Hajime Akimoto

Experimental study on transition of flow pattern and phase distribution in upward air–water two-phase flow along a large vertical pipe

Abstract In order to investigate the dependency of gas–liquid two-phase flow on pipe scale, the transition characteristics of flow pattern and phase distribution were studied experimentally in upward air–water two-phase flow along a large vertical pipe (inner diameter D: 0.2 m, the ratio of pipe length to diameter L/D: 61.5). The experiments were conducted under the flow rate: 0.03 m/s ≤ superficial air velocity (at top of test section) ≤ 4.7 m/s, 0.06 m/s ≤ superficial water velocity JL ≤ 1.06 m/s. Flow pattern was observed and measurements were performed on axial differential pressure, phase distribution, bubble size and bubble and water velocities. The scale effect was discussed with small-scale data (D: 0.025–0.038 m). The flow conditions at which coalescence starts are almost the same as those found in small-scale pipes, but no large bubbles are observed in the region L/D 20. The churn flow is dominant in the large vertical pipe under the conditions where small-scale pipes have slug flow. The transition of phase distribution corresponds to the change of flow pattern. Large coalescent bubbles affect the phase distribution as similar to small-scale pipes but the following remarks are concluded as the scale effect: (1) under a low JL where small-scale pipes have a wall-peak phase distribution, a core-peak phase distribution is established, where some large eddies including bubble clusters fill up the pipe, (2) the large coalescent bubbles are developed along the test section via the churn bubbly flow where the phase distribution is a core peak one, whereas Taylor bubbles in small-scale pipes are generated at the vicinity of gas–liquid mixing region or are developed from the bubbly flow with a wall-peak phase distribution, (3) the wall-peak in the large vertical pipe is lower even under the same bubble size. The lower peak is considered to be related to the lower radial velocity gradient of water and the larger turbulent dispersion force.

An experimental study on developing air-water two-phase flow along a large vertical pipe : Effect of air injection method

Abstract The flow structure in a developing air-water two-phase flow was investigated experimentally along a large vertical pipe (inner diameter, Dh: 0.48 m, ratio of length of flow path L to Dh: about 4.2). Two air injection methods (porous sinter injection and nozzle injection) were adopted to realize an extremely different flow structure in the developing region. The flow rate condition in the test section was as follows: superficial air velocity: 0.02–0.87 m s (at atmospheric pressure) and superficial water velocity: 0.01–0.2 0.01–0.2 m/s, which covers the range of bubbly to slug flow in a small-scale pipe (Dh ⩽ about 0.05 m). No air slugs occupying the flow path were recognized in this experiment regardless of the air injection methods even under the condition where slug flow is realized in the small-scale pipe. In the lower half of the test section, the axial distribution of sectional differential pressure and the radial distribution of local void fraction showed peculiar distributions depending on the air injection methods. However, in the upper half of the test section, the effects of the air injection methods are small in respect of the shapes of the differential pressure distribution and the phase distribution. The comparison of sectional void fraction near the top of the test section with Kataokas correlation indicated that the distribution parameter of the drift-flux model should be modeled including the effect of Dh and the bubble size distribution is affected by the air injection methods. The bubble size distribution is considered to be affected also by L D h based on comparison of results with Hills correlation.

Model Development for Bubble Turbulent Diffusion and Bubble Diameter in Large Vertical Pipes

Multi-dimensional analyses have been expected recently with expanding computation resources for gas-liquid two- phase flow analyses of advanced nuclear systems such as passive safety systems and natural-circulation-type reactors. However, the applicability of previous constitutive equations for multi-dimensional analyses has not been fully investigated especially for the effects of flow path scale because the equations have been assessed for small-scale experiments. In this study, we analyzed the scale effects by the multi-dimensional two-fluid model code using data in 38 mm and 200 mm diameter pipes. We clarified a key-parameter to model the scale effects and developed models for the effects on phase distribution. The scale effects can be classified by the relative relationship between bubble diameter db and turbulent length scale lT . Bubble-induced turbulence is increased under that db is smaller than lT and bubble coalescence is predominated rather than breakup under that lT is about three times larger than db and under higher void fraction. Based on these findings, we established new models for bubble turbulent diffusion and bubble diameter. The applicability was promising through assessments against the 38 mm and 200 mm pipes under different flow rates and against databases for 60 mm, 100 mm and 480 mm pipes.

Critical Power Correlation for Axially Uniformly Heated Tight-Lattice Bundles

Abstract Critical power experiments were carried out, and the critical power correlation for axially uniformly heated tight bundles has been derived based on the present experimental data and data sets measured by the Bettis Atomic Power Laboratory. The shape of the test section simulates the fuel assembly of the reduced-moderation water reactor (RMWR), which is a water-cooled breeder reactor with a core of the tight triangular fuel rod arrangement. The obtained correlation covers the following conditions: channel geometry (triangular arrangement bundle of 7 to 20 rods, 6.6 to 12.3 mm in rod diameter, 1.0- to 2.3-mm gap between rods, 1.37 to 1.8 m in heated length), mass velocity of 100 to 2500 kg/(m2s), inlet quality of –0.2 to 0, pressure of 2 to 8.5 MPa, and radial peaking factor of 0.98 to 1.5, which include uniform, center-peak, and liner transverse heat flux distribution data. An excellent agreement was obtained between the developed correlation and data (371 points) within an error of ±4.6%.

