2 kg of water is contained in the vertical frictionless piston cylinde device.Th

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najamanukaposted 8 years ago

2 kg of water is contained in the vertical frictionless piston cylinde device.The mass of the...

piston is such that the H2O exists as a saturated liquid at 10 Mpa. there is heat transfer to the cylinder until the piston reaches some stops, at which point the total volume is 0.02 m^3. there is addition heat transfer to the water until the H2O exists as a saturated vapor. Determine the total heat transfer and the total work(kj). and show the T-V diagram.

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MickSposted 8 years ago

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najamanukaposted 8 years ago

!!!! this is not a homework question.......................................

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Ben Evansposted 8 years ago

How did the vapor expand.

Did it expand by adiabatic, isentropic, isobaric thermodynamic cycles.?  I am assuming that this would be a Rankine cycle but for sake of ease we will assume isobaric as that is what it appears that you are pointing towards.

It is easier if we just look at the work performed by the piston and the heat difference between the initial water and the saturated steam.

Assuming this is ideal (no heat loss and there are many other assumptions to make this ideal)

If we assume a piston area of .02 meters and a stroke of 1 meter we will have 200kj of work.  The area of the piston face and stroke do not matter in this calculation because if the area decreased then stroke is increased proportionally.  The pressure is a force per unit area so this becomes simply the the total displacement x pressure.  Now one thing to note this is not likely to expand over constant pressure.

Now the question I have to calculate the amount of energy that was required to heat the saturated water to saturated steam is:

Is this exhausted at an atmospheric pressure?  or maybe this is exhausted at 10 Mpa, which I will again assume because this then becomes a simple question that can be derived from a steam table.  You can find the difference in heat and that will give you the to energy required to take this from a staturated liquid to a saturated steam at 10 Mpa.  I would do this but it is little tedious.

So the energy to heat the water + the energy for the movement of the piston is the total energy that needs to be put into the system to create the work just described.  Since we do not know how this is going to expand ie (adiabatic, isentropic, isobaric thermodynamic), we can't draw a TV diagram.  Also I dont have the graphics

I hope this helped.  Most of it now should be simple.  Just look up a table for saturated liquid and saturated steam at 10 Mpa and it is an easy calculation.  The TV can be drawn and I would assume it is isobaric and there are good examples on the net.

One other note is a system would not typically behave this way however you may look up Rankine cycle which is used for steam engines if you want a better idea of this type of cycle.

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