I am attempting some convection analysis for research I am doing. I was wondering someone could kindly confirm whether I am on the right track. The calculation is based up air being passed over a flat surface and the heat exchange taking place between the 'air' and ' surface'.

I have the following issues...

I have read books that say that hc for air by forced convection is normally within 10 - 10^3 range, however all my calculations seem to result in around 4-5 is this correct as per my example below?.

The example i have based this upon is a 1m2 surface 1m x 1m dimensions and I want to know how long it would take to change the temperature of the surface material. My approach using the example below shows that if I had to add 460 kJ to change the temperature and then calculate the time, would it be the following to determine time to add the energy.....

460000 J / 66.42 j.s = 6925 secs = 1.92 hours, is this correct?

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1 Answer 1


You have the right general approach, and the numbers look reasonable. Your airspeed isn't very fast, so I'm not surprised that the convection coefficient you calculate is less than 10 W m-2 K-1.

Note, however, that you're applying the heat flux you calculated at a plate temperature of 288 K for all other temperatures as well. In reality, the convective flux will decrease as the plate heats up to the airflow temperature. Try using the lumped capacitance model. You end up getting a time constant (not a total time) of 6925 s; that is, every 6925 s, the temperature different is reduced by (1-1/e) = 73%. Because the heat flux magnitude affects the plate temperature but also depends on the plate temperature, this temperature approaches the airflow temperature asymptotically. People often assume that the process is essentially complete after 2-3 time constants have passed. Does this make sense?


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