Question

Hi all Referring to the formula for heat energy loss through convection:

dQ/dt=h*A*(Tw-Tair)

q=hc*A*dT

q= heat transfer per unit time W (energy/time) (E=W*t)

h= convective heat transfer coefficient of the process W/m² °C

A= heat transfer area of the surface dT= temperature difference between surface Tw and the bulk fluid Tair , °C (bulk fluid is the surrounding medium eg. air)

What is the correct value of h, thermal constant for air to use? these figures vary widely.

Answer 1

I hate to break it to you but the convective coefficient of heat transfer is not a constant value for any given media.

For air the value varies generally from 10 - 100 in standard units. It is dependent on various factors like the temperature of surrounding air, the velocity of air surrounding the body, forced or free convection and amount of humidity in the air. It may be possible that the convective heat transfer coefficient may vary as we move away from the surface, the geometrical arrangement of the surface also plays a role.

So I would say that there no constant value of convective heat transfer coefficient that can considered for air and in most cases the value is provided with. Generally the variation of heat transfer coefficient of air is considered with variation in temperature of air.

https://quickfield.com/natural_convection.htm

This link I provided is a site which calculates heat transfer coefficients. This will give you an idea on how many factors are required to calculate the convective heat transfer coefficient of a medium.

Hope I could help you with your doubt.