Numerical analysis of rapid solidification of NiTi alloy: Influence of boundary conditions / Numerična analiza hitrega strjevanja NiTi zlitine: vpliv robnih pogojev
Matej Zadravec, Primož Ternik, Rebeka Rudolf, Milan Svetec
ANALI PAZU, 4 (2014), št. / No. 2, strani / pages 82-88
Celotno besedilo / Full text (PDF)
Abstract: The objective of the present work is to analyze influence of temperature boundary conditions on the solidification process of NiTi alloy. Alloy is taken as an incompressible fluid where the heat is transferred by conduction and convection, including the thermal phase change phenomenon. The last one is modelled by the improvement procedure, so-called enthalpy-porosity formulation, where the liquid-solid mushy zone is treated as a porous zone with porosity equal to the liquid fraction. The numerical model is based on the finite volume method in body fitted coordinates with a SIMPLER scheme to couple the pressure and velocity fields. Simulation presents solidification for two cooling cases on the cylindrical part, where in the first case the adiabatic boundary condition is considered and in the second case the convective cooling over the wall is present. The results are presented for the velocity and temperature field as well as for the NiTi mass fraction during the solidification process. Results shows that velocity and temperature field is strongly affected by the different cooling condition on the cylindrical wall and therefore the solidification process of the alloy. Analysis shows that cooling on the cylindrical part is one of the major parameters for alloy solidification and therefore should not be neglected.
Key words: Solidification, Finite volume method, Phase change.
Povzetek: Delo obravnava vpliv temperaturnih robnih pogojev na strjevanje NiTi zlitine s pomočjo numerične simulacije. Zlitina je obravnavana kot nestisljiva tekočina, kjer se toplota prenaša s prevajanjem in prestopom, vključno s pojavom fazne spremembe. Numerično modeliranje fazne spremembe je izvedeno z izboljšanim pristopom, imenovanim formulacija entalpija-poroznost. Pri tem pristopu je območje vmesnega faznega stanja tekoče-trdno obravnavano kot porozno območje s poroznostjo, ki je enaka deležu tekoče faze. Numerični model temelji na metodi končnih volumnov, pri čemer sta tlačno in hitrostno polje povezana s SIMPLER algoritmom. Numerična simulacija podaja strjevanje NiTi zlitine v geometriji epruvete, ki je v prvem primeru po cilindričnem delu oboda idealno izolirana (adiabatni pogoj) in v drugem primeru se preko te površine hladi zaradi okoliškega zraka. V rezultatih so predstavljeni hitrostno in temperaturno polje, kakor tudi deleži posameznih faz NiTi zlitine med procesom strjevanja. Iz rezultatov je razviden precejšen vpliv različnega robnega pogoja na hitrostno in temperaturno polje, kar posledično vpliva tudi na strjevanje zlitine. Analiza je pokazala, da je ohlajanje preko stene epruvete eden izmed vplivnejših parametrov strjevanja zlitine in ga zato ne smemo zanemariti.
Ključne besede: strjevanje, metoda končnih volumnov, fazna sprememba.