Determining Film Coefficient 1

[LiamBlair]

Dear all.

Currently i am investigating cooldown times from a multilayered pipe. I need help in finding an experiment to determine the film coeffcient of polypropylene, is this worth doing or should i just take a conservative value found in the (large) range given online?

Thanking in advance.

 

[25362]

For the homopolymer Lange's Handbook of Chemistry gives a value of 0.12 W/(m.K) for thermal conductivity, and 0.44-0.46 cal/g for specific heat. I believe both are taken at ambient conditions. The only wide range-like thermal property given therein is the coefficient of linear thermal expansion: 81-100 x 10^-6/^oC.

For copolymers the values differ a bit.

 

[corus]

The film coefficient is a measure of heat loss from a surface due to forced convection. It is independent of the material.

corus

 

[Latexman]

I'm a little confused on what you are doing. Can you explain in more detail what you are doing and what information you are trying to get?

Good luck,
Latexman

 

[25362]

To corus, is it possible that LiamBlair meant the heat transferred through the polypropylene film, not the loss by convection by the air film surrounding the multilayered pipe ?

 

[LiamBlair]

Thanks for the attention/tips!

what i'd like to know is how much heat is tranferred away from the surface of the multilayered pipe. Since the top layer is injected moulded polypropylene, it is 'cooling rate' and 'strain rate' dependent.due to shrinkage with cooling, the level of strain depends on the rate of surface cooling. the problem in hand is the pipe is cooled with water flowing across it, and i'm trying to find a value of film coefficient for computational modelling.(see Polymer Engineering Forum/Field Joint Failure for more detailed info.) Previous values for forced convection with water were in the range of (if i remember right) 20-3000 and most likely incorporated turbulent and laminar flow. I was wondering if anyone knew of specfic codes/guidlines/experiments to determining this value or even better aimed towards flow across a pipe? Thanking in advance for any advice/direction pointing.

 

[magicme]

the *average* convection coefficient on the external surface of a cylinder (pipe) i cross flow is discussed in the book "fundamentals of heat and mass transfer" by incropera and dewitt, chapter 7.

there are several complex equations. the simple one is....

H = k * C * (Re^m) * (Pr^.33) / D

where k = external fluid conductivity
Re = reynolds number of external fluid
Pr = Prandtl number
D = external diameter
C and m vary with Re, but average values are....
C = m = .5 ( but they vary considerably)

there are other equations discussed that better fit various ranges of Pr and Re.

daveleo

 

[25362]

At the end it appears corus was right. LiamBlair is indeed after a coefficient of heat transfer for the external film of water flowing across the pipe probable in a trickle (cascade) cooler.