Makoto Hishida

Studies on molecular diffusion and natural convection in a multicomponent gas system

Abstract Experimental and numerical studies have been carried out on the combined phenomena of molecular diffusion and natural convection with a graphite oxidation reaction in a multicomponent gas system to investigate the process of air ingress into a reverse U-shaped tube consisting of one side heated and the other side cooled pipes. The range of the Grashof number based on the height of the tube was about 3.7 × 10 9 Gr L 11 . One-dimensional basic equations for continuity, momentum conservation, energy conservation of the gas mixture, and mass conservation of the gas species are numerically solved to obtain concentration changes in the gas species and the onset time of the natural circulation of air. The experimental results showed that air entered the tube due to molecular diffusion and a very weak natural convection of the multicomponent gas mixed prior to the onset of the natural circulation of air. The calculated results are in good agreement with the experimental ones regarding the concentration changes in the gas species and the onset time of the natural circulation of air.

Study on air ingress during an early stage of a primary-pipe rupture accident of a high-temperature gas-cooled reactor

Abstract A primary-pipe rupture accident is one of the design-based accidents of the HTTR. As the first step of our final goal of predicting the multicomponent gas flow in a reactor during the early stages of the accident, the present paper aims at studying experimentally and analytically, the basic features of air ingress and gas transportation by transient molecular diffusion and the transient natural convection of a two-component gas mixture. The present paper comprises two main parts. The first part deals with analytical and experimental studies on N2 ingress (corresponding to air ingress) and gas transportation by molecular diffusion and the one-dimensional natural convection of an He-N2 two-component gas mixture in a reverse-U-shaped tube. Analytical and experimental results are discussed on the N2 mole fraction change with time after the simulated pipe rupture and on the initiation time of the natural circulation of pure N2. The second part deals with a preliminary simulation test of air ingress during the early stages of the accident. The test is performed with a very simple model of the reactor. The experimental results are discussed on the change in mole fraction of air with time and on the initiation time of the natural circulation of pure air.

Studies on diffusion and natural convection of two-component gases

Abstract A primary-pipe rupture accident is one of the design-based accidents of a high-temperature engineering test reactor (HTTR), which is being developed at JAERI. When the primary pipe ruptures, air is expected to enter into the reactor core from the breach by molecular diffusion and natural convection. In order to investigate the process of air ingress during the early stage of the primary-pipe rupture accident, experimental and analytical studies are performed on the conjugate phenomenon of the transient molecular diffusion and natural convection of a two-component gas mixture in two test sections, a reverse-U-shaped tube and a test model simulating simply the reactor. One-dimensional basic equations for continuity and momentum conservation are numerically solved to obtain a concentration change of gas species and an initiation time of a natural circulation of pure nitrogen in the reverse-U-shaped tube. Moreover, a modified numerical solution is proposed to reduce the computing time. A one-dimensional flow net work model is employed to calculate the transport process of air in the test model simulating the reactor. The calculated results agree well with the experimental ones on the concentration change of gas species and the initiation time of the natural circulation of pure nitrogen or pure air.

Experimental studies on the thermal and hydraulic performance of the fuel stack of the VHTR: Part I. HENDEL single-channel tests with uniform heat flux

The Helium Engineering Demonstration Loop (HENDEL) was constructed for a large-scale component test of the very high temperature gas-cooled reactor (VHTR) under simulated reactor operating conditions. The fuel stack test section (T1) of HENDEL simulates the fuel stack of the VHTR core to investigate thermal and hydraulic performance. n nExperimental results showed that the turbulent heat transfer characteristics were maintained down to a low Reynolds number of about 2000. Friction factors and heat transfer coefficients for the fuel rod were found to be higher than those in a concentric smooth annular channel.

