Undergrad Regarding when to consider both laminar and turbulent flow

[FebinThomas]

TL;DR

For flow over a flat plate when should i assume turbulent flow all over the plate and use the related equations(nusselt number) and when to use the laminar and turbulent equation(Nusslet number)

Hello,i am just a college student who has a little confusion on when to consider both laminar and turbulent flow -say over a flat plate,should i assume turbulent flow all over the plate and use the related equations(nusselt number) or use the laminar and turbulent equation(Nusslet number) but if so there i couldn't find an equation for finding local heat transfer coefficient(hx) for laminar and turbulent condition.

QUESTION

Here should i consider laminar&turbelnt conditio or just assume turbulent all over.

I hope i have included the relevant information for understanding my question.

 

[erobz]

I believe whether or not its turbulent or laminar, or mixed is based on the Reynolds number?

 

[Chestermiller]

What is the value of the Re at L=2m in your example? Compare this with the transition from laminar to turbulent flow which occurs in the range Re=3x10^5 to Re=3x10^6.

 

[FebinThomas]

I believe whether or not its turbulent or laminar, or mixed is based on the Reynolds number?

yes,but Re greater than (say for flat plate)5*10^5 is going to be turbulent right if it is mixed when its going to be turbulent.

 

[FebinThomas]

What is the value of the Re at L=2m in your example? Compare this with the transition from laminar to turbulent flow which occurs in the range Re=3x10^5 to Re=3x10^6.

its about 27 *10^5 sir.
at x=2

 

[Chestermiller]

its about 27 *10^5 sir.
at x=2

If th is calculated value is correct, then it sounds like most of the heat transfer is in the transition region. So there is going to be significant uncertainty in the values of the heat transfer coefficients.

 

[BWV]

Wouldn’t 0.25 m/s be too slow for turbulence?

 

[Chestermiller]

At 300 K, the density of liquid propane is 0.493 gm/cc and the viscosity is 0.001 gm/cm-sec. So, under these conditions, I calculate a Re of about 2.5x10^6 at L = 200 cm, in agreement with your calculation. Most of the plate would be judged to be in the transition zone.