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February 6, 2010 10:29am UTC in response to Roopesh Mohandas Syamaladevi

Re: apparent specific heat

Hi

I would use something like:

Cp = Cp_frozen*(T<=Tf) +
Cp_latent heat*ice fraction/DT*(T-Tm')/(Tf-Tm')*(T>Tm’)*(T<Tf) +
Cp_unfrozen*(T>Tf)

Then you must combine, if you need to smoothen the "(T-Tm')/(Tf-Tm')", a "heaviside" or a "sine step" function. I prefere "sine" step as these are smooth up to to 2-3 derivatives without overshoots, but you must write them out first, a little agebra

PLS CHECK/PLOT this function out carefully, I might well have got the signs or the bolean wrong.

Often I define these expressions as functions in COMSOL as you can then easily check them with the "plot" button

Good luck
Ivar

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February 16, 2010 6:41am UTC in response to Roopesh Mohandas Syamaladevi

Re: apparent specific heat

Hi

good to hear that you found a way with smoothened functions, in V4 (soon there I have heard) the step standard function proposed are already "smoothened", still you must check that your time range does take a few point within the transition region, othervise it might skip over it (even fully I had the other day some short pulse with a low repetition cacle and the solvr just ignored several of them, so some hand tuning and a careful verification is mandatory.

Conserning heat exchanges over boundaries, when I do not want to use full fluidic models, induced convection fully simulated, I mostly set a Q heat loss factor (2W/K/m^2 for calme situations, 5W/K/m^2 and 2-4 times this for blow conditions, as "rule of thumb", but you must set a "true" external temperature, whatever that is).

Mostly this works quite OK but is rather empirical. For certain cases, such as small air gaps (mm and below)and "now blow", air behaves differently as convective exchange does manage to start up and you can get much lower heat exchange values, ar becomes a good isolator, I know you can use other tricks too and have larger air gaps while blocking the convective buildup, such as a "hairy surface" with fibres standing out, they do not even need to be very dense, finally it depends also on the direction of gravity w.r.t shell dimensions then I use only standfard convection of air with no "air" motion (just "bulk air as any bulk material).

Again all his depends on your experience and the details you want tu put in, not t sa the time you can spend simulatng ;)

Hope this helps on the way
Good luck
Ivar

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March 9, 2010 12:57pm UTC in response to Justin Chiu

Re: apparent specific heat

Hi

Thanks for the corrections, you are right I typed it out too quickly ;) I have corrected the "<"

The ice fraction is without units so I have a T[K] unit issue, I obviously need to revise my stuff ;)
I edited and added a "/DT" on my first reply, remains how to define correctly DT

furthermore if you need to express this as an energy there is a *rho [kg/m^3] missing and an implicit volume [m^3] from the integration (depending on how this is implemented in COMSOL) that is neither not explicitely writen out

Ivar

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August 23, 2010 9:44pm UTC in response to Thomas Helbing

Re: apparent specific heat

Hello,

I have found this forum and it was so useful for me because I had the same problem about how type the apparent specific heat of phase change material in COMSOL.
But I've got interest in "sine" step or the function thay you metionated above, i don't know how to use it, but i understood that you can use it for check the function of cp, am I wrong??

I have a material that change phase from 313 to 316 K. The melting cp is 60.5 kJ/kgK and the cp from solid state is 2.5 kJ/kgK, and I type this into the program

Cp= 2500*(T<313)+(60500*(T>313)*(T<316))+2500*(T>316)

It works, but now i would like to plot the function in order to check the latent heat, if the integral of that cp function is the latent heat of my material i could validate the formula. My model is in 2D in COMSOL 3.5a.

I hope I've been clear,
thank you so much.

Ivette.

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