1. Convection Cooling Metal Plates
By Kelli Fredrickson and Adam Bilodeau

2. Objectives Analyzing the heat treatment of steel
Run an experiment that could be compared to convection models
In Material Science, it is well known that different cooling rates of carbon steels affect the material properties
We wanted to see how much of a difference in cooling there was between forced and free convection on carbon steel plates

3. Preliminary Setup Measured the ambient temperature of the room
Measured air velocity created by fan (for forced convection) with an anemometer (see picture on right)
Heated oven and plates up slightly past 500˚C

4. Forced Convection cooling
Experiment
Forced Convection cooling
(air flow from red fan)
Free Convection cooling
All temperature measurements done with an infrared thermometer
Temperature measured first in the oven
Temperature measurements taken every 5-10 seconds as each plate cooled

5. Results
Measured the cooling rate of the plates until they cooled below 50˚C
Forced convection cooling took 82 seconds
Free convection cooling took 160 seconds

6. ASSUMPTIONS (possible reasons for error)
Analytic Analysis
ASSUMPTIONS (possible reasons for error)
Assumed gray surface
Emissive radiation approximated with equivalent convection coefficient
Average surface temperature is good enough for emissivity approximations and all other steel properties
Film temperature constant for air properties
No other air movement in room (air conditioning, etc.)
Conduction from table negligible
No heat lost during movement between oven and table
From analysis, there was no free convection associated with the forced convection
Radiation must be considered however
t = 298 seconds calculated for free (vs seconds observed)
t = 149 seconds calculated for forced (vs. 82 seconds observed)
See Appendix for process

7. Why do we care?
Material properties after cooling will be affected by rate of cooling
Almost a factor of 2 between how long it took from forced vs. free convection
Faster cooling from other methods such as quenching
From Material Science for Engineers: An Introduction by Will D. Callister, Jr. and David G. Rethwisch

8. Appendix Free Forced Found Grashof number NuL from (9.21) and (9.20)
Radiation (same for both), used convection equivalent
Found Grashof number
NuL from (9.21) and (9.20)
h from NuL = hL/k
htot from convection and radiation
Find Biot for lumped capacitance
Bi < 0.1
Time from lumped capacitance
Forced
Radiation (same for both), used convection equivalent
Found Reynolds number
Re2 >> Gr so only forced convection
Check L/xc < 1.05
NuL from (7.30)
h from NuL = hL/k
htot from convection and radiation
Find Biot for lumped capacitance
Bi < 0.1
Time from lumped capacitance