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Heat transfer co-efficient of air
2

Heat transfer co-efficient of air

Heat transfer co-efficient of air

(OP)
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

RE: Heat transfer co-efficient of air

This information should be available in your heat transfer texts.

TTFN



RE: Heat transfer co-efficient of air

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

RE: Heat transfer co-efficient of air


For a horizontal plate facing upward, J.P. Holman gives
values for free convection from surfaces to air, W/(m2×oC), table 7-2:

h = 1.32(ΔT/L)0.75 for laminar cases, and
h = 1.43(ΔT)1/3 for turbulent cases

where

ΔT = Twall-T , oC
L = horizontal dimension, m

Turbulence is for Gr.Pr > 109

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

h = 0.61(ΔT/L2)0.2

RE: Heat transfer co-efficient of air

Holman is a good source.

RE: Heat transfer co-efficient of air

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 (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"

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