THERMODYNAMICS ! HELP
Rasbawy
me_hw2.pdf
ME 370 - Fall 2013 - HW #2
2.1
a) Sketch the device, summarize the “knowns”, and then write out the general form for the conservation of mass and conservation of energy; simplify them for this problem, and then clearly state any assumptions you have made.
1 kg of water is contained in a vertical cylinder, initially at ~ 1.25 atm pressure with a quality of 20%. The cylinder is sealed with a frictionless piston resting on fixed “stops”; weights are placed on top of it. Heat is transferred to the water until the water pressure reaches 200 kPa, and then the piston lifts off the stops. The piston rises, while compressing a linear spring, until it hits a second set of restraining stops. When the piston first reaches the second stops, the pressure has increased to 500 kPa and the volume has doubled. The process continues, adding more thermal energy until the pressure finally reaches 1.0 MPa.
b) Sketch a P-v diagram for this sequence of processes. Number each significant state for reference.
c) Determine the pressure, temperature, volume, internal energy and quality (if appropriate) for each reference state in the overall sequence of processes, and summarize your results in a table.
d) Determine the work associated with each process and summarize your results in a table (even if the value(s) are zero!)
e) Determine the heat transferred for each process and summarize in a table.
f) If the mass of water were doubled, what would happen to the values of the thermodynamic properties?
g) If the mass of water were doubled, what would happen to the magnitude of the work and the heat transfer for each of the process steps?
2.2 Water is enclosed in a rigid container (i.e. no piston!) with an electric heating element inside. The initial temperature of the water is 30 °C and the quality is 10%. The water is heated until the quality reaches 100%.
a) Sketch the device, summarize the “knowns”, and then write out the general form for the conservation of mass and conservation of energy; simplify them for this problem, and then clearly state any assumptions you have made.
b) Sketch a T-v diagram for this process. Number each significant state for reference.
c) Determine the pressure, temperature, volume, and internal energy for the initial and final states; summarize your results, in a table.
d) How much work is there for this process? Discuss.
e) How much heat transfer is there for this process? Discuss.
f) What property remains constant during this process?
1.3
a) Sketch the device, summarize the “knowns”, and then write out the general form for the conservation of mass and conservation of energy; simplify them for this problem, and then clearly state any assumptions you have made.
Water with a quality of 70% is contained in a cylinder with a weight on a piston that maintains a water pressure of 500 kPa. The water is heated until the temperature reaches 250 °C.
b) Sketch a T-v diagram for this process. Number each significant state for reference.
c) Determine the pressure, temperature, volume, and internal energy for the initial and final states; summarize your results, in a table.
d) How much work is there for this process? Discuss.
e) How much heat transfer is there for this process? Discuss.
f) What property remains constant during this process?
