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Porous domain energy calculation and specific heat capacity temperature dependent 

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November 28, 2017, 13:27 
Porous domain energy calculation and specific heat capacity temperature dependent

#1 
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VB
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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, 17:29 

#2 
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Glenn Horrocks
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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, 17:45 

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VB
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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, 17:54 

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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, 18:06 

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VB
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November 28, 2017, 18:12 

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Erik
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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, 18:25 

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November 29, 2017, 05:29 
VolumeInt , Energy

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November 29, 2017, 11:15 

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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, 14:24 

#10 
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VB
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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.


Tags 
energy function, integral computing, porous domain, specific heats, temperature dependent 
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