Menu

Discussion Forum

New Discussion
Filter

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Time to reach steady state

Posted Jul 27, 2016, 11:22 a.m. EDT Heat Transfer & Phase Change 4 Replies

Krish
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Hi,

I'm trying to calculate the time to reach steady state in a heat transfer transient analysis. Please let me know if anybody has an idea?

Thanks,
Krish
4 Replies Last Post Aug 2, 2016, 7:36 p.m. EDT
Jeff Hiller COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 8 years ago Jul 28, 2016, 4:59 p.m. EDT
Hello Krish,
First, you can perform a stationary analysis to determine what the steady state is.
Second, you compare the time-dependent analysis results to the stationary analysis results.
How you define "reaching steady state" is up to you and will depend on what you're trying to get out of that information. It could be defined as the time when the temperature at a particular point has reached, say, 99% of its final value. It could be defined as the time when the temperature at a specific point does not increase by more than, say, 1 Kelvin per hour. It could be defined as the time when 99% of the energy at steady state has entered, or left, the system. It could be defined as the time when the average temperature has reached 99% of it's final value. etc. You get my point: there are various possible definitions.
Typically you'll just look at a graph and read off the curve when your criterion is reached, although you could automate this (but it may not be worth the effort, frankly).
Best,
Jeff
Hello Krish, First, you can perform a stationary analysis to determine what the steady state is. Second, you compare the time-dependent analysis results to the stationary analysis results. How you define "reaching steady state" is up to you and will depend on what you're trying to get out of that information. It could be defined as the time when the temperature at a particular point has reached, say, 99% of its final value. It could be defined as the time when the temperature at a specific point does not increase by more than, say, 1 Kelvin per hour. It could be defined as the time when 99% of the energy at steady state has entered, or left, the system. It could be defined as the time when the average temperature has reached 99% of it's final value. etc. You get my point: there are various possible definitions. Typically you'll just look at a graph and read off the curve when your criterion is reached, although you could automate this (but it may not be worth the effort, frankly). Best, Jeff
Krish
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 8 years ago Jul 28, 2016, 9:13 p.m. EDT
Hello Jeff,

Thank you for your response. Suppose, the steady state is defined as the time when the temperature at a specific point does not increase by more than, say, 1 Kelvin per hour.

Could you please tell me how to define it in COMSOL?

Thanks
Krish
Hello Jeff, Thank you for your response. Suppose, the steady state is defined as the time when the temperature at a specific point does not increase by more than, say, 1 Kelvin per hour. Could you please tell me how to define it in COMSOL? Thanks Krish
Jeff Hiller COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 8 years ago Jul 29, 2016, 11:31 a.m. EDT
You will not need to define anything in COMSOL to achieve this. Rather, I am suggesting you look at the time-plot of the temperature at the point in question and by inspection determine when its slope is less than 1K/hour. You could also plot its time derivative and by inspection determine when that graph drops below 1K/hour. Nothing particularly fancy here.

If you really insist on automating this (and, as mentioned above this is not something I'd encourage you to do), you could add your own equation (using the Coefficient Form mathematics interface, for instance) to integrate in time a boolean that's equal to 1 when the time derivative of the temperature at the point in question is higher than 1K/hour. When that boolean switches to zero, the variable for your equation will settle at the time you're looking for. As the French would say, this approach is like "using a jackhammer to crush strawberries".
Best,
Jeff
You will not need to define anything in COMSOL to achieve this. Rather, I am suggesting you look at the time-plot of the temperature at the point in question and by inspection determine when its slope is less than 1K/hour. You could also plot its time derivative and by inspection determine when that graph drops below 1K/hour. Nothing particularly fancy here. If you really insist on automating this (and, as mentioned above this is not something I'd encourage you to do), you could add your own equation (using the Coefficient Form mathematics interface, for instance) to integrate in time a boolean that's equal to 1 when the time derivative of the temperature at the point in question is higher than 1K/hour. When that boolean switches to zero, the variable for your equation will settle at the time you're looking for. As the French would say, this approach is like "using a jackhammer to crush strawberries". Best, Jeff
Krish
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 8 years ago Aug 2, 2016, 7:36 p.m. EDT
Thank you, Jeff.
Thank you, Jeff.

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content