Thermodynamics- Mass and Energy Analysis Of Control Volumes

Question 1

What is the conservation of mass principle?

Answer 1

Mass, like energy, is a conserved property so cannot be created or destroyed during a process.

Question 2

What happens to mass in a closed system and control volumes?

Answer 2

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.

Question 3

Why does the velocity of a fluid flowing through a pipe vary across the cross-sectional area?

Answer 3

The velocity is lower at the walls of the pipe due to friction.

Question 4

Formula for average velocity of fluid flowing through a pipe

Answer 4

1/CSA times integral of normal velocity with respect to CSA from 0 to CSA.

Question 5

Formula for mass flow rate

Answer 5

Density times average velocity times CSA (Ac)

Question 6

Formula for volumetric flow rate

Answer 6

Integral of normal velocity with respect to CSA from 0 to CSA. Basically CSA times average velocity.

Question 7

Relation between mass flow rate and volumetric flow rate

Answer 7

Mass flow rate is density times volumetric flow rate

Question 8

During a steady-flow process does the total amount of mass contained in a control volume change with time?

Answer 8

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

Question 9

Is there a conservation of volume principle?

Answer 9

No

Question 10

When is volumes flow rate conserved?

Answer 10

For a steady flow of liquids because they are essentially incompressible.

Question 11

What is flow work/energy?

Answer 11

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.

Question 12

Formula for flow work

Answer 12

Pressure times CSA times distance moved. Equal to pressure times volume of fluid moving for kJ.

Question 13

What is the energy of a non-flowing fluid (e)?

Answer 13

The internal energy plus kinetic and potential energy

Question 14

What is the energy of a flowing fluid (θ)?

Answer 14

Flow energy plus internal energy plus kinetic and potential energy.
Flow energy plus internal energy is enthalpy.

Question 15

Formula for rate of energy transport for a flowing fluid

Answer 15

Mass flow rate times θ. When kinetic and potential energies are negligible is just mass flow rate times enthalpy.

Question 16

Overall energy balance equation.

Answer 16

Q•(net in) plus Σm•(θ1) = W•(net out) plus Σm•(θ2)

Question 17

What is a nozzle and what does it do?

Answer 17

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.

Question 18

What is a diffuser and what does it do?

Answer 18

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.

Question 19

What does a compressor do.

Answer 19

It can compress the gas to very high pressures. Work is supplied to it from an external source through a rotating shaft.

Question 20

What does a fan do?

Answer 20

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.

Question 21

What does a pump do?

Answer 21

Works like a compressor for liquids instead of gases. Work is supplied to it from an external source through a rotating shaft.

Question 22

How does a turbine work?

Answer 22

As the fluid passes through the turbine, the turbine produces shaft work.