Auswahl der wissenschaftlichen Literatur zum Thema „Melt pool convection“

To effectively interpret the fluid flow dynamics in the molten metal pool, a numerical model was established. The moving repetitive Gaussian laser pulse is irradiated in the work piece. The consideration of laser scanning speed makes the transport phenomena complex. The continuity and momentum equations are solved to get the flow velocity of the molten metal in the melt pool. The energy equation is solved to know the temperature field in the work piece. The algebraic equations obtained after discretization of the governing equations by Finite Volume Method (FVM) are then solved by the Tri Diagonal Matrix Method. Enthalpy-porosity technique is used to capture the position of the melt front which determines the shape of the melt pool. Marangoni convection is considered to know its effect on the shape of the melt pool. The surface tension coefficient is taken as both positive and negative value while calculating the Marangoni force. The two possible cases will cause the Marangoni force to distort the flow dynamics in the melt pool . It's dominance over the buoyancy force in controlling the melt pool shape is focused in the present study. Further, the present model will present an insight to the consequences of laser scanning velocity over the melt pool dimensions and shape.

The formation of melt and its spread in materials is the focus of many high temperature processes, for example, in laser welding and cutting. Surface active elements alter the surface tension gradient and therefore influence melt penetration depth and pool width. This study describes the application of direct laser interference patterning (DLIP) for structuring steel surfaces with diverse contents of the surface active element sulphur, which affects the melt convection pattern and the pool shape during the process. The laser fluence used is varied to analyse the different topographic features that can be produced depending on the absorbed laser intensity and the sulphur concentration. The results show that single peak geometries can be produced on substrates with sulphur contents lower than 300 ppm, while structures with split peaks form on higher sulphur content steels. The peak formation is explained using related conceptions of thermocapillary convection in weld pools. Numerical simulations based on a smoothed particle hydrodynamics (SPH) model are employed to further investigate the influence of the sulphur content in steel on the melt pool convection during nanosecond single-pulsed DLIP.

Melt pool geometry and thermal behavior control are essential in obtaining consistent building performances, such as geometrical accuracy, microstructure, and residual stress. In this paper, a three dimensional model is developed to predict the thermal behavior and geometry of the melt pool in the laser material interaction process. The evolution of the melt pool and effects of the process parameters are investigated through the simulations with stationary and moving laser beam cases. The roles of the convection and surface deformation on the heat dissipation and melt pool geometry are revealed by dimensionless analysis. The melt pool shape and fluid flow are considerably affected by interfacial forces such as thermocapillary force, surface tension, and recoil vapor pressure. Quantitative comparison of interfacial forces indicates that recoil vapor pressure is dominant under the melt pool center while thermocapillary force and surface tension are more important at the periphery of the melt pool. For verification purposes, the complementary metal oxide semiconductor camera has been utilized to acquire the melt pool image online and the melt pool geometries are measured by cross sectioning the samples obtained at various process conditions. Comparison of the experimental data and model prediction shows a good agreement.

The phenomenon of turbulent natural convection in a horizontal heat-generating melt layer with solidification taking place at the cooled upper and lower boundaries is investigated theoretically. The objective is to determine the transient behavior of the crust at the upper and lower surfaces and the effect of crust formation on the turbulent natural convection process in the melt layer. Various surface temperatures, latent heats, and the heat source strengths are considered along with the effects of the Stefan number and Rayleigh number. Special attention is given to the interaction between the melt pool heat transfer and the crust dynamics. Numerical results are presented for the transient crust thickness, transient temperature distribution, eddy heat transport, and the heat transfer characteristics at the solid-liquid interface during the freezing process. The present study provides basic information needed to predict the transient behavior of a melt pool in a reactor lower head following a severe core-meltdown accident.