Numerical Analysis of Buoyancy-Driven Exchange Flow with Regard to an HTTR Air Ingress Accident

A three-dimensional thermal-hydraulic code using boundary-fitted coordinates systems has been developed to predict incompressible flows with complex geometries and large variations of physical properties. This code has been applied to a buoyancy-driven exchange flow in an enclosed space consisting of an upper and lower hemisphere connected with a circular vertical pipe. The computational results have been compared with experiments. It was found that the computed heat transfer rate was smaller than that obtained from the experimental correlation in a single hemisphere at large Rayleigh number. This may be attributed to the effect on the flow behavior of a large variation of gas properties. Unsteady and asymmetric flow patterns such as observed in the experiments were numerically obtained in the vertical pipe.

Researches on air ingress accidents of the HTTR

Abstract This paper deals with experimental and analytical studies which have been performed to understand air ingress processes during primary-pipe and stand-pipe rupture accidents of the HTTR. Air ingress processes are summarized in the following. During the first stage of the primary-pipe rupture accident, air enters the reactor by molecular diffusion, a very small natural convection of gas mixture and local natural convections with high velocity. On the other hand, air enters the reactor by natural circulation of gas mixture in the second stage. During the stand-pipe rupture accident, air enters the reactor by He/air exchange flow in the first stage, and by natural convection of gas mixture in the second stage.

Helium-air exchange flow through annular and round tubes

Air ingress by buoyancy-driven exchange flow occurs during a standpipe rupture accident in a high-temperature engineering test reactor (HTTR). The exchange flow of helium and air through annular and round tubes is investigated. The method of mass increment is applied to measure the exchange flow rate. A test cylinder with a small tube on the top is used for the experiment. The following results were obtained: The exchange velocity is largest for the short vertical round tube as compared with the orifice and long tube. In the annular tube, the exchange-velocity or the volumetric exchange flow rate decreases with the equivalent diameter of the annular passage under 6 mm. The annular tube is effective to reduce the air ingress flow rate from the broken standpipe of the HTTR. In the inclined round tube, the inclination angle for the maximum densimetric Froude number decreases with the increase of the length-to-diameter ratio of the tube for the helium-air system. On the other hand, this angle remains almost constant for the water-brine system. Flow visualization results indicate that the exchange flows through the inclined round tubes take place smoothly and stably in the separated passage of the tube. The flow pattern in themorexa0» vertical annular tube seems to be similar to that in the inclined round tube.«xa0less

Visualization of Buoyancy-Driven Exchange Flow in an Enclosure by Numerical Simulation

The purpose of this paper is to investigate the phenomena buoyancy-driven exchange flow in an enclosure. A three-dimensional computer code using the Boundary-Fitted Coordinates method was applied to realistic simulation of curved boundary shape. The computed results were visualized by a graphic work station. Unsteady and asymmetric flow patterns such as observed in the experiments were obtained.

Local heat transfer coefficient of a ribbed surface.

The local heat transfer coefficient of a ribbed surface was experimentally measured on the base surface between two ribs. Two-dimensional square bars were attached to the lower wall of a parallel channel, the upper wall being a smooth wall. Experiments were carried out using air flow at atmospheric pressure and room temperature. The Reynolds number ranged from 3×103∼1×105, the ratio of pitch to height of the rib from 2.5 to 60 and the ratio of the height of the rib to the hydraulic diameter of the parallel channel from 0.0324 to 0.174. The distance from the position of the maximum local Nusselt number to the rib, and the local Nusselt number upstream and downstream from the maximum position were experimentally studied.

Thermohydraulic characteristics of coolant gas flow through a control rod channel in a very high temperature gas cooled reactor

Abstract A calculation code was developed to evaluate the thermohydraulic performance of a coolant flow through a control rod channel in a very high temperature gas cooled reactor (VHTR) and a high temperature engineering test reactor (HTTR). A one-dimensional flow network model was employed in the present calculation code. The calculated results agreed well with the experimental ones on the flow rate distribution and the total pressure loss in an isothermal coolant flow. The thermohydraulic characteristics of the HTTR control rod channel were evaluated by the code under various conditions, including the normal operating conditions of a HTTR.