Thermodynamics problem

[DiFerrara]

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...

 

 

[studiot]

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?

[DiFerrara]

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

[Endy0816]

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, 2017 by Endy0816

[DiFerrara]

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

 

I'll try it, thanks for the advice !

[imatfaal]

!

Moderator Note

 

challenge to thermodynamics moved to its own thread in speculations