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June 6, 2017, 14:14 |
illogical result !!
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#1 |
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dilaw meda
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hello every one, this is a nozzle called ''lobed nozzle'', it has a circular shape at the inlet and a corrugated shape at the exit, in my case, the area of the inlet is 314 mm² . the area of the exit is 213 mm² .
experimentally, the velocity at the center of the inlet area was 7.5 m/s and when i mesured the velocity at the center of the exit it was 4.5 !!! numérically it was an other thing , when i set inlet velocity at 7.5 it gave me 10 m/s at the exit which i think is a logical result if we consider the continuity equation. now i don't know what to believe because i repeated the experimental many times and i get the same result each time. There is something wrong here but i didnt find it noting that i used the ''SST K-omega'' model and i set the the walls of the nozzle as '' no slip WALL'' in BC Last edited by medaouarwalid; June 7, 2017 at 09:20. |
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June 7, 2017, 09:13 |
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#2 |
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Blanco
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What about the density? Are you injecting a gas?
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June 7, 2017, 09:19 |
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#3 |
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dilaw meda
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June 7, 2017, 09:26 |
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#4 |
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Blanco
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Ok, so density can change between inlet section and outlet section, and all we know is that mflux=dens*A*vel=const. Therefore it is possible that experimental velocity is correct (!). Talking about your simulation: have you double checked the gas properties you're using in the model? Are you using ideal gas or what? Are the wall boundaries adiabatic, fixed T, etc.? What is the distance between inlet and outlet of the nozzle?
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June 7, 2017, 09:42 |
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#5 | |
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dilaw meda
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Quote:
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June 7, 2017, 10:13 |
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#6 | |
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Blanco
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Quote:
I don't know which program you're using, so I suppose that your "default" settings for wall are consistent with adiabatic walls. Another question: is the nozzle "representative" of a small hair dryer in your intentions? If yes, then the "inlet" section is filled with cold air while hot air flows through the outlet section, or I am missing something? |
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June 7, 2017, 10:42 |
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#7 | |
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dilaw meda
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Quote:
second i am studying the effect of the geometrie of that nozzle on the augmentation of mixing the blowing air with the ambiant air, so the hair dryer is tool to give me hot hair , the outlet of the blow dryer is equal to the inlet of the nozzle. Also, i want to say that i mesure the inlet velocity afther that i joined the nozzle inlet to the blow dryer exit . |
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June 7, 2017, 10:59 |
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#8 |
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dilaw meda
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I went back to the lab and i did other mesurements. This time i mesured the central velocity not only in the inlet and outlet of the nozzle but also in different points of the centerline of the nozzle , here the new results:
- the velocity was 7.5 m/s only in the point of the grid of the exit of the blower ( see image 3 blower +nozzle ) , - when i go down a little bit from there , the velocity decreases until 3.15 and stay like this in the cylindre shape of the nozzle until it reaches the corrugated shape of the nozzle - then it begins to increase until it reaches 4.6 m/s at the exit of the nozzle !!! Any explications please Last edited by medaouarwalid; June 7, 2017 at 13:42. |
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June 8, 2017, 05:22 |
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#9 |
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Blanco
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Now it makes sense to me:
1- the velocity you measured at the center of the nozzle inlet section is not representative of average inlet flow, because it is influenced by the grid and other stuff you have in the blower near its outlet section. I really think the velocity distribution is all but uniform in the nozzle inlet section. 2- going down along the centerline of the nozzle, the disuniformities you have tends to fade out and a more uniform flow is achieved, therefore the 3.15 m/s you mesured on the centerline "could" be considered representative of the mean flow though a nozzle section 3- the velocity you measured on the outlet section of the nozzle is 4.6 m/s in the centerline, I suppose. Here again I don't expect a very uniform velocity distribution on the outlet section, because of the particular geometry, but the continuity equation tells us that we're very close to theory: 3.15 * 314 / 213 = 4.64 m/s and you have measured 4.6 m/s... It makes sense. Btw: how are you measuring the local velocity? You have also to consider the influence you have on the flowfield. |
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June 8, 2017, 08:11 |
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#10 |
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Filippo Maria Denaro
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The setting of the numerical parameters at the inlet is very important... doing RANS you should prescribe a statistically averaged velocity profile. Consider also the requirement of the turbulence model varaibles at the inlet.
What is the geometry before of the inlet of the nozzle? |
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June 8, 2017, 12:56 |
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#11 | |
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dilaw meda
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June 8, 2017, 12:59 |
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#12 | |
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dilaw meda
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Thank you for helping me ,m. Cordially |
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June 8, 2017, 13:03 |
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#13 | |
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Filippo Maria Denaro
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If you use the incompressible formulation you can prescribe either the velocity or the pressure, what measurement about the pressure do you have? |
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June 8, 2017, 13:07 |
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#14 | |
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dilaw meda
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June 8, 2017, 13:14 |
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#15 | |
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Filippo Maria Denaro
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The key is that if you use RANS, you should measure a statistically averaged profile at the inlet (at least a sufficient number of points)... Are you able to do such a measure on the plane at the inlet? |
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June 8, 2017, 13:21 |
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#16 | |
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dilaw meda
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The TSI VELOCICALC® Multi-Function Ventilation Meter 9565-) has a probe wich dont allow me to do much mesurements at that plane ( i think you mean the grid) , maybe at the center and near to the nozzle walls thats all |
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June 8, 2017, 13:23 |
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#17 | |
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Filippo Maria Denaro
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Quote:
would be more simple to do the measure in the cone plane before the nozzle?? |
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June 8, 2017, 13:25 |
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#18 |
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dilaw meda
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June 8, 2017, 13:46 |
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#19 |
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Filippo Maria Denaro
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June 8, 2017, 13:48 |
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#20 |
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dilaw meda
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