Friday, December 12, 2008

TE 303 - Please give Dr.B some feedback about Thermo-CD

Well, the final is over and the holidays are almost here. I hope you had a great semester and learned a lot.

If you would be so kind as to share your opinions about Thermo-CD or ideas about how I could improve the program or workbook, that would be greatly appreciated. You can make your comments anonymously, but they would be more credible if you chose to give your name. Whether you give your name or not is up to you. I will NOT contact you in either case. You can always send questions or comments directly to me if you prefer:

Best of luck to all of you in the years ahead !

Happy Holidays,
Dr. B

Wednesday, December 03, 2008

TE 303 - HW #8, P1 - Inventor Claim for Silicon Chip - 20 pts

An inventor claims to have created a silicon chip whose surface temperature is no more than 60 °C. The silicon chip has dimensions of 5 mm on each side with a 1 mm thickness, and is embedded in a ceramic substrate. At steady state, the chip has an electrical power input of 0.225 W. The top surface of the chip is exposed to a coolant whose temperature is 20 °C. The heat transfer coefficient for convection (h) between the coolant and chip is 105 W/m2•K. Evaluate the inventor's claim using the following:

a.) First Law Energy Balance, assuming that the heat transfer by conduction between the chip and substrate is negligible.

b.) Second Law Energy Balance: Calculate entropy generation.

In your analysis section, be sure to comment on what is the main source of entropy generation in this system.

No old comments.

TE 303 - HW #8, P2 - Maximum Work From an Adiabatic Turbine - 10 pts

Steam enters an adiabatic turbine at 800 psia and 900oF and leaves at a pressure of 40 psia. Determine the maximum amount of work that can be delivered by this turbine.

No old comments.

TE 303 - HW #8, P3 - Lost Work in a Heat Exchanger - 15 pts

A well-insulated shell-and-tube heat exchanger is used to heat water (CP = 4.18 kJ/kg-K) in the tubes from 20oC to 70oC at a rate of 4.5 kg/s. Heat is supplied by hot oil (CP = 2.30 kJ/kg-K) that enters the shell side at 170oC at a rate of 10 kg/s.

Disregarding any heat loss from the heat exchanger, determine...

a.) The exit temperature of the oil.
b.) The rate of entropy generation in the heat exchanger.
c.) The rate at which work is lost due to the irreversible nature of heat transfer in this process in kW. Assume the surroundings are at 20oC.

3 Old Comments

TE 303 - HW #8, P4 - Entropy Generation and Lost Work in a Nozzle - 20 pts

Oxygen, O2, enters a nozzle operating at steady-state at 3.8 MPa, 387oC and 10 m/s. At the nozzle exit, the conditions are 150 kPa, 37oC and 790 m/s.

a.) For a system that encloses the nozzle only, determine the heat transfer (kJ/kg) and the change in specific entropy (kJ/kg-K), both per kg of oxygen flowing through the nozzle. What additional information would be required to evaluate the rate of entropy production in this process ?

b.) Using an enlarged system boundary that includes the nozzle and a portion of its immediate surroundings, evaluate the rate of entropy generation (kJ/kg-K) and the rate of lost work (kJ/kg), both per kg of oxygen flowing through the nozzle. Assume that heat exchange at the enlarged system boundary takes place at the ambient temperature, 20oC.

Treat O2 as an ideal gas with variable heat capacities. Verify that the ideal gas assumption is valid.

2 Old Comments

TE 303 - HW #8, P5 - Entropy Balance for a Power Plant - 25 pts

The figure to the right shows a simple vapor power plant operating at steady state with water as the working fluid. Data at key locations are given in the figure. The mass flow rate of the water circulating through the components is 109 kg/s. Stray heat transfer and kinetic and potential energy effects can be ignored.

Determine the following:

a.) The mass flow rate of the cooling water, in kg/s.
b.) The thermal efficiency.
c.) The rates of entropy production, each in kW/K, for the turbine, condenser, and pump.

In your analysis section, place the components in rank order, beginning with the component contributing most to inefficient operation of the overall system.

No old comments.