S.Z. Qiu

Development of a solidification model for TEXAS-VI code and application to FARO L14 analysis

Abstract A molten fuel breakup model that considers solidification effects is proposed in this paper. Both the effect of a solid crust layer and the effect of thermal stresses on the fuel particle fragmentation are taken into account in this model. This solidification model predicts the transient temperature profile and crust layer thickness of the fuel particle by numerically solving the Fourier heat conduction equation under specific initial and boundary conditions. This fuel particle breakup model and transient temperature profile model were incorporated into the TEXAS fuel-coolant interaction (FCI) model; this revised TEXAS FCI model is called TEXAS-VI. This paper compares TEXAS-VI to the FARO L14 experiment (FARO L14), for which fuel-coolant mixing and quench data have been published. The FARO L14 pressure history, liquid water pool temperature, and vapor temperature were found to be in good agreement with the revised model predictions. This mixing behavior will also have an impact on FCI explosion energetics. The solidification effect is under investigation for energetics.

Analysis of KROTOS Steam Explosion Experiments Using the Improved Fuel-Coolant-Interaction Code TEXAS-VI

Abstract In the present study, we propose a new fragmentation criterion for the explosion phase to take account of the effect of partial fuel melt solidification on the rapid fragmentation process. This new criterion judges whether or not the explosive fragmentation can occur by comparing the impact stress induced by vapor film collapse and water jet impingement with the fracture toughness of the corium crust layer. The fragmentation criterion was incorporated into the revised Thermal EXplosion Analysis Simulation (TEXAS) fuel-coolant-interaction (FCI) model TEXAS-VI and combined with the previously proposed fuel particle solidification model and the fragmentation criterion for the mixing phase. TEXAS-VI was compared to KROTOS alumina test K-44 and corium tests K-52 and K-53, and good agreement was obtained. The simulation results indicate that TEXAS-VI has the capability to consider the effect of partial solidification for both the mixing and the explosion phases of the FCI process and can capture the effect of fuel solidification, which reduces corium-water explosion energetics. Experiments K-52 and K-53 also demonstrate the ability of TEXAS-VI to model the effects of ambient pressure on energetics.

Study on the Onset of Nucleate Boiling in Narrow Annular Channel by Using Wavelet and Wavelet Neural Network

In this study, the local modulus maxima of cubic B-spline wavelet transform are introduced to determine the location of onset of nucleate boiling (ONB). Wavelet transformation has the ability of representing a function and revealing the properties of the function in the joint local regions of the time frequency space. Based on wavelet and artificial neural network, a Wavelet Neural Network (WNN) model predicting ONB for upward flow in vertical narrow annuli with bilateral heating has been developed. The WNN mode combining the properties of the wavelet transform and the advantages of Artificial Neural Networks (ANN) has some advantages of solving non-linear problem. The methods of establishing the model and training of wavelet neural network are discussed particularly in the article. The ONB prediction is investigated by WNN with distilled water flowing upward through narrow annular channels with 0.95 mm, 1.5 mm and 2.0mm gaps, respectively. The WNN prediction results have a good agreement with experimental data. At last, the main parametric trends of the ONB are analyzed by applying WNN. The influences of system pressure, mass flow velocity and wall superheat on ONB are obtained. Simulation and analysis results show that the network model can effectually predict ONB.Copyright

Analysis of Flow Instabilities in Forced-Convection Steam Generator

Because of the practical importance of two-phase instabilities, substantial efforts have been made to date to understand the physical phenomena governing such instabilities and to develop computational tools to model the dynamics. The purpose of this paper is to present a numerical model for the analysis of flow-induced instabilities in forced-convection steam generator. The model is based on the assumption of homogenous two-phase flow and thermodynamic equilibrium of the phases. The thermal capacity of the heater wall has been included in the analysis. The model is used to analysis of flow instabilities in steam generator and to study the effects of system pressure, mass flux, inlet temperature and inlet/outlet restriction, gap size, the ratio of do/di and the ratio of qi/qo on system behavior.Copyright

Numerical Simulation on Collapse of Vapor Bubble Using Particle Method

In this paper, the collapse of a void bubble filled with vapor content is numerically investigated using a novel moving particle semi-implicit with meshless advection by flow-directional local grid (MPS-MAFL) method. The interfacial velocity, collapse time, bubble shape variation, peak pressure, rebound bubble radius, and other interesting parameters were obtained and are discussed profoundly. The vapor bubble undergoes several cycles of oscillation with reduced amplitude during the whole collapse process, which is similar to cavitation bubble collapse. The computational results show that the bubble collapse time is linearly proportional to the initial bubble size, which agrees with the Rayleigh equation. The minimum rebound bubble radius ratio is less affected by initial bubble size for a large bubble. Comparison work was also conducted against experimental data by Board and Kimpton. The comparison revealed that the MPS method supplied with an adiabatic compression assumption for vapor content is more suitable to evaluate the collapse behaviors of a low-pressure vapor bubble. This work is helpful for further application of the moving particle semi-implicit with meshless advection using flow-directional local grid (MPS-MAFL) method to solving complicated bubble dynamics.

