Real world heat transfer?

Question

I would appreciate it if someone can advise me if my idea of heat transfer is correct. I would like to transport frozen products and below is my idea of insulating them.

From what Ive found online there are 3 modes of heat transfer: conduction, convection and radiation.

To limit heat transfer by conduction, my plan is to place my products inside a corrugated cardboard box lined with 0.75inch EPS expanded polystyrene foam sheets on all sides. My guess is that conduction on the bottom would contribute the most heat because my products are packaged in a way that the bottom is flat (and in constant contact with the bottom foam) and the top is mostly air, so I would probably increase the base to 1.5inch EPS foam.

To limit heat transfer by convection, I would use a cardboard box that has as little empty space inside the box as possible when filled. I was also planning to place 1/4in flat ice packs but Im not sure whether to place it on the top or bottom of the box. If I were to place it on the top of the box would there be an 'air current'(convection) since cold air is more dense? If I were to place it on the bottom of the box would the top part of my product melt?

As for radiation I wouldn't worry too much because at the most an aluminum foil layer would suffice since it reflect 95%? of heat by radiation and foil is light.

Obviously I don't have enough real world data so general speculation would suffice. I live in a tropical country, usually around 34C in the afternoon. Let's also assume 2 scenarios, one where the box is exposed to the sun and 1 where it isn't. PS shipping company doesn't allow dry ice.

Basically I would like to know if:

1. Will conduction contribute the most heat transfer overall?
2. Am I right to assume heat conduction mainly comes through the bottom of the box?
3. Should I place ice pack on the top or bottom of the box?

Answer 1

Radiative heat transfer will be minimal, even without the aluminum foil.

Almost all losses through the walls of the box will be conductive. All walls will participate more or less equally if the box is completely full, or nearly so. The idea of conduction through the bottom can be easily tested by doubling the bottom wall thickness and seeing if the time-to-melt a block of ice of fixed size inside the box changes. Convective losses will be small compared to conductive losses.

In any case, best place for the ice is on top of the load. Recommend you put a folded blanket in the bottom of the box to absorb the melt water during use. Doubling the foam wall thickness should cut the conduction loss in half.