Functional surfaces characterised by periodic microstructures are sought in numerous technological applications. Direct laser interference patterning (DLIP) is a technique that allows the fabrication of microscopic periodic features on different materials, e.g., metals. The mechanisms effective during nanosecond pulsed DLIP of metal surfaces are not yet fully understood. In the present investigation, the heat transfer and fluid flow occurring in the metal substrate during the DLIP process are simulated using a smoothed particle hydrodynamics (SPH) methodology. The melt pool convection, driven by surface tension gradients constituting shear stresses according to the Marangoni boundary condition, is solved by an incompressible SPH (ISPH) method. The DLIP simulations reveal a distinct behaviour of the considered substrate materials stainless steel and high-purity aluminium. In particular, the aluminium substrate exhibits a considerably deeper melt pool and remarkable velocity magnitudes of the thermocapillary flow during the patterning process. On the other hand, convection is less pronounced in the processing of stainless steel, whereas the surface temperature is consistently higher. Marangoni convection is therefore a conceivable effective mechanism in the structuring of aluminium at moderate fluences. The different character of the melt pool flow during DLIP of stainless steel and aluminium is confirmed by experimental observations.

This thesis aims to provide the thermal condition of melt pool convection by CFD simulation, which is important to the assessment of the invessel melt retention (IVR) strategy widely adopted in Generation III pressurized water reactors (PWRs). As a severe accident mitigation measure, the IVR strategy is realized through external cooling of the lower head of a reactor pressure vessel (RPV). To achieve the coolability and retention of the corium pool in the RPV lower head, the heat flux at the outer surface of the vessel should be less than the critical heat flux (CHF) of boiling around the lower head. Under such condition, the integrity of the RPV is guaranteed by the adequate thickness of the unmelted vessel wall. The thesis work starts from the selection and validation of a turbulence model in the CFD computational tool chosen (Fluent). Afterwards a numerical model is set up for estimation of melt pool heat transfer of a reference PWR with the power capacity of 1000 MWe, including a mesh sensitivity study. Based on the numerical model of a twolayer melt pool, four tasks are carried out to investigate the effects of Zr oxidation ratio, Fe content, and radiation emissivity on heat flux profiles, as well as the focus effect under extreme conditions. Selection and validation of the turbulence model are conducted by comparing the simulation results of different turbulence models with the DNS data on the convection of volumetrically heated fluid layer bounded by rigid isothermal horizontal walls at equal temperature. The internal Rayleigh numbers of the flow reach up to 10e6. The comparison shows a good agreement of the SST k-ω turbulence model results with the DNS data. The simulations with the Zr oxidation ratio of 0, 0.2 and 0.5, correspondingly, the oxide layer of 1.389m, 1.467m and 1.580m, and the metal layer of 0.705m, 0.646m and 0.561m in height, show that, the temperature of the oxide layer will increase with Zr oxidation ratio, while the temperature of the metal layer will decrease resulting in more heat transfer through the oxide layer sidewall and less top radiation. Nevertheless, the effect of the Zr oxidation ratio is not pronounced in the range of 00.5. The simulations with the Fe mass of 22t, 33t and 45t, and respective height of the metal layer of 0.462m, 0.568m and 0.646m, show that, the inner metal layer will significantly increase the temperatures of both the metal layer and the oxide layer. The percentage of heat transfer at the oxide layer sidewall will increase to supplement the reduction of that at the metal layer. The simulations with the radiation emissivity of 0.2, 0.35, 0.45 and 0.7 show that, the emissivity below 0.45 has an impact on heat transfer, and the temperatures and sidewall heat flux of both the oxide layer and the metal layer will increase with decreasing emissivity. The impact is negligible when the emissivity is above 0.45. The simulations under the hypothetically extreme conditions with either an adiabatic top boundary or a very thin metal layer show the focusing effect may occur, i.e., the heat flux through the metal sidewall is larger than that in the oxide layer. But the local high heat flux is flattened by the vessel wall with good heat conductivity. In summary, the simulations demonstrate that, except for the cases