Gas / Liquid Interface Heat Transfer Coefficient

[ChemEngSquirrel]

I have a water vessel with N2 blanket. If the water is at 5deg C and minus 7 deg C nitrogen enters the vessel, I am interested in how long it takes to reach a uniform vessel temperature.

By ignoring external heat transfer i see heat transfer as occurring across the gas / liquid interface. How can i estimate the gas / liquid interface heat transfer coefficient?

Thanks.

 

[Latexman]

A decision/an assumption is needed on the controlling mechanism - natural convection (no/low flow), forced convection (flow), radiation (doubt it). Is it a steady state or unsteady state issue? On one or both sides of the interface? Then, pick the right htc correlation and calculate the htc on both sides of the interface.

Concerned about ice formation and possible problems from that? Are these problems important or a nuisance? Do a what if. High N2 flow. No water flow. If important, just heat up the N2 a bit and forget about it.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

Given that this is probably not a steady state heat exchange, you really need to either integrate or use a transient analysis program as the heat transfer will be continually varying as the temperature difference reduces and if the level starts to move up and down as well then regular doses of low temp N2 will also vary the heat transfer. Add to that the effect of ambient temperature / sun etc on the tank or vessel and I think you have far too any variables to give you any real idea about what this time is.

If I was you I would work out your max volume of N2 and its heat capacity and mass and the min volume of water and its heat capacity and mass then sum the energy and divide again to work out temperature of the two on the basis that nothing changes and equilibrium eventually occurs. Given the huge density difference between water and blanketing N2, my guess is that the water temp will change by about half a degree at most.

You might be "interested" to know the time, but sometimes things are just not worth calcualting. Just do some experiments on your actual vessel.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way