Flashcards in Test 3 Deck (40)
5–1C When is the flow through a control volume steady?
5-1C Flow through a control volume is steady when it involves no changes with time at any specified position.
5–2C Define mass and volume flow rates. How are they
related to each other?
5-2C Mass flow rate is the amount of mass flowing through a cross-section per unit time whereas the volume flow rate is the amount of volume flowing through a cross-section per unit time.
5–3C Does the amount of mass entering a control volume
have to be equal to the amount of mass leaving during an
5-3C The amount of mass or energy entering a control volume does not have to be equal to the amount of mass or energy leaving during an unsteady-flow process.
5–4C Consider a device with one inlet and one outlet. If the
volume flow rates at the inlet and at the outlet are the same,
is the flow through this device necessarily steady? Why?
5-4C No, a flow with the same volume flow rate at the inlet and the exit is not necessarily steady (unless the density is
constant). To be steady, the mass flow rate through the device must remain constant.
5–17C What is flow energy? Do fluids at rest possess any
5-17C Flow energy or flow work is the energy needed to push a fluid into or out of a control volume. Fluids at rest do not possess any flow energy.
5–18C How do the energies of a flowing fluid and a fluid
at rest compare? Name the specific forms of energy associated
with each case.
5-18C Flowing fluids possess flow energy in addition to the forms of energy a fluid at rest possesses. The total energy of a fluid at rest consists of internal, kinetic, and potential energies. The total energy of a flowing fluid consists of internal, kinetic, potential, and flow energies.
5–23C A diffuser is an adiabatic device that decreases the
kinetic energy of the fluid by slowing it down. What happens
to this lost kinetic energy?
5-23C It is mostly converted to internal energy as shown by a rise in the fluid temperature.
5–24C The kinetic energy of a fluid increases as it is accelerated
in an adiabatic nozzle. Where does this energy come from?
5-24C The kinetic energy of a fluid increases at the expense of the internal energy as evidenced by a decrease in the fluid temperature.
5–40C Consider an air compressor operating steadily. How
would you compare the volume flow rates of the air at the
compressor inlet and exit?
5-40C The volume flow rate at the compressor inlet will be greater than that at the compressor exit.
5–41C Will the temperature of air rise as it is compressed
by an adiabatic compressor? Why?
5-41C Yes. Because energy (in the form of shaft work) is being added to the air.
5–42C Somebody proposes the following system to cool a
house in the summer: Compress the regular outdoor air, let it cool back to the outdoor temperature, pass it through a turbine, and discharge the cold air leaving the turbine into the house. From a thermodynamic point of view, is the proposed system sound?
5–58C Why are throttling devices commonly used in refrigeration and air-conditioning applications?
5-58C Because usually there is a large temperature drop associated with the throttling process.
5–59C Would you expect the temperature of air to drop as it undergoes a steady-flow throttling process? Explain.
5-59C No. Because air is an ideal gas and h = h(T) for ideal gases. Thus if h remains constant, so does the temperature.
5–60C Would you expect the temperature of a liquid to
change as it is throttled? Explain
5-60C If it remains in the liquid phase, no. But if some of the liquid vaporizes during throttling, then yes.
5–61C During a throttling process, the temperature of a
fluid drops from 30 to 2208C. Can this process occur adiabatically?
5–68C Consider a steady-flow mixing process. Under what conditions will the energy transported into the control volume
by the incoming streams be equal to the energy transported out of it by the outgoing stream?
5-68C Under the conditions of no heat and work interactions between the mixing chamber and the surrounding medium.
5–69C Consider a steady-flow heat exchanger involving
two different fluid streams. Under what conditions will the
amount of heat lost by one fluid be equal to the amount of
heat gained by the other?
5-69C Under the conditions of no heat and work interactions between the heat exchanger and the surrounding medium.
5–70C When two fluid streams are mixed in a mixing
chamber, can the mixture temperature be lower than the temperature of both streams? Explain.
5-70C Yes, if the mixing chamber is losing heat to the surrounding medium.
6–1C Describe an imaginary process that violates both the
first and the second laws of thermodynamics.
6-1C Transferring 5 kWh of heat to an electric resistance wire and producing 6 kWh of electricity.
6–2C Describe an imaginary process that satisfies the first
law but violates the second law of thermodynamics.
6-2C Transferring 5 kWh of heat to an electric resistance wire and producing 5 kWh of electricity.
6–3C Describe an imaginary process that satisfies the second law but violates the first law of thermodynamics.
6-3C An electric resistance heater which consumes 5 kWh of electricity and supplies 6 kWh of heat to a room.
6–4C An experimentalist claims to have raised the temperature of a small amount of water to 1508C by transferring heat from high-pressure steam at 1208C. Is this a reasonable claim? Why? Assume no refrigerator or heat pump is used in the process.
6-4C No. Heat cannot flow from a low-temperature medium to a higher temperature medium.
6–5C What is a thermal energy reservoir? Give some examples
6-5C A thermal-energy reservoir is a body that can supply or absorb finite quantities of heat isothermally. Some examples are the oceans, the lakes, and the atmosphere.
6–6C Consider the process of baking potatoes in a conventional oven. Can the hot air in the oven be treated as a thermal energy reservoir? Explain.
6-6C Yes. Because the temperature of the oven remains constant no matter how much heat is transferred to the potatoes.
6–7C What are the characteristics of all heat engines?
6-7C Heat engines are cyclic devices that receive heat from a source, convert some of it to work, and reject the rest to a sink.
6–8C What is the Kelvin–Planck expression of the second
law of thermodynamics?
6-8C It is expressed as "No heat engine can exchange heat with a single reservoir, and produce an equivalent amount of work".
6–9C Is it possible for a heat engine to operate without
rejecting any waste heat to a low-temperature reservoir?
6-9C No. Such an engine violates the Kelvin-Planck statement of the second law of thermodynamics.
6–10C Baseboard heaters are basically electric resistance
heaters and are frequently used in space heating. A home
owner claims that her 5-year-old baseboard heaters have a conversion efficiency of 100 percent. Is this claim in violation of any thermodynamic laws? Explain.
6-10C No. Because 100% of the work can be converted to heat.
6–11C Does a heat engine that has a thermal efficiency of
100 percent necessarily violate (a) the first law and (b) the
second law of thermodynamics? Explain.
6-11C (a) No, (b) Yes. According to the second law, no heat engine can have and efficiency of 100%.