under extreme conditions, the heat fluxes of the melt pools in all other cases are significantly lower than the CHF of external cooling of the lower head. Therefore, the safety margin of the IVR strategy of the PWR chosen is seems sufficient. However, due to some limitations (e.g., simplification and assumptions) in the simulation cases and coupling of different influential factors, as indicated by the present study, the precise predictions of heat flux under all scenarios are still difficult. Therefore, the conclusions could not be generalized to the other conditions or other configurations of the molten pools. By discussing the model and simplifications/assumptions adopted in this work, the improvement directions of the numerical model and other perspectives are proposed at the end of the thesis.
Denna avhandling syftar till att tillhandahålla det termiska tillståndet för smältbassängskonvektion genom CFD-simulering, vilket är viktigt för bedömningen av IVR-strategin som allmänt antagits i tryckvattenreaktorer (PWR) i Generation III. Som en åtgärd för att mildra allvarliga olyckor realiseras IVR-strategin genom extern kylning av det nedre huvudet av ett reaktortryckkärl (RPV). För att uppnå kylbarhet och kvarhållning av koriumbassängen i det nedre RPV-huvudet bör värmeflöde vid den yttre ytan av kärlet vara mindre än det kritiska värmeflödet (CHF) som kokar runt det nedre huvudet. Under sådant tillstånd garanteras RPV: s integritet av den osmälta kärlväggens tillräckliga tjocklek. Examensarbetet startar från valet och valideringen av en turbulensmodell i det valda CFD-beräkningsverktyget (Fluent). Därefter sätts en numerisk modell upp för uppskattning av smältbassängens värmeöverföring av en referens PWR med en effektkapacitet på 1000 MWe, inklusive en nätkänslighetsstudie. Baserat på den numeriska modellen för en tvålagers smältbassäng utförs fyra uppgifter för att undersöka effekterna av Zr-oxidationsförhållande, Fe-innehåll och strålningsemissivitet på värmeflödesprofiler, liksom fokuseffekten under extrema förhållanden. Val och validering av turbulensmodellen utförs genom att jämföra simuleringsresultaten för olika turbulensmodeller med DNS-data för konvektionen av volymetriskt uppvärmt fluidskikt avgränsat av styva isoterma horisontella väggar vid lika temperatur. De interna Rayleigh-siffrorna i flödet når upp till 10e6. Jämförelsen visar att SST k-ω turbulensmodellresultaten överensstämmer med DNS-data. Simuleringarna med Zr-oxidationsförhållandet 0, 0,2 och 0,5, motsvarande oxidskiktet på 1,389 m, 1,467 m och 1,580 m, och metallskiktet på 0,705 m, 0,664 m och 0,561 m i höjd, visar att temperaturen av oxidskiktet kommer att öka med Zr-oxidationsförhållandet, medan metallskiktets temperatur kommer att minska vilket resulterar i mer värmeöverföring genom oxidskiktets sidovägg och mindre toppstrålning. Ändå är effekten av Zr-oxidationsförhållandet inte uttalad i intervallet 00,5. Simuleringarna med Fe-massan på 22t, 33t och 45t och respektive höjd av metallskiktet på 0,462m, 0,568m och 0,664m visar att det inre metallskiktet avsevärt kommer att öka temperaturerna för både metallskiktet och oxiden lager. Andelen värmeöverföring vid oxidskiktets sidovägg ökar för att komplettera minskningen av den vid metallskiktet. Simuleringarna med strålningsemissiviteten 0,2, 0,35, 0,45 och 0,7 visar att emissiviteten under 0,45 påverkar värmeöverföringen, och temperaturerna och sidoväggens värmeflöde för både oxidskiktet och metallskiktet kommer att öka med minskande emissivitet. Effekten är försumbar när strålningen är över 0,45. Simuleringarna under de hypotetiskt extrema förhållandena med antingen en adiabatisk övre gräns eller ett mycket tunt metallskikt visar att fokuseringseffekten kan uppstå, dvs. värmeflödet genom metallsidan är större än det i oxidskiktet. Men det lokala höga värmeflödet plattas ut av kärlväggen med god värmeledningsförmåga. Sammanfattningsvis visar simuleringarna att, förutom fall under extrema förhållanden, är värmeflödet från smältpoolerna i alla andra fall betydligt lägre än CHF för extern kylning av nedre huvudet. Därför verkar säkerhetsmarginalen för IVR-strategin för den valda PWR tillräcklig. På grund av vissa begränsningar (t.ex. förenkling och antaganden) i simuleringsfall och koppling av olika inflytelserika faktorer, vilket indikeras av den aktuella studien, är de exakta förutsägelserna av värmeflöde under alla scenarier fortfarande svåra. Därför kunde slutsatserna inte generaliseras till de andra förhållandena eller andra konfigurationer av de smälta poolerna. Genom att diskutera modellen och förenklingar / antaganden som antagits i detta arbete föreslås förbättringsriktningarna för den numeriska modellen och andra perspektiv i slutet av avhandlingen.

