# Mechanical Engineering Ph.D. Prelim. Qualifying Exam

Mechanical Engineering Ph.D. Prelim. Qualifying Exam

Thermodynamics, January 2004

You are required to do four out of five problems.

Clearly indicate which four problems you are selecting. Show all the work on exam sheets provided.

Write your student ID on each sheet. Do not write your name on any sheet.

Student ID Number______

1. A proposal is made to use geothermal supply of hot water to operate a steam turbine as shown. The high pressure water at 190C and 1.6 MPa is throttled into a flash evaporator chamber, which forms liquid and vapor at a lower pressure of 500 Kpa. The liquid is discarded while saturated vapor at 500KPa feeds the turbine. Steam exits the turbine at 20kPa and 95% quality. The turbine produces 2MW of power, find (a) the flow rate of hot water per unit time into the flash evaporator (b) Entropy change for the process in the evaporator flash chamber.

Mechanical Engineering Ph.D. Prelim. Qualifying Exam.

Thermodynamics, January 2004

You are required to do four out of five problems.

Clearly indicate which four problems you are selecting. Show all the work on exam sheets provided.

Write your student ID on each sheet. Do not write your name on any sheet.

Student ID Number______

2. A rigid tank initially contains 0.2 m3 of saturated liquid R-134a and 0.1 m3 of saturated vapor R-134a at -4C. A valve on the bottom of tank is connected to a line flowing R-134a at 12C, 1.0 MPa. A pressure relief valve on top of tank is set at 700kPa (pressure in the tank cannot exceed 700kPa, extra R-134a escapes as vapor through the valve). The valve at bottom is opened to allow 20 kg of R_134a to enter the tank, then valve is closed. Heat is transferred slowly to the tank, until the final mass in the tank is 95 kg, (extra mass escapes through relief valve at the top), at which point the process is stopped.

a. How much mass left the relief valve

b. What is the heat transfer to the tank

Mechanical Engineering Ph.D. Prelim. Qualifying Exam.

Thermodynamics, January 2004

You are required to do four out of five problems.

Clearly indicate which four problems you are selecting. Show all the work on exam sheets provided.

Write your student ID on each sheet. Do not write your name on any sheet.

Student ID Number______

3. A small gas turbine uses C8H18 (liquid) for fuel, and 300% theoretical air. The air and fuel enter at 25C and products leave at 1000K. The output of the engine and fuel consumption are measured, and it is found that specific fuel consumption is 0.30 kg/s per megawatt (1000 kJ/s) output. Determine the heat transfer from the engine (gas turbine) per kg of fuel consumption. Assume complete combustion.

Mechanical Engineering Ph.D. Prelim. Qualifying Exam.

Thermodynamics, January 2004

You are required to do four out of five problems.

Clearly indicate which four problems you are selecting. Show all the work on exam sheets provided.

Write your student ID on each sheet. Do not write your name on any sheet.

Student ID Number______

4 Consider a nuclear power plant generating 750 MW (75,000 kJ/s), the reactor temp is 315C and a river with water temperature of 20C is available as heat sink.

(1) What is the maximum possible thermal efficiency of the plant and minimum rate at which heat must be discarded to the river (40%)

(2) If the actual thermal efficiency of the plant is 60% of the maximum possible efficiency, at what rate heat must be discarded to river and what is the temperature rise of river water with a flow rate of 165 m3/s (Water density 1000 kg/m3) ? (60%)

Mechanical Engineering Ph.D. Prelim. Qualifying Exam.

Thermodynamics, January 2004

You are required to do four out of five problems.

Clearly indicate which four problems you are selecting. Show all the work on exam sheets provided.

Write your student ID on each sheet. Do not write your name on any sheet.

Student ID Number______

5. An insulated tank contains 0.5 m3 of saturated water vapor at 700 kPa is connected to an initially evacuated, insulated piston-cylinder device. The mass of piston is such that a pressure of 200 kPa is required to raise it. Now valve is opened slightly and part of steam flows from the tank to the cylinder, raising the piston. The process stops, when the pressure in the tank A has dropped to 200 kPa. Assuming the steam that remains in the tank undergoes a reversible adiabatic process, determine the final temperature (a) in the rigid tank (b) in the cylinder.