CFD analysis of a turbulent jet behavior induced by a steam jet discharge through a single hole in a subcooled water tank

A computational fluid dynamics (CFD) analysis for a turbulent jet flow induced by a steam jet discharged into a subcooled water pool was performed for 10 s of transients to investigate whether the currently available CFD codes can be suitably used as a tool to investigate the applicability of the existing semi-analytical correlations to a condensing jet-induced turbulent jet and to analyze the thermal-hydraulic behavior, such as global circulation and local hot spot, in a condensation pool for advanced light water reactors. As for the numerical experiment, a series of sensitivity calculations was conducted systematically to elucidate the major factors which can cause different analysis results by varying the mesh distributions, numerical models for a convection term and an eddy viscosity term. The effect of a difference in the velocity and the temperature distribution in a region between the sparger and the pool wall has not been observed in the afore-mentioned sensitivity calculations. The comparison of the CFD results with the test data shows that the CFD analysis does not accurately simulate the local phenomenon of a turbulent jet existing downstream of a steam jet. It was found that the value of the turbulent intensity at the inlet of the turbulent jet region is the most important factor because it can determine the boundary of a turbulent jet through a momentum diffusion process in a radial direction. The comparison of the CFD results with the test data shows that the CFD analysis can accurately simulate the local phenomenon of a turbulent jet existing downstream of a steam jet only when the CFD analysis reflects the physics of a turbulent jet.     

CFD simulation of steam jet-induced thermal mixing in subcooled water pool

This study examined the IRWST thermal mixing phenomena induced by a steam jet in a subcooled water pool. Due to the limitation of the current CFD code to simulate condensation, the steam condensation region model was developed to evaluate the thermal mixing phenomena. Within this region, all the steam was condensed into water, and the steam mass and energy inputs were treated as the source. This calculation was treated using single-phase CFD methods. The benchmark calculation for a thermal mixing experiment in the water tank was performed to develop an optimized 3D evaluation methodology of the thermal-hydraulic behavior in APR1400 IRWST. Steam discharge through the sparger and condensation phenomenon was modeled with the choking flow and thermal mixing model in the quenching tank using CFX11.Three types of thermal mixing experiments, local phenomena test, thermal mixing tests in cylindrical water pool and annulus water pool, were designed to provide data representative of the behavior of the prototype for CFD simulations of the thermal-hydraulic behavior in IRWST. A comparison of the calculated and experimentally measured temperature profiles showed some disagreement particularly around the sparger. The main reason for this disagreement was caused by the difference in the test and simulating conditions at the tank wall. However, moving away from the sparger, the trends of the temperature rise became similar to that in the experiment. Despite these problems, this model is the best way of evaluating the thermal mixing phenomena caused by a steam jet in a subcooled water pool.     

Experimental study on a steam-driven turbulent jet in subcooled water

This study was designed to investigate the behaviour of a steam-driven turbulent jet in subcooled water. The total pressures along the axis of the jet were measured under various steam mass fluxes, water temperatures and nozzle sizes. The results indicated that the lifting-pressure coefficient decreased with inlet steam pressure and water temperature, and an optimal lifting-pressure coefficient could be obtained at pressure ratio 0.228. Then an empirical correlation based on the influencing factors was given to fit the position of the maximum pressure. Moreover, the velocity profiles in the turbulent jet region were obtained, and the results showed the self-similarity property of the velocity profiles. An empirical correlation was suggested to correlate the jet half width obtained from the velocity profiles.     

CFD analysis for thermal mixing in a subcooled water tank under a high steam mass flux discharge condition

A Computational Fluid Dynamics (CFD) analysis for a thermal mixing test was performed for 30 s to develop the methodology for a numerical analysis of the thermal mixing between steam and subcooled water and to apply it to Advanced Power Reactor 1400 MWe (APR1400). In the CFD analysis, the steam condensation phenomenon by a direct contact was simulated by the so-called condensation region model. Thermal mixing phenomenon in the subcooled water tank was treated as an incompressible flow, a free surface flow between the air and the water, and a turbulent flow, which are implemented in the CFX4.4. The comparison of the CFD results with the test data showed a good agreement as a whole, but a small local temperature difference was found at some locations. A sensitivity analysis was performed to find the reason of the temperature difference. The commercial CFD code of CFX4.4 together with the condensation region model can simulate the thermal mixing behavior reasonably well when a sufficient number of mesh distributions and a proper numerical method are selected.     

