everyday-lifeheat conductionthermodynamics
Aluminium being such a good conductor, how is it possible that it is helping me keep my food warm ?? Because ultimately it should conduct the heat that is inside to the outside for exchange and should have no effect (maybe even cool it faster by increasing the surface area).
Then why is it that we wrap our food with aluminium foil ? How does it keep my food warm ?
I We did the experiment. (Early results indicate that dipping may win, though the final conclusion is uncertain.)
There were four trials, each lasting 10 minutes. Boiling water was poured into the canning jar, and the spoon was taken from the ice bath and placed into the jar. A temperature reading was taken once a minute. After each trial the water was poured back into the boiling pot and the spoon was placed back into the ice bath.
Method: Final Temp.
1. No Spoon 151 F
2. Spoon in, no motion 149 F
3. Spoon stirring 147 F
4. Spoon dipping 143 F
Temperature readings have an uncertainty of $\pm1\,\mathrm{^\circ F}$.
Red line: no Spoon
Green line: Spoon in, no motion
Aqua line: Stirring
Blue line: Dipping
$$\begin{array}{|c|cl|cl|cl|cl|} \hline \text{Min} & \text{No Spoon} & & \text{Spoon} & & \text{Stirring} & & \text{Dipping} \\ \hline & \text{°F} & \text{°C} & \text{°F} & \text{°C} & \text{°F} & \text{°C} & \text{°F} & \text{°C} \\ \hline 1' & 180 & 82.22 & 175 & 79.44 & 175 & 79.44 & 177 & 80.56 \\ 2' & 174 & 78.89 & 172 & 77.78 & 171 & 77.22 & 173 & 78.33 \\ 3' & 171 & 77.22 & 168 & 75.56 & 167 & 75 & 168 & 75.56 \\ 4' & 168 & 75.56 & 165 & 73.89 & 164 & 73.33 & 164 & 73.33 \\ 5' & 164 & 73.33 & 162 & 72.22 & 161 & 71.67 & 160 & 71.11 \\ 6' & 161 & 71.67 & 160 & 71.11 & 158 & 70 & 156 & 68.89 \\ 7' & 158 & 70 & 156 & 68.89 & 155 & 68.33 & 152 & 66.67 \\ 8' & 155 & 68.33 & 153 & 67.22 & 152 & 66.67 & 149 & 65 \\ 9' & 153 & 67.22 & 151 & 66.11 & 150 & 65.56 & 146 & 63.33 \\ 10' & 151 & 66.11 & 149 & 65 & 147 & 63.89 & 143 & 61.67 \\ \hline \end{array}$$
You want large area of the ice to improve heat transfer. And you want good thermal contact between the ice and the contents of the box.
For this reason, crushing the ice and leaving the water in would be good. Gently stirring the contents would be the finishing touch (heat transfer by convection is much more efficient than by conduction, and the work done in stirring will be quite small compared to the benefits).
As for the other points: wrapping in foil limits convection -> bad. Single block of ice limits area -> bad. Draining the water means that the heat capacity of the convection medium (air instead of water) is lower -> less ability to carry heat from A to B, bad.
The point that crushing the ice does work is true but irrelevant. The question was about rate of cooling, not maximum final amount of heat extracted. Assuming that the contents of the box will end up at 0°C with some ice still solid, the work done will not materially change the outcome.
Best Answer
Being a shiny surface the aluminium sheet reflects radiant heat and reduces the heat loss by radiation by as much as $90\%$.
Being impermeable the sheet stops the movement of hot air from the vicinity of the surface of the food into the surrounding by convection currents.
This also has the effect of reducing the rate at which water evaporates from the surface of the food, evaporation requiring an input heat from the food.
However as you point out aluminium is a good conductor of heat and so does not reduce heat loss by this mechanism although it does trap a layer of air between the sheet and the food.
This does reduce the loss of heat by conduction as air is a bad conductor of heat.
You may have seen these properties of reduced heat loss at the end of a marathon with the use of "space blankets"?