Fine Fragmentation Process Research Articles

Mechanisms Governing Fine Fragmentation of Hot Melt Immersed in Cold Water

Hypotheses about the mechanisms governing fragmentation of superheated liquid metal droplets falling into cold water are analyzed. It is shown that a physical model based on the cavitation–acoustic mechanism governing fine fragmentation of melt under steam explosion conditions is likely the most suitable one for consistently describing the fragmentation of both low-melting and refractory metals. For checking this conjecture, special experiments for studying the processes triggered when cold (20°С) water comes into contact with a heated surface and for measuring the pressure impulses (arising both in coolant and in the hot body) accompanying the coolant flashing were carried out using liquid metal (tin and steel) droplets and superheated solid steel bodies. The working substance temperatures were varied in the range from 200 to 1600°С. The results obtained from the performed experiments are not in contradiction with the melt fine fragmentation process represented by the cavitation–acoustic model. It is shown that the acoustic waves generated during explosive growth of bubbles on a hot surface propagate in the solid body and are alternating in nature. Their intensity (including that at negative pressure values) differs only slightly in the modulus from the pressure impulses measured in the coolant and is sufficient for finely fragmenting the droplets. It is experimentally found—with the use of a conductance measuring technique—that the transition from the coolant film to bubble boiling mode is preceded by a short-term (lasting a few milliseconds) process involving intense interaction of waves at the steam–liquid boundary with the heated surface. The signal from the conductance measuring transducer was subjected to a wavelet analysis for different values of the heated surface temperature. The study results testify that high-frequency (several tens of kilohertz) pulsations of electric current are generated in the preburnout region with their characteristics varying (toward increasing the amplitude and intensity) with time as the heating and heated media come closer into contact with each other. A probabilistic process development scenario is suggested.

Simulations of KROTOS alumina and corium steam explosion experiments: Applicability of the improved solidification influence modelling

Abstract The KROTOS steam explosion experiments revealed important differences in the energy conversion efficiency among the melts. The differences observed in the experiments are due to the differences in the jet fragmentation, the melt droplet solidification and the void production. These are limiting processes in the steam explosion phenomenon and have to be modelled adequately in fuel–coolant interaction codes. The presented research focuses on the solidification effect which inhibits the fine fragmentation of the melt droplets and therefore influences directly the strength of the steam explosion. The solidification influence modelling in fuel–coolant interaction codes is strongly related to the modelling of the temperature profile inside the melt droplets and to the modelling of the mechanical effect of the formed crust on the fine fragmentation process. An improved model was developed and implemented into the exploratory version of the MC3D code. This enables a more accurate prediction of the amount of droplets participating in the fine fragmentation process during the explosion phase. The general applicability of the proposed improved modelling and its potential for reliable extrapolations of experimental findings to reactor conditions was assessed. For the assessment the KROTOS alumina K44 and corium K53 explosion experiments were selected. The simulations support the key role of the solidification in the steam explosion phenomenon. The application of the improved modelling enhances the geometrical extrapolation capabilities of fuel–coolant interaction codes to cover reactor cases.

Improved solidification influence modelling for Eulerian fuel–coolant interaction codes

Steam explosion experiments revealed important differences in the efficiency between simulant alumina and oxidic corium melts. The experimentally observed differences are importantly attributed to the differences in the melt droplets solidification and void production, which are limiting phenomena in the steam explosion process and have to be adequately modelled in fuel–coolant interaction codes. This article focuses on the modelling of the solidification effect. An improved solidification influence modelling approach for Eulerian fuel–coolant interaction codes was developed and is presented herein. The solidification influence modelling in fuel–coolant interaction codes is strongly related to the modelling of the temperature profile and the mechanical effect of the crust on the fragmentation process. Therefore the first objective was to introduce an improved temperature profile modelling and a fragmentation criterion for partly solidified droplets. The fragmentation criterion was based on the established modified Weber number, which considers the crust stiffness as a stabilizing force acting to retain the crust under presence of the hydrodynamic forces. The modified Weber number was validated on experimental data. The application of the developed improved solidification influence modelling enables an improved determination of the melt droplet mass, which can be efficiently involved in the fine fragmentation during the steam explosion process. Additionally, also the void production modelling is improved, because it is strongly related to the temperature profile modelling in the frame of the solidification influence modelling. Therefore the second objective was to enable an improved solidification influence modelling in codes with an Eulerian formulation of the droplet field. Two additional transported model parameters based on the most important droplets features regarding the fuel–coolant interaction behaviour, were derived. First, the crust stiffness was considered as an important property, because it enables the correct prediction of the amount of droplets participating in the fine fragmentation process during the explosion phase. Second, the heat flux from the droplet interior to the surface was considered as an important feature, because it enables to improve the surface temperature determination and reflects the history of the droplet's cooling. The last objective was to implement the improved solidification influence modelling into the Eulerian code MC3D. The first demonstrative simulations with the implemented modelling are promising and are showing improvements in the simulation capability.