Condensation regime diagram for supersonic/sonic steam jet in subcooled water

This study was designed to determine the behaviour of the flow patterns of supersonic and sonic steam jet condensation in subcooled water. The effects of steam mass flux, water temperature and pressure ratio on the flow pattern were discussed. The results indicated that the flow pattern was not only affected by steam mass flux and water temperature, but also affected significantly by pressure ratio. The expansion and contraction ratios of the steam plume were influenced significantly by pressure ratio, and a correlation for predicting the expansion and contraction ratio was given based on the theoretical expansion ratio of sonic steam jet. At relatively low steam mass flux and high water temperature, the transition of flow pattern from stable jet to unstable jet was observed, and a criterion of stable-unstable jet transition was given. Moreover, the three-dimensional condensation regime maps, including stable steam plume shapes and unstable jet, for sonic and supersonic steam jet under different conditions were discussed. Four different shapes of the steam plume occurred in sonic steam jet regime map, and six different shapes of the steam plume occurred in supersonic steam jet regime map. The regime maps were also validated against the previous experiments.     

Film boiling from a vertical jet of fuel entering a water pool

The rate of removal of coolant is examined by means of quasi-steady film boiling from a jet of fuel falling into a water pool during a core meltdown accident in a light water reactor. A two-layer turbulence model in the vapor film is used. Entrainment of droplets due to the high vapor velocity is taken into account, which, for a jet of diameter 0.5 m, increases the removal rate by factor of 5 at 0.7 m behind the leading edge. However, the water removal rate is only of the order of kilograms/second, and the total amount of coolant removed is still small compared to the coolant inventory in the lower plenum.     

Effect of turbulent natural convection on sodium pool combustion in the steam generator building of a fast breeder reactor

A computational model is proposed to simulate sodium pool combustion considering the effect of turbulent natural convection in a vented enclosure of the steam generator building (SGB) of a fast breeder reactor. The model is validated by comparing the simulated results with the experimental results available in literature for sodium pool combustion in a CSTF vessel. After validation, the effects of vents and the location of the pool on the burning rate of sodium and the associated heat transfer to the walls are studied in an enclosure comparable in size to one floor of the steam generator building. In the presence of ventilation, the burning rate of sodium increases, but the total heat transferred to the walls of the enclosure is reduced. It is also found that the burning rate of sodium pool and the heat transfer to the walls of the enclosures vary significantly with the location of sodium pool.     

Simulating the mixing of helium and air by mixing salt water and fresh water in a scaled enclosure

We are attempting to establish scaling laws to simulate the mixing of helium (a simulant for hydrogen) with air in a large-scale enclosure by mixing salt water and fresh water, in a small-scale enclosure. This will allow us to assess the mixing of gases in a nuclear reactor containment using relatively small-scale liquid-mixing experiments. The scope of our current work does not cover the integrated effects of different mechanisms of gas mixing expected to prevail during postulated LOCA (loss-of-coolant accident) scenarios. The work is limited to mixing caused by jet inertia and buoyancy forces. Within this scope, we have identified the dominant scaling laws, and tested them by conducting gas-mixing experiments in a large-scale enclosure and liquid-mixing experiments in a small-scale enclosure. The experimental results demonstrate the validity of the scaling laws.     

Large-Eddy Simulation study of turbulent mixing in a T-junction

A potential cause of thermal fatigue failures in energy cooling systems is identified with cyclic stresses imposed on a piping system. These are generated due to temperature changes in regions where cold and hot flows are intensively mixed together. A typical situation for such mixing appears in turbulent flow through a T-junction, which is investigated here using Large-Eddy Simulations (LES). In general, LES is well capable in capturing the mixing phenomena and accompanied turbulent flow fluctuations in a T-junction. An assessment of the accuracy of LES predictions is made for the applied Vreman subgrid-scale model through a direct comparison with the available experimental results. In particular, an estimation of the minimal mesh-resolution requirements for LES is examined on the basis of the complementary RANS simulations. This estimation is based on the characteristics turbulent scales (e.g., Taylor micro-scale) that can be computed from LES or RANS simulations.     

