The Effect of The Natural Convection on The Transition from Columnar to Equiaxed Crystals:

1981 ◽  
Vol 9 ◽  
Author(s):  
Hasse Fredriksson
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Solidification Front ◽  
Growth Process ◽  
Solidification Process ◽  
Temperature Gradients ◽  
Metal Alloys ◽  
Dendritic Solidification ◽  
Kinetics Of ◽  
Solidification Of Metal

ABSTRACTDuring dendritic solidification of metal alloys there normally occur temperature gradients in the liquid ahead of the solidification front. The gradient causes a natural convection ahead of the front. The natural convection causes a heat transfer from the interior of the liquid, which gives the possibility for crystals to grow ahead of the solidification front. The growth of these crystals is determined by the cooling rate and the kinetics of the solidification process. The paper will deal with the theory behind the growth of free crystals ahead of a solidification front. Some experimental results will be given. The effect of gravity on the growth process will. be discussed.

scholarly journals Numerical investigation of heat transfer and solidification in a cavity filled with molten metal alloys in the presence of thermodiffusion

2021 ◽  
Author(s):  
Elham Jafar-Salehi
Keyword(s):  
Natural Convection ◽  
Molten Metal ◽  
Solidification Process ◽  
Thermosolutal Convection ◽  
Metal Alloys ◽  
Strong Mixing ◽  
Slight Variation ◽  
Fe Model ◽  
Molten Metals ◽  
Bottom Heating

The objective of this research was to study the solidification process of binary molten metals. This study was conducted in three phases. One was to estimate the thermodiffusion factor, two was to simulate the effect of natural convection and radiation on the velocity, temperature, and concentration distributions, and the third was to investigate the solidification process of binary molten metals using the proposed thermodiffusion factors. The proposed expression for the estimation of thermodiffusion factor was based on the physical properties of the mixture constituents. The estimated thermodiffusion factor was used to study thermosolutal convection in a quartz enclosure filled with molten Sn-Bi alloy. Two simulations were carried out: top heating and bottom heating. The sidewalls in both cases were exposed to convection and radiation. Numerical results show that in the case of top heating, the distribution of temperature and concentration are linear, but species segregation occurs due to the thermodiffusion effect. In the bottom heating case, boundary-driven convective flow develops with a large Rayleigh number (Ra) where an increase in the Ra number negates thermodiffusion due to the development of strong mixing. The results of these simulations showed that the effect of convection and radiation are negligible. In phase three, finite element method (FE) was employed to investigate the effect of thermodiffusion during vertical solidification of binary molten metal alloys with bottom cooling. The systems considered here are tin-bismuth (Sn-Bi), tin-cadmium (Sn-Cd), tin-zinc (Sn-Zn), tin-lead (Sn-Pb), tin-gallium (Sn-Ga), and bismuth-lead (Bi-Pb) binary molten metals. The geometry under study was a cylindrical cavity. The FE model was constructed using a 2D axisymmetric element to represent a 3D cyclindrical model. Two cases were studied: one without and one with the effect of thermodiffusion. The simulation including thermodiffusion showed slight variation from the simulation without thermodiffusion, in that thermodiffusion causes a slightly faster solidification and a more uniform concentration distribution if the thermodiffusion coefficient is greater than zero (DT > 0). The main object of this research is development of a more accurate thermodiffusion factor, and applying it in a numerical simulation to study its effects on radiation, natural convection, and solidification processes.

scholarly journals Numerical investigation of heat transfer and solidification in a cavity filled with molten metal alloys in the presence of thermodiffusion

2021 ◽  
Author(s):  
Elham Jafar-Salehi
Keyword(s):  
Natural Convection ◽  
Molten Metal ◽  
Solidification Process ◽  
Thermosolutal Convection ◽  
Metal Alloys ◽  
Strong Mixing ◽  
Slight Variation ◽  
Fe Model ◽  
Molten Metals ◽  
Bottom Heating

