Shumpei Funatani

Experiment and Numerical Analysis of Control Method of Natural Circulation by Injection of Helium Gas

This study is to investigate a control method of the natural circulation of the air by the injection of helium gas. A depressurization is the one of the design-basis accidents of a Very High Temperature Reactor (VHTR). When the primary pipe rupture accident occurs in the VHTR, the air is predicted to enter into the reactor pressure vessel from the breach and oxidize in-core graphite structures. Finally, it seems to be probable that the natural circulation flow of the air in the reactor pressure vessel produce continuously. In order to predict or analyze the air ingress phenomenon during the depressurization accident of the VHTR, it is important to develop the method for prevention of air ingress during the accident. In this study, the air ingress process is discussed by comparing the experimental and analytical results of the reverse U-shaped channel which has parallel channels.The experiment of the natural circulation using a circular tube consisted of the reverse U-shaped type has been carried out. The vertical channel is consisted of the one side heated and the other side cooled pipe. The experimental apparatus is filled with the air and one side vertical tube is heated. A very small amount of helium gas is injected from the top of the channel. The velocity and the mole fraction of each gas are also calculated by using heat and mass transfer numerical analysis of multi-component gas.The result shows that the numerical analysis is considered to be well simulated the experiment. The natural circulation of the air has very weak velocity after the injection of helium gas. About 780 seconds later, the natural circulation suddenly produces. The natural circulation flow of the air can be controlled by the method of helium gas injection. The mechanism of the phenomenon is found that mole fraction is changed by the molecular diffusion and the very weak circulation.Copyright

Study on Mixing Process of Two Component Gases in a Vertical Fluid Layer: Effectiveness on One-Dimensional Natural Circulation

This study is to investigate an effect of natural convection or natural circulation on a transport process by molecular diffusion in a stratified fluid layer consisting of two component gases. There are many experiment and analysis regarding natural convection or natural circulation in the vertical slot. However, there are few studies on natural convection or circulation and molecular diffusion in the stratified fluid layer consisting of two component gases. It was confirmed that these phenomena appear when the depressurization accident occurs in the very high temperature reactor (VHTR). Therefore it is important to evaluate the transport and mixing processes during the depressurization accident of the VHTR.The experiment has been performed regarding the combined phenomena of molecular diffusion and natural convection or circulation in a two parallel vertical slots filled with two component gases. The vertical slot consists of the one side heated wall and the other side cooled wall. The other slot consists of the two cooled walls. The dimension of heated wall is 500mm×200mm and thickness is 3mm. The width of the slot is 20mm and the aspect ratio is 25. Combination of nitrogen(N2)/argon(Ar), neon(Ne)/argon(Ar), helium(He)/nitrogen(N2) and helum(He)/argon(N2) was used as the first step of the experiment of the two component gases. The density change of the gas mixture and the gas temperature distribution in the slots were obtained. The mixing process of the heavier gas from the bottom side of the slot filled with the lighter gas was discussed in this paper.The experimental results showed that the transport phenomena by the molecular diffusion were influenced by the localized natural convection or circulation of the gas mixture in the stratified fluid layer. From the experimental results, it was found that the mixing process by molecular diffusion was affected significantly by the natural convection or circulation induced by the slight temperature difference between both vertical walls.© 2012 ASME

Study on Control Method of Natural Circulation by Injection of Helium Gas

This study is to investigate a control method of natural circulation of air by injection of helium gas. A depressurization accident is one of the design-basis accidents of a Very High Temperature Reactor (VHTR). When the primary pipe rupture accident occurs in the VHTR, air is expected to enter into the reactor pressure vessel from the breach and oxidize in-core graphite structures. Finally, it seems to be probable that the natural circulation flow of air in the reactor pressure vessel produce continuously. In order to predict or analyze the air ingress phenomena during the depressurization accident of the VHTR, therefore, it is important to develop the method for prevention of air ingress during the accident.The experiment has been carried out regarding natural circulation using a circular tube consisting of the loop type or the reverse U-shaped type. The vertical channel consists of the one side heated tube and the other side cooled tube. The experimental results were obtained as follows. When the temperature difference between the vertical tubes was kept at 52K, the velocity of natural circulation flow became about 12cm/s. During this steady state, a small amount of helium injected to the channel. Then, the flow velocity of natural circulation suddenly decreased. The volume of injected helium is about 3% of the total volume of the channel. The velocity became around zero. After 1500 seconds elapsed, the natural circulation suddenly produced again. The experimental results show that the natural circulation flow of air can be controlled by the method of helium gas injection. This paper also discusses an overview of the method for the prevention of air ingress during the primary pipes rupture accident.Copyright