Acoustics impact on steam dryer induced by main steam line break in a boiling water reactor

In boiling water reactor (BWR) design, significant acoustic pressure loads impact the steam dryer hood as a result of the main steam line break outside containment (MSLB) event. When a main steam line breaks, it is assumed that the pipe instantaneously breaks completely open to the ambient environment (double-ended guillotine break). Due to the huge pressure difference between the inside of the reactor pressure vessel (RPV) and surrounding ambient environment, a shock wave will form at the break point and burst into the surrounding environment. At the same time, an expansion wave will travel upstream through the main steam line to the RPV, which results in a pressure reduction on the outside of the steam dryer hood. This expansion wave will create a substantial pressure difference between the two sides of the steam dryer hood with a resultant high stress on the hood. This differential pressure load is the acoustic load used in the structure design evaluations for this event. A key design basis requirement for the steam dryer is to maintain structural integrity during transient, and accident conditions. Demonstration that the steam dryers meet this design basis requires a calculation of the magnitude of the acoustic load on the steam dryer during a MSLB. In this study, computational fluid dynamics (CFD) is used as an alternate calculation method to investigate the phenomenon of MSLB. Transient simulations with fine time steps were carried out. The results show that CFD is a useful tool to provide additional information on the acoustic load as compared to the traditional methods. From the CFD results, the minimum pressure value and its distribution area at different flow times was identified. Through the modeling, an understanding of the detailed transient flow field, particularly the acoustic pressure field near the dryer hood during the MSLB was achieved.     

Two-phase turbulent mixing and buoyancy drift in rod bundles

This paper describes the development of generalized relationships for single- and two-phase intersubchannel turbulent mixing in vertical and horizontal flows, and lateral buoyancy drift in horizontal flows.The relationships for turbulent mixing, together with a recommended one for void drift, have been implemented in a subchannel thermalhydraulics code, and assessed using a range of data on enthalpy migration in vertical steam–water flows under BWR and PWR diabatic conditions. The intent of this assessment was to optimize these relationships to give the best agreement with the enthalpy migration data for vertical flows. The optimized turbulent mixing relationships were then used as a basis to benchmark a proposed buoyancy drift model to give the best predictions of void and enthalpy migration data in horizontal flows typical of PHWR CANDU¹ reactor operation under normal and off-normal conditions.Overall, the optimized turbulent mixing and buoyancy drift relationships have been found to predict the available data quite well, and generally better and more consistently than currently used models. This is expected to result in more accurate calculations of subchannel distributions of phasic flows, and hence, in improved predictions of critical heat flux (CHF).     

Pressure distribution due to a steam bubble collapse in a BWR pressure suppression pool

At the end of a steam blowdown into a pressure suppression pool severe steam bubble collapses occur. Because the condenser pipes are arranged in a regular pattern. There are two limit loading conditions: the simultaneous collapse with the same intensity at each condenser pipe which is mostly discussed and a strong collapse only at one pipe or a small group of pipes. Since the influence of the wall elasticity on the pressure input due to the steam bubble collapse is unessentially small, the complete bubble-fluid-structure problem may be solved in two steps: First the bubble-fluid interaction in the rigid containment and later on the fluid-structure interaction in the elastic containment due to the pressure input by the bubble-fluid system. The bubble-fluid problem is also separated in the analysis for the pressure time history and that for the pressure distribution. If compressibility effects are neglected, the steam bubble surface is to idealize as a sphere during the whole collapse time for analyzing the pressure time history. The pressure distribution is fixed then by the potential due to a stationary source in the bubble center. The structure system is solved as usual in structural dynamics by the eigenmode response analysis in which the fluid-structure system is substituted by some uncoupled mass spring oscillators.The aim of this paper is to clarify the behaviour of the bubble-fluid-structure system by simple models and by separating the problem into the important global behaviour and some additional localized effects. This seems to be necessary since the pool geometry especially for steel containments and also the analysis for the structural system, which consists of different shells is too complicated. The influence of different idealisations on the pressure distribution in the rigid containment and on the eigenmodes of the fluid-structure system is presented. Formulas are derived for estimating the eigenfrequencies, the eigenmodes and the pressure distribution as well as the pressure reduction by the wall elasticity. Furthermore the similarity rules for transfering the results from small experimental facilities to the complex ring containment are explained. Analytical and experimental results are compared and how the maximum pressure distribution in steel and concrete containments differ from each other. Finally the following engineering concept for a stepwise analysis is confirmed: The resulting wall loads due to the fluid-pressure and the inertia wall forces may be determined by a simple fluid-structure model and then the stresses due to these loads by a refined shell analysis.     

