Keep the Heat B/Thermodynamics C

[Paleofreakazoid]

Anyone else get the thermo scores from Nationals last year?

[chalker]

Chalker - Any thoughts on the legality of paraffin based phase change materials......?

To what purpose would you use them? Note 3.d. prohibits any energy sources in the device being used to warm the water, thus a 'preloaded' phase change material would likely fall into that category.

[iwonder]

I think it's more for the prediction score, assuming you use paraffin that changes phases at a temp a little lower then your beaker temp, wouldn't the phase change itself hold the temperature of the beaker fairly constant? Of course, it's late and I'm tired and probably forgetting something here...

[chalker]

I think it's more for the prediction score, assuming you use paraffin that changes phases at a temp a little lower then your beaker temp, wouldn't the phase change itself hold the temperature of the beaker fairly constant? Of course, it's late and I'm tired and probably forgetting something here...

I'm not sure I follow. My understanding of phase change materials is they maintain a near constant temperature while transitioning from one phase to another (in fact almost all materials do that to a certain degree, boiling water and freezing water are common examples). Phase change materials just have a higher heat of fusion, meaning they can absorb or release more energy than typical substances.

When going from liquid to solid, they release heat (which I believe is prohibited by the rules as indicated above).

When going from solid to liquid they absorb heat (while maintaining a constant temperature). Thus they in essence act as a fixed temperature heat sink. However, there is a key aspect of the rules that I think makes this a moot point - that all device components must start at essentially room temperature.

Thus, let's assume for a second that room temp is 70 degrees F (I'll use F for the sake of familiarity here), and the water beaker starts at 150 F. There is a phase change material in the device with a transition temper of 120 F. The hope would be that the water heats it up enough to get it to 120 F, at which point it acts as a constant temp heat sink for the water.

If we look at this over time, you have 3 potential phases in my mind:

1. Beaker dropping temp from 150 F while device is increasing from 70 F to 120 F
2. Device staying at 120 F due to the phase change while beaker continues to drop to 120 F
3. Both beaker and device dropping below 120 F to room temp while the room acts as the ultimate heat sink

The reason I say 'potential', is it would take some very special planning to ensure step 2 happens. The beaker needs to have enough thermal energy to get the device up to 120 F without dropping to that temp itself. Even if that happens, the beaker isn't actively producing heat, thus it will continue to drop in temp until it too reaches 120 F. I suspect the time spent in step 2 is going to be very momentary, since there will be no active heat generation in the beaker.

Looking at this from the beaker's perspective, I suspect the cooling curve will be very similar to as if there was no phase-change material present. Thus, I don't see the purpose of it. Am I missing something?

[iwonder]

Yeah... After sleeping a little while I realize that my brain wasn't quite functioning.. But it seemed like such a nice idea!

[sciencegeek999]

Anyone know where you can get straw bale?

[Soccer_Girl]

I don't know if you've ever heard of it, but I think Agway's has straw bale. I'm not sure, though, depends on the Agway's in your area. Or maybe you don't even have one...

[sciencegeek999]

Sorry, never heard of it.

[Flavorflav]

This time of year they sell them at garden stores and farm stands around here, for decoration.

[retired1]

Many places that sell pumpkins for Halloween will have straw bales for display purposes. Many feed and seed stores will also have it as well as the garden shop in some of the big box stores.