Homework-4

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University Of Arizona *

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230

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Mechanical Engineering

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Feb 20, 2024

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Thermodynamics - AME 230 Student name : Date : Homework Assignment No 4 I. Indicate whether the following statements are true or false 1. Mass flux is the time rate of mass flow per unit area. A: True B: False 2. For steady-state operation all properties are unchanging in time. A: True B: False 3. Mass flow rate is the product of density, area, and velocity for 1D flow. A: True B: False 4. As velocity decreases in a diffuser, pressure decreases. A: True B: False 5. Volumetric flow rate is expressed in units of kg/s. A: True B: False 6. At steady state, conservation of mass asserts the total rate at which mass enters the control volume equals the total rate at which mass exits. A: True B: False 7. A mixing chamber is a direct-contact heat exchanger A: True B: False 8. As velocity decreases in a diffuser, pressure decreases. A: True B: False 9. For a one-inlet, one-exit control volume at steady state, the mass flow rates at the inlet and exit are equal but the inlet and exit volumetric flow rates may not be equal. A: True B: False 10. Mass flow rate is the time rate of change of mass within the control volume. 1

Thermodynamics - AME 230 A: True B: False 11. The mass rate balance is defined as the sum of all inlet mass flow rates minus the sum of all outlet mass flow rates. A: True B: False 12. For one-dimensional flow, mass flow rate is the product of density, area, and velocity. A: True B: False 13. As velocity decreases in a diffuser, pressure decreases. A: True B: False 14. Flow work is the work done on a flowing stream by a paddle wheel or piston. A: True B: False II. Checking understanding 1. The time rate of mass flow per unit area is called Answer: 2. _________-________means all properties are unchanging in time. 3. A system consists of liquid water in equilibrium with a gaseous mixture of air and water vapor. How many phases are present? Does the system consist of a pure substance? Explain. Answer: 4. Does the red line in the picture below follow a constant pressure? 2

Thermodynamics - AME 230 Answer: 5. A device in which power is developed as a result of a gas or liquid passing through a set of blades attached to a shaft free to rotate. Answer: 6. A device in which work is done on a gas to increase the pressure and/or elevation. Answer: 7. A device in which work is done on a liquid to increase the pressure and/or elevation. Answer: 8. A flow passage of varying cross-sectional area in which the velocity of a gas or liquid increases in the direction of flow. Answer: 9. A flow passage of varying cross-sectional area in which the velocity of a gas or liquid decreases in the direction of flow. Answer: 10. A device where two moving fluid streams exchange heat without mixing. Answer: 11. A flow-restricting device that causes a significant pressure drop in the fluid. Answer: 3

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Thermodynamics - AME 230 III. Steam enters a turbine operating at steady state with a mass flow rate of 10 kg/min, a specific enthalpy of 3100 kJ/kg, and a velocity of 30 m/s. At the exit, the specific enthalpy is 2300 kJ/kg and the velocity is 45 m/s. The elevation of the inlet is 3 m higher than at the exit. Heat transfer from the turbine to the surroundings occurs at a rate of 1.1 kJ per kg of steam flowing. Let g = 9.81 m/s 2 . Determine the power developed by the turbine, in kW. (Please, use problem-solving techniques for this problem, KNOWN, FIND,…) 4

Thermodynamics - AME 230 IV. Separate streams of air and water flow through the compressor and heat exchanger arrangement shown in figure below. Steady-state operating data are provided in the figure. Heat transfer with the surroundings can be neglected, as can all kinetic and potential energy effects. The air is modeled as an ideal gas. The water can be assumed incompressible, so h ≈ h f (T) . Determine a. the total power required by both compressors, in kW. b. the mass flow rate of the water, in kg/s. (Please, use problem-solving techniques for this problem, KNOWN, FIND,…) Hint: You need to use Table A-22 to find h 1 , h 2 , h 3 , and h 4 . You need to use Table A-2 to find h 5 ≈ h f5 and h 6 ≈ h f6 5

Thermodynamics - AME 230 6

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