The objective of this research was to study the solidification process of binary molten metals. This study was conducted in three phases. One was to estimate the thermodiffusion factor, two was to simulate the effect of natural convection and radiation on the velocity, temperature, and concentration distributions, and the third was to investigate the solidification process of binary molten metals using the proposed thermodiffusion factors. The proposed expression for the estimation of thermodiffusion factor was based on the physical properties of the mixture constituents. The estimated thermodiffusion factor was used to study thermosolutal convection in a quartz enclosure filled with molten Sn-Bi alloy. Two simulations were carried out: top heating and bottom heating. The sidewalls in both cases were exposed to convection and radiation. Numerical results show that in the case of top heating, the distribution of temperature and concentration are linear, but species segregation occurs due to the thermodiffusion effect. In the bottom heating case, boundary-driven convective flow develops with a large Rayleigh number (Ra) where an increase in the Ra number negates thermodiffusion due to the development of strong mixing. The results of these simulations showed that the effect of convection and radiation are negligible. In phase three, finite element method (FE) was employed to investigate the effect of thermodiffusion during vertical solidification of binary molten metal alloys with bottom cooling. The systems considered here are tin-bismuth (Sn-Bi), tin-cadmium (Sn-Cd), tin-zinc (Sn-Zn), tin-lead (Sn-Pb), tin-gallium (Sn-Ga), and bismuth-lead (Bi-Pb) binary molten metals. The geometry under study was a cylindrical cavity. The FE model was constructed using a 2D axisymmetric element to represent a 3D cyclindrical model. Two cases were studied: one without and one with the effect of thermodiffusion. The simulation including thermodiffusion showed slight variation from the simulation without thermodiffusion, in that thermodiffusion causes a slightly faster solidification and a more uniform concentration distribution if the thermodiffusion coefficient is greater than zero (DT > 0). The main object of this research is development of a more accurate thermodiffusion factor, and applying it in a numerical simulation to study its effects on radiation, natural convection, and solidification processes.

Transient Numerical Analysis of Different Finned Tube Designs for Use in Latent Heat Thermal Energy Storage Devices

2015 ◽  
Author(s):  
Anton Beck ◽  
Martin Koller ◽  
Heimo Walter ◽  
Michael Hameter
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Numerical Analysis ◽  
Numerical Investigation ◽  
Sodium Nitrate ◽  
Solidification Process ◽  
Melting Process ◽  
Finned Tube ◽  
Heat Transfer Mechanism ◽  
Melting And Solidification

In this paper the results of a numerical investigation of the melting and solidification process of sodium nitrate, which is used as phase change material, will be presented. For the heat transfer to the sodium nitrate different finned tube designs, namely helical-, transversal- and longitudinal finned tubes, are used. The numerical results of the melting and solidification process for the different design cases will be compared. The numerical analysis of the melting process has shown that apart from the first period of the charging process natural convection is the dominant heat transfer mechanism. The numerical analysis of the melting process has also shown that for a fast melting process heat exchanger tubes should be designed in such a way that an unrestricted natural convection is guaranteed. The numerical investigation for the solidification process has shown that the dominant heat transfer mechanism is heat conduction. The investigation has also shown that the solidification front grows more uniformly from the tube surface to the outer shell compared to the melting front. Therefore no significant differences between the different tube designs are detected concerning the solidification process.

scholarly journals Analytical Study of the Solidification of a Phase Change Material in an Annular Space

Energies ◽  
2020 ◽  
Vol 13 (21) ◽  
pp. 5561
Author(s):  
Zygmunt Lipnicki ◽  
Tomasz Małolepszy
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Phase Change Material ◽  
Solidification Front ◽  
Solidification Process ◽  
Theoretical Research ◽  
Graphical Form ◽  
Annular Space ◽  
Conjugate System ◽  
Change Material

In this study, the process of the solidification of a PCM (phase change material) liquid in an annular space was analytically investigated with the use of a simplified quasi-steady-state model. This model described the phase change phenomenon with the cylindrical solidification front and with the solidification liquid overheated above the solidification temperature. One of the important novelties of the applied model was the determination of the coefficient of the heat transfer between the liquid and the solidified layer on the solidification surface, which was calculated as a function of the location of the solidification front. A method for calculating the variable coefficient of heat transfer on the surface of the solidification front during the solidification process is presented. The contact layer between the cold wall and the solidified layer was incorporated into the model and played an important role. The theoretical analytical method describing the solidification process based on the quasi-steady model was used in the study. Moreover, the main problem considered in this work could be reduced to a conjugate system of differential equations, allowing it to be solved numerically. From this perspective, the influence of various dimensionless parameters on the solidification process could be clearly seen. The obtained numerical results are presented in graphical form. The results of the theoretical research were compared with the experimental research of one of the author’s earlier works and they showed a significant agreement. Finally, the simple analytical approach presented in this work can be used for designing annular heat accumulators.

Melting in Rectangular Enclosures: Experiments and Numerical Simulations

1985 ◽  
Vol 107 (4) ◽  
pp. 794-803 ◽  
Author(s):  
C. Be´nard ◽  
D. Gobin ◽  
F. Martinez
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Transfer Process ◽  
Solid Phase ◽  
Solid Material ◽  
Melting Process ◽  
Heat Transfer Process ◽  
Numerical Code ◽  
Melting Front ◽  
Kinetics Of

This paper presents a numerical and experimental analysis of the heat transfer process that takes place while melting a solid material, in a rectangular enclosure. Natural convection is present in the melt layer, and the solid phase is assumed to be isothermal. Very detailed and precise experimental results are given that are used to validate a particularly rapid numerical code. Some insights into the kinetics of the melting process lead to a deeper understanding of the coupling between convection and phase change and allow us to propose a simple algebraic correlation that predicts the time evolution of the melting front to within 5 percent.

