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Air volume vs temperature?

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- Thread starter gary350
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- #1

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Air volume vs temperature?

- #2

olivermsun

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- #3

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You need to use the ideal gas law: PV = nRT. This should give you a good approximation of how much the volume will expand, since you can assume the pressure before and after the temperature change are both going to be equal to the ambient (atmospheric?) pressure. Remember to use the correct value of R depending on the units you use; 0.08206 if you're using liters and amtomspheres; 8.314 if you're using pascals and cubic meters.

You will need to solve for n (moles of 'air molecules') using the initial state of your system before solving for the final volume.

Oh and don't forget that the temperature must be converted to absolute temperature in Kelvins.

- #4

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Let us assume that air is an ideal gas.

We can use $V1/T1=V2/T2$ we have $deltaT=T2-T1$

We can use $V1/T1=V2/T2$ we have $deltaT=T2-T1$

- #5

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http://www.youtube.com/watch?v=J56D56rQV-Q

http://www.youtube.com/watch?v=iSBXcn4u4es

http://www.youtube.com/watch?v=NBpRl5eF3fc

- #6

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Actually yeah, that's a lot more straightforward than what I said.Let us assume that air is an ideal gas.

We can use $V1/T1=V2/T2$ we have $deltaT=T2-T1$

- #7

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- #8

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So the ratio of initial and final temperature would give you the ratio of initial and final volumes; T2/T1 = V2/V1 or V2 = (V1T2)/T1

Since you're using a ratio you shouldn't need to convert temperature to Kelvins... I think. I would just to be safe though

- #9

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I can calculate volume.

Don't now n?

Don't know R?

I can measure temperature to a point plus or minus maybe 100 degrees.

This 1 cylinder has 2 pistons. The piston in the center of the cylinder does not tough the sides or top of bottom. There is a space of about 1/16" all the way round the displacer piston. When the displacer piston is up all the air is at the hot end of the cylinder about 600 degrees. Hot air expands pushing the power piston up. When displacer piston is at the bottom of the cylinder all the air is at the top COLD end of the cylinder where it cools contracts and sucks the power piston down. This is a 1 cycle beta stirling engine. After the engine runs for several minutes the top of the cylinder gets hot too I am guessing maybe 300 or 400 degrees. Once the engine is warmed up there is a temperature difference from bottom to top of maybe 200 or 300 degrees. Maybe I need to buy a laser thermometer to check both ends for exact temperature while the engine runs. Next engine I want a larger diameter piston and air or water cooling for the top. I put air cooling on 1 engine already it runs better. I did a few manual test pushing the displacer piston up/down by hand and measuring how much the power piston moves I am not getting accurate measurements.

- #10

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If I understand you correctly, you have a temperature gradient from one end of the cylinder to the other. In a relatively small (i.e. not huge) cylinder it's hard to imagine why air at one end would be drastically different from air at the other end. Perhaps the metal at each end has different temperature, but the gas inside the cylinder would move around very quickly to equalize any considerable temperature or pressure difference.

Sorry if I'm misunderstanding something, perhaps a diagram of the inside of the cylinder would help? I'm not terribly familiar with stirling engines.

- #11

jbriggs444

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You need the temperature expressed on an absolute scale. e.g. Kelvin or Rankine. Not Celsius or Fahrenheit.Since you're using a ratio you shouldn't need to convert temperature to Kelvins...

- #12

Nidum

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The air in the engine is going through a cycle controlled by the motions of two pistons with a roughly 90 phase angle between them and by the effects of heat addition and extraction .

The volume of the contained air at particular point in the cycle of an engine of known geometry only depends on the positions of the pistons .

To work out the details of pressure variations in the engine you need to analyse the complete engine .

Start by getting to understand the theoretical thermodynamic cycle for a Stirling engine .

The volume of the contained air at particular point in the cycle of an engine of known geometry only depends on the positions of the pistons .

To work out the details of pressure variations in the engine you need to analyse the complete engine .

Start by getting to understand the theoretical thermodynamic cycle for a Stirling engine .

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- #13

Nidum

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https://en.wikipedia.org/wiki/Stirling_engine#Theory

Those grooves are called labyrinth seals . More common in rotary applications but they can work reasonably well in reciprocating applications .

Those grooves are called labyrinth seals . More common in rotary applications but they can work reasonably well in reciprocating applications .

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- #14

Kaminska Zakrzewska

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See the video.

- #16

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What percentage does air volume change with a temperature change of 100 degrees F.

- #17

olivermsun

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Depends what percentage 100°F is of the original temperature.

- #18

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70 degrees to 170 degrees

- #19

olivermsun

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So convert to absolute temperature (e.g., Kelvins) and use the Ideal Gas Law.

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