Flow Simulation

prince_mech · March 13, 2017, 23:57

Hi guys!!
I want to run a simulation on a duct of non frost fridge. Would someone please suggest me how many iteration i will do for a static setup. i want to calculate the average flow and temperature distribution in the duct.
Thanks

piu58 · March 14, 2017, 00:46

I see two ways to come to an answer:
1) Let a simulation run and look after some simulated time whether the total thermical engergy still changes .
2) Calculate the Prandl number for your case. This gives you an idea how fast things change.

prince_mech · March 14, 2017, 05:06

i didn't catch it properly. will you please elaborate

piu58 · March 14, 2017, 11:42

I recommend:

- calculate the Reynold number for the outer flow. For that you need some idea of the velocity $U_{\infty}$ .
- calculate the Prandt number for the outer flow. You need the kinematic viscosity and the thermal diffusivity for that, which you may get from tables. For air is Pr~0.7 and for water at 20 deg Pr~7.
- calculate the Nußelt number

$Nu \sim 0.037 Re ^{0.8} Pr ^{0.43}$

With the help of the Nußelt number you may calculate the maximal thermal flow (at starting temperature difference). Using the the heat capacity you may calculate how long it lasts to exchange all the energy. The real time will be longer, depending on what remaining difference you accept. But you get at least an idea in which range this time will be.

prince_mech · March 15, 2017, 04:32

thank you so much for your answer. i am on it

March 13, 2017, 23:57	Flow Simulation	#1
prince_mech Member Prince Hossain Join Date: Feb 2017 Posts: 32 Rep Power: 9	Hi guys!! I want to run a simulation on a duct of non frost fridge. Would someone please suggest me how many iteration i will do for a static setup. i want to calculate the average flow and temperature distribution in the duct. Thanks

March 14, 2017, 00:46		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	I see two ways to come to an answer: 1) Let a simulation run and look after some simulated time whether the total thermical engergy still changes . 2) Calculate the Prandl number for your case. This gives you an idea how fast things change. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

March 14, 2017, 11:42		#4
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	I recommend: - calculate the Reynold number for the outer flow. For that you need some idea of the velocity $U_{\infty}$ . - calculate the Prandt number for the outer flow. You need the kinematic viscosity and the thermal diffusivity for that, which you may get from tables. For air is Pr~0.7 and for water at 20 deg Pr~7. - calculate the Nußelt number $Nu \sim 0.037 Re ^{0.8} Pr ^{0.43}$ With the help of the Nußelt number you may calculate the maximal thermal flow (at starting temperature difference). Using the the heat capacity you may calculate how long it lasts to exchange all the energy. The real time will be longer, depending on what remaining difference you accept. But you get at least an idea in which range this time will be. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

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March 14, 2017, 05:06		#3
prince_mech Member Prince Hossain Join Date: Feb 2017 Posts: 32 Rep Power: 9	i didn't catch it properly. will you please elaborate

March 15, 2017, 04:32		#5
prince_mech Member Prince Hossain Join Date: Feb 2017 Posts: 32 Rep Power: 9	thank you so much for your answer. i am on it