Convection Cooling Metal Plates By Kelli Fredrickson and Adam Bilodeau

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

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

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

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

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. 160 seconds observed) t = 149 seconds calculated for forced (vs. 82 seconds observed) See Appendix for process

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

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