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Absolute pressure not increasing in closed volume with a constant heat source

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Old   July 9, 2021, 07:37
Post Absolute pressure not increasing in closed volume with a constant heat source
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trashcanman
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Hello everyone,

I'll firstly describe the setup:

I'm modelling a Big box (closed volume), with a wall in the middle of it with divides it into 2 different compartments.

This makes it a box with two fluid volumes, both modeled as air (default settings and density governed by ideal gas law).

In this box, in one compartment, there is a cylinder with a very very high constant heat source (W/m3).

The point is to evaluate the pressure build up inside this compartment, and then open a section of the middle wall when a certain pressure condition is met (I've already done this part using UDFs+Scheme in another model).

All the walls in this model are adiabatic.

The problem however, is that even though I have a huge heat source (1E11 W/m3), the absolute pressure of the air does not hardly increase (not even 1kPa) even though the air around the cylinder reaches 9000K-10000K temperatures.
This makes no sense, since with the ideal gas law in a closed volume, for an increase temperature i should see a pressure increase.

Naturally, all my solid-fluid interfaces (cylinder-air) are coupled (conformal mesh) and the model and solver settings are shown bellow.

Models
------

Model Settings
------------------------------------------------------------------
Space 3D
Time Unsteady, 1st-Order Implicit
Viscous Realizable k-epsilon turbulence model
Wall Treatment Standard Wall Functions
Heat Transfer Enabled
Solidification and Melting Disabled
Radiation P1 Model
Species Disabled
Coupled Dispersed Phase Disabled
NOx Pollutants Disabled
SOx Pollutants Disabled
Soot Disabled
Mercury Pollutants Disabled
Structure Disabled
Acoustics Disabled
Eulerian Wall Film Disabled
Potential/Li-ion Battery Disabled
Multiphase Disabled

olver Settings
---------------

Equations

Equation Solved
-------------------
Flow yes
Turbulence yes
Energy yes
P1 yes

Numerics

Numeric Enabled
---------------------------------------
Absolute Velocity Formulation yes

Unsteady Calculation Parameters


-------------------------------------
Time Step (s) 0.001
Max. Iterations Per Time Step 30

Relaxation

Variable Relaxation Factor
----------------------------------------------
Pressure 0.3
Density 0.69999999
Body Forces 0.69999999
Momentum 0.7
Turbulent Kinetic Energy 0.8
Turbulent Dissipation Rate 0.8
Turbulent Viscosity 1
Energy 0.69999999
P1 0.69999999

Linear Solver

Variable Solver Type Termination Criterion Residual Reduction Tolerance
-----------------------------------------------------------------------------------------------
Pressure V-Cycle 0.1
X-Momentum Flexible 0.1 0.7
Y-Momentum Flexible 0.1 0.7
Z-Momentum Flexible 0.1 0.7
Turbulent Kinetic Energy Flexible 0.1 0.7
Turbulent Dissipation Rate Flexible 0.1 0.7
Energy F-Cycle 0.1
P1 Flexible 0.1 0.7

Pressure-Velocity Coupling

Parameter Value
------------------
Type SIMPLE

Discretization Scheme

Variable Scheme
------------------------------------------------
Pressure Second Order
Density Second Order Upwind
Momentum Second Order Upwind
Turbulent Kinetic Energy First Order Upwind
Turbulent Dissipation Rate First Order Upwind
Energy Second Order Upwind

Solution Limits

Quantity Limit
---------------------------------------
Minimum Absolute Pressure 1
Maximum Absolute Pressure 5e+10
Minimum Temperature 1
Maximum Temperature 200000
Minimum Turb. Kinetic Energy 1e-14
Minimum Turb. Dissipation Rate 1e-20
Maximum Turb. Viscosity Ratio 100000



Any clues to why the pressure isn't increasing?

Thank you for your time
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Old   July 14, 2021, 17:36
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Erik
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That is strange.
Are you sure the ideal gas model is being used for density?
Are you sure The interfaces have Heat Transfer activated?
What is your reference pressure? Is it constant? Pressure is calculated based off this, so if it is variable, check your absolute pressure instead.
Are you using thermal energy, or the correct total energy model?
Have you simulated enough time to see a pressure change?
Have you checked if mass is being conserved, and not leaving? Try checking volumeInt(Density)@yourDomain to see the total mass.
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Old   July 15, 2021, 04:08
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Hi evcelica, first of all thank you for taking the time.

I'm going to try and answer your questions in order:

1) Yes the ideal gas model is being used.

2) The solid-fluid interfaces have heat transfer activated, I have flux reports on the heat transfer rate at those boundaries and I can confirm that. Plus the fluid in the compartment which has the heat source increases it's temperature.

3) That is a good question. I set my operating pressure to 0 and initialized my model at atmosferic pressure at both compartments (reminder that both compartments are not connected at the moment, only after a pressure condition is met the wall that connects them is switched to an interior bc).

That being said, should I change the reference pressure location to other coordinates where the pressure is known, for example the other compartment that is initiated at atmosferic pressure? I didn't realize that could be done in Fluent.

4) I don't think I understood this question. I'm activating the energy equation and applying a energy source in the solid cylinder through the Cell Zone Conditions tab (activated energy source in w/m3). Is that what you meant?

5) I think so, I simulated 0.5s which by what i've seen in other papers for similar cases should be more than enough to get a decent pressure increase.

6) Yes, mass is being conserved since I checked the Mass Flux report. The air in the heated compartment has no leaks, so it can only escape to the other compartment when the pressure condition is met (wall dividing the two compartments switches to interior).

Thank you for your insight
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Old   August 2, 2021, 05:19
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Replying to increase visibility
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closed domain, conjugate heat transfer, fluent, overpressure

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