Just started an experiment to compare the use of steel to aluminium as a material for the inner pot of the DF.
Idea was suggested by prof. Worsely (DF research meeting 5/7/10) as steel has a higher emissivity than aluminium and therefore would enable more efficient heat removal from inner chamber.
However we know there is also heat transfer occurring in the opposite direction (i.e from the outside-in) therefore would this not increase the rate of heat transfer in this direction as well?
So we decided to run this experiment to find out..
2 Desert Fridges (DFs) set up in oven kept at approx 30 oC.
One DF with aluminium inner pot (coka cola can) and other with steel inner pot (pepsi can). Used a magnet to verify which is steel/aluminium.
Porous clay (terracotta) flower pots used for outer pots both DFs. Sand from Swansea bay.
100ml water added to sand in each DF.
Logging temperatures inside each DF and ambient oven temp simultaneously for 24 hours (3 min log interval) using thermocouples (AU:T1 , AU:T2, AWE:T1) immersed in water (to give better contact and increase thermal mass to reduce fluctuations)
Results
The experiment was conduced as set out above, starting on 16/7/10 and ran for more than a day. Below is a graph of the results which has been plotted from the measurement recorded by data logging equipment.
Oven temperature: Initially increases as door was left open for some short period, while the thermocouples were put in place which caused the temperature to drop from the level it was originally being maintained at 29.8oC (air temp). When experiment started, door closed and temperature returned to set level (not quite - actually only went up 26-27). Also, it may have taken time for the water which the thermocouples were immersed in to heat up. Not sure why oven didnt maintain constant temperature - maybe rubbish thermostat control.
Aluminium: very poor performance (temperature difference of little more than a single oC) but consistently followed. Dont know why the results were so much worse than the previous experiment conducted using an aluminum inner pot (which gave a temperature difference of 10oC)
Steel: Definitely performed much better than aluminum by any measure (max 6oC cooler than ambient), except consistency - why is there such a massive swing in temperature?!
Maybe the tip of the thermocouple was wetted but not properly immersed in the water in the can. So the little residual water evaporated thus causing the initial cooling. Then once this residual water had completely dried it began to heat up with the ambient oven temperature. Then the main evaporate cooling process may have kicked in which caused the next phase of cooling. Then the water in the sand and outer clay pot dried out so the temperature began to increase towards the surrounding oven temperature again. Just a guess but decided to re-run this experiment to check if these were consistent results or just down to some experimental error as suggested.
Humidity was recorded at beginning and end of experiment at 56%.
Next experiment
Just started (18/7/10 at 15:41) next run where the oven temperature has been increased (to approx 350 C. ). The outer pots used in the first run will be switched (i.e the outer pot used for he alumimum can in the first run will be used for the steel can in the second run and visa-versa) as I noticed on of the pots seemed more porous than the other. The position of the DFs in the oven (LHS/RHS) have also been switched. Theoretically the switch should have no effect on the results as the purpose is to prove that the only variable is the material of the inner pot.
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