1,720,998 research outputs found
Numerical investigation on water deteriorated turbulent heat transfer regime in vertical upward heated flow in circular tube
No full textMixed convection in vertical upward heated flow has been intensively studied in the last few years since it reduces the heat transfer capability of the system in the deteriorated turbulent heat transfer (DTHT) regime. Several mixed turbulent heat transfer correlations have been proposed, but the best correlation to be used in DTHT regime for water in vertical upward heated flow is yet to be determined. Furthermore, the relation between momentum flux and heat flux in mixed convection regime is still unclear. This paper presents comparison of several mixed turbulent heat transfer correlations performance of water up-flow in a circular tube by numerical approach using v(2-)-f turbulence model. The existing correlation of Symolon et al. showed the best agreement with the numerical analysis results in predicting the heat transfer capability of a water-cooled system while Petulchov and Polyakov correlation showed a good agreement to correlate the momentum and heat transport behavior in the mixed convection turbulent regime
Numerical Investigation of a Centrifugal Compressor for Supercritical CO2 as a Working Fluid
No full textThe supercritical carbon dioxide (S-CO2) Brayton cycle is considered as a strong candidate for power conversion systems. This includes concentrated solar power, coal power, bottoming cycle to fuel cells, and the next generation nuclear systems. In the previous studies, it was identified that the compressor consumes very small compressing work as operating condition approaches to the critical point. Thus, smaller amount of input work contributes to the enhancement of overall cycle efficiency. To achieve an efficient S-CO2 cycle, one of the major technical challenges exists in the compressor design. At KAIST, a research team is conducting a S-CO2 compressor tests to obtain fundamental data for advanced compressor design and to measure the performance of the compressor near the critical point. The measurements reveal the S-CO2 fluid to have properties of gases and liquids at the same time, but in regards to compressibility and density variation, its behavior is much closer to the liquid rather than gas near the critical point. In this paper, a CFD analysis of S-CO2 centrifugal compressor with the full geometry including diffuser and volute is presented. The numerical results are compared to the experimental data from KAIST SCO2 Pressurizing Experiment facility. A 3D grid was generated starting from the model of the compressor full geometry provided by the manufacturer. Furthermore, a property table of CO2 was generated by an in-house code and implemented to the CFD code. Then the performance characteristic of S-CO2 compressor is investigated in terms of compressor efficiency and pressure ratio. Additional flow variables inside the compressor such as velocity, pressure and viscosity are also investigated to help understanding the main reason behind the relatively higher compressor efficiency near the critical point compared to other flow conditions far from this region. In general acceptable results in comparison to the experiment are obtained (order of error from 0.5 to 7% for the compressor efficiency). Hence, the current CFD results should be able to provide additional and detailed information to be used for design enhancements of the compressor for S-CO2 Brayton power cycle
Computational investigation into heat transfer coefficients of randomly packed pebbles in flowing FLiBe
No full textUsing CFD simulations, this study presents: (1) LES and RANS comparisons for a unit cell geometry of heat transferring pebbles in flowing FLiBe; (2) statistical distributions of pebble heat transfer coefficients due to the random packing; (3) development of a heat transfer coefficient correlation for randomly-packed heat transferring pebbles in flowing FLiBe in the representative Fluoride High-temperature Reactor (FHR) operating condition; and (4) comparison of the developed heat transfer coefficient correlation with existing ones obtained in different fluids and pebble sizes. The LES and RANS comparison has shown that the k-ω Shear Stress Transport (SST) with low Reynolds number approach performance gives the best result in comparison to the two eddy-viscosity based k-ε and k-ω SST models.
Investigation of the pressure vessel lower head potential failure under IVR-ERVC condition during a severe accident scenario in APR1400 reactors
No full textIn the event of a core meltdown in a high-power reactor, the integrity of the reactor pressure vessel is presumably protected by severe accident mitigation systems such as in-vessel retention external reactor vessel cooling (IVRERVC). However, in the late phase of the accident, two possible locations on the RPV are prone to failure: the location of the focusing effect and location of in-core instrument penetration. These two potential points of damage in the RPV are investigated in this study. A numerical model for the prediction of the natural convection, melting, and solidification processes for IVR-ERVC is presented. The model is based on the enthalpy-porosity approach with an extension for continuous liquid fraction function. The model is implemented in open-source field operation and manipulation (OpenFOAM) computational fluid dynamic code to produce a new solver which is based on the combination of conjugate heat transfer solver and buoyant-driven natural convection solver and the new solver is validated against the melting Gallium experimental test, in-core instrumentation failure experimental test, and BALI experimental test. This numerical model is applied for the investigation of the RPV rupture at the location of the focusing effect and in-core instrumentation penetrations. Severe ablations of the cladding and the weld materials are observed at a heat load of about similar to 1800 K which is expected to lead to the ejection of the penetration tubes if the force holding the penetration tube in place is lower than the force exerted by the system pressure. Subsequently, a two-layer IVR configuration is assessed and the integrity of the RPV is found not to be compromised under external reactor vessel cooling. However, in the case of a boiling crisis, the temperature of the ex-vessel wall is expected to rise quickly and this is simulated by increasing the exvessel wall temperature. The RPV is found to fail near the beltline due to a phenomenon known as focusing effect when the ex-vessel wall temperature rises above 1200 K.
