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Old   July 7, 2020, 08:07
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hi,
I am using ANSYS-CFX to model a propane reaction. Heat of combustion of propane(ΔcHo) is 50 MJ/kg but i want change manually ΔcHo of propane in CFX. Can you help me?
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Old   July 7, 2020, 08:32
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The heat of combustion is not a material property, but the result of a reaction between materials.

If you look at the material data for Propane in ANSYS CFX you can see enthalpy reference accounts for the heat of formation.

I wonder why you would like to change such a value, perhaps you may want to adjust the heat of formation of your fuel?

Hope the above helps,
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Old   July 7, 2020, 08:46
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Tanks for your help.Yes i want change heat of formation of fuel. I want set for example 20 MJ/kg for propane. Heat of combustion for propane by default is 50 MJ/kg but i want change manually. In CFX in select propane w2D for reaction but i want change heat of fuel for propane. I hope explain good.
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Old   July 7, 2020, 15:18
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Go to the Propane material definition, and check the Reference State of the material

If the heat of the fuel changes from 50MJ to 20MJ, I doubt it is the same material.

Also, keep in mind ANSYS CFX does not use the heat of combustion, or heat of fuel explicitly anywhere in the combustion model. It uses thermodynamic properties of the material. The ANSYS CFX implementation of the energy equation does not use any split of the enthalpy material definition.
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Last edited by Opaque; July 7, 2020 at 15:22. Reason: Complete message
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Old   July 8, 2020, 00:23
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Hi,
I checked Material Properties of Propane and found Reference Specific Enthalpy .Is this same ΔcH0 or heat of fuel?
In fact i want the propane to burn at a lower latent heat.
Tanks a lot for your Answer
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Old   July 8, 2020, 08:32
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My advice at this point is to read the documentation, and complement it with a thermodynamics book and look the heat of formation for propane.

Heat of formation is NOT heat of combustion.
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Old   July 8, 2020, 08:38
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yes Heat of formation is NOT heat of combustion.I can not find Standard Enthalpy Materials in CFX?can you help me?
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Old   July 8, 2020, 11:02
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Sorry.. Do not understand your last question?

"Standard Enthalpy Material". What do you mean by that?
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Old   July 11, 2020, 01:04
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Hi,
i explain otherwise.I have e reaction for Propane and heat of combustion is 24.2 MJ/kg.
I don't know how to set 24.2 MJ/k in CFX?
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Old   July 12, 2020, 18:59
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My advice is to go back to the fundamentals and you will see how it comes out immediately

Write down the formula for the heat of combustion of the specific reaction you have. You must make clear assumptions, for example, complete or incomplete reaction. Which other materials are present, and what is the total composition:

Assumptions for the equation below:
1 - Complete combustion
2 - Constant specific heat

Say

A * Propane + B * O2 + C * N2 --> D * CO2 + E * H2O + F * N2

Write down the heat of combustion formula as a function of the enthalpies defined as:

h_material = h_material_ref + Cp_material (T - T_ref)

Once you work out the heat of combustion, it should be a function of the reference enthalpy of your type Propane and the adiabatic flame temperature for the value you have.

Hope the above helps,
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Old   July 13, 2020, 04:19
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Hi dear Opaque,
tanks a lot.
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