Hi scienceforums !

 

it's my first time in this forum. I need your help for a problem in thermodynamics. I just started this field so my knowledge it's limit for now. My problem :

 

 

Steam in a cylinder-piston arrangement expands from an initial condition of 5 bar, 80% dry to a final state of 2 bar, dry saturated steam.

 

The initial volume of steam is 0.8 m³. The process of expansion can be represented by a straight line on the p-V diagram.

 

the mass of steam in the system ? [kg]

 

the work done by the system ? [kJ]

 

the change in energy of the system ? [kJ]

 

the heat absorbed by the system ? [kJ]

 

 

Maybe it's a simple problem but really I tried, nothing to do... I'm waiting your help...

 

 

Welcome to Science Forums.

 

You should definitely start by reading this here.

 

http://www.scienceforums.net/topic/75772-read-this-before-posting-in-homework-help/

 

and then following the instructions.

 

What have you done so far towards the problem?

thanks for the links, before start calcul first I think all work interaction is of the expansion type and only thermal energy change is significant.

 

For the initial property, I know the volume and pressure. with this information I go to the "saturated water and steam table" the look the properties of this condition

 

I find for 5 bar presasure :

 

151.85 C boiling point, 2.669 kg/m³ steam density, 2.3289 kJ/kg K specific heat, 2747.54 kJ/kg specific enthalpy of steam, 640.12 kJ/kg specific enthalpy of liquid water, 2107.42 kJ/kg latent heat of vaporization

 

- the mass of steam in the system ? steam density x volume initiale <=> 2.669 kg/m³x 0.8 m³= 2.1352 kg for the mass of steam

 

I'm here, the problem is I didn't understand what means "80% dry to a final state of 2 bar"

 

thanks for the help

There can be liquid water droplets in the steam.

 

 

The steam dryness fraction is used to quantify the amount of water within steam. If steam contains 10% water by mass, it's said to be 90% dry, or have a dryness fraction of 0.9.

 

http://www.tlv.com/global/US/steam-theory/wet-steam-dry-steam.html

 

Link above provides a good overview along with the equations.

 

 

There's other issues, but the impact of water hitting/being hit by the turbine blades is the main one that I recall.

Edited June 12, 20178 yr by Endy0816

oh nice links thanks I understand better this story of dry, you open my eyes hahah

 

I'll try it, thanks for the advice !

!

Moderator Note

 

challenge to thermodynamics moved to its own thread in speculations