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Finding the state in thermodynamics

  • #1
Hi Im having trouble finding the state of a process in thermodynamics from the question for example I have a question that asks



10-kg of R-134a at 300 kPa fills a rigid container
whose volume is 14 L. Determine the temperature and total
enthalpy in the container. The container is now heated until
the pressure is 600 kPa. Determine the temperature and total
enthalpy when the heating is completed.

I dont want to know how to solve the problem I just need help understanding it. The solution states that it this solution is a mixture and thus the temperature is the saturation temperature at the given pressure.

What I would like to know is how do we know from what is given that this is a mixture.

Thanks a lot.
 

Answers and Replies

  • #2
20,111
4,191
Hi Im having trouble finding the state of a process in thermodynamics from the question for example I have a question that asks



10-kg of R-134a at 300 kPa fills a rigid container
whose volume is 14 L. Determine the temperature and total
enthalpy in the container. The container is now heated until
the pressure is 600 kPa. Determine the temperature and total
enthalpy when the heating is completed.

I dont want to know how to solve the problem I just need help understanding it. The solution states that it this solution is a mixture and thus the temperature is the saturation temperature at the given pressure.

What I would like to know is how do we know from what is given that this is a mixture.

Thanks a lot.
Do you have graphs with temperature, pressure, and enthalpy for 134a? What kind of graph is it? If not, do you have tables, including specific volumes of saturated liquid and vapor? If you look up a density of 10/14 kg/L at 300 kPa, you will find that this density is between that of the saturated vapor and the saturated liquid at this pressure.
 
  • #3
I have access to the tables but the tables doesnt have the information at 300 kPa I had to use interpolation to find all the values but from the solution it seems to be obvious that the solution is a mixture here is what is written in the solution

'' The initial state is determined to be a mixture, and thus the temperature is the saturation temperature at the given pressure. From tables A-12 by interpolation.''

It seems that before any work is done its known that its a mixture, why is that?
 
  • #4
20,111
4,191
I have access to the tables but the tables doesnt have the information at 300 kPa I had to use interpolation to find all the values but from the solution it seems to be obvious that the solution is a mixture here is what is written in the solution

'' The initial state is determined to be a mixture, and thus the temperature is the saturation temperature at the given pressure. From tables A-12 by interpolation.''

It seems that before any work is done its known that its a mixture, why is that?
Go back to your tables and interpolate to get the density of the saturated liquid at 300 kPa and the density of the saturated vapor at 300 kPa. You will find that your bulk density of 10/14=0.714 gm/cc will lie between that of the saturated liquid and the saturated vapor. This means that you must have a mixture of saturated liquid and saturated vapor. Superheated vapor will have a density less than that of the saturated vapor, and subheated liquid will have a density greater than 0.714.

Chet
 

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