Assume water flows outside the metal holder at certain flow rate. Is the above situation practically possible? Whether the lesser melting point material will not melt? Any body know some practical applications like the above question? And finally how to find the thickness of the metal holder?
Hakeem,
Yes, this is possible. There are a number of parameters that one has to consider to ensure that the crucible/holder does not melt.
a) Relative magnitude of the mass of the crucible and the melt.
b) The specific heat capacity and thermal conductivity of the above
c) The rate at which the crucible is artificially cooled.
You may see some erosion from the holder, depending somewhat on the relative conductivity of the melt and the crucible - as well as the rate at which the crucible is cooled.
A thought experiment. Take a cubic metre of tin with a small depression in its top-side.
Pour a blob of molten aluminium into the depression - and watch. The aluminium (which is less dense than tin) will simply sit there - cooling rapidly by conduction. Some tin will probably melt, locally, as the thermal conductivity of aluminium is higher than that of tin - if one were to actively cool the tin, then one might not loose much of the crucible.
Summary: have a high cooling flow, a small mass of 'melt' and you'll be able to get away with using a lower melting-point material.
May I ask what sort of situation (mass, materials, etc.) you are considering?
There are methods for surface treatment of metal bars which are introduced at high temperature with high speed tubes with strong water flows. I do not know if support has a lower melting temperature, but in practice the molten metal does not touch the media as it forms film-boiling, which under certain conditions can sustain the metal.
Hakeem,
Yes, this is possible. There are a number of parameters that one has to consider to ensure that the crucible/holder does not melt.
a) Relative magnitude of the mass of the crucible and the melt.
b) The specific heat capacity and thermal conductivity of the above
c) The rate at which the crucible is artificially cooled.
You may see some erosion from the holder, depending somewhat on the relative conductivity of the melt and the crucible - as well as the rate at which the crucible is cooled.
A thought experiment. Take a cubic metre of tin with a small depression in its top-side.
Pour a blob of molten aluminium into the depression - and watch. The aluminium (which is less dense than tin) will simply sit there - cooling rapidly by conduction. Some tin will probably melt, locally, as the thermal conductivity of aluminium is higher than that of tin - if one were to actively cool the tin, then one might not loose much of the crucible.
Summary: have a high cooling flow, a small mass of 'melt' and you'll be able to get away with using a lower melting-point material.
May I ask what sort of situation (mass, materials, etc.) you are considering?
I agree with James. It totally depends on heat content of the hot substance, heat transfer duration, and amount of cooling available externally. Other day, while working in lab, I had few observations while soldering few electronic components.
The soldering flux usually melts at ~ 400- 450 deg C. While soldering, few blobs of molten metal fell in couple of occasions on an acrylic sheet (melting point : 160 deg C) at room temperature.
The solid blob impacts on the sheet, spreads, exchanges heat with surrounding and the sheet ; and solidifies within few milliseconds.
Even when the melt has a very high temperature, due to small heat content and rapid heat dissipation to a large body of cooling available, nothing happens to the acrylic sheet (less melting point substance).
James has elaborated the considerations to be taken. This question appears to qualify for an exercise problem in basic heat transfer. To estimate the crucible thickness, values of specific heat, conductivity, quantity of molten metal and temperatures etc may be required. Then Flow rate of the coolant is required to be determined to keep the temperature of the crucible surface below 1500 deg. Some empirical relations if available may be used.
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