Study on point of net vapor generation by neutron radiography in subcooled boiling flow along narrow rectangular channels with short heated length

Abstract Point of net vapor generation (PNVG) was investigated based on the void fraction dataset obtained by high-frame-rate neutron radiography. The test channels used in the experiment were rectangular channels heated from one side with channel gap of 3 and 5 mm, channel width of 30 mm, and heated length of 100 mm. In this study, we discuss on (1) the determination of the instantaneous and time-averaged PNVG, (2) the effects of system parameters on PNVG, (3) the applicability of existing PNVG correlations to the channel with a short heated length, and (4) the effect of the PNVG in critical heat flux (CHF) model. The following results were obtained: (a) the effects of system parameters on the thermal equilibrium quality at the PNVG were small under the present conditions, (b) existing PNVG correlations tended to underestimate the thermal equilibrium quality at the PNVG in the channel with a short heated length, and (c) the prediction accuracy of Katto’s CHF model could be improved significantly by using the accurate PNVG.

Void fraction measurement in subcooled-boiling flow using high-frame-rate neutron radiography

Abstract A high-frame-rate neutron radiography (NR) technique was applied to measure the void fraction distribution in forced-convective subcooled-boiling flow. The focus was experimental technique and error estimation of the high-frame-rate NR. The results of void fraction measurement in the boiling flow were described. Measurement errors on instantaneous and time-averaged void fractions were evaluated experimentally and analytically. Measurement errors were within 18 and 2% for instantaneous void fraction (measurement time is 0.89 ms), and time-averaged void fraction, respectively. The void fraction distribution of subcooled boiling was measured using atmospheric-pressure water in rectangular channels with channel width 30 mm, heated length 100 mm, channel gap 3 and 5 mm, inlet water subcooling from 10 to 30 K, and mass velocity ranging from 240 to 2000 kg/(m2·s). One side of the channel was heated homogeneously. Instantaneous void fraction and time-averaged void fraction distribution were measured parametrically. The effects of flow parameters on void fraction were investigated.

SCTF-III test plan and recent SCTF-III test results☆

Abstract A test plan of the Slab Core Test Facility with Core-III (SCTF-III) has been clarified. The previous SCTF-III tests simulating PWRs with combined-injection-type ECCS indicated the following results on the thermal-hydraulics in the full-radius core: 1. (1) two-region separation and multidimensional thermal-hydraulics both before and after reflood initiation. 2. (2) practically no core cooling in the region far from the water downflow region before reflood initiation in spite of good core cooling in the water downflow region. 3. (3) good core cooling even in the two-phase upflow region after bottom reflood initiation. 4. (4) large fall-back flow rate in comparison with the prediction by a typical one-dimensional correlation.

Critical Power Correlation for Tight-Lattice Rod Bundles

Developing design correlation for the prediction of critical power in rod bundles is indispensable for R&D of Reduced-Moderation Water Reactor (RMWR) which adopts a triangular tight-lattice fuel rod configuration and axially double-humped-heated profile. In this research, critical power correlation for tight-lattice rod bundles is newly proposed using 7-rod axially uniform-heated data, 7-rod and 37-rod axially double-humped-heated data at Japan Atomic Energy Research Institute (JAERI). For comparatively high mass velocity region, the correlation is written in local critical heat flux-critical quality type. For low mass velocity region, it is written in critical quality-annular flow length type. The standard deviation of ECPR (Experimental Critical Power Ratio) to the whole JAERI data (694 data points) is 6%. The correlation is verified by Bettis Atomic Power Laboratory data (177 points, standard deviation: 7.7%). The correlation is confirmed being able to give good prediction for the effects of mass velocity, inlet temperature, pressure and heated equivalent diameter on critical power. The applicable range of the correlation is: Rod number lower than 37, rod gap from 1.0 to 2.29 mm, heated length from 1.26 to 1.8 m, mass velocity from 150 to 2,000 kg/m2·s and pressure from 2 to 11 MPa.

Three-dimensional analysis of water-vapor void fraction in a fusion experimental reactor under water ingress

Abstract An integrated Ingress-of-Coolant Event (ICE) test facility was constructed to demonstrate that the International Thermonuclear Experimental Reactor (ITER) safety design approach and design parameters for the ICE are adequate. The integrated ICE test facility simulates the actual ITER components with a scaling factor of 1/1600. Before the integrated ICE experiments the water–vapor two-phase flow characteristics inside the test facility during the ICE were predicted by the three-dimensional numerical simulations using the modified Transient Reactor Analysis Code (TRAC). From the present study it was clarified numerically that the actual ITER safety approach during the ICE is adequate and the present numerical method is very effective to predict the water–vapor void fraction during the ICE.

Downcomer Effective Water Head during Reflood in Postulated PWR LOCA

A study was conducted of the downcomer effective water head which is the only driving force to supply emergency core coolant into core during reflood phase in a PWR loss-of-coolant accident. With a full height downcomer simulator, effective water head experiments were carried out under 1 atm to investigate the applicability of the correlation for void fraction for evaluating the effective water head as well as to investigate the effect of the scale factor. As the results, the effect of the scale factor (the gap of the downcomer) was revealed to be significant, that is, the smaller gap gives the smaller effective water head. From the comparison of predictions based on the correlation for void fraction with the experimental results, it was revealed that (1) for a slow effective water head change expected in the reflood phase, the used correlation gives a good prediction for the experimental results and that (2) for a rapid change of effective water head, however, more elaborate investigation is needed. It w...