Thread: Heat transfer for air

Hi guys

I need to know what value to use for a Free convective heat transfer coefficient for air in a cylinder of diameter 0.0127 metres and length of 0.1778 metres.

I know the value is generally beetween 5 and 25 W/(m²K) but I jus want it to be as accurate as possible for my scenario.

Also what does it mean to have a high Free convective heat transfer coefficient for air rather than a low one?

Thanks.

Originally Posted by ChaosD.Ace

Hi guys

I need to know what value to use for a Free convective heat transfer coefficient for air in a cylinder of diameter 0.0127 metres and length of 0.1778 metres.

I know the value is generally beetween 5 and 25 W/(m²K) but I jus want it to be as accurate as possible for my scenario.

Also what does it mean to have a high Free convective heat transfer coefficient for air rather than a low one?

Thanks.

I find the Engineering Toolbox website quite good for this kind of query: Convective Heat Transfer

A high coefficient means faster heat transfer, so better cooling or heating. I'm not a heating and ventilating engineer, but it looks to me as if the reason for the variation of values is that the equation does not build in the velocity of the convecting air. I can imagine the ability of the buoyancy effect to accelerate the air will depend on a number of design factors.

I think this is really an engineering problem rather than a physics one - you may get more help if you post it under Engineering.

The length is more than 10 time the diameter, so you could probably consider it a long horizontal cylinder such as discussed in Section 17, Natural Convection from a Long Horizontal Cylinder, (assuming it is a horizontal cylinder) here:
http://s3.amazonaws.com/suncam/npdocs/119.pdf

I am no expert on this matter but I would have thought that the the heat flow depends upon whether the convection current is laminar or turbulent, and this would depend on the temperature difference between the solid and fluid involved. The value of "heat transfer coefficient" would be expected to depend on which of these two situations applies.

Assuming that the "heat transfer coefficient" is the constant which appears in Newton's law of cooling, a high value would mean a greater rate of heat transfer.

Thanks a lot guys for all the quick replies they were really helpfull and I know what I am doing now.