1. Ch. 4 The first law of thermodynamics: Control Volume

2. 4.1 Thermodynamic analysis of control volumes
Control Surface
Steady: No change with time
 Unsteady, Transient
Uniform: No change with location

3. Thermodynamic analysis of control volumes
Conservation of Mass Principle
Mass and Volume Flow Rates
Mass Flow Rate: Amount of mass flowing through a cross section per unit time
Total
mass entering CV
mass leaving
Net change
In mass within - =

4. Mass Flow Rate
It is proportional to the cross-sectional area A, the density , the velocity V of the fluid.
Vn: Velocity component normal to dA
Vav : Average fluid velocity normal to A

5. Volume Flow Rate
Volume of the fluid flowing through a cross-section per unit time
The mass and volume flow rate:

6. Conservation of Energy Principle
In closed system
Net heat(Q) – Work(W) = Energy change(E)
When mass enters or leaves a control volume, the energy increase or decrease within the control volume.

7. Conservation of Energy Principle
The conservation of energy equation for a control volume
The energy required ot push fluid into or out of a control volume is called the flow work, or flow energy.

8. Flow Work
If the fluid pressure is P and the cross-sectional area of the fluid element A, the force applied on the fluid element by the imaginary piston F = PA.
The work done in pushing the fluid element
Wflow = FL = PAL =PV

9. Total Energy of a Flowing Fluid
Three Parts of total energy of a simple compressible system: Internal, Kinetic, and Potential energies.
On a unit mass basis,
Total energy of a flowing fluid on a unit mass basis ()

10. Total Energy of a Flowing Fluid
By the definition of enthalpy (h = u + Pv):
: Methalphy Metaenthalpy, which means beyond enthalphy

11. Steady Flow Process
Process during which a fluid flows through a control volume steadily
Steady flow devices: Turbines, compressors, and nozzles operating for long periods of time under the same conditions
Steady means no change with time.

12. Steady Flow Process
No properties (volume, mass, total energy) within the control volume change with time. The boundary work is zero for steady flow systems since Vcv is constant.
No properties change at the boundaries of the control volume with time. The mass flow rate of the fluid at an opening must remain constant during a steady flow process.
The heat and work interactions between a steady flow system and its surrounding do not change with time

13. Conservation of Mass
During a steady flow process, the total amount of mass in CV does not change with time. (Entering Mass = Leaving Mass)

14. Conservation of Energy
Water Heater under steady operation:
The water stream will experience an increase in its total energy as it flows through the water heater, which is equal to the electric energy supplied to the water minus the heat losses.

15. Conservation of Energy
 = h + ke + pe

16. Single-Stream Steady-Flow Systems
One inlet and one exit
Mass flow rate through the entire CV remains constant
For unit-mass basis

17. Single-Stream Steady-Flow SystemsIf
Then
Rate of heat transfer b/w CV and its surroundings
= Power (work done per unit time)

18. Some Steady-Flow Engineering Device
Nozzles and Diffusers
Jet engines, rockets, spacecraft, garden hoses
Nozzle: Device to increase the velocity of a fluid
Diffuser: Device to increase the pressure of a fluid by slowing it down.

19. Some Steady-Flow Engineering Device
Turbines
Steam, gas, hydroelectric power plants
As the fluid passes through the turbine, work is done against the blades which are attached to the shaft.
Compressors
Device to increase the pressure of a fluid

20. Some Steady-Flow Engineering Device
Throttling Valves
Adjustable valves, capillary tