[asking]

Oh, geniuses of the interwebs, please help me resolve a question.

I am writing science curriculum for kids.

The students heat some water until it creates a jet of steam, which drives a pinwheel. The purpose is to get them thinking about energy transfer and get them ready for the lesson where they learn how a power plant works (steam turbine).

My editor wants me to have them measure the speed of rotation of the pinwheel as a function of the amount of heat applied to the water. I think this is unlikely to produce a result, since the steam will not be under much pressure and the temperature of the water will top out at 100 degrees C.

I think the water can't get any hotter and if you lower the temp so it's not boiling vigorously, it won't produce enough steam pressure to drive the pinwheel. You might see some differences between very small changes in heat input, but students will not likely get enough precision to get consistent results.

Am I wrong?

I tried to test this at home and my pinwheel was too badly designed to turn, so I'll go buy one tomorrow. Meanwhile... what do you all think?
Thanks!

[Perito]

While it's true that as long as there's water in the kettle (or whatever), the temperature of the water can't get above 100 C (at atmospheric pressure and assuming everything is at eqilibrium). It's also true that the rate of evaporation depends on the amount of heat applied to the water. The amount of steam that is produced when everything has been heated to 100 C, can thus be calculated by dividing the heat applied (usually in Joules, but possibly in other units) by the latent heat of vaporization of water (aka enthalpy of vaporization, usually in Joules/gram or in other units).

The speed of rotation of a pinwheel will be heavily influenced by friction and, in fact, overcoming that friction is the purpose of continually applying steam to the vanes. Without friction, the vanes would continually accelerate as steam pressure is applied to them. Estimating the friction of a pinwheel is not an easy task. It would be best measured directly.

In a turbine, in addition to friction, you are expending energy to produce electricity. This produces a (large) drag on the turbine that has to be overcome by the steam.

[asking]

Thank you. This is very helpful. I think I understand about 85% of what you said (I dimly remember enthalpy and not liking it).

So the answer is that it all depends?
If friction were low, which it probably won't be, then maybe you'd see a result?

This is for 13 year olds in low income school districts, which means their teachers will not understand most of what you said. I am not even allowed to explain entropy without using the word. They would not be measuring friction, of course.

Nice avatar!

[Perito]

Actually, Enthalpy is easier to understand (by far) than Entropy.

Enthalpy of vaporization - Wikipedia, the free encyclopedia

[asking]

Sheesh. Is that all? Why do I remember enthalphy as mysterious?

This leads me back to the lack of clarity of so much teaching...

But now I understand your explanation better. Thanks for leading me by the nose to Wikipedia.

Since friction will increase as the vanes turn faster in some probably nonlinear fashion, there also won't be a linear relation between heat applied and pinwheel speed. Is this correct?

I don't see any practical way to measure friction for all the different speeds. Am I missing something?

[KISS]

What you can do to get your pressure jet is to use a pressure cooker. The weight acts as a pressure relief valve, so no tubing.

But you will get a nice jet of steam and turning the burners up increases the rate of evaporation and you are going through an orifice.

Use a non-contact optical speed measuring device to measure the RPM of the pin wheel. If it's slow enough, then count the revolutions.

I could probably come up with a cheap way to measure revolutions using an optocoupler, battery and small counter module.

[asking]

Hey, Kiss,
A pressure cooker is a great idea!

My only concern would be if the schools can afford to buy a half dozen pressure cookers for this one lesson. These are low-income schools. The kids are just regular kids, but there's no money. Although there seems to be money to design curriculum... hmmm

Thanks for the suggestions (and offer?). But if they can't count revolutions by sight, I doubt this will happen unless what you have in mind is VERY cheap.

Students can work in groups of 4 or 5, so maybe 6-8 groups times per unit cost. Total materials budget for 4 weeks is only $500 for everything. That's not per group. This is a physical science class for 13 year olds. It's sad because they are at an age when they could really understand a lot.

[KISS]

With experiments like this, it might be best to have it as a demonstration all because of the safeties involved.

Note some of what's mentioned in this table: Properties of Saturated Steam - SI Units

i.e.

Steam at atmospheric pressure has little or no value.

A commercial non-contact RPM meter is about $60. Uses a piece of reflective tape. Doubt I can do much better than that.

You could almost do this with a cylinder of Nitrogen, a regulator and a needle valve.