Use of the TREPAM hot wire quenching test results for modelling heat transfer between fuel and coolant in FCI codes

Steam explosions are the result of the constrained thermal expansion of heated coolant by the hot fuel fragments resulting of a fine fragmentation process. Results of the hot wire quenching experiment TREPAM are used to investigate the fragments cooling rate which are seen to be smaller than the generally assumed values used in steam explosion codes. As the amount of heat to produce hot coolant (vapor in subcritical conditions or pseudo-vapor in supercritical conditions) cannot be easily obtained experimentally, a simple model using experimental results is presented to provide such an information.

Fine fragmentation of molten droplet in highly subcooled water due to vapor explosion observed by X-ray radiography

Experimental investigation of the fine fragmentation process during vapor explosion was conducted in a single drop system employing continuous high-speed X-ray radiography and photography. A molten tin drop of 0.7 g at 1000 °C was dropped into a water pool, at highly subcooled temperatures of about 20 °C, and the explosion was triggered by an external shock pulse of about 1 MPa. X-ray radiographs show that a shell of finely fragmented melt particles accelerates to the vapor bubble boundary during the period of vapor bubble expansion in the explosion process. For tests with highly subcooled coolant, a local explosion due to an external trigger pulse resulted in the stratified explosion along the melt surface. An analysis to estimate mixing depth during the stratified explosion indicated that about 20% of droplet mass was fragmented due to the stratified explosion with a 1 MPa trigger pulse. Finely particles generated during the vapor explosion process were quantified from high-speed X-ray radiography images.

Analysis of bubble dynamics in explosive boiling of droplet with fine fragmentation

If a high temperature liquid comes into contact with cold and volatile liquid, rapid (or explosive) evaporation may occur spontaneously or triggered by the impact of a pressure wave. This event generating a shock wave is called a steam explosion. It involves many multiphase flow and heat transfer phenomena. One of the more important phenomena in a steam explosion is the fine fragmentation of the hot liquid, which determines the explosive heat transfer from the hot liquid to the cold liquid and the vaporisation rate of the cold liquid. When a small hot single drop (∼1 mm) interacts with the coolant, a vapour bubble is formed around the drop. It was observed experimentally that these vapour bubbles grow and collapse. During this process, the small hot droplet fragments and generates finer particles. To understand the fine fragmentation process during a steam explosion, in this study, this phenomenon was examined by using non-linear stability analysis of vapour bubble dynamics based on a concept developed by Inoue et al. [Chem. Eng. Commun. 118 (1992) 189]. From the analysis, it was observed that higher spherical modes were very much unstable during collapse process, which decided the size of the fragmented particles. Vapour shell between molten metal and coolant was considered unstable if the amplitude of one of the spherical modes was greater than vapour shell thickness. In addition, the mass of fragmented particles during each cycle of vapour bubble dynamics was predicted from the analysis. The calculated results were found to be in reasonable agreement with the previously reported [J. Non-Equil. Thermodyn. 13 (1988) 27] experimental results.

