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about the steady and unsteady simulations

Dear Sir or Madam,

I meet some problems about the simulations of fluid flow in channels. When you do some simulations related to fluid flow, how do you judge whether it is a steady issue or not? For example, some references reported that the single-phase unsteady flow in T-shaped channels (Two miscible liquids contact in T-junction inlet structure, the main channel, depth: 0.3 mm, width: 0.6 mm and length: 4 mm) will begin to occur even when Re number reach 240 (they call this flow as laminar transient flow; if Re > 500, the flow will be in the transition to turbulence). For me, the solution of steady laminar flow is still available even when Re number reaches 600 in straight channels (the dimensions of these channels are larger and longer than those in reference; depth: 0.5 mm; width: 0.5 mm and length: 10 mm). And the simulation results show that if the entrance effect of the inlet zone is eliminated, the pressure drop in these straight microchannels accord well with that predicted by the conventional laminar flow theory when Re number is larger than 240 (for example, Re =300). Why can I get the solution of steady flow even under such relatively high Re number? During these several days I think about this issue and have doubts on it. I like to listen to your suggestions. Who could give me some hints? Thank you!



2 Replies Last Post Mar 12, 2015, 3:41 AM EDT
COMSOL Moderator

Hello Yuanhai Su

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Posted: 3 years ago Mar 11, 2015, 11:48 AM EDT

Dear Sir or Madam,

I meet some problems about the simulations of fluid flow in channels. When you do some simulations related to fluid flow, how do you judge whether it is a steady issue or not? For example, some references reported that the single-phase unsteady flow in T-shaped channels (Two miscible liquids contact in T-junction inlet structure, the main channel, depth: 0.3 mm, width: 0.6 mm and length: 4 mm) will begin to occur even when Re number reach 240 (they call this flow as laminar transient flow; if Re > 500, the flow will be in the transition to turbulence). For me, the solution of steady laminar flow is still available even when Re number reaches 600 in straight channels (the dimensions of these channels are larger and longer than those in reference; depth: 0.5 mm; width: 0.5 mm and length: 10 mm). And the simulation results show that if the entrance effect of the inlet zone is eliminated, the pressure drop in these straight microchannels accord well with that predicted by the conventional laminar flow theory when Re number is larger than 240 (for example, Re =300). Why can I get the solution of steady flow even under such relatively high Re number? During these several days I think about this issue and have doubts on it. I like to listen to your suggestions. Who could give me some hints? Thank you!


Hi,
I have the same problem. Did you find a solution ?
Thanks in advance.
Mehrez
[QUOTE] Dear Sir or Madam, I meet some problems about the simulations of fluid flow in channels. When you do some simulations related to fluid flow, how do you judge whether it is a steady issue or not? For example, some references reported that the single-phase unsteady flow in T-shaped channels (Two miscible liquids contact in T-junction inlet structure, the main channel, depth: 0.3 mm, width: 0.6 mm and length: 4 mm) will begin to occur even when Re number reach 240 (they call this flow as laminar transient flow; if Re > 500, the flow will be in the transition to turbulence). For me, the solution of steady laminar flow is still available even when Re number reaches 600 in straight channels (the dimensions of these channels are larger and longer than those in reference; depth: 0.5 mm; width: 0.5 mm and length: 10 mm). And the simulation results show that if the entrance effect of the inlet zone is eliminated, the pressure drop in these straight microchannels accord well with that predicted by the conventional laminar flow theory when Re number is larger than 240 (for example, Re =300). Why can I get the solution of steady flow even under such relatively high Re number? During these several days I think about this issue and have doubts on it. I like to listen to your suggestions. Who could give me some hints? Thank you! [/QUOTE] Hi, I have the same problem. Did you find a solution ? Thanks in advance. Mehrez

Lasse Murtomäki
Posted: 3 years ago Mar 12, 2015, 3:41 AM EDT
In the chemical engineering class I was taught that flow is laminar when Re < 2100 (McCabe & Smith, Unit Operations of Chemical Engineering, McGraw-Hill, 1976, p. 52).

br
Lasse
In the chemical engineering class I was taught that flow is laminar when Re < 2100 (McCabe & Smith, Unit Operations of Chemical Engineering, McGraw-Hill, 1976, p. 52). br Lasse

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