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Two stroke exhaust simulation

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Old   November 14, 2018, 05:24
Default Two stroke exhaust simulation
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Juanjo Garcia
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Hello everyone,

I am trying to set-up a transient simulation of a two stroke engine in Star CCM+ to see the pressure wave created. The geometry of the exhaust is similar to the one shown in the following link.

https://www.diymotofix.com/blog/how-...t-system-works

My models are:
- Implicit Unsteady
- Segregated Fluid Entalphy
- Turbulent (k-epsilon)
- Ideal gas
- Three dimensional

My boundary conditions are:
- Inlet: mass flow inlet (0.01 kg/s at 700K)
- Outlet: pressure outlet (0.0 Pa)

Moreover, the inlet boundary conditions are pulse inputs, simulating the opening of the exhaust valve during 0.001s. To do so, I have created field functions with the following sintaxis:
($Time > 0.001s) ? 0 : value (where value depends on the field, i.e. temperature or mass flow).

The simulation runs without problem, but when looking at results, the temperature has a strange behaviour. The temperature is at 700K in the inlet, but it quickly decreases to ambient temperature at a short distance of the input, as shown in the figure below.

Captura de pantalla (244).jpg

So, my question is, do I need to change any other parameters?

Thank you
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Old   November 14, 2018, 16:36
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André
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What are the boundary conditions of the walls? Also if this is a transient simulation, what are your initial conditions? Probably you are starting with the entire domain at 300K which may or may not be what you are looking for.
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Old   November 14, 2018, 17:33
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Thank you for your reply.

For the wall, I tried adiabatic, which is the default option. Aftee that, I also tried Heat Flux and Temperature, with similar results.

Initially, the domain is at 300K, ambient temperature, but this temperature should not disipate the flow temperature so quick, right?

I was wondering if the problem cames because the domain is full of the same air at lower temperature, which rapidily disipates the exhaust gas. Maybe I can set-up a simulation with different gases or start the simulation with the domain wmpty of air, but I don't know if it is possible.

Any ideas?
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Old   November 14, 2018, 17:58
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Well depends on how long you ran the simulation. I think it would be OK to either start at 300K and let it run for a while, or perhaps start at a higher gas temperature which would mean the exhaust is already heated up and would take less time to reach a "steady-state" condition.
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