Specific volume of refrigerant 134a

Thermodynamics: Homework #3

Due: September 15 in class

Be sure to follow the format given in the syllabus. Use Tables when possible. Consult the YouTube channel.

Q.1 Combustion in a 4–stroke engine may be modeled as a constant-volume heat addition process with a fixed mass of air in the cylinder before and after combustion. Consider an engine with cylinder conditions of 1241◦R and 18 atm before combustion, and 3600◦R after it. Determine the pressure after the combustion process.

Q.2 A fixed quantity of refrigerant R-134a gas has a pressure 160 psia and specific volume 0.3479 ft 3

lbm . Find the temperature

modeling the refrigerant as a van der Waals gas. What is the temperature if we modeled the refrigerant as an IG.

Q.3 Modeling a substance: A blob of H2O gas has p = 0.2 MPa and T = 555 ◦C. What is the specific volume

a. from experiment?

b. modeling the Blob as ideal?

c. modeling the Blob as a van der Waals gas?

What is the % error incurred by the IG model?

Q.4 Introduction to the tabular EOS:

a) What is the specific volume of H2O liquid when p = 23 MPa and T = 160 ◦C?

ans: Table A-7 & v=interp1([20 30],[0.0010886 0.0010823],23) % m^3/kg

b) What is the specific volume of R134a gas when p = 0.55 MPa and T = 120◦C? ans: Table A-13 & v=interp1([.5 .6],[0.061687 0.050997],.55) % m^3/kg

c) What is the specific volume of R134a gas when p = 0.55 MPa and T = 125◦C?

d) What is the temperature of R134a gas when p = 0.9 MPa and v = .029700 m 3

kg ?

e) What is the temperature of R134a gas when p = 0.85 MPa and v = .029700 m 3

kg ?

Hint: try table creation!

f) What is the pressure of H2O gas when T = 965 ◦C and v = .06510 m

3

kg ?

g) What is the pressure of H2O liquid when T = 188 ◦C and v = .0011200 m

3

kg ?

Q.5 The vapor-pressure curve: The average atmospheric pressure in Denver (elevation 1610 m) is 83.4 kPa. Determine the temperature at which water boils in Denver. Sketch by hand and use the vapor pressure curve.

Q.6 Incompressible assumption: In our class thought experiment isobarically boiling water, we saw that the liquid phase is almost incompressible. More precisely, the specific volume of the liquid at a given temperature doesn’t change appreciably near the vapor pressure curve from the saturated liquid specific volume. In the absence of compressed liquid data, one approximates the the specific volume of a compressed liquid at a given T by vf . What is the specific volume of water at 20◦C and 100 kPa? Check: The specific volume of R-134a at p = 970 kPa and T = 38◦C is approximately 0.0008657 m

3

kg .

Q.7 For each problem fill in the missing thermodynamic property of R134a. If the state lies on the vapor pressure curve, give the range of possible specific volumes.

T [◦C] p [kPa] v [ m 3

kg ] phase

-12 320 30 .0065

550 x = 1 60 600

Note that a quality of 0 or 1 indicates that the phase is mixed and saturated; i.e. is boiling but still all liquid or all gas, respectively.

http://en.wikipedia.org/wiki/Otto_cycle