Observation of Flow Phenomena during Dendritic Solidification

Research output: ThesisDoctoral Thesis

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Abstract

The current dissertation presents the “Observation of flow phenomena during dendritic solidification” of a hypereutectic ammonium chloride alloy, in a cast cell cooled homogeneously from the side and bottom walls. The main aim of this thesis work is to establish an experimental benchmark with well controlled temperature boundaries for numerical simulations. The temperature was measured via thermocouples fixed in the mould and also temperature fields were extracted using Light Induced Fluorescence technique (LIF). A large interest was given to flow characterisation during solidification and especially its interaction with solidification. Additionally the measurements of the mushy zone thickness and estimation of total volume of mush developed in the cast cell during solidification was performed. The innovation brought in this work is the use of the dual Particle Image Velocimetry (PIV) technique to explore simultaneously the melt flow and the equiaxed crystals motion during the solidification. Different flow regimes were reported and flow velocities were extracted independently of equiaxed crystals velocities. At the beginning of the cooling process a steady convection pattern is formed, then the solutal buoyancy destabilized the steady convection flow into multiple chaotic cells, the flow transitioned from a 2D to a 3D turbulent regime. This regime is progressively replaced by a horizontal stratified flow, called meander flow. The meander flow was found to be a new flow phenomena occurring when double diffusive convection is triggered by solidification. The solidification type observed in most experiments was mainly columnar. The reproducibility of these experimental data was very good and therefore they constitute an excellent benchmark for the validation of numerical models. However some experiments, under specific conditions, presented a rain fall of equiaxed crystals in the bulk melt and sometimes even freckles have been observed in the mushy region. In these cases the average melt flow velocity was related with the velocity of the columnar front and it was confirmed that for high velocity flow and high columnar front velocity, equiaxed crystal occurred. Furthermore a clear correlation between the strength of the flow and the occurrence of equiaxed crystals was observed: when equiaxed crystal occurred in large amount, a pick in the flow kinetic energy (KE) was measured. The analysis of the results suggests strongly a fragmentation origin of the equiaxed crystals occurrence. A coupling mechanism between the hydrodynamics and the generation and growth of the equiaxed crystals was proposed. The transition from purely columnar growth to a strongly equiaxed rains (CET) was found to be initiated by the magnitude of the flow intensity driven by the equiaxed crystals, and the liberation and the transport of the fragments by the same flow recirculating within the mushy zone. In these experiments where equiaxed crystals were observed, the relative velocity of a single crystal was extracted for different cases and plotted versus the crystals diameter. The results show an increase of the relative velocity with the measured size and a power law was found to fit these experimental data. Observation of spectacular equiaxed avalanches in a large scale cast cell in laboratory, creating conspicuous equiaxed layers at the bottom of the cast cell, encourages us to believe that this phenomenon can be at the origin of the equiaxed zone often noticed in large industrial castings. The experimental data presented in this work were used to validate several multi-phase models. The solidification front and evolution of mushy zone thickness during solidification were calculated with reasonable accuracy using an Eulerian-Eulerian model. Evolution of the columnar dendrite envelope, the primary arm spacing and more complex flow phenomena such as the meandering flow were predicted by a
Translated title of the contributionBeobachtung von Strömungsphänomenen während der dendritischen Erstarrung
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Schumacher, Peter, Assessor B (internal)
  • Ludwig, Andreas, Assessor A (internal)
Publication statusPublished - 2018

Bibliographical note

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Keywords

  • Solidification
  • Flow
  • Turbulence
  • Natural Convection
  • Columnar
  • Equiaxed crystals
  • Macrosegregation
  • Ammonium Chloride
  • Mushy Zone
  • Freckels
  • Channel Segregation
  • Particle Image Velocimetry

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