Our understanding of how magma-poor rifts accommodate strain remains limited largely due to sparse geophysical observations from these rift systems. To better understand magma-poor rifting processes, chapter 1 of this dissertation is focused on investigating the lithosphere-asthenosphere interactions beneath the Malawi Rift, a segment of the magma-poor Western Branch of the East African Rift (EAR). Chapter 2 and 3 are focused on investigating the sources of melt beneath the Rungwe Volcanic Province (RVP), an anomalous volcanic center located at the northern tip of the Malawi Rift. In chapter 1, we use the lithospheric structure of the Malawi Rift derived from the World Gravity Model 2012 to constrain three-dimensional (3D) numerical models of lithosphere-asthenosphere interactions, which indicate ~3 cm/yr asthenospheric upwelling beneath the thin lithosphere (115-125 km) of the northern Malawi Rift and the RVP from lithospheric modulated convection (LMC) that is decoupling from surface motions. We suggest that the asthenospheric upwelling may generate decompression melts which weakens the lithosphere thereby enabling extension. The source of asthenospheric melt for the RVP is still contentious. Some studies suggest the asthenospheric melt beneath the RVP arises from thermal perturbations in the upper mantle associated with plume head materials, while others propose decompression melting from upwelling asthenosphere due to LMC where the lithosphere is thin. Chapter 2 of this dissertation is focused on testing the hypothesis that asthenospheric melt feeding the RVP can be generated from LMC using realistic constraints on the mantle potential temperature (Tp). We develop a 3D thermomechanical model of LMC beneath the RVP and the entire Malawi Rift that incorporates melt generation. We find decompression melt associated with LMC upwelling (~3 cm/yr) occurs at a maximum depth of ~150 km localized beneath the RVP. Studies of volcanic rock samples from the RVP indicate plume signatures which are enigmatic since the RVP is highly localized, unlike the large igneous provinces in the Eastern Branch of the EAR. In chapter 3, we test the hypothesis that the melt beneath the RVP is generated from plume materials. We investigate melt generation from plume-lithosphere interactions (PLI) beneath the RVP by developing a 3D seismic tomography-based convection (TBC) model beneath the RVP. The seismic constraints indicate excess temperatures of ~250 K in the sublithospheric mantle beneath the RVP suggesting the presence of a plume. We find a relatively fast upwelling (~10 cm/yr) beneath the RVP which we interpret as a rising plume. The TBC upwelling generates decompression melt (~0.25 %) at a maximum depth of ~200 km beneath the RVP where the lithosphere is thinnest (~100 km). Our results demonstrate that an excess heat source from may be plume materials is necessary for melt generation in the sublithospheric mantle beneath the RVP because passive asthenospheric upwelling of ambient mantle will require a higher than normal Tp to generate melt. Studies of volcanic rock samples from the RVP indicate plume signatures which are enigmatic since the RVP is highly localized, unlike the large igneous provinces in the Eastern Branch of the EAR. In chapter 3, we test the hypothesis that the melt beneath the RVP is generated from plume materials. We investigate melt generation from plume-lithosphere interactions (PLI) beneath the RVP by developing a 3D seismic tomography-based convection (TBC) model beneath the RVP. The seismic constraints indicate excess temperatures of ≈ 250K in the sublithospheric mantle beneath the RVP suggesting the presence of a plume. We find a relatively fast upwelling (≈10 cm/yr) beneath the RVP which we interpret as a rising plume. The TBC upwelling generates decompression melt (≈0.25 %) at a maximum depth of ≈200 km beneath the RVP where the lithosphere is thinnest (≈100 km). Our results demonstrate that an excess heat source from may be plume materials is necessary for melt generation in the sublithospheric mantle beneath the RVP because passive asthenospheric upwelling of ambient mantle will require a higher than normal Tp to generate melt.