Studies of various single phase natural circulation systems for small and medium sized reactor design
No full textPassive safety is a primary motive behind the development of small and medium sized reactors of various coolants. After the Fukushima accident, there is an increased interest in a nuclear reactor's reliance on passive safety systems. Most of the existing passive systems, regardless of the reactor type, utilize buoyancy force to drive the cooling flow. Hence, it is essential to evaluate if the naturally developed cooling flow is sufficient to maintain the heated surface temperature of the fuel elements below the design limit. Evaluating passively driven flows can be quite a challenging task in both two phase natural circulation systems and also in single phase natural circulation systems. Previous research works have found that single phase heat transfer can be deteriorated and becomes uncertain when the driving force of a system is shifted from external force (forced convection) to self generated buoyancy force or a combination of both (natural or mixed convection). In this paper, single phase gas, water, and liquid metal reactors with passive systems are reviewed briefly. A simple theoretical analysis of each reactor type is performed to find the tendency of the shift in the operating heat transfer regime into the deteriorated region. The analysis results show that single phase water system can maintain operation within the forced convection regime but the operating regime gets closer to the deteriorating heat transfer regime as the system's physical size reduces from a large nuclear power plant to the small and medium reactor scale. The gas cooled system has a high tendency to operate in the deteriorated heat transfer regime when the driving force changes from forced to natural. Meanwhile the liquid metal system demonstrates more margins to operate outside from the deteriorated heat transfer region compared to the two other fluid systems. However further studies are needed to clearly identify the boundaries of the deteriorated heat transfer regime for each coolant since the deterioration greatly depends on the thermophysical properties variation of the coolant and the near-wall flow behavior of the coolant with respect to temperature change. (C) 2013 Elsevier B.V. All rights reserved
Study of Safety Injection Tank Performance Using MARS and CFD
No full textThe APR1400 has four Safety Injection Tanks with fluidic devices to control the flow passively. Because of the complex flow behavior, it is hard to predict the performance of the fluidic device. Therefore, CFD and MARS analyses were performed to compare with experimental results. The accumulator model predicted the results quite well but was not able to model nitrogen entrainment. The pipe model showed little difference in the flow but nitrogen entrainment was too high. CFD results will be introduced in the conference
Numerical Analysis of Single-Phase Thermal Hydraulic Parameters Along Nanostructured Coating Film
No full textIn typical pressurized water reactors, zirconium alloys are used as cladding material for the fuel. However, zircalloy is known to face problems with the high temperature steam, due to the chemical process of oxidation, the oxygen molecules will be separated from the water molecules of the coolant leading to hydrogen gas releases. Recently, a research team at KAIST, South Korea suggested a methodology to fabricate nanoporous oxide layer with the aim of preventing the zircalloy outer surface from reacting with the coolant. Although, this new proposal offers a better solution to prevent the potential hydrogen gas generation, it is still not well understood how the nanoporous-layer is going to affect the convective heat transfer rates between the coolant and the fuel. In fact, on one hand the low conductivity of the oxide layer is expected to reduce the conduction heat transfer within the cladding material; but on the other hand, the nanopores on the oxide layer might act as an effective surface roughness, hence affecting both the hydrodynamic and thermal fields within the coolant channels. In this study, a CFD analysis is carried out to investigate the influence of this nanoporous layer on the convective heat transfer rate and pressure drop coefficient. A detailed 2-D steady-state numerical analysis on single-phase model is performed using Star-CCM+ code. The study is conducted using pores with a diameter of 30 to 100 nm. The results obtained from these predictions are then compared with the ones obtained in the case of the smooth surface. Therefore, the main objectives of the present study are to examine the effect of this nanopourous layer on the thermal hydraulic parameters and to produce the corresponding correlations to be used in the system scale thermal-hydraulic codes
COMPUTATIONAL STUDY OF THE SAFETY INJECTION TANK PERFORMANCE
No full textInside the Safety Injection Tanks (SIT) of APR1400s, fluidic devices (FD) are installed to passively control the mass flow discharged from the SIT, thus, eliminating the need for low pressure safety injection pumps. As passive safety mechanisms are emphasized more and more nowadays, it has become more important to understand the flow structure and the loss mechanism within the fluidic device. However, current computational fluid dynamics (CFD) calculations have had limited success in predicting the fluid flow accurately. This study proposes to find a more exact result using CFD and more realistic modeling. Preliminary CFD results match quite well with the experiment data. Comprehensive CFD results also match well with the experiment results and show interesting patterns which was analyzed using Fast Fourier Transform(FFT). Calculation is still in progress but current status shows promising result
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