Modeling of Melt Droplet Fragmentation Following Vapor Film Destabilization by a Trigger Pulse

In hypothetical Severe Accident studies for a PWR, a large amount of molten corium may be poured into water. There is then a risk of Steam Explosion. After the premixing sequence in which the melt is more or less dispersed into water, a fine fragmentation process may start, which can lead to an escalation. Such an event is generally triggered by the destabilization of the vapor film surrounding the hot melt droplets. In this paper, an attempt to describe all the successive processes leading to this fine fragmentation is presented.First, a critical analysis of previous models is performed, allowing us to propose a new sequence of events. As in the previous models, the film destabilization leads to the growth of cold liquid peaks induced by Rayleigh Taylor instability. As these peaks have a smaller density than the drop, they do not penetrate into the hot drop. At the cold liquid-hot liquid contacts, transient heat transfer leads to the explosive boiling of a small amount of coolant. The generated local pressurization deforms the hot melt interface. This can produce fine fragments from the filaments issued from the melt. Some of them may reach the vapor-coolant interface where intense and rapid vaporization occurs. A large bubble then develops, and a new fragmentation sequence may again appear at the bubble collapse. The present model is supported by experimental results.

Environmental changes in the Tibetan Plateau and surrounding areas — Preface

There are basically two kinds of water vapour explosions, viz. confined and unconfined. Confined explosions can occur in various situations in industry. Unconfined explosions typically occur when very hot molten metal accidentally makes contact with water. In order to explain the extremely rapid production of water vapour that can then occur, “fine fragmentation” of the molten metal must take place. The paper first presents some early suggestions for the nature of the fine fragmentation process. In more recent theories the Weber number, We, is used as a measure of expected extent of molten-metal fragmentation in a given situation. Film boiling plays a central role. Attempts at developing mathematical models of unconfined water vapour explosions have been made. The next section of the paper is devoted to case histories of unconfined water vapour explosions. Then various kinds of confined water vapour explosions are discussed, including water boiler explosions, water vapour BLEVEs, and water vapour explosions in the paper industry. Some case histories of confined water vapour explosions are also given. The final section of the paper is devoted to measures for preventing and mitigating water vapour explosions in industry.

Quantitative experiments on phreatomagmatic explosions

An experimental set-up for controlled generation of thermal explosions (Fuel Coolant Interactions) has been built by an interdisciplinary research group of volcanologists, physicists and engineering scientists: The TEE-Haus (Thermal Explosion Experiment). During the last 3 years more than 500 experimental runs were performed. Aim of the project was not to build a “mini volcano” but to produce igniteable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects. Such small scale mixtures, however, most probably also occur during phreatomagmatic events in nature. Mixtures of water and magma on a large scale may be modelled by combination of small elements. Earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively. Therefore, a carbonate melt (approx. 0.3 kg of 1:1 mass ratio Na 2CO 3 / K 2CO 3) was used for initial experimental series because of easy handling and more “magma similarity”. By injection of water into this melt explosive mixtures were produced and ignited (self-triggering). Explosive interactions occurred in a wide melt-temperature range (720 to 1040°C). At melt temperatures exceeding 1000°C, CO 2 increasingly degassed. Presence of non-condensible gas bubbles in the melt strongly reduced the explosivity. The intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities, because of more intensive mixing. A minimum injection velocity was found to be approx. 0.7 m s −1. Experiments with silicate melts, derived from diverse remelted volcanic rocks were carried out with a slightly modified set-up. Explosions were ignited by an additional trigger: a shock wave of approx. 8 J. In volcanic systems, shock waves of this magnitude are abundant (volcanic tremor, etc.). Strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750°C. The produced explosions exerted repulsion forces of up to 24,000 N with a typical pulse duration of 1 ms (carbonate melt) resp. 1.5 ms (silicate melts). The ejection velocity was found to be in a range between 200 and 400 m s −1. Explosion energy values up to 500 J were calculated. The maximum explosion pressure was calculated to be in the range between 10 and 100 MPa. Only part of the melt (so-called interactive melt) reacts explosively with injected water. Most of the melt is passively ejected by and after the thermal explosion. Different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other. Particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20 μm to 180 μm. The amount of interactive melt is proportional to the intensity of the explosion. The maximum amount of water vapourized at the explosive interaction (so-called interactive water) was estimated. Thus, the water/melt mass ratio for explosive interactions could be calculated. The values range between 1 25 and 1 35 (carbonate melt) resp. between 1 6 and 1 25 (silicate melts).