HW-1: Due Tuesday 13 Jun 2017 by 2:00:00 pm EDT to Your Division s GradeScope Site

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1 HW-1: Due Tuesday 13 Jun 2017 by 2:00:00 pm EDT to A residential ceiling fan is shown in the photograph below. It consists of an electric motor, the fan blades, and the light. Sketch each of these three items as parts of a block diagram and then indicate all energy transfer processes using arrows. Combine the three items into a single unit, sketch it, and then indicate all energy transfer processes (again using arrows).

2 HW-2: Due Friday 16 Jun 2017 by 2:00:00 pm EDT to Return to problem HW-1. Using your sketches label each of your energy transfer processes as either work, W, or heat transfer, Q. o For work indicate if it s electrical work, moving boundary work, rotating shaft work, or spring work. o For heat transfer locate the higher temperature point(s) and the lower temperature point(s).

3 HW-3: Due Friday 16 Jun 2017 by 2:00:00 pm EDT to When fully inflated, the hospital mattress shown below is 203 cm long, 91 cm wide, and 20.3 cm tall. Before inflation, the air pressure inside the bed is 1 bar and the volume 18.5 l. As the air expands, its pressure inside the bed and its volume follow the relationship pv 1.4 = C, where C is some constant. Calculate the final air volume. Report your answer in m 3. What quantity of boundary work is required to inflate the bed to its full volume. Report your answer in kj.

4 HW-4: Due Friday 16 Jun 2017 by 2:00:00 pm EDT to Residential garden hose comes in 3/8 (10 mm), ½ (13 mm), 5/8 (16 mm) and ¾ (19 mm) inside diameter. A 25 ft (7.62 m) length of 13 mm garden hose will flow 91 l/min when provided with a 370 kpa (40 psig) supply of water. Compute the water mass flow rate for the 7.62 m long-13 mm diameter case. Report you answer in kg/s. Compute the water velocity for the same case. Report your answer in m/s. If the water velocity for the 7.62 m long-19 mm diameter case was the same as that for the 7.62 m long-13 mm diameter case, what is the new mass flow rate? Report your answer in l/sec.

5 HW-7: Due Friday 23 Jun 2017 by 2:00:00 pm EDT A tank of water contains an electrical resistance heater and vaned stirrer to keep the water temperature uniform throughout. A torque of 5 N-m is required to rotate the stirrer at 20 rpm. A current of 2 A experiences a 10 V drop in potential as it flows through the heater. Compute the power delivered to the water. Report your answer in kw. HW-8: Due Friday 23 Jun 2017 by 2:00:00 pm EDT Sketch p- diagrams for the following processes undergone by a gas or vapor: constant volume constant pressure p 1.4 = constant p constant Will any of the process diagrams be different for either a liquid or a solid? HW-9: Due Friday 23 Jun 2017 by 2:00:00 pm EDT A high pressure-high temperature liquid can be made to flash boil by rapidly lowering its pressure. The pressure drop can be caused by flowing the liquid though a throttling valve, or other restriction. In the case of a perfect throttling valve, there is no energy lost from the flow during the flashing process. For an input temperature of 200 C and 20 bar, what will be the temperature when the pressure drops to 10 bar?

6 HW-5: Due Tuesday 20 Jun 2017 by 2:00:00 pm EDT Draw EFDs for the single unit defined at the bottom of HW-1, for the bed described in HW-3, and for the garden hose of HW-4. When constructing your EFDs clearly describe the system you chose, clearly mark the system boundary, and make certain you have considered all possible mass and energy flow terms. After drawing your EFDs identify which terms in the 1 st Law and CoM must be retained to perform the corresponding energy accounting. Be certain to list all assumptions you make when eliminating 1 st Law and/or CoM terms. HW-6: Due Tuesday 20 Jun 2017 by 2:00:00 pm EDT Use Gibb s Phase Rule to determine how many independent, intensive properties there are for: 1) Pure liquid having only one chemical component 2) Pure vapor having only one chemical component 3) Pure gas having only one chemical component 4) Liquid-vapor mixture of only one chemical component 5) Solid-liquid-vapor mixture of only one chemical component 6) A mixture of two chemically different gases

7 HW-10: Due Tuesday 27 June 2017 by 2:00:00 pm EDT A turbocharger like the one shown in Chapter 3 of the LectureBook has the following operating conditions: Rotational speed = 90,000 rpm Compressor inlet air is at 45 C and 1.0 bar Compressor pressure ratio = air mass flow rate of 0.25 kg/s Compressor air outlet temperature is 145 C Turbine inlet stream is at 900 C and 1.6 bar Turbine exhaust is stream is at 850 C and 1.1 bar Determine the heat transfer associated with this system. HW-11: Due Tuesday 27 June 2017 by 2:00:00 pm EDT A reciprocating compressor is one example of a piston-cylinder device. The Captain model ME-3 is a one-cylinder unit having a bore and stroke of 56.9 x 41.7 mm and operating at 720 rpm. Calculate the power required compress air from atmospheric pressure to 6.12 bars if the process follows p 1.4 = C, where C is some constant. How close is your result to the manufacturer stated value of 0.5 hp (1 hp = 746 W)? Does your agreement improve if the polytropic exponent is reduced to 1.2?

8 HW-12: Due Thursday 29 June 2017 by 2:00:00 pm EDT The heat exchanger from a desktop computer is shown in the photograph below. This device conducts thermal energy from a CPU chip along the six Cu arms. The Al fins in turn conduct energy from the Cu arms and are cooled by air flowing over them. The air stream has a flow rate of 0.1 m 3 /min, an inlet temperature of 30 C, and an exit temperature of 45 C. Al fins Cu arm Computer chip Air flow Compute the rate of heat transfer from the computer chip. HW-13: Due Thursday 23 June 2017 by 2:00:00 pm EDT A centrifugal compressor (rotating machinery) has air flowing in at 1.6 bar and 400 K. The outflow is 26 bar and 850 K. Compute the specific shaft power associated with this compression process.