Numerical Research on Oscillation of Two-Phase Flow Under Rolling Motion

Two-phase flow instability and dynamics of a parallel multichannel system has been theoretically studied under periodic excitation induced by rolling motion in the present research. Based on the homogeneous flow model considering the rolling motion, the parallel multichannel model and system control equations are established by using the control volume integrating method. Gear method is used to solve the system control equations. The influences of the inlet, upward sections, and heating power on the flow instability under rolling motion have been analyzed. The marginal stability boundary (MSB) under rolling motion condition is obtained. The unstable regions occur in both low and high equilibrium quality and inlet subcooling regions. The multiplied period phenomenon occurs in the high equilibrium quality region and the chaos phenomenon appears on the right of MSB. The concept of stability space is presented.© 2010 ASME

Research on Two-Phase Flow Instability in Parallel Multi-Channel Under Rolling Motion Condition

Two-phase flow instability of the parallel multi-channel system has been studied under rolling motion condition in this paper. Based on the homogeneous flow model with considering the rolling motion condition, the parallel multi-channel model is established by using the control volume integrating method. Gear method is used to solve the system equations. The influences of the inlet and upward sections and the heating power on the flow instability under rolling motion condition have been analyzed. The marginal stability boundary (MSB) under rolling motion condition is obtained and the unstable regions occur in both low and high equilibrium quality regions. The region with low inlet subcooling is also instable. In high equilibrium quality region, the multiplied period phenomenon is found and the chaotic phenomenon appears at the MSB. The oscillation part of mass flow rate (amplitude) may be averaged into other channels so that the influence of rolling motion is weakened. But the stability of multi-channel system is independent of the channel number and the increase of the channel number could only make the amplitude more uniformity in channels.Copyright

Prediction of Post-Dryout Heat Transfer in Vertical Annular Channels Using Artificial Neural Network Method

Experimental investigations on post-dryout heat transfer in 10×8.1, 10×7 and 10×6mm annular test sections have been carried out under low-pressure and low mass flow rate conditions. An Artificial Neural Network (ANN) was trained successfully based on the experimental data for predicting the average post-dryout Nusselt number. Based on the ANN, the effects of gap size, pressure, steam Reynolds number, Reg , inlet quality, xi , Prandtl number, (Prg )W, and the ratio of heat flux of inner-tube to that of outer-tube, qi /qo , on post-dryout heat transfer were analyzed, respectively. In present study, Nusselt number in annular channels with big gap size is larger than that in annular channels with small gap size. Nusselt number increases significantly in 1.5mm and 2.0mm annular channels while it is almost constant in 0.95mm annular channel with increasing pressure or qi /qo . Nusselt number increases with Reg in case of 0.95mm and 1.5mm gap sizes. However, Nusselt number in 2.0mm annular channel firstly increases and then decreases with increasing Reg . Nusselt number decreases with increasing inlet quality under all three annular channels condition. Nusselt number decreases significantly with increasing (Prg )W when (Prg )W is less than 1.5. The changes of Nusselt number in 1.5mm or 2.0mm annular channels are larger than that in 0.95mm annular channel.Copyright

Development of Simulation Codes for Some Accidents in Pressurized Water Reactors

An analysis code has been developed for evaluating the transient thermo-hydraulic behaviors of the pressurized water reactor system. A series of mathematical and physical models is considered in this code, such as the point reactor neutron kinetics model, the heat transfer model, the friction model, the thermo-physical property model and so on. All possible flow and heat transfer conditions in some accidents have been considered and their corresponding models are supplied. Gear’s method is adopted for a better solution to the stiff equations. In this paper, some general accidents in the pressurized water reactors have been investigated, including the station blackout accident (SBO), the loss of flow accident (LOFA), the loss of feed water accident (LOFWA) and the reactivity insertion accident (RIA). The calculated results have been verified by the RELAP5/Mod3 and the results are satisfactory.Copyright

Experimental Research on Dryout Point of Flow Boiling in Narrow Annuli

An experimental research on the dryout point of flow boiling in narrow annuli is conducted under low mass flux with 1.5mm and 1.0mm gap, respectively. Distilled water is used as work fluid, the range of pressure is from 2.0 to 4.0MPa and that of mass flux is 26.0∼69.0 kg/(m2 · s). The relation of CHF and critical qualities with mass flux and pressure are revealed. It is found that the critical qualities decrease with the mass flux and increase with the inlet qualities in externally heated annuli. Under the same conditions critical qualities in outer tube are always larger than that in inner tube. Кутателадзе’s correlations is cited and modified to predict the location of dryout and proved to be not a proper one. Considering in detail the effects of the geometry of annuli and heat flux on dryout, an empirical correction is finally developed to predict dryout point in narrow annuli under low mass flux condition which has a good agreement with experimental data.Copyright