• Transactions of the Korean Society of Automotive Engineers
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• v.7 no.8
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• pp.143-149
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• 1999

In this study , the new method which is used by magneticfluid for undercooling ends and continuous ice making is introduced to ice making in the ice stroage system and its validity is studied experimentally . This study made sure how shape control of magniticfluid and instant explosure method can effect on the undercooling degree and it exchange by time change at the moments of undercooling ends and acquired the fundamental knowledge for control method about undercooling ends.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.5
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• pp.698-707
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• 1998

A simplified model for predicting microsegregation during columnar-dendritic solidification of binary alloys is developed, in which back diffusion, dendrite arm coarsening and dendrite tip undercooling are simultaneously incorporated. The inclusion of tip undercooling is accomplished by modifying the initial conditions of the existing solute diffusion model, in such forms that tip undercooling depresses the beginning of solidification below the liquidus temperature, and that the secondary arm spacing evolves in accordance with the minimum undercooling theory. Sample calculations for the well-known benchmark system show that the present predictions not only consist with the extablished limiting cases, but also agree favorably with the available experimental data within a reasonable tolerance. In particular, a typical decreasing trend in the eutectic fraction at high cooling rates is successfully resolved. Comparison of the individual and combined effects of characteristic parameters in reference with the limiting cases reveals the interactions among parameters. Every parameter plays the role of reducing the eutectic fraction, and the degree of influence depends primarily on the cooling rate. Coarsening enhances the effect of tip undercooling, while suppressing that of back diffusion. A vigorous back diffusion seems to restrain the apperance of the undercooling effect. Overall, each contribution of the three parameters to microsegregation is estimated to be of the same order, which suffices to justify the present study.

• Journal of Welding and Joining
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• v.17 no.4
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• pp.60-68
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• 1999

The previously developed two dimensional model was modified in order to predict more accurately the degree of microsegregation and eutectic fraction on in weld metal whose solidification rate is very fast. The model employed the same assumptions with previous model but considered of a tip undercooling. The previously predicted microsegregation and eutectic fraction has the discrepancies between simulated and examined results in the weld metal solidification. The experiments for the weld metal solidification of 2024 A1 and Fe-Ni alloy were carried out in order to examine the reasonability and feasibility of this modified model. The concentration profile of the solute and eutectic fraction predicted by the simulation agreed well with those found from experimental works. According to the results, it was believed that the dendrite tip undercooling considered in the modified model be reasonable for predicting the degree of microsegregation more accurately in weld metla solidification. In the GTA welds, degree of dendrite-tip undercooling increases with increasing solidification rage(welding speed). This serves to increase the concentration of dendrite core and thus result in reducing the degree of segregation. And solid state diffusion(back diffusion) during solidification is very low in the weld metal solidification so that little additional homogenization of solute occurs during solidification. With consideration of tip undercooling this modified model can predict exactly degree of microsegregation and eutectic fraction from slow solidification(casting) to fast solidification(welding).

• 한국신재생에너지학회:학술대회논문집
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• 2006.06a
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• pp.391-394
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• 2006

Growth characteristics of methane-propane clathrate hydrate, growing under different undercooling conditions, was investigated. After the water within pressurized vessel was fully saturated with guest gas molecules by agitation, medium was rapidly undercooled and maintained at the constant temperature. The growth of hydrate was always Initiated with film formations at the upper bounding surface of liquid pool. The visual observation using microscope revealed detailed features of subsequent crystal nucleation, migration, growth and interference occurring within liquid pool. A number of small crystals ascended and settled at the hydrate film. When undercooling was small $({\Delta}T=3.2K)$, some of the settled crystals slowly grew into faceted columns. As the undercooling increased, the downward growth of crystals underneath the hydrate film became dendritic and occurred with greater rate and with finer arm spacing. The shapes of the floating crystals were diverse and included octahedron and triangular or hexagonal platelet When the undercooling was small, the octahedral crystals were found dominant. As the undercooling increased, the shape of the floating crystals also became dendritic. The detailed characteristics of floating crystals were reported in this study.

• Korean Chemical Engineering Research
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• v.45 no.4
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• pp.400-409
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• 2007

Morphology of methane/propane clathrate hydrate crystal was investigated under different undercooling conditions. After the water pressurized with compound guest gas was fully saturated by agitation, medium within the vessel was rapidly undercooled and maintained at the constant temperature while the visual observations using microscope revealed detailed features of subsequent crystal nucleation, migration, growth and interference occurring within liquid pool. The growth of hydrate was always initiated with film formations at the bounding surface between bulk gas and liquid regions under all tested experimental conditions. Then a number of small crystals ascended, some of which settled beneath the hydrate film. When undercooling was relatively small, some of the settled crystals slowly grew into faceted columns. As the undercooling increased, the downward growth of crystals underneath the hydrate film became dendritic and occurred with greater rate and with finer arm spacing. The shapes of the floating crystals within liquid pool were diverse and included octahedron and triangular or hexagonal platelet. When the undercooling was small, the octahedral crystals were found dominant. As the undercooling increased, the shape of the floating crystals also became dendritic. The detailed growth characteristics of floating crystals are reported focused on the influences caused by undercooling and memory effect.

