Hideki Nariai

Solubility of CO2 and density of CO2 hydrate at 30 MPa

Data needed for evaluation of ocean CO2-sequestering technologies include the solubility of CO2 and density of CO2 hydrate at 30 MPa. Theses were measured using a high-pressure facility suitable for simulating pressure and temperature conditions 3000 m beneath the ocean surface. The solubility decreases linearly with decreasing water temperature, which is opposite to the temperature dependence in the non-hydrate region. The measured solubility agrees with the published value for the non-hydrate region at an equilibrium temperature of 12 °C. The hydrate density was estimated from pressure drop, temperature increase, and concentration changes, followed by precipitation of CO2 hydrate. The estimated CO2 hydrate density varies from 1.11 to 1.09, depending on the fraction of CO2 in hydrate form.

Evaluation of subcooled critical heat flux correlations for tubes with and without internal twisted tapes

Abstract Eleven correlations and models for critical heat flux (CHF) of subcooled flow boiling in water were evaluated. Both a direct substitution method (DSM) and a heat balance condition method (HBM) were compared in the evaluations. The HBM was recommended as a better prediction method in the present study. For straight tubes under uniform heating conditions, the correlations of the Gunther, Knoebel, modified Tong, W-2, and Tong-75, and also the Celata and Weisman-Pei models were confirmed to give reasonably good predictions. Among them, the Celata model was the best with respect to accuracy. For swirl flow under uniform heating conditions. Tong-75-I (involving modification of the water velocity parameter) and Nariai-Inasaka correlations were confirmed to give reasonably good predictions, even though their predictions were too low for the CHF under non-uniform heating conditions.

Fluid and pressure oscillations occuring at direct contact condensation of steam flow with cold water

Abstract Pressure and fluid oscillations at pressure suppression containment and cold leg flow oscillations at Emergency Core Cooling Water Injection in water-cooled nuclear power reactors are studied concerning flow oscillations occuring at direct contact condensation of steam flow with cold water. Classifications and mechanisms of these oscillation patterns are presented. Linear frequency analyses are developed in explaining the oscillation frequency and oscillation threshold of both condensation oscillation and plug oscillation. Oscillation frequencies at chugging and at ON-OFF oscillation are examined with analytical models developed for such large amplitude oscillations. Analytical results with these models are compared with the data by simulation experiments. Discussions are made focusing on the applicability of the analyses.

Thermal-hydraulic behavior of a marine reactor during oscillations

Abstract The effect of ship motion, such as heaving and rolling, on the thermal-hydraulic behavior of marine reactors was investigated. The COBRA-IV-I CODE was modified to analyse the thermal-hydraulic performance on the critical heat flux under oscillating acceleration conditions. The critical heat flux in the code was verified experimentally using freon as a comparison. The Critical Heat Flux Ratio (CHFR) at the hottest channel of the PWR subchannel was analysed using the same code. A system code RETRAN-02/MOD2-GRAV was developed by improving RETRAN-02/MOD2 to simulate the thermal hydraulic transient under ship motion. It was verified by comparison using the experimental results of both two-phase natural circulation flow under heaving motion and single-phase natural circulation flow at an inclined attitude. The code was used to analyse reactor plant behavior in the nuclear ship Mutsu. Natural circulation flow during rolling motion was investigated experimentally. The characteristics of loop flow and core flow rates were clarified. The core flow rate correlated well with the Reynolds number of rolling motion.

Evaluation of heat-transfer coefficient at direct-contact condensation of cold water and steam

The estimation of the heat transfer coefficient at the direct-contact condensation of cold water and steam is a very hard task since the phenoma are essentially undsteady and the interface motion is so complicated that an exact estimation of its area is almost impossible. The present study shows the heat transfer coefficient evaluated experimentally by assuming simple interface shapes for complicated surfaces and estimated those through comparison of the numerical analyses to the data of experiments related to the loss of coolant accidents of light water reactors. At chugging, the heat transfer coefficient reached up to 2 × 106W/(m2 K). At condensation oscillation, it ranged between 105–106 W/(m2 K). At a jet region of cold water injected into the steam flow in a pipe or the stationary steam in a vessel, the value was around 2 × 105W/(m2K), and at the surface of stratified flow, it was between 3 × 103–3 × 104W/(m2K).

