Two stroke exhaust simulation
 

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.

Attachment 66756

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

Thank you

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.

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?

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.