Chapter 5 Flashcards
Conservation of Mass
Mass, like energy, is a conserved property, and it cannot be created or destroyed during a process
Closed systems
The mass of the system remain constant during a process
Control volumes
Mass can cross the boundaries, and so we must keep tract of the amount of mass entering and leaving the control volume
The mass change due to energy change
Is negligible
The average speed through a cross section
Average velocity
Volume of fluid flowing through a cross section per unit time
Volume flow rate
The conservation of mass principle for a control volume
The net mass transfer to or from a control volume during a time interval (change in t) is equal to the net change (increase or decrease) in the total mass within the control volume during change in t
During a steady flow process, the total amount of mass contained within a control volume
Does not change with time (Mcv = constant)
The total amount of mass entering a control volume equal the total amount of mass leaving it
Single stream
Nozzles, diffusers, turbines, compressors, pumps
One inlet and one outlet
Steady flow of liquids - Incompressible flow
The volume flow rates, as well as the mass flow rates, remain constant since liquids are essentially incompressible substances
During a steady-flow process
Volume flow rates are not necessarily conserved although mass flow rates are
The work (or energy) required to push the mass into or out of the control volume. This work is necessary for maintaining a continuous flow through a control volume.
Flow work, flow energy
A process during which a fluid flows through a control volume steadily
Steady-flow process
Steady flow process - control volume
Mass and energy remain constant
Steady-flow process - fluid properties at an inlet or exit
Remain constant (do not change with time)
Under steady operation, ____ are the only forms of work a simple compressible system may involve
Shaft work and electrical work
Nozzles and diffusers are commonly utilized in
Jet engines, rockets, spacecraft, garden hoses
Nozzle
Device that increase the velocity of a fluid at the expense of pressure
Diffuser
Device that increase the pressure of a fluid by slowing it down
Cross-sectional area of a nozzle
Decreases in the flow direction for subsonic flows and increases for supersonic flows. Opposite for diffusers
Nozzles and diffusers are shaped so that they cause…
Large changes in fluid velocities and thus kinetic energies
Turbine
Drives the electric generator in steam, gas, or hydroelectric power plants
As fluid passes through the turbine…
Work is done against the blade, which are attached to the shaft. This, the shaft rotates, and the turbine produces work
Compressors, pumps, fans
Devices used to increase the pressure of a fluid
Work is supplied to these devices from an external source through a rotating shaft
Fan
Increases the pressure of a gas slightly and is mainly used to mobilize a gas
Compressor
Capable of compressing the gas to very high pressures
Pumps
Work very much like compressors except that they handle liquids instead of gases
Throttling valves
Are any kind of flow-restricting devices that cause a significant pressure drop in the fluid
Internal Energy + Flow Energy = Constant
Enthalpy = Constant
What is the difference between a turbine and a throttling valve?
The pressure drop in the fluid is often accompanied by a drop in temperature and for that reason throttling devices are commonly used in refrigeration and air-conditioning applications
The temperature of an ideal gas during a throttling process
Does not change
In engineering applications, the section where the mixing process takes place is commonly referred to as a ____
Mixing chamber
Conservation of mass principle for a mixing chamber
Requires that the sum of the incoming mass flow rates equal the mass flow rate of the outgoing mixture
Heat exchangers
Devices where two moving fluid streams exchange heat without mixing
Flow through a pipe or a duct usually satisfies the ____
Steady-flow conditions
Processes that change within the control volume with time
Transient-flow processes, unsteady processes
Most unsteady-flow processes can be represented reasonably well by ____ process
Uniform-flow
Uniform-flow process
The fluid flow at any inlet or exit is uniform and steady, and thus the fluid properties do not change with time or position over the cross section of an inlet or exit. If they do, they are averaged and treated as constants for the entire process
___ of a control volume may change during an unsteady-flow process
Shape and size
The energy equation of a uniform-flow system reduces to that of a _____ when all the inlets and exits are closed
Closed system
A uniform-flow system may involve…
Electrical, shaft, and boundary work all at once
