bd:
Problems with dry ice:
1) Obtaining, maintaining, handling in a probable 100F environment (Speedweek).
2) “putting the heat exchanger in dry ice” would result in a gaseous boundary around the exchanger which has very poor heat conductivity--nil heat transfer into the gas. And the gas has only 1/5 the specific heat capacity of water. Advisable to further analyze and/or test this in the lab before committing to it.
3) Possible condensation issues inside the (electrical equipment-dense) vehicle.
Questions:
If the motors are happy at 65C/149F, where does the “320F” temperature cited come from?
Traction control for the “instant torque” motors?
Suggestion:
Do an evaluation of the energy expended (heat generated) for a realistic run down the course. Full power likely cannot be utilized in the initial stages of acceleration, and heavy power consumption only in the last. Less time to generate heat.
Comment - I would question the 28 gallons. I know of 1100 hp full bodied cars running with less than that amount, starting at ambient temperatures--not iced, as you would be able to do (although that might raise the condensation problem unless insulated).
Those were the 3 main concerns I had with dry ice as well.
1) We have a local shipping company that provides both dry ice and liquid nitrogen, so obtaining it wouldn't be difficult. I've talked to people that use dry ice for camping and things that though, and they said they've kept dry ice wrapped in newspaper for 3-4 days at the bottom of a coleman ice chest, and since it is used a lot in shipping, I feel fairly comfortable with it maintaining for a few days for Speedweek.
2) For the sublimation/gas layer issue, I was considering placing the radiator/heat exchanger at the bottom of the box with the dry ice on top of it, so as it sublimates, it is basically gravity-fed onto the heat exchanger. There would obviously be some loss in contact area, but it will be minimized. I was thinking of boring holes through the dry ice to allow the gas to vent out the top.
3) The box would need to be sealed, except for a vent hose that would route to the tail of the car.
Just for reference, the heat exchanger/ radiator will be at the back of the car while the batteries will be in front of the driver's compartment. We have one pancake motor in the hub of each wheel.
Sorry, for being unclear; the INLET temperature of the dielectric oil is specified to be 65C. After the oil passes through all four motors, it has been heated to 160C.
The cooling system for the motor controllers and batteries will be a separate circuit since the controllers require antifreeze. The heat loads for that circuit are much less though.
When we were figuring out the power and torque needed, we had done some analysis to figure how far we will be to reach top speeds and everything, but by no means is it perfect, and we hadn't applied it to our cooling system. Do you have a general timeline or list of "events" at each mile marker you go by? I've attached some excerpts from my report that shows my calculations for the heat load, temperatures, ice water volume, and ice water weight. The 3.74 cu. ft. in my report is where the 28 gallons came from.
If anybody would like more details of my application and how I've come to certain conclusions, I can email the initial report as a pdf, but the file is too large to upload to the forum.
Again, thanks to everyone for the feedback. It helps a lot to have to get other perspectives and to have to check over my own thoughts.