Performance Evaluation of Ground Source Heat Pump Using Direct Expansion Method: -In Case of Hot Water Supply System-@@@-給湯システムの場合-

1. ABSTRACT This paper describes the experimental results and the performance of ground source heat pump using the direct expansion method. Ground source has potential energy, which does not come from the earth center, but from the sun, at local places. This kind of energy is clean and does not affect the environment. The underground temperatures are more stable than air temperature. The temperature beneath the about 10 m from the surface maintains a nearly constant temperature about 15-20°C. Ground Source Heat Pump (GSHP) absorbs energy from the earth by applying this constant temperature. Authors paid attention to shallower depth than 100 m under the ground surface. In order to obtain the basic data, authors tried to experiment on GSHP based on direct expansion. In this experiment, performance evaluation was carried out during 48 hours operation. The performance of the ground sources heat pump system was evaluated by coefficient of performance (COP), which is determined by the ratio of heat exchange rate to consumption power of the compressor. The COP approached to 12.3 in cooling mode and 4.2 in heating mode. Totally, COP of GSHP with direct expansion method is higher than that in airsource type heat exchanger or indirect heat exchange method. In addition, long term test operations have to be considered for practical application in the near future.

Development of Technique for Controlling Natural Circulation Flow by Using Helium Gas

The purpose of this study is to investigate a control method of natural circulation flow of air by injection of helium gas. A depressurization accident by the primary pipe rupture is one of the design-basis accidents of a Very High Temperature Reactor (VHTR). When the double coaxial duct connecting between a reactor core and an intermediate heat exchanger (IHX) module breaks, air is expected to enter the reactor pressure vessel from the breach and oxidize in-core graphite structures. Then, it seems to be probable that the natural circulation flow of air in the reactor pressure vessel produce continuously. In such condition, injection of helium gas into the channel by a passive method can prevent occurrence of the natural circulation flow of air in the reactor pressure vessel. Therefore, it is thought that oxidation of in-core graphite structures by air ingress can be prevented by establishing this method.The experiment has been carried out regarding the natural circulation flow using a circular tube consisting of a reverse U-shaped type. The vertical channel consists of one side heated tube and the other side cooled tube. The experimental procedure is as follows. Firstly, the apparatus is filled with air and one vertical tube is heated. Then, natural circulation of air will be produced in the channel. After the steady state is established, a small amount of helium gas is injected from the top of the channel. The velocity, mole fraction, temperature of gas, and temperature of the tube wall are measured during the experiment. The results were obtained as follows. When the temperature difference between the both vertical tubes was kept at about 60K, the velocity of the natural circulation flow of air was measured about 0.17m/s. During a steady state, a small amount of helium gas was injected into the channel. When the volume of injected helium gas is about 5.7% of the total volume of the channel, the velocity of the natural circulation flow of air became around zero. After 810 seconds elapsed, the natural circulation flow suddenly produced again. The natural circulation flow of air can be controlled by injecting of helium gas.Copyright

Study on the Effect of One-Dimensional Natural Circulation on the Mixing Process of Two Component Gases

This study is to investigate the effect of one-dimensional natural circulation on the mixing process of two component gases by evaluating the onset time of natural circulation through the apparatus under the stable density stratified fluid layer.The experimental apparatus consists of a reverse U-shaped vertical slot and a storage tank. The left side vertical slot consists of the heated wall and the cooled wall. The right side vertical slot consists of the two cooled walls. Temperature difference between the vertical walls was set to 50, 70, and 100 K. In this study, the combination of the two component gases is He/Ar and density ratio of each component is 1.4/10.The heavy gas was filled with the storage tank and light gas was filled with the reverse U-shaped vertical slot. Before the experiment starts, the localized natural convection was generated in the heated side vertical slot. After the experiment starts, the heavy gas will be transported to the slot by the molecular diffusion and natural convection. And then, natural circulation occurs abruptly through the reverse U-shaped passage. The mixing process of two component gases and the onset time of natural circulation in the vertical fluid layer were affected not only by the localized natural convection but also by the molecular diffusion.The wall and gas temperatures were measured by thermocouples and the velocity of natural convection was measured to evaluate the characteristics of the mixing process and the natural convection.These experimental results show that generation time of natural circulation was affected by molecular diffusion and localized natural convection. When the two components of gases have large density ratios and large Gr numbers, the mixing process of two components of gases was affected by more intensively molecular diffusion than localized natural convection when temperature difference was 50K. The mixing process of two component gas was affected by more intensively localized natural convection than molecular diffusion when temperature difference was 70 to 100K. However, two component gases were affected by more intensively molecular diffusion than localized natural convection at small density ratios and small Gr numbers.© 2014 ASME