Doctor of Philosophy
Studies suggest the presence of hot, melted rock deep in the continents makes them weaker and easier to break apart, however, our understanding of how continents with less melted rock break apart remains limited largely due to sparse geophysical observations from these dry areas. To better understand how continents with less melted rock break apart, chapter 1 of this dissertation is focused on investigating the interactions between the rigid part of the Earth, called lithosphere, and the underlying lower viscosity rock layer called asthenosphere beneath the Malawi Rift, a segment of the magma-poor Western Branch of the East African Rift (EAR). Chapter 2 and 3 are focused on investigating the sources of melt beneath the Rungwe Volcanic Province (RVP), an anomalous volcanic center located at the northern tip of the Malawi Rift. In chapter 1, we use the lithospheric structure of the Malawi Rift derived from gravity data to constrain three-dimensional (3-D) numerical models of lithosphere-asthenosphere interactions, which indicate ~3 cm/yr asthenospheric upwelling beneath the thin lithosphere (115-125 km) of the northern Malawi Rift and the RVP that does not seem to drive movements at the surface. We suggest that the asthenospheric upwelling may generate melted rock which weakens the lithosphere thereby enabling extension. However, the source of asthenospheric melt for the RVP is still contentious. Some studies suggest the asthenospheric melt beneath the RVP arises from thermal perturbations in the upper mantle associated with rising mantle rocks or plume head materials, while others propose melting occurs from upwelling asthenosphere due to lithospheric modulated convection (LMC) where the lithosphere is thin. Chapter 2 of this dissertation is focused on testing the hypothesis that asthenospheric melt feeding the RVP can be generated from LMC. We develop a 3D thermomechanical model of LMC beneath the RVP and the entire Malawi Rift that incorporates melt generation. We find decompression melt associated with LMC upwelling (~3 cm/yr) occurs at a maximum depth of ~150 km localized beneath the RVP. Studies of volcanic rock samples from the RVP indicate plume signatures which are enigmatic since the RVP is highly localized, unlike the large igneous provinces in the Eastern Branch of the EAR. In chapter 3, we investigate melt generation from plume-lithosphere interactions (PLI) beneath the RVP. We develop a 3D model of convection using information from seismology we call tomography-based convection (TBC) beneath the RVP. The seismic data indicate excess temperatures of ~250 K beneath the RVP suggesting the presence of a plume. We find a relatively fast upwelling (~10 cm/yr) beneath the RVP which we interpret as a rising plume. The TBC upwelling generates decompression melt at a maximum depth of ~200 km beneath the RVP. Our results demonstrate that an excess heat source from may be plume materials is necessary for melt generation in the sublithospheric mantle beneath the RVP because passive asthenospheric upwelling of ambient mantle will require a higher than normal mantle potential temperatures to generate melt.