Prediction of critical heat flux for subcooled flow boiling

A theoretical critical heat flux (CHF) prediction model is developed for the subcooled flow boiling based on the liquid sublayer dryout mechanism. The model is tested over a large data bank (about 2482 points), which is characterized by covering almost the entire physics scope, showing a general good accuracy. Parametric trends of the CHF in terms of mass flux, pressure, subcooling, channel diameter and ratio of heated length to diameter are studied with the aim of not only indicating the trends, but also giving the theoretical interpret. The model also shows good adaptation to non-uniform heating, twist tape insert and non-water (nitrogen and refrigerant 113) system.

Boundaries between regimes of pressure oscillation induced by steam condensation in pressure suppression containment

Abstract Boundaries between various regimes of pressure oscillation, occurring when steam is made to condense in pool water through vent tubes, were analyzed. “Bubbling”, characterized by low-frequency oscillation only, occurs between temperature boundaries of low- and high-frequency components which are obtained by the linear stability analysis for movement of the steam-water interface. The boundary between small chugging (consisting of low and high frequencies) and condensation oscillation (high frequency only) correlates well with the Hodgson Number to control pulsating gas flow. The threshold of internal chugging, characterized by periodic back flow of pool water into vent tubes, appears when the eq. term relating to the sucking up of pool water due to steam condensation balances the term relating to the pushing down of the interface due to supplied steam.

Self-Triggering Mechanism of Vapor Explosions for a Molten Tin and Water System

The self-triggering mechanism of vapor explosions was investigated analytically and experimentally using molten tin and water. First, we modeled a simple droplet system consisting of a hot liquid droplet in a pool of cold liquid. Then, to model the self-triggering mechanism, we assumed that an instability (i.e. perturbed oscillation) in the vapor/cold-liquid interface produces a collapse of the vapor film, which in turn would produce a vapor explosion. To investigate the stability of perturbed oscillations in a vapor film, we did a linear stability analysis of a vapor film surrounding a hot liquid. We found that there was a region of film stability in the cold-liquid temperature where spontaneous vapor explosions did not occur. To validate our model, we experimentally determined the thermal interaction zone (TIZ) in which spontaneous vapor explosions occur. The occurrence conditions for spontaneous vapor explosions were investigated for molten tin, as the hot liquid, dropped into a water pool, as the cold...

Pressure and Fluid Oscillations in Vent System due to Steam Condensation, (I): Experimental Results and Analysis Model for Chugging

Pressure and fluid oscillation in vent tubes and a header induced by steam condensation were measured with a test apparatus. Pressure oscillation consists of low-, middle- and high-frequency components. The frequencies measured in the present apparatus are around 2–8Hz, 15 Hz and 100–150 Hz for low, middle and high frequency respectively. The chugging phenomenon occurs in a certain range of steam flow rate. When the amplitude of fluid oscillation becomes maximum, the amplitude of pressure oscillation in the header also becomes maximum. High frequency component is predominant in the pressure oscillation in vent tubes. When the temperature of pool water becomes lower, the amplitude becomes larger. As the temperature of pool water gets higher, high-frequency component of pressure oscillation disappears, and middle, low frequency in order. Based on the experimental facts mentioned above, the theoretical analysis was conducted by considering the header as one volume and by modeling chugging as one-dimensional ...

The Trigger Mechanism of Vapor Explosion

In the present study, trigger mechanisms of the vapor explosion are experimentally investigated. The interfacial behavior between high temperature molten liquid and low temperature water are experimentally investigated by using a molten material droplet and external pressure pulse. As the results, it is indicated that spontaneous vapor explosion hardly occur in high temperature water near saturation temperature since vapor film is stable. The vapor explosion can occur even in high temperature water near saturation temperature in case that the external pressure pulse is applied to high temperature molten material. Vapor explosion can not occur when the interfacial temperature between the molten material and water is lower than the material melting temperature, even if the vapor film around the molten material is collapsed by the external pressure pulse. It is clarified that the impossibility of the trigger process for the vapor explosion can be judged by comparing the interfacial temperature and the molten material temperature. The results obtained in the present experiments are applied to the results of the large-scale experiments using uranium dioxide. The results indicate that the possibility of the vapor explosion of the uranium dioxide and water under the present LWR operational condition is extremely unlikely. It should be noted that the present criteria should be applicable in case that the melting temperature does not decrease by containing the metal component.