Heat Transfer and Fluid Flow Characteristics of One Side Heated Vertical Rectangular Channel Inserting Copper Wire

In the Very High Temperature Reactor (VHTR) which is a next generation nuclear reactor system, ceramics and graphite are used as a fuel coating material and a core structural material, respectively. Even if the depressurization accident occurs and the reactor power goes up instantly, the temperature of the core will change slowly. This is because the thermal capacity of the core is so large. Therefore, the VHTR system can passively remove the decay heat of the core by natural convection and radiation from the surface of the reactor pressure vessel (RPV). The objective of this study is to not only investigate heat transfer characteristics of natural convection of a one-side heated vertical channel inserting the porous materials with high porosity but also develop the passive cooling system for the Very-High-Temperature Reactor (VHTR). An experiment was carried out using the one-side heated vertical rectangular channel. In order to obtain the heat transfer and fluid flow characteristics of the vertical channel inserting porous material, we have also carried out a numerical analysis using a commercial CFD code. This paper describes thermal performances of the one-side heated vertical rectangular channel inserting copper wire with high porosity.Copyright

Study on Mixing Process of Two Component Gases in a Stable Stratified Fluid Layer: Effectiveness on Natural Convection

This study is to investigate an effect of natural convection or natural circulation on a transport process by molecular diffusion in a stratified fluid layer consisting of two component gases. There are many experiment and analysis regarding natural convection or natural circulation in the vertical slot. However, there are few studies on natural convection or circulation and molecular diffusion in the stratified fluid layer consisting of two component gases. It was confirmed that these phenomena appear when the depressurization accident occurs in the very high temperature reactor (VHTR). Therefore it is important to evaluate the transport and mixing processes during the depressurization accident of the VHTR.The experiment has been performed regarding the combined phenomena of molecular diffusion and natural convection or circulation in a two parallel vertical slots filled with two component gases. The vertical slot consists of the one side heated wall and the other side cooled wall. The other slot consists of the two cooled walls. The dimension of heated wall is 500mm×200mm and thickness is 3mm. The width of the slot is 20mm and the aspect ratio is 25. Combination of nitrogen (N2)/argon(Ar), neon(Ne)/Ar, helium(He)/N2 and He/Ar was used as the first step of the experiment of the two component gases. The density change of the gas mixture and the gas temperature distribution in the slots were obtained. The mixing process of the heavier gas from the bottom side of the slot filled with the lighter gas was discussed in this paper.The experimental results showed that the transport phenomena by the molecular diffusion were influenced by the localized natural convection or circulation of the gas mixture in the stratified fluid layer. From the experimental results, it was found that the mixing process by molecular diffusion was affected significantly by the natural convection or circulation induced by the slight temperature difference between both vertical walls.Copyright

Heat Transfer Characteristics of One Side Heated Vertical Rectangular Channel Inserting Porous Material

The objective of this study is to not only investigate heat transfer characteristics of natural convection of a one-side heated vertical channel inserting the porous materials with high porosity but also develop the passive cooling system for the Very-High-Temperature Reactor (VHTR). An experiment and analysis was carried out using the one-side heated vertical rectangular channel. From the results obtained in the experiment and analysis, it was found that an amount of removed heat by forced convection using the copper wire (porosity>0.996) was about 15% higher than that without the wire. It was also found that the amount of transferred heat from the heated wall will be increased even if the heat removed by natural convection. Furthermore, the ratio between the amounts of heat removed of the rectangular channel with the porous material and without the porous material increases with increasing temperature of the channel wall. In order to obtain the heat transfer and fluid flow characteristics of the vertical channel inserting porous material, we have also carried out a numerical analysis using a commercial CFD code. This paper describes thermal performances of the one-side heated vertical rectangular channel inserting copper wire with high porosity. From a view point of economical and safety characteristics, the passive cooling system should be designed for the VHTR as the best way of the system. Therefore, the gas cooling system by natural convection is the one of candidate system.Copyright