Convection Cooling Metal Plates By Kelli Fredrickson and
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
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 Temperature of Air Cooled Steel Plate 600 Free Convection Forced Convection 500 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 400 Temperature (°C) Room temperature 300 200 100 0 0 20 40 60 80 100 Time (s) 120 140 160 180 200
Analytic Analysis ASSUMPTIONS (possible reasons for error) From analysis, there was no free convection associated with the forced convection Assumed gray surface Emissive radiation approximated with equivalent convection coefficient Average surface temperature is good enough for emissivity approximations and all other steel properties Radiation must be considered however t = 298 seconds calculated for free (vs. 160 seconds observed) t = 149 seconds calculated forced (vs. 82 seconds observed) Film temperature constant for air properties See Appendix for process No other air movement in room (air conditioning, etc. ) Conduction from table negligible No heat lost during movement between oven and table
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 Forced Free Radiation (same for both), used convection equivalent Found Reynolds number Re 2 >> Gr so only forced convection Check L/xc < 1. 05 Nu. L from (7. 30) h from Nu. L = h. L/k htot from convection and radiation Find Biot for lumped capacitance Bi < 0. 1 Time from lumped capacitance Radiation (same for both), used convection equivalent Found Grashof number Nu. L from (9. 21) and (9. 20) h from Nu. L = h. L/k htot from convection and radiation Find Biot for lumped capacitance Bi < 0. 1 Time from lumped capacitance
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