Small Supersonic Nozzle
 

Hi All,

I am working on a physical test set-up and CFD analysis, which is featuring a supersonic nozzle of small dimension (Øthroat 0.5 mm, Øfinal 0.53 mm). And all of a sudden I realised that the dimensions of the nozzle is approaching the actual thickness of a shock wave (Approx 10^-2 mm according to my old lecture notes). Does anyone know of any references for this type of problem, or experience of small supersonic nozzles? I am at present not able to work out what will happen, having said that the analysis seems to indicate that it will work OK. I guess I will find out also when conducting the experiments.

Regards, Paal

Re: Small Supersonic Nozzle
 

Re: Small Supersonic Nozzle
 

Sorry about the blank message!

Hi Paul,

I'm pretty sure that a shock wave is typcially only a few mean free paths in thickness (usually > 1E-4m although dependent on density of course). I think the main issue is if your flow is rarefied. What is your approximate knudsen number for this flow? Are you using a commerical or academic CFD code?

Re: Small Supersonic Nozzle
 

Hi Michael,

Thanks for the response;

I have to admit that I had not calculated the knudsen number in advance. The Knudsen number is estimated to 2.8*10^-6 so I guess I should be OK. I have also found another criterion for continuum saying: M < 0.01sqrt(Re), which my case also satisfies by some margin. Next challange I guess is the production of the Nozzle for the experimental part...

I am using a commercial code for this case(Star-CD), it has been behaving quite well, I will now go on to verify my CFD results with the experimental set-up. It is all done in an industrial setting.

Once again thanks for very useful suggestions.

Regards, Paal