Multiple Choice: choose all that apply.

If none of the choices can be expressed with confidence, indicate that this is unanswerable.

1. (2pts) Indicate whether the following statements are True  (T) or False (F), or Neither (N) if their validity cannot be determined given the information.

A.  If there are no losses or irreversibilities in a refrigeration cycle, the theoretical maximum coefficient of performance is 100%.

B.  The theoretical maximum efficiency of a power cycle occurs when the magnitude of heat rejection is zero (when QOUT=0).

C. An entropy change must always occur in any process involving the addition of thermal energy by heat transfer

D.  If a process is adiabatic, then there can be no entropy change.

2. (2pts) A mass of saturated vapor ammonia is in a closed rigid container. If this mass undergoes a constant temperature process that results in a slight decrease in          pressure, what will the phase become?

(a)  compressed liquid

(d) saturated vapor

(b) saturated liquid

(e) superheated vapor

(c) liquid-vapor mixture

(f) unanswerable

3. (2pts) Choose any of the following that may be used to approximate the specific enthalpy of a compressed liquid refrigerant R134a?

(choose all that apply and assume the cycle remains operable with the change)

a)  the specific enthalpy of a saturated liquid R134a at the same temperature

b)  the specific enthalpy of a saturated liquid R134a at the same pressure

c)  the product of the pressure in kPa times the specific volume of saturated liquid R134a at the same pressure

d)  none of these

4-5 Short answer (provide all information requested)

4. (2pts) Is it possible for a closed system to experience a decrease in entropy?

If it is possible what must occur?

If it is not possible, explain why.

(1-2 sentences– not an essay)

5. (2pts) An ideal Rankine cycle (vapor power cycle) uses water as a working fluid. The cycle is located down south where in the past the weather has reached a record low down to −20˚C (20 below zero) and a record high up to +38˚C.

Provide a suggestion for the condenser pressure (bar) and explain the reasoning for your choice. (1-2 sentences – not an essay)

6-8 Short Work Problem (show all work to receive credit)

6. (3pts) A heat engine receives heat from a source at 1000K and rejects waste heat to a sink at 350K. The engine produces power at a rate of 150 kW .

Determine the theoretical minimum rate of heat rejection from this cycle .

7. (3 pts) 25 kg of Air (R=0.287kJ/kgK) experiences an isentropic expansion in a closed system.

The air is initially at 325 kPa and at 500K.

The final temperature of the air is 350K.

Using variable specific heats, determine:

a. The heat transfer to the air

b. The entropy production of the air

c. The final pressure of the air

8. (4 pts) In a closed adiabatic system 25 kg of superheated vapor water experiences an expansion from 20 bar at 400˚C to 10 bar at 360˚C

What is the entropy produced in this process (kJ/K)?

9-13 Work Problems (show all work to receive credit)

9. (5pts) 5.0 kg of air (R=0.287kJ/kgK) in a closed system experiences a constant temperature compression at 1200K so that Pv=Constant.

The initial air pressure is 400 kPa and the final air pressure is 600 kPa.

Determine

a. The work done by the air in this process (kJ).

b. The heat transfer to the air in this process (kJ).

10.(5pts) Water as steam steadily enters a nozzle at a mass flow rate of 12kg/s. The water enters with a negligible inlet velocity at 40 bar and at 400°C.         The water exits with an average velocity of 400 m/s at 15 bar and at 280°C.

Estimate the rate of heat transfer (kW) to the steam during this process.

11.(3pts) An ideal refrigeration cycle cools a refrigerated space that is maintained at 2˚C. The cycle uses R134a which enters the compressor at a mass flow rate of 0.8 kg/s.   The system operates between 1.2 bar and 4 bar.

a. Sketch and label the system

b. Sketch and label the T-s plot

c. Determine the rate of heat removal from the refrigerated space (KW). (do not evaluate the entire system- only evaluate as indicated above)

12.(3pts) An air power cycle is represented by a Brayton cycle .

The compressor and the turbine have isentropic efficiencies of 80%. Air steadily enters the compressor at 6.0 kg/s at 300K and at 1 bar. Air (R=0.287kJ/kgK) enters the turbine at 1200K and at 12 bar.

a. Sketch and label the system

b. Sketch and label the T-s plot

c. What is the rate of heat addition required in this cycle (KW)?

(use the Cold Air Standard Analysis) (do not evaluate the entire system- only evaluate as indicated above)