In this paper, we have analysed an effect of quantum nature of the hydrogen molecule on its thermodynamic and transport properties using molecular dynamics (MD) method based on the path integral method. We performed NVE constant MD simulation and the quantum effect on the molecular mechanism was analysed. The simulation results were compared with experimental data. As a result, we clarified that the quantum nature makes the virial pressure larger than in classical mechanics and taking account the quantum nature makes smaller intermolecular interaction energy and larger repulsive force than classical representation. Besides, we have confirmed that the path-integral-based MD method well reproduces the thermal conductivity and quantum effect on the transport properties is also large.

Backflow vortex behaviours in contra-rotating axial flow pump at low flow rates

Backflow usually exists at the inlet of rotors of many turbomachines at low flow rates. In counter-rotating rotors applied for the axial flow pump, such vortical structures are also able to form. In our previous researches, some broad-banded pressure fluctuations in low frequency range have been observed between front and rear rotors, but we have not yet been able to explain what causes such phenomenon. In this study, in order to find out the causes of low frequency components at low flow rates, unsteady numerical simulations for the whole front and rear rotors are conducted, and casing pressure is experimentally measured at the inlet and outlet of front rotor and the inlet of rear rotor. It is found that vortical structures exist between front and rear rotors at below 40% of design flow rate. These vortices seem to be the result of shear layer instability at the impingement location of the exiting flow from front rotor to the backflow of rear rotor. The behaviours of these backflow vortices and their interaction with front rotor contribute the low frequency components in pressure fluctuations observed at low flow rates.

A molecular dynamics study of nuclear quantum effect on diffusivity of hydrogen molecule

In this paper, the nuclear quantum effect of the hydrogen molecule on its diffusivity was analyzed using the molecular dynamics (MD) method. The centroid MD (CMD) method was applied to reproduce the time evolution of the molecules. The diffusion coefficient of hydrogen was calculated using the Green-Kubo method over a wide temperature region, and the temperature dependence of the quantum effect of the hydrogen molecule on its diffusivity was addressed. The calculated results were compared with classical MD results based on the principle of corresponding state (PCS). It was confirmed that the difference in the diffusion coefficient calculated in the CMD and classical MD methods was small, and the PCS appears to be satisfied on the temperature dependence of the diffusion coefficient, even though the quantum effect of the hydrogen molecules was taken into account. It was clarified that this result did not suggest that the quantum effect on the diffusivity of the hydrogen molecule was small but that the two changes in the intermolecular interaction of hydrogen due to the quantum effect offset each other. Moreover, it was found that this tendency was related to the temperature dependence of the ratio of the kinetic energy of the quantum fluctuational motion to the classical kinetic energy.

Performance Prediction of Darrieus-Type Hydroturbine with Inlet Nozzle Operated in Open Water Channels

Small hydropower is one of the renewable energies and is expected to be effectively used for local supply of electricity. We have developed Darrieus-type hydro-turbine systems, and among them, the Darrieus-turbine with a weir and a nozzle installed upstream of turbine is, so far, in success to obtain more output power by gathering all water into the turbine. However, there can several cases exist, in which installing the weir covering all the flow channel width is unrealistic, and in such cases, the turbine should be put alone in open channels without upstream weir. Since the output power is very small in such a utilization of small hydropower, it is important to derive more power for the cost reduction. In the present study, we parametrically investigate the preferable shape of the inlet nozzle for the Darrieus-type hydroturbine operated in an open flow channel. Experimental investigation is carried out in the open channel in our lab. Tested inlet nozzles are composed of two flat plates with the various nozzle converging angles and nozzle outlet (runner inlet) widths with the nozzle inlet width kept constant. As a result, the turbine with the nozzles having large converging angle and wide outlet width generates higher power. Two-dimensional unsteady numerical simulation is also carried out to qualitatively understand the flow mechanism leading to the better performance of turbine. Since the depth, the width and the flow rate in the real open flow channels are different from place to place and, in some cases from time to time, it is also important to predict the onsite performance of the hydroturbine from the lab experiment at planning stage. One-dimensional stream-tube model is developed for this purpose, in which the Darrieus-type hydroturbine with the inlet nozzle is considered as an actuator-disk modelled based on our experimental and numerical results.

A simple cavitation model for unsteady simulation and its application to cavitating flow in two-dimensional convergent-divergent nozzle

In this paper, a simple cavitation model is developed under the framework of homogeneousone-fluid model, in which the perfect mixture of liquid and vapor phases is assumed. In most of conventional models, the vapor phase is considered as a dispersed phase against the liquid phase as a continuous phase, while in the present model, two extreme conditions are considered: for low void fraction, dispersed vapor bubbles in continuous liquid phase, while for high void fraction, dispersed droplets in continuous vapor phase. The growth of bubbles and droplets are taken into account in the mass transfer between vapor and liquid phases, and are switched according to the local void fraction. The model is applied for the simulation of cavitating flow in a two-dimensional convergent-divergent nozzle, and the result is compared with that using a conventional model. To enhance the unsteadiness of cavitation due to the instability at the cavity interphase, the turbulent shear stress is modified depending upon the continuous phases in combination with the proposed cavitation model, which drastically reduces the turbulent viscosity for high void fraction region. As a result, the unsteadiness of cavitation observed in experiments is well reproduced.

A molecular dynamics analysis of homogeneous bubble nucleation with a noncondensable gas in cryogenic cavitation inception

In this paper, homogeneous bubble nucleation in liquid oxygen (as one of the cryogenic fluids) with a noncondensable gas of nitrogen or that of helium was investigated using molecular dynamics method employing a fitted Lennard-Jones potential. We evaluated the influence of nitrogen gas and helium gas on the SATuration line (SAT) and the spinodal line as the thermodynamic limit of stability (TLS), and on the kinetic limit of stability (KLS) defined from a bubble nucleation rate. As a result, it was obtained that the influence of the noncondensable gases on the SAT and the TLS was negligible at molar fraction less than 1% although helium gas had several times stronger action to decrease the KLS compared with nitrogen gas. On the other hand, it was also indicated that the actual influence of both noncondensable gases on the cavitation inception in liquid oxygen might be negligible at least at standard conditions where the fluid starts to flow around or less than the atmospheric pressure.

A molecular dynamics study of nuclear quantum effect on the diffusion of hydrogen in condensed phase

In this paper, the quantum effect of hydrogen molecule on its diffusivity is analyzed using Molecular Dynamics (MD) method. The path integral centroid MD (CMD) method is applied for the reproduction method of time evolution of the molecules. The diffusion coefficient of liquid hydrogen is calculated using the Green-Kubo method. The simulation is performed at wide temperature region and the temperature dependence of the quantum effect of hydrogen molecule is addressed. The calculation results are compared with those of classical MD results. As a result, it is confirmed that the diffusivity of hydrogen molecule is changed depending on temperature by the quantum effect. It is clarified that this result can be explained that the dominant factor by quantum effect on the diffusivity of hydrogen changes from the swollening the potential to the shallowing the potential well around 30 K. Moreover, it is found that this tendency is related to the temperature dependency of the ratio of the quantum kinetic energy and classical kinetic energy.