Effective heat capacity - Phase change does not work!!!

papteo · January 15, 2013, 10:29

Hi,

I am trying to simulate a PCM with the CFX. To simulate the phase change , i wrote an expression varying the heat capacity depending on its temperature in order to implement the effective heat capacity for the melting phase. The normal value of the specific heat capacity is 1200 J/(kg K) and during the melting area 100.000 (J/kg K). My problem is that I do not obeserve any change at the temperature during the phase change while the Cp has the right value according to my simulation! The temperature should have remained constant for a certain time or at least the temperature increase should have been smoother

I am attaching also two photos from the post-processing which show the Cp and Temperature over time!

Can anyone help me with this problem?

step · January 15, 2013, 12:00

Hi there,

I would also be interested in a solution. I've found some bits of literature that suggest an approach using an effective heat capacity to account for the energy necessary for a phase change.

Any ideas?

ghorrocks · January 15, 2013, 21:57

What are you actually modelling? How are you driving the temperature?

papteo · January 16, 2013, 04:49

Hi Glenn,

thank you very much for your interest. What I am actually doing is to test the principle of the effective heat capacity for a phase change material. I am modelling a cube (PCM-solid) which has an initial temperature of 45 °C and one side is heated up with 10.000 W/m² . The PCM has a very high Cp value within the melting field (e.g. 50-60 °C). As mentioned above my goal is to monitor a constant temperature within this temperature field. Instead the temperature is rising without observing any change at the temperature contrary to the Cp value as the pictures show.

Do you have any ideas why this is happening?

Thank you very much in advance

ghorrocks · January 16, 2013, 17:45

Obviously your variable CP is not beign applied. Check that it is the CEL properly. Also check that the table CFX uses to model variable properties is appropriate to capture the non-linear bits of your Cp curve. You might need to tighten the table temperature limits, or put more points in the table.

papteo · January 17, 2013, 10:20

Thank you very much Glenn for your advice. I just increased the number of the points and the problem was solved.

Once again thank you for your help and quick answer!

jjdd2011 · October 23, 2013, 22:16

Thank you for your help. Just wonder where is the table CFX used to model variable properties.

ghorrocks · October 24, 2013, 06:27

It is internally generated. You define how many points the table has, but do not access the table directly. You can see its result by looking at the material properties it ends up with in the post processor if you like.

jjdd2011 · October 31, 2013, 06:15

Thanks a lot. After I have posted the question, I realized that it can be in material property setting. I changed the number of points. It worked. Great help.

January 15, 2013, 12:00		#2
step New Member Join Date: Jun 2009 Posts: 9 Rep Power: 17	Hi there, I would also be interested in a solution. I've found some bits of literature that suggest an approach using an effective heat capacity to account for the energy necessary for a phase change. Any ideas? papteo likes this.

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January 15, 2013, 21:57		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	What are you actually modelling? How are you driving the temperature?

January 16, 2013, 04:49		#4
papteo Member Theodoros Papadopoulos Join Date: Mar 2011 Posts: 36 Rep Power: 15	Hi Glenn, thank you very much for your interest. What I am actually doing is to test the principle of the effective heat capacity for a phase change material. I am modelling a cube (PCM-solid) which has an initial temperature of 45 °C and one side is heated up with 10.000 W/m² . The PCM has a very high Cp value within the melting field (e.g. 50-60 °C). As mentioned above my goal is to monitor a constant temperature within this temperature field. Instead the temperature is rising without observing any change at the temperature contrary to the Cp value as the pictures show. Do you have any ideas why this is happening? Thank you very much in advance

January 16, 2013, 17:45		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Obviously your variable CP is not beign applied. Check that it is the CEL properly. Also check that the table CFX uses to model variable properties is appropriate to capture the non-linear bits of your Cp curve. You might need to tighten the table temperature limits, or put more points in the table.

January 17, 2013, 10:20		#6
papteo Member Theodoros Papadopoulos Join Date: Mar 2011 Posts: 36 Rep Power: 15	Thank you very much Glenn for your advice. I just increased the number of the points and the problem was solved. Once again thank you for your help and quick answer!

October 23, 2013, 22:16		#7
jjdd2011 New Member John Join Date: Dec 2011 Posts: 2 Rep Power: 0	Thank you for your help. Just wonder where is the table CFX used to model variable properties.

October 24, 2013, 06:27		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	It is internally generated. You define how many points the table has, but do not access the table directly. You can see its result by looking at the material properties it ends up with in the post processor if you like.

October 31, 2013, 06:15		#9
jjdd2011 New Member John Join Date: Dec 2011 Posts: 2 Rep Power: 0	Thanks a lot. After I have posted the question, I realized that it can be in material property setting. I changed the number of points. It worked. Great help.