I am actually working on engines exhaust gases and for the purposes of the experiment, the exhaust gas flow (initially turbulent) should be laminar. So i was wondering how to kill "the turbulence". Unfortunately, the lab room is not big enough to consider pipes longer than 2-2,5 meters.
Dear Omar Moussa ,
You can introduce flow straighteners to kill the local turbulence.
Check the experimental methodology of these two works. You will get the idea.
Article Effect of burner diameter and diluents on the extinction str...
Conference Paper On extinction Strain rate of Counterflow diffusion flames.
Hi Omar,
I suppose a priori you have some design limitations, such as proportionality of dimensions, type of material, which would be pertinent to know before suggesting something more specific. What are you modeling? What kind of conditions, dimensions? What speed range or pressure drop are you considering? If you are modeling an engine exhaust, I would suggest you balancing back pressure and high speed to avoid turbulence (as this is a consequence of an inefficient drop in exhaust flow caused by lack of speed).
may be the flow is not discharged into the free atmosphere, but into the chamber where conditions can be changed (pressure, temperature ...)
Add some resistance to the loop like an orifice or restrictions so that the overall velocity reduces
Some suggestions for hot turbulent exhausts - Increase the duct diameter to reduce velocity until Reynolds number is below laminar threshold, alternately provide cooling in the upstream duct (fins, venturi air induction) to cool the mixture to increase density, reduce volume and hence reduce velocity. For a longer pipe consider a loop of 5.0m or even 7.5m in your 2.5m available space as done with the early VW 5 cylinder Kombi (rear engine led exhaust to vehicle front and then back to the rear to achieve muffling and exhaust cooling).
Adding a sketch with dimensions and all possible conditions for the pipe you have given in your question; could help the answering person for a better and accurate response.
I wonder if the geometric requirements to achieve that Reynolds value are compatible with the nature of the gas flow emission you are analyzing (mass flow, temperature, viscosity, thermal gradient along the pipe, etc.).
It could be that a "theoretical geometry" was defined without real technical sense.
Why do you need the regime to be laminar?
I agree with Syed. If you put some flow straightners such as honeycomb or wire mesh in the flow passage, there is a high of turbulent flow will transform into laminar flow. In case of wire mesh, denser the wire mesh, smooth is the flow.
It might be so weird, but, in my view increase flow velocity to stream up the regime
Dear all, there are some additives such as polymers and surfactants which are able to reduce or even eliminate turbulence, so that smooth flow is achieved. Drag reduction is one aspect to play on. My Regards
Hello
By adding a perforated plate somewhere after inlet you can have a uniform velocity profile. Perhaps it helps you also for killing turbulence. We did it one time for an experiment and it works well in the uniform velocity profile as well as reducing inlet velocity.
I would go with variable area. Create a diffuser section at outlet, which decreases velocity, increases the pressure and as other researchers suggested go with honey comb. We should play with drag. Increase it and decrease velocity. I think diffuser with shock at inlet section may do it.
Hi,
I see what you are talking about is entrance length. In laminar flow, the entrance region. The flow is not fully developed either hydrodynamics boundary layer or thermal boundary layer until some length depends on the Reynolds number and Prandtl number. As far as I know, It's easier to make flow in turbulent than laminar if dealing with entrance length. The pressure gradient and heat transfer rate are constant during the turbulent flow. It's interesting. For CFD, we just extend the pipe easily.
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