Thermodynamics- Mass and Energy Analysis Of Control Volumes Flashcards
What is the conservation of mass principle?
Mass, like energy, is a conserved property so cannot be created or destroyed during a process.
What happens to mass in a closed system and control volumes?
In a closed system, the mass of the system remains constant during a process.in a control volume, mass can cross the boundaries so we must keep track of the amount of mass entering and leaving the control volume.
Why does the velocity of a fluid flowing through a pipe vary across the cross-sectional area?
The velocity is lower at the walls of the pipe due to friction.
Formula for average velocity of fluid flowing through a pipe
1/CSA times integral of normal velocity with respect to CSA from 0 to CSA.
Formula for mass flow rate
Density times average velocity times CSA (Ac)
Formula for volumetric flow rate
Integral of normal velocity with respect to CSA from 0 to CSA. Basically CSA times average velocity.
Relation between mass flow rate and volumetric flow rate
Mass flow rate is density times volumetric flow rate
During a steady-flow process does the total amount of mass contained in a control volume change with time?
No because the system is at a steady state. For an unsteady flow then mass flow rate 2 minus mass flow rate 1 is dm/dt
Is there a conservation of volume principle?
No
When is volumes flow rate conserved?
For a steady flow of liquids because they are essentially incompressible.
What is flow work/energy?
The work (or energy) required to push the mass into or out of the control volume. The work is necessary for maintaining a continuous flow through a control volume.
Formula for flow work
Pressure times CSA times distance moved. Equal to pressure times volume of fluid moving for kJ.
What is the energy of a non-flowing fluid (e)?
The internal energy plus kinetic and potential energy
What is the energy of a flowing fluid (θ)?
Flow energy plus internal energy plus kinetic and potential energy.
Flow energy plus internal energy is enthalpy.
Formula for rate of energy transport for a flowing fluid
Mass flow rate times θ. When kinetic and potential energies are negligible is just mass flow rate times enthalpy.
Overall energy balance equation.
Q•(net in) plus Σm•(θ1) = W•(net out) plus Σm•(θ2)
What is a nozzle and what does it do?
A device that increases the fluid velocity at the expense of pressure. Entry CSA is greater than exit CSA for subsonic flows (opposite for supersonic flows). The most important change in in energy is in kinetic energy.
What is a diffuser and what does it do?
A device that increases the pressure of a fluid by slowing it down. Entry CSA is less than exit CSA for subsonic flows (opposite for supersonic flows). The most important change in in energy is in kinetic energy.
What does a compressor do.
It can compress the gas to very high pressures. Work is supplied to it from an external source through a rotating shaft.
What does a fan do?
Increases the pressure of a gas slightly and is mainly used to mobilise a gas. Work is supplied to it from an external source through a rotating shaft.
What does a pump do?
Works like a compressor for liquids instead of gases. Work is supplied to it from an external source through a rotating shaft.
How does a turbine work?
As the fluid passes through the turbine, the turbine produces shaft work.