9 HW-14: Due Wednesday 5 July 2017 by 2:00:00 pm EDT Patients undergoing anesthesia prior and during surgery often require an external means of breathing. Part of that apparatus is the bellows, which is often shown on television and in movies. See figure below. Bellows The bellows is nothing more than a piston-cylinder device, as shown in the next figure.

10 Bellows-piston/cylinder The piston-cylinder interacts with air/anesthesia gases being inhaled and exhaled by the patient in a cyclic process such as the one shown in the -p diagram below.

11 Your task is to compute the work associated with piston motion. do that you will need to approximate the respiration process curves. Start with a constant volume pressure rise from p=0 cm H 2 O until p=8 cm H 2 O, followed by a straight line until p=16 cm H 2 O. The straight line has the form p = v where is in ml and p in cm H 2 O. The last segment is polytropic with p = * where p and have the same units as for the straight line.

12 HW-15: Due Wednesday 5 July 2017 by 2:00:00 pm EDT Less industrially developed regions of the world often lack energy and communications infrastructure. The energy infrastructure is less of an issue if there is significant solar heating nearby. In that case, low capacity power plants can be created. Consider the situation shown below. Solar collector/boiler Pump Turbine Condenser Liquid water at 0.1 bar and 35 C enters the pump where it s compressed to 3 bar. It emerges from the solar collector/boiler at 3 bar and 160 C. The turbine exit condition is 0.1 bar and saturated vapor. Compute the energy interaction for each of the four components. Find the net specific power. Divide the net specific power by the solar collector/boiler specific heat transfer rate to get the cycle energy efficiency. What happens to solar energy that s not converted to work?

13 HW-18: Due Friday 14 July 2017 by 2:00:00 pm EDT Revisit HW-15 and determine the rate of entropy generation for each of the four components. The surroundings temperature is 40 C. HW-19: Due Friday 14 July 2017 by 2:00:00 pm EDT Revisit HW-17 and determine the isentropic efficiencies for the turbine and compressor. HW-20: Due Friday 14 July 2017 by 2:00:00 pm EDT We have often used p 1.4 = C to describe compression or expansion of a gas. Assume the gas is air and determine if the polytropic exponent value of 1.4 describes and isentropic process. accomplish this, assume an initial pressure (say 1 bar) and temperature (say 300 K) and a final pressure 10 times the initial one. Use the polytropic expression to calculate the final temperature, and then the ideal gas tables to compute s.

14 HW-16: Due Tuesday 11 July 2017 by 2:00:00 pm EDT Revisit HW-5 and HW-7 and use your EFDs to decide where the entropy generation is non-zero. Then determine the rate of entropy generation for HW-7. The surroundings temperature is the same as that for the heat exchanger outlet. HW-17: Due Tuesday 11 July 2017 by 2:00:00 pm EDT Revisit HW-10 and determine the entropy generation rate for the turbocharger as your system. You should assume the surrounding temperature is the same as that for the compressor inlet.

15 HW-21: Due Friday 21 July 2017 by 2:00:00 pm EDT Air enters a nozzle at a temperature of 800 K and a pressure 0f 10 bar. The exhaust pressure is 0.5 bar. What is the exhaust velocity for the isentropic case? What is the exhaust velocity if the nozzle isentropic efficiency is 92%? HW-22: Due Friday 21 July 2017 by 2:00:00 pm EDT Water enters a pump at 1 bar and 20 C. It s pressure is raised to 25 bar. What is the exit temperature if the water undergoes an isentropic process? Compute the corresponding stead yflow, reversible, mechanical work. HW-23: Due Friday 21 July 2017 by 2:00:00 pm EDT A Rankine cycle power plant has a pump whose isentropic efficiency is 75% and a turbine whose isentropic efficiency is 97%. The pump receives saturated liquid water at 0.1 bar. The boiler outlet stream is as 40 bar and 400 C. Compute the cycle energy efficiency and compare it one whose pump and turbine are isentropic.

16 HW-24: Due Tuesday 25 July 2017 by 2:00:00 pm EDT A Rankine cycle power plant comprises an 80% efficient pump, a boiler/reheater, two 94% efficient turbines, and a condenser. The pump inlet state is saturated liquid at 0.1 bar. The high pressure turbine inlet state is 40 bar and 600 C while the low pressure turbine sees steam at 25 bar and 560 C. Compute the cycle efficiency Sketch the processes on T-s and p-n diagrams Determine which piece of rotating machinery and which HXer have the highest specific entropy generation rates. HW-25: Due Tuesday 25 July 2017 by 2:00:00 pm EDT A vapor compression heat pump uses R-134a as the working fluid. The evaporator operates at 10 C and the condenser outlet temperature is 35 C. The compressor is 87% efficient. Compute the system CoP. Compare the specific entropy generation in the throttle with that in the compressor. What do you conclude?

17 HW-26: Due Thursday 27 July 2017 by 2:00:00 pm EDT An IC engine contains air that undergoes the following processes: 1. An isentropic compression starting at 1 bar and 300 K with a compression ratio of Constant volume heat addition until the temperature reaches 1600 K. 3. An isentropic expansion until the specific volume equals that of the starting state. 4. A constant volume heat rejection back to the initial pressure and temperature. Find the cycle thermal efficiency.