Heat transfer co-efficient of air 2

[chinmoy123]

All
I have been using a heat transfer co-efficient h = 5 W/m^2 K for free convections from a flat surface. Does anyone have a graph or literature which gives the h values of air relative to air velocity and temperature for a flat surface?
Thanks in advance
Chinmoy

 

[IRstuff]

This information should be available in your heat transfer texts.

TTFN

 

[MortenA]

The HYSYS manual gives the following formula for pipes:

h=k/d*0.25*Re^(0.6)*Pr^(0.38)

where k is air conductivity and d is pipediameter and the velocity is the velocity of the wind. he refer to an article by J.P. Holman (1989) - maybe that a lead?

Perry has something for a flat plate (sec 10 p 16 in 6ed)- but only valid for laminar flow, Re up to 1000-100000 and Pr>0.6.

Here Nu=^0.648*Re^0.5*Pr^0.333


Best regards

Morten

 

[25362]

For a horizontal plate facing upward, J.P. Holman gives
values for free convection from surfaces to air, W/(m[sup]2[/sup][×][sup]o[/sup]C), table 7-2:

h = 1.32([Δ]T/L)[sup]0.75[/sup] for laminar cases, and
h = 1.43([Δ]T)[sup]1/3[/sup] for turbulent cases

where

[Δ]T = T[sub]wall[/sub]-T[sub][∞][/sub] , [sup]o[/sup]C
L = horizontal dimension, m

Turbulence is for Gr.Pr > 10[sup]9[/sup]

For a horizontal heated plate facing downward or cooled plate facing upward:

h = 0.61([Δ]T/L[sup]2[/sup])[sup]0.2[/sup]

 

[gwolf]

Holman is a good source.

 

[cymro]

Lots of bona fide equipment manufacturers give practical figures for such heat losses, for example Fig. 2.9.1 in the Spirax Sarco Learning Centre (

http://www.spiraxsarco.com/learn)

gives a pretty good graph showing heat loss against temp. diff. for horizontal and vertical flat surfaces. See Module 2.9, "Energy consumption of tanks & vats"