碩士
國立中山大學
機械與機電工程學系研究所
99
The molten pool shape and thermocapillary convection during melting or welding of metals or alloys are self-consistently predicted from scale analysis. Determination of the molten pool shape and transport variables is crucial due to its close relationship with the strength and properties of the fusion zone. In this work, surface tension coefficient is considered to be negative, indicating an outward surface flow, whereas high Prandtl number represents a thinner thickness of the thermal boundary layer than that of momentum boundary layer. Since Marangoni number is usually very high, the domain of scaling is divided into the hot, intermediate and cold corner regions, boundary layers on the solid-liquid interface and ahead of the melting front. The results find that the width and depth of the pool, peak and secondary surface velocity, and maximum temperatures in the hot and cold corner regions can be explicitly and separately determined as functions of working variables or Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The scaled results agree with numerical data, different combinations among scaled equations, and available experimental data.

The Tibetan Plateau (TP) is subjected to strong interactions among the atmosphere, hydrosphere, cryosphere, and biosphere. The Plateau exerts huge thermal forcing on the mid-troposphere over the mid-latitude of the Northern Hemisphere during spring and summer. This region also contains the headwaters of major rivers in Asia and provides a large portion of the water resources used for economic activities in adjacent regions. Since the beginning of the 1980s, the TP has undergone evident climate changes, with overall surface air warming and moistening, solar dimming, and decrease in wind speed. Surface warming, which depends on elevation and its horizontal pattern (warming in most of the TP but cooling in the westernmost TP), was consistent with glacial changes. Accompanying the warming was air moistening, with a sudden increase in precipitable water in 1998. Both triggered more deep clouds, which resulted in solar dimming. Surface wind speed declined from the 1970s and started to recover in 2002, as a result of atmospheric circulation adjustment caused by the differential surface warming between Asian high latitudes and low latitudes.The climate changes over the TP have changed energy and water cycles and has thus reshaped the local environment. Thermal forcing over the TP has weakened. The warming and decrease in wind speed lowered the Bowen ratio and has led to less surface sensible heating. Atmospheric radiative cooling has been enhanced, mainly through outgoing longwave emission from the warming planetary system and slightly enhanced solar radiation reflection. The trend in both energy terms has contributed to the weakening of thermal forcing over the Plateau. The water cycle has been significantly altered by the climate changes. The monsoon-impacted region (i.e., the southern and eastern regions of the TP) has received less precipitation, more evaporation, less soil moisture and less runoff, which has resulted in the general shrinkage of lakes and pools in this region, although glacier melt has increased. The region dominated by westerlies (i.e., central, northern and western regions of the TP) received more precipitation, more evaporation, more soil moisture and more runoff, which together with more glacier melt resulted in the general expansion of lakes in this region. The overall wetting in the TP is due to both the warmer and moister conditions at the surface, which increased convective available potential energy and may eventually depend on decadal variability of atmospheric circulations such as Atlantic Multi-decadal Oscillation and an intensified Siberian High. The drying process in the southern region is perhaps related to the expansion of Hadley circulation. All these processes have not been well understood.

Laser material processing has becoming a rapid developing technology due to the flexibility of laser tool. Melt pool is the main product from the interaction between laser and material and its features has a great impact on the heat transfer, solidification behavior, and defects formation. Thus, understanding changes to melt pool flow is essential to obtain good fabricated product. This chapter presents a review of the experimental studies on melt pool flow dynamics for laser welding and laser additive manufacturing. The mechanisms of melt pool convection and its principal affecting factors are first presented. Researches on melt flow visualization using direct and indirect experimental methods are then reviewed and discussed.

In the last phase of the core degradation, an oxidic melt pool of mainly UO2, ZrO2, and unoxidized Zircaloy and stain-less steel will form in the lower head of the RPV (Theofanous et al., 1996). A molten metal layer (composed mainly of Fe and Zr) will rest on the top of the crust of the oxidic pool. A thin oxidic crust layer of frozen core material is formed on the vessel’s inside wall. In this bounding configuration, thermal loads to the RPV walls are determined by natural convection heat transfer driven by internal heat sources. Decay heat from fission products is assumed to be generated uniformly within the oxidic pool and generally no heat generation is considered in the upper metallic layer. For example, in a hypothetical severe accident scenario for an AP600-like reactor, the following values can be expected: volumetric heat generation Qv ∼ 1 MW/m3, volume of the oxidic pool V ∼ 10 m3, radius R = 2 m, temperatures in the oxidic pool T ∼ 2700°C, temperatures in the metal layer T ∼ 2000°C, maximum depth ratio of the metal layer to the oxidic pool L12 ∼ 0:3, properties of the oxidic pool, depending on melt composition, as characterized by the Prandtl number, Pr ∼ 0:6, properties of the metallic layer Pr < 0:1, the intensity of convective motion, as characterized by the Rayleigh number, Ra ∼ 1015–1016 (Theofanous et al., 1996). The time scale of core melt pool formation is estimated as 1/2 to 1 hour (Sehgal, 1999). Indeed, these estimates could vary, depending very much on the accident scenario and the type of reactor.

