Nusselt number 2

[Amine A]

Hello ,
For a turbulent flow ,to determine the nusselt number.. anyone knows how to calculate the fluid viscosity at the bulk fluid temperature and the fluid viscosity at the heat-transfer boundary surface temperature , please ?

 

[1503-44]

See ASTM D341 calculate viscosity at an intermediate temperature given viscosities at two other temperatures. This might work. I have not checked it.

https://www.paragon-sci.com/d341

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[mskds545]

Viscosity is a physical property. You don't calculate it, you measure it. For most fluids you will be able to find published data, and viscosity is included in the tables in the back of most introductory heat transfer textbooks. They often list the kinematic viscosity. You want the dynamic viscosity, which is (kinematic viscosity)*(density). Viscosity is a function of temperature, so it will be different in the bulk and at the wall.

-mskds545

 

[hacksaw]

Use the upstrean viscosity.

 

[georgeverghese]

Nnu = 0.023 (Nre^0.8). (NPr^0.3).(μb/µw)^0.14

Ts = (Tw + Tb)/2 ; Ts is used for the calculation of Nre and NPr. To get Ts, you have to run a few trials: Assume Tw, then calculate Ts and then calculate Nnu, which then gives you hi. To verify the trial value of Tw, you have to calculate ho and then U at that location and then calculate what the corresponding value of Tw is. Compare this with trial value of Tw. For details, pls refer to DQ Kern - Process Heat Transfer.

 

[Amine A]

Thank you all !

 

[davefitz]

The standard correlation provided by georgeverhese works in nearly all heating applications, but fails in some unique applications. For example, in the case of supercritical fluids where the bulk fluid is slightly below the critical temperature while the wall temperature is above the critical temperature, radical differences in fluid properties between the 2 temperatures would require a different treatment. I believe the Nusselt method implies the assumption that fluid properties are linearly varying vs temperature in the film layer.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[1503-44]

mskds Get serious.

Viscosity is a physical property. You don't calculate it, you measure it.

I'm an engineer, not a physical chemist. If you model viscosity vs temp, well yes you can calculate the viscosity given a temperature and in the process save yourself quite a lot of time by not being forced to measure it every time you need a quick estimate of a pressure drop in a pipe or something. Many fluids have already known viscosity to temperature relationships. You might want to try it smoetimes. I guarantee you'll save a lot of time.

 

[mskds545]

I'm not sure why you found it necessary to take a rude tone with me, but perhaps you missed where I said that there is published data available for most fluids and recommended the OP check the tables in the back of a heat transfer text. I am also an engineer, by the way, and I am quite adept at making quick estimates. But thanks for that advice.

 

[1503-44]

Sorry about the tone. It was condescending, but IMO not rude. I just thought your comment made no sense. Engineers don't often measure anything, if some kind of adept model already exists. I measured viscosity in uni fluids lab exactly one time in my entire life and that was only to verify our prexisting model. Example: Structural engineers don't usually build something and then measure beam deflections. So in fact the exact opposite of your comment is true. Right? So where did your comment come from.

 

[mskds545]

So, was it that I said "you measure it" rather than "someone measures it" that bothered you? The next sentence stated that data is already available for most fluids and you can look it up. The poster asked a question and I tried to help. Other people offered alternate answers. I hope one of those answers was useful to the person that asked, even if it wasn't mine. That's what the forum is for. And sometimes when I'm designing something with an unusual feature, I do need to measure things. Generally I design things with simple hand calculations and sketches or engineering software, but sometime I need data for the model. Depending what I need, I will either do this myself or find other people to do it for me. I have even occasionally had reason to measure beam deflections. Not all engineers do the same thing or have the same sorts of problems to solve. Some engineers that I know measure things daily, but that isn't my cup of tea.