• Journal of Powder Materials
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• v.1 no.1
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• pp.85-94
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• 1994

In order to predict droplet velocity and temperature profiles and fractional solidification with flight distance during spray forming, the Newtonian heat transfer formulation has been coupled with the classical heterogeneous nucleation and the specific solidification process. It has been demonstrated that the thermal profile of the droplet in flight is significantly affected by process parameters such as droplet size, initial gas velocity, undercooling. As the droplet size and/or the initial gas velocity increase, the onset and completion of solidification are shifted to greater flight distances and the solidification process also extends over a wider range of flight distances. The amounts of solid fractions formed during recoalescence, segregated solidification and eutectic solidification are insensitive to droplet size and initial gas velocity whereas those are strongly affected by the degree of undercooling. There are good linear relations between the undercooling and the corresponding solid fractions generated during recoalesced, segregated and eutectic stages.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.9
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• pp.2404-2412
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• 1994

A mathematical model has been developed for predicting kinematic, thermal, and solidification histories of atomized droplets during flight. Liquid droplet convective cooling, recalescence, equilibrium-state solidification, and solid-phase cooling were taken into account in the analysis of the solidification process. The spherical shell model was adopted where the heterogeneous nucleation is initiated from the whole surface of a droplet. The growth rate of the solid-liquid interface was determined from the theory of crystal growth kinetics with undercooling caused by the rapid solidification. The solid fraction after recalescence was obtained by using the integral method. The thermal responses of atomized droplets to gas velocity, particle size variation, and degree of undercooling were investigated through the parametric studies. It is possible to evaluate the solid fraction of the droplet according to flight distance and time in terms of a dimensionless parameter derived from the overall energy balance of the system. It is also found that the solid fraction at the end of recalescence is not dependent on the droplet size and nozzle exit velocity but on the degree of subcooling.

• Journal of Korea Foundry Society
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• v.13 no.5
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• pp.462-475
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• 1993

A numerical analysis on the microstructural evolutions of microcellular and cellular ${\alpha}-aluminum$ phase in the gas-atomized Al-8wt. pct droplets was represented. The 2-dimensional non-Newtonian heat transfer and the dendritic growth theory in the undercooled melt were combined under the assumptions of a point nucleation on droplet surface and the macroscopically smooth solid-liquid interface enveloping the cell tips. It reproduced the main characteristic features of the reported microstructures quite well. It predicted a considerable volume fraction of segregation-free region in a droplet smaller than $l0{\mu}m$ if an initial undercooling larger than 100K is given. The volume fractions of the microcellular region($g_A$) and the sum of the microcellular and cellular region($g_a$) were predicted as functions of the heat transfer coefficient, h and initial undercooling, ${\triangle}T$. It was shown that $g_A$ and $g_a$, in the typical gas-atomization processes with $h=0.1-1.0W/cm^2K$, are dominated by ${\triangle}T$ and h, respectively, but for h larger than $4.0W/cm^2K$, a fully microcellular structure can be obtained irrespective of the initial undercooling.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.433-434
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• 2006

Monosized germanium micro particles are prepared by a newly developed Pulsated Orifice Ejection Method. The obtained particles are categorized into two kinds of the microstructures as refined and coarse ones. The morphological difference is estimated to be determined by the undercooling level during nucleation. Actually, the increase in the temperature of the melt was effective in coarsening the microstructure, because the temperature of the melt intensely relates to the undercooling level. The transition temperature of coarse and refined microstructures is found to be 1300-1350K. Furthermore, a triggered nucleation could improve the crystallinity of the particles in the short separation.

• Journal of Korea Foundry Society
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• v.11 no.4
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• pp.314-322
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• 1991

In this study, Na, Li, Sb, Sr were added as the modifiers to improve the mechanical proper ties of Al-Si alloys and their effects were compared one another Thermal analysis was carried out to measure the degree of undercooling, that is, the basic mechanism of modifying effect. The measurements of holding time and analyses of components of Al-Si alloys were carried out to compare the fading of the modifiers. Greater degree of undercooling was measured from the modified alloy than the normal alloys. In the modified alloy, the shape of Si was changed into a fine fiber phase or a lamellar phase from the coarse plate phase. There are not modifying effect of Sb below the $1.5^{\circ}C$ /sec of cooling rate. As the cooling rate increases, similar effects appears in the non-modified alloys like the modified alloys. Na and Li were faded in short time and were difficult to control on the melt, but Sb and Sr were scarcely faded for 6 hours of holding and were easy to control on the melt. Na and Sr are the good modifiers in terms of microstructure. Sb is the best modifier, but Na, Li are the inadequate modifiers in terms of fading.