Motohiko Nishimura

Numerical study on mixing of oscillating quasi-planar jets with low Reynolds number turbulent stress and heat flux equation models

A comparative investigation was conducted on the mechanistic numerical simulation of mixing of non-isothermal, quasi-planar jets incorporating low Reynolds number turbulent stress and heat flux equation models (LRSFM) and an experiment. A water test facility with three vertical jets, the unheated in between two heated jets, simulated convective mixing and temperature fluctuations expected at the outlet of a liquid metal fast reactor core. The LRSFM and a two-equation k-e turbulence model were applied to the simulation. The LRSFM showed good agreement with the experiment, with respect to mean profiles and reproduced the oscillatory motion of the jetting flow, while the k-e model under-predicted the mixing effect, such that a transverse mean temperature difference remained well downstream. Specifically the LRSFM results indicated that the influence of turbulence on mixing was of second order importance in the present flow, while the contribution by coherent phenomena, mainly associated with the periodic oscillation of the present jets, promoted the mixing. These results were evident in both the numerical simulation with LRSFM and in the experiment.

Natural Convection Heat Transfer in the Horizontal Dry Storage System for the LWR Spent Fuel Assemblies

A heat transfer and flow visualization experiment was conducted with a one-fifth scale model simulating a dry shielded canister (DSC) with 24 PWR spent fuel assemblies in order to elucidate the heat transfer characteristics and the velocity distribution for natural convection inside a DSC filled with air or water at atmospheric pressure. It was found that the average heat transfer coefficients were proportional to the one-fourth power of the Rayleigh number despite the complicated geometry inside the DSC. Flow patterns inside the DSC were visualized clearly through a digital image processing system. The velocity distributions inside the DSC were obtained quantitatively from the Particle Tracking Velocimetry. In comparison with the results of a two-dimentional thermal hydraulic analysis, computed flow patterns were similar to the experimental results and the computational temperature distributions on the sleeve surfaces agreed well with the experiments within 8%, except at the top point of the center gap. ...

Prediction of forced gas flows in circular tubes at high heat fluxes accompanied by laminarization

The application of high heat fluxes to turbulent gas flows causes significant variation of the gas properties due to the large temperature increase, invalidating the use of design relations such as the popular Dittus-Boelter correlation and of conventional turbulence models. To develop turbulence models for such flows, the predictive capabilities of a Reynolds stress equation model (RSM) with turbulence heat flux equations and of a model were examined. We have employed the turbulence stress-thermal energy gradient production, in the pressure-temperature fluctuation gradient correlation, for the turbulence heat flux equations, in order to improve the accuracy of predictions for heat transfer accompanied by laminarization. The model has already been validated for gas flows in annular tubes; in this study its applicability to a circular tube was examined. Validations were performed by comparison of predictions with constant fluid properties at fully-established conditions and to experimental measurements extending to laminarization conditions. The turbulence models both predict laminar and turbulent results and the approximate transition Reynolds number for the fully-established flow at low heat fluxes. The results predicted with both models also agreed well with data on laminarizing flows. Thus, the models presented here are capable of predicting the flow reasonably from low to high heat fluxes.

Computational Fluid Dynamics Simulations and Experiments for Reduction of Oil Churning Loss and Windage Loss in Aeroengine Transmission Gears

The demand for power generation capacity has increased considerably due to the electric drive of cabin air conditioners and commercial aircraft engines. It is estimated that power losses may increase in the accessory gearbox due to generators and pumps that augment fuel consumption. To reduce these losses, a computational fluid dynamics simulation technique that analyzes oil churning and windage losses was developed and improvements were made to the shrouds of bevel gears, which have large losses in the gearbox. It was revealed experimentally that shrouding reduced losses up to 36% as compared to unshrouded gears.

CFD Simulation for Reduction of Oil Churning Loss and Windage Loss on Aeroengine Transmission Gears

In recent years, the demand for power generation capacity has increased considerably due to the electric drive of air conditioners and so on in the engines of civil aircraft. Therefore, it is estimated that power losses may increase in the gearbox because of generators and pumps that in turn augment fuel consumption. To understand the phenomena of losses in the gearbox and to reduce these losses, Computational Fluid Dynamics (CFD) simulation that analyzes oil churning loss and windage loss was developed and improvements were made to the shroud of bevel gears. The CFD agreed with experimental results on a bevel gearbox of a 100-seater aircraft. And, it was shown that the suppression of momentum transfer from the rotating gears to oil clusters is of importance. In addition, it was revealed that the loss was reduced up to 36% compared to non-shrouded gears by shrouding in the experiments. This CFD simulation can be applied to many types of gearboxes that have spur gears, bearings and seals.Copyright

A study of the heat transfer characteristics for a horizontal dry storage system for LWR spent fuel assemblies

Heat transfer experiments were carried out for natural convection in an enclosure, simulating a dry shielded canister with 24 PWR spent fuel assemblies. The objectives of this study were to investigate the thermal hydraulic characteristics of natural convection in the canister and to establish an evaluation method for heat removal from the canister under combined natural convection and thermal radiation. In these experiments, a neon and nitrogen gas mixture was used as a working fluid, based on a thermal hydraulic similarity, in order to simulate exactly the ratio of natural convection and thermal radiation in the experimental apparatus to that in an actual system. The results were analyzed in detail by using a thermal hydraulic analysis computation. It was found that the temperature difference ΔT in the canister was proportional to P−0.3 and the average heat transfer coefficient correlated approximately with the equation: Nu = 0.07Ra1/4.

Development of Induction Melting System With Active Insulator for Radioactive Solid Waste

The melting treatment is suitable for reducing the volume of the wastes because of the high volume reduction ratio (the volume reduction ratio is the initial volume to the volume after treatment). We have developed a new high-frequency induction melting system for the low-level radioactive miscellaneous solid wastes. The non-conductive ceramic canister and a heat loss compensator (Active insulator) were used in this new system. It is difficult to melt a large amount of the non-metallic materials with the canister. We solved this problem by using the active insulator, which was made of the conductive material. Melting performance confirmation tests were performed in the medium-scale melting system. Based on the result of the medium-scale melting test and multi-dimensional thermal-hydraulic analysis, the full-scale melting system was designed and constructed. We performed the melting tests using the full-scale melting system. the volume ratio of the non-metallic wastes at the re-solidification was more than 70%. Behavior of nuclides was also investigated with non-radioactive Co and Cs tracers. The residual ratio of Co and Cs were 97%, 58%, respectively.Copyright