# Porous domain energy calculation and specific heat capacity temperature dependent

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 November 28, 2017, 12:27 Porous domain energy calculation and specific heat capacity temperature dependent #1 Member   VB Join Date: Jul 2016 Posts: 35 Rep Power: 9 Hello people, I have a simple model where porous domain is inbetween two fluid domains. Hot fluid is given as Input. I am using Specific heat capacities for fluid and solid material in porous domain to be temperature dependent. Also the density of fluid is temperature dependent. Energy calculation formula used 'Integral over (mass_flow*C_p*DeltaT )dt. Is there a way to calculate C_p for each cell?(in ansys cfx pre or post). The opearating temperature range is about 800°C, so I do not want to take single average C_p value as it varies quite significantly between the temp range and compromises the Energy calculation. In function Calculator I tried Function:volumeInt , Location: porous, Variable:Specific Heat Capacity at Constant Pressure. What volumeInt(SHC)@Domain returns actually? Ultimately I need to calculate Energy in porous region/solid, where it is calculated using C_p that is temperature dependent. Any suggestion?

 November 28, 2017, 16:29 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,727 Rep Power: 143 In the material model make Cp/specific heat a function of temperature. It is simple as that. But be aware that if you are specifying density as well you need to make it thermodynamically consistent across the temperature range.

November 28, 2017, 16:45
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VB
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Quote:
 Originally Posted by ghorrocks In the material model make Cp/specific heat a function of temperature. It is simple as that. But be aware that if you are specifying density as well you need to make it thermodynamically consistent across the temperature range.
Hi. What do you mean by make specific heat, a function of temperature in material model? I have specified Cp as temperature dependent in expression -CFX Pre for porous solid domain but I am sure you are not specifing this, are you?

I have completed my simulation run using C_p of Porous domain as temperature dependent(C_p is temperature dependent for both solid and fluid materials in porous). I need the formula 'Integral over (mass_flow*C_p*DeltaT )dt for Energy calculation in porous domain where C_p is not constant. To put in other way, Is there a way to access C_p during each iteration/each node so that energy of total porous domain is calculated at each time step or at the end of simulation, either one is fine?

 November 28, 2017, 16:54 #4 Senior Member   Join Date: Jun 2009 Posts: 1,815 Rep Power: 32 Not clear what you are trying to compute? When working with varying specific heat capacities, you are better off using energy quantities directly than simplified formulas unless you want to keep the thermodynamics consistency yourself. For example, the integral over volume of static enthalpy, i.e. volumeInt ( density * static enthalpy)@Volume of Interest

November 28, 2017, 17:06
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VB
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Quote:
 Originally Posted by Opaque Not clear what you are trying to compute? When working with varying specific heat capacities, you are better off using energy quantities directly than simplified formulas unless you want to keep the thermodynamics consistency yourself. For example, the integral over volume of static enthalpy, i.e. volumeInt ( density * static enthalpy)@Volume of Interest
Hi. I need to calculate Energy contained in porous domain after my simulation or during the run, either one is fine. "you are better off using energy quantities directly than simplified formulas" - I would love to keep it simple . The formula that I mentioned is used to compute Energy in each node and in turn the whole domain. (eg., in Open foam or similar software where we have access to each node/cell in a domain, unlike Ansys). I believe Ansys is even simpler and accurate in Energy calculations over interested domain. What this returns? "volumeInt ( density * static enthalpy)@Volume of Interest"

November 28, 2017, 17:12
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Erik
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Quote:
 Originally Posted by ghorrocks But be aware that if you are specifying density as well you need to make it thermodynamically consistent across the temperature range.
Pay attention to this, you can't specify both density and specific heat as variable unless you are very sure you have no thermodynamic inconsistencies.

Ways to get around this include:
Use the Boussinesq approximation for buoyant flow,
Use the ideal gas model,
Use a real gas model
Use an RGP table.

You can get some weird stuff happening with the energy equations if you specify both density and Specific heat and functions of temperature.

November 28, 2017, 17:25
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VB
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 Originally Posted by evcelica Pay attention to this, you can't specify both density and specific heat as variable unless you are very sure you have no thermodynamic inconsistencies. Ways to get around this include: Use the Boussinesq approximation for buoyant flow, Use the ideal gas model, Use a real gas model Use an RGP table. You can get some weird stuff happening with the energy equations if you specify both density and Specific heat and functions of temperature.
Hi Evcelica, Thank you for the suggestion. I have defined density and specific heat in such a way it does'nt behave weird in the temp range. I believed so. now the simulation has been completed. How do you calculate Energy in each domains now? I need this Energy value to cross check and few other stuffs.

November 29, 2017, 04:29
VolumeInt , Energy
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VB
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Quote:
 Originally Posted by Vishnu_bharathi Hi. I need to calculate Energy contained in porous domain after my simulation or during the run, either one is fine. "you are better off using energy quantities directly than simplified formulas" - I would love to keep it simple . The formula that I mentioned is used to compute Energy in each node and in turn the whole domain. (eg., in Open foam or similar software where we have access to each node/cell in a domain, unlike Ansys). I believe Ansys is even simpler and accurate in Energy calculations over interested domain. What this returns? "volumeInt ( density * static enthalpy)@Volume of Interest"
Update: In order to verify this expression before using in Porous domain: I used volumeInt(density*static enthalpy)@my domain and compared with Energy calculated by m(mass)*C_p*dt. Both have difference. Why do I have difference here? Note: Here, the domain has constanct specific heat capacity and density, its a solid domain.

 November 29, 2017, 10:15 #9 Senior Member   Join Date: Jun 2009 Posts: 1,815 Rep Power: 32 What is the reference temperature of the solid material definition? Static Enthalpy = Integral from Tref to T of (Cp dT). It is only Static Enthalpy = Cp * (T - T_ref) uniform Cp Simplicity is plagued by the assumptions along the way. Assumptions are usually forgotten or implied and violated.

 November 29, 2017, 13:24 #10 Member   VB Join Date: Jul 2016 Posts: 35 Rep Power: 9 I formula volumeInt(density*Static Enthalpy)@domain worked. I am able to arrive the Energy in Joules. I have some other problem will post as new thread.