In-vessel-retention (IVR) has become an important subject of severe accident mitigation strategy. Up to now, many experimental and numerical investigations have been performed on the natural convection characteristics in melt pools with volumetric heating. But these studies are limited to the melt pools under static condition. As floating nuclear reactors become increasingly popular among both commercial and military ships, for successful application of IVR in this occasion, research should be done on the heat transfer characteristics of melt pool under moving conditions. Currently, the specially-designed facility is under construction in Xi'an China for the relevant experiment and numerical studies are performed beforehand. In this paper, a hemisphere with an inner radius of 0.5m, similar to LIVE experimental facility, is chosen to simulate the melt pool. Its flow behaviors under periodic rolling condition are simulated by means of CFD calculation. This work may cast a light on the melt pool characteristics under moving conditions and could be further evaluated by future experimental data.

In this paper, we present a modified k-ε model capable of addressing turbulent molten metal-pool convection in the presence of a continuously evolving phase-change interface during a laser melting process. The phase change aspects of the present problem are addressed using a modified enthalpy-porosity technique. The k-ε model is suitably modified to account for the morphology of the solid-liquid interface. A three-dimensional mathematical model is subsequently utilised to simulate a typical laser melting process with high power, where effects of turbulent transport can actually be realised. In order to investigate these effects on laser molten pool convection, simulations with laminar transport and turbulent transport are carried out for same problem parameters. Finally, results of the simulation using the present turbulence model are compared with the results of laminar simulation with same problem parameters. Significant effects of turbulent transport on penetration and the geometrical features of the molten pool are observed which is an outcome of the thermal history of the pool. The thermal history in turn determines the microstructure of the work piece, which finally governs the mechanical properties of the work piece.

The COPRA experiments were performed to study the natural convection heat transfer behavior in a large-scale homogeneous melt pool inside the reactor pressure vessel lower plenum. The test section consists of a two-dimensional 1/4 circular slice with an inner radius of 2.2 m. A non-eutectic binary mixture 20%NaNO3-80%KNO3 was selected as melt simulant in the previous tests and the Rayleigh number of the melt pool reached up to 1016. In this paper, the working fluid was a eutectic binary mixture 50%NaNO3-50%KNO3. The melt pool temperature, heat flux distribution and crust thickness were obtained in the experiments with different heating powers. Results from the eutectic molten salt tests can be applied for posttest calculations and comparative analyses.

Abstract Pulsed Laser Surface Melting (pLSM) is a technique that offers an efficient way to modify the geometry surfaces without any addition or removal of material. In pLSM, an incident laser beam melts a small region on the surface and induces surface tension and viscosity-driven flows that modify the surface geometry. Initial surface geometry plays an important role in deciding the melt pool flows and shape as it governs the initial surface tension acting on the melt pool. In this paper, we present a systematic numerical study that captures the effects of initial geometries using a two-dimensional axisymmetric model. The results show that geometries with higher curvatures result in deeper melt pools and higher surface displacement because higher fluid velocities aid the convection heat transfer. Additionally, we define a modified capillary number (CaM) which elegantly captures these effects.

