quiz 4

Conservation of mass

Mass cannot be created or destroyed during a process

Closed systems and mass

mass of a system remains constant during a process

control volumes and mass

mass can cross the boundaries, so we must keep track of the amount of mass entering and leaving the control volume

mass during steady flow process

During a steady-flow process, the total amount of mass contained within a

control volume does not change with time (m_CV= constant).

5 engineering devices that involve a single stream (One inlet and one outlet)

1. nozzles

2. diffusers

3. turbines

4. compressors

5. pumps

Flow Work/Energy

The work (or energy) required to push the mass into or out of the control volume

Flow work/energy is necessary for

maintaining a continuous flow through a

control volume.

Steady-flow process

A process during

which a fluid flows through a control

volume steadily.

Many engineering systems (e.g. steam

power plant) work at

steady-state

steady-flow (SSSF)

Steady state

mass and energy of a control volume remains constant with time

Steady flow

Fluid properties at inlet or exit remain constant with time

shaft work and electrical work:

are the only forms of work a simple compressible system may involve. (No moving boundary work)

Kinetic energy of a fluid at very high velocities

even small changes can cause significant changes in the kinetic energy of the fluid

steady flow devices

engineering devices that operate essentially under the same conditions for long periods of time

4 examples of steady flow devices

1. turbines

2. compressors

3. heat exchangers

4. pumps

nozzle

device that increases the

velocity of a fluid at the expense of

pressure.

diffuser

device that increases the

pressure of a fluid by slowing it down.

nozzle and diffuser in subsonic and supersonic flows

The cross-sectional area of a nozzle

decreases in the flow direction for

subsonic flows and increases for

supersonic flows. The reverse is true for

diffusers.

turbine

drives the electric generator In

steam, gas, or hydroelectric power plants.

• As the fluid passes through the turbine,

  work is done against the blades, which are

  attached to the shaft. As a result, the shaft

  rotates, and the turbine produces work.

3 devices that are used to increase pressure of a fluid and how

1. compressors

2. pumps

3. fans

Work is supplied to these devices from an external source thru 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

capable of compressing the

liquid to very high pressures.

Throttling valves

any kind of flow-restricting devices that cause a significant

pressure drop in the fluid.

turbine vs throttling valce

throttling devices are commonly used in refrigeration and air-

conditioning applications.

mixing chamber

the section

where the mixing process takes place

The conservation of mass principle for a

mixing chamber requires

the sum of

the incoming mass flow rates equal the

mass flow rate of the outgoing mixture.