Analysis of heat transfer and flow characteristics in turbulent impinging jet

Jet impingement technique is characterized by a high heat removal capability. As such it has been proposed as a cooling method for the helium cooled divertor, a high-heat flux component of the future fusion reactor called DEMO. Since power plant efficiency depends on the divertor’s heat removal capability it has to meet certain demands, i.e. high-heat transfer and low pressure drop.In this paper local heat transfer and flow characteristics of an axis-symmetric impinging jet are analyzed numerically using the RANS approach and eddy viscosity type SST turbulence model. Turbulence models and heat transfer predictions are validated on the free jet impingement experiment (Baughn and Shimizu, 1989). Since the numerical results are affected by the turbulence model the influence of the turbulent production is investigated in particular.The validated numerical model is further applied to analyze the effect of the nozzle inlet shape on the heat removal capability and pressure drop in the confined impinging jet. Two different nozzle inlet parameters are tested; chamfer angle θ and chamfer depth L_ch. The numerical results are compared with the experimental data (Brignoni and Garimella, 2000).     

Low-quality and subcooled film boiling of water at elevated pressures

Stable film boiling heat transfer data have been obtained in an 8.9 mm ID tube at pressures from 2 to 9 MPa. These data were obtained at low-quality and subcooled conditions, over a mass flux range of 0.11 to 2.75 Mg m⁻² s⁻¹. Excessive film boiling surface temperatures were avoided by using the hot patch technique. Contrary to the high-quality data, the low-quality data showed a decrease in heat transfer coefficient with an increase in quality. The film boiling data were compared with existing film boiling correlations. None of these were found to be satisfactory.     

Unsteady numerical simulations of the single-phase turbulent mixing between two channels connected by a narrow gap

This paper focuses on the unsteady numerical simulation of the turbulent flow in two types of geometry containing a narrow gap with the explicit algebraic Reynolds stress model. The model was first validated through the comparison of simulation results inside a rectangular channel containing a cylinder and the corresponding experimental data. The structures of the oscillation were correctly reproduced. Simulation of turbulent mixing between circular channels connected by a narrow gap was carried out with the validated model. Because of the influence of the strong anisotropic turbulent flow in the gap region, the mixing rate was dramatically enhanced by the cyclic and almost periodic flow pulsation. The calculation results of the turbulent mixing rate showed good agreement with the experiment and the maximum error was less than 15%.     

Spectroscopic measurements in Fe-plasma produced by pulsed laser ablation

A Nd:Yag laser is employed to irradiate thick Fe targets placed in vacuum. The obtained non-equilibrium Fe-plasma is investigated with various analytical techniques. An electrostatic ion energy analyzer (IEA) and a ring ion collector (ICR) are employed, in time-of-flight configuration, to monitor in situ the ejected ions from the plasma along the normal direction to the target surface and to determine the plasma core temperature and the ion energy distributions.The visible plasma emission, detected with an optical spectrometer, permitted to evaluate the electronic temperature and density and the fluence threshold of the visible light emission. The spectroscopy measurements of ions and photons and the fast CCD plasma images are employed to evaluate the temperature gradients in the laser-generated plasma plume.     

Experimental study on the Reynolds number dependence of turbulent mixing in a rod bundle

An experimental study for Reynolds number dependence of the turbulent mixing between fuel-bundle subchannels, was performed. The measurements were done on a triangular array bundle with a 1.20 pitch to diameter relation and 10 mm rod diameter, in a low-pressure water loop, at Reynolds numbers between 1.4 × 10³ and 1.3 × 10⁵.The high accuracy of the results was obtained by improving a thermal tracing technique recently developed. The Reynolds exponent on the mixing rate correlation was obtained with two-digit accuracy for Reynolds numbers greater than 3 × 10³. It was also found a marked increase in the mixing rate for lower Reynolds numbers.The weak theoretical base of the accepted Reynolds dependence was pointed out in light of the later findings, as well as its ambiguous supporting experimental data.The present results also provide indirect information about dominant large scale flow pulsations at different flow regimes.     

Calculation of JAERI's jet discharge experiment by KfK's code DRIX-2D

This report describes the computed results by Kernforschungszentrum Karlsruhe's (KfK) code DRIX-2D in regard of the Japan Atomic Energy Research Institute's (JAERI) jet discharge test.Comparisons of the pressure in a test pipe, reaction thrust force and pressure on the target disk due to a sudden jet impingement are made between experimental and calculational results. Taking the influence of a time-dependent break of the rupture disk into consideration, close agreement was obtained between experimental and computed results with respect to the reaction thrust force and to the pressure in the test pipe. But the pressure on the target disk calculated by DRIX-2D was lower than that in the experiment.