Natural convection in a volumetrically heated fluid is of significant importance related to the In-Vessel Retention (IVR) issue. The COPRA facility is designed to simulate the lower plenum of reactor vessel at 1:1 scale for the Chinese advanced PWR, which is a two-dimensional 1/4 circular slice structure with an inner radius of 2.2 m. Water was used as simulant in COPRA-L1 tests. Due to the full scale geometry, The Rayleigh numbers within the corium pools could reach to 1016, matching those in the prototypical situation during severe accident. Due to the high Rayleigh numbers and high turbulence in the melt pool, we choose the Large-Eddy Simulation (LES) method to do the numerical calculations. The LES method can better capture the heat transfer characteristic in a high-Rayleigh number volumetrically heated liquid pool. The numerical results of pool temperature and heat flux distribution were compared with those from COPRA-L1 tests.

This paper investigates effects of natural and Marangoni convection on the resultant solidification microstructure in the scanning laser epitaxy (SLE) process. SLE is a laser-based additive manufacturing process that is being developed at the Georgia Institute of Technology for the additive manufacturing of nickel-base superalloys components with equiaxed, directionally-solidified or single-crystal microstructures through the laser melting of alloy powders onto superalloy substrates. A combined thermal and fluid flow model of the system simulates a heat source moving over a powder bed and dynamically adjusts the thermophysical property values. The geometrical and thermal parameters of the simulated laser melt pool are used to predict the solidification behavior of the alloy. The effects of natural and Marangoni convection on the resultant microstructure are evaluated through comparison with a pure conduction model. Inclusion of Marangoni effect produces shallower melt pools compared to a pure conduction model. A detailed flow analysis provides insights into the flow characteristics of the powder, the structure of rotational vortices created in the melt pool, and the solidification phenomena in the melt pool. The modeling results are compared with measurements and observation through real-time thermal imaging and video microscopy to understand the flow phenomenon. In contrast to the single weld-bead approach, the raster scan in SLE allows every position in melt pool to be visited twice by the solid-liquid interface as the scan source progresses. To properly address this situation, time tracking is incorporated into the model to correctly couple the microstructure prediction model. An optimization study is carried out to evaluate the critical values of the transition parameters that govern the columnar-to-equiaxed transition (CET) and the oriented-to-misoriented (OMT) transition. This work is sponsored by the Office of Naval Research through grant N00014-11-1-0670.

In the frame of severe accident research for the second and the third generation of nuclear power plants, some aspects of the concrete cavity ablation during the molten corium–concrete interaction are still remaining issues. The determination of heat transfer along the interfacial region between the molten corium pool and the ablating basemat concrete is crucial for the assessment of concrete ablation progression and eventually the basemat melt-through. For the purpose of experimental investigation of thermal-hydraulics inside a liquid pool agitated by gas bubbles, the CLARA project has been launched jointly by CEA, EDF, IRSN, GDF-Suez and SARNET. The CLARA experiments are performed using simulant materials and they reveal the influence of superficial gas velocity, liquid viscosity and pool geometry on the heat transfer coefficient between the internally heated liquid pool and vertical and horizontal pool walls maintained at uniform temperature. The first test campaign has been conducted with the smallest pool configuration (50 cm × 25 cm × 25 cm). The tests have been performed with liquids covering a wide range of dynamic viscosity from approximately 1 mPa s to 10000 mPa s. This paper presents some preliminary conclusions deduced from the experiments which involve a liquid pool with the gas injection only from the bottom plate. A comparison with existing models for the assessment of heat transfer has also been carried out.

Directed Metal/Material Deposition (DMD) process is one of additive manufacturing processes based on laser cladding process. A full understanding of laser cladding process is a must to make the DMD process consistent and robust. A two dimensional mathematical model of laser cladding was developed to understand the influence of fluid flow to the mixing, dilution, and deposition dimension, incorporating melting, solidification, and evaporation phenomena. The fluid flow in the melt pool driven by thermal capillary convection and energy balance at liquid-vapor and solid-liquid interface was investigated and the impact of the droplets on the melt pool shape and ripple was studied. Dynamic motion, development of melt pool and the formation of cladding layer were simulated.