Heat transfer enhancement in solidification process by change of fins arrangements in a heat exchanger containing phase-change materials

2019 ◽  
Vol 29 (5) ◽  
pp. 1741-1755 ◽  
Author(s):  
Mohammad M. Hosseini ◽  
Asghar B. Rahimi
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Energy Storage ◽  
Heat Exchanger ◽  
Phase Change ◽  
Thermal Energy ◽  
Thermal Energy Storage ◽  
Solidification Process ◽  
Solidification Time ◽  
Content Type

Purpose Reducing discrepancy between energy demand and supply has been a controversial issue among researchers. Thermal energy storage is a technique to decrease this difference to increase the thermal efficiency of systems. Latent heat thermal energy storage has interested many researchers over the past few decades because of its high thermal energy density and constant operating temperature. The purpose of this paper is to provide a numerical study of the solidification process in a triplex tube heat exchanger containing phase change material (PCM) RT82. Design/methodology/approach A two-dimensional transient model was generated using finite volume method and regarding enthalpy-porosity technique. After that, a detailed and systematic approach has been presented to modify longitudinal fins’ configuration to enhance heat transfer rate in PCMs and reducing solidification time. The numerical results of this study have been validated by reference experimental results. Findings The ultimate model reduced solidification time up to 21.1 per cent of the Reference model which is a substantial improvement. Moreover, after testing different arrangements of rectangular fins and studying the flow pattern of liquid PCM during solidification, two general criteria was introduced so that engineers can reach the highest rate of heat transfer for a specified value of total surface area of fins. Finally, the effect of considering natural convection during solidification was studied, and the results showed that disregarding natural convection slows down the solidification process remarkably in comparison with experimental results and in fact, this assumption generates non-real estimation of solidification process. Originality/value The arrangement of the fins to have the best possible solidification time is the novelty in this paper.

scholarly journals Convective Heat Transfer During Melting in a Solar LHTES

2021 ◽  
Vol 11 (3) ◽  
pp. 7181-7186
Author(s):  
F. Z. Mecieb ◽  
F. García Bermejo ◽  
J. P. Solano Fernández ◽  
S. Laouedj
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Melting Rate ◽  
Melting Time ◽  
Heat Sources ◽  
Stored Energy ◽  
Numerical Investigations ◽  
Coupling Strategy ◽  
Kinetics Of ◽  
Change Material

Melting combined with natural convection in a shell and Latent Thermal Energy Storage (LHTES) tube driven by a solar collector was analyzed numerically in the present work. This work's particularity lies in the fact that the HTF temperature varies at each moment following the solar irradiance curve. A program (UDF) has been developed and integrated into Ansys to meet this requirement. The use of this coupling strategy allows obtaining realistic unsteady LHTES results. Several numerical investigations were carried out to analyze the effect of the heat sources' power on the accumulator's performance. The obtained results show that natural convection considerably influences the heat transfer as well as the melting kinetics of the Phase Change Material (PCM). Besides, the results show that increasing the heat transfer fluid's thermal load can increase the melting rate of the PCM and the stored energy and reduce the entire melting time.

scholarly journals Heat transfer and entropy generation for natural convection by adiabatic obstacles inside a cavity heated on the left side

2020 ◽  
Vol 48 (4) ◽  
pp. 825-832
Author(s):  
Jamal Baliti ◽  
Mohamed Hssikou ◽  
Youssef Elguennouni ◽  
Ahmed Moussaoui ◽  
Mohammed Alaoui
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Natural Convection ◽  
Entropy Generation ◽  
Temperature Gradients ◽  
Large Temperature ◽  
Square Cavity ◽  
Transfer Rates ◽  
Thermal Fields ◽  
Rayleigh Numbers

By using finite difference method, the problem of heat transfer and entropy generation for natural convection of a fluid inside a square cavity with inner adiabatic bodies has been investigated numerically. Calculations have been made for Rayleigh numbers ranging from 102 to 5·104 for two obstacles with different heights. Results are presented as streamlines, isotherm contours and Nusselt number for Prandtl number of 0.71 (assuming the cavity is filled with air). The obtained results demonstrate the effects of pertinent parameters on the fluid flow, thermal fields and heat transfer inside the cavity. The results show that the heat transfer rates generally increase with the shrink of the obstacle size and with the increase of Rayleigh number. The entropy generation is higher at locations with large temperature gradients. Excellent agreement is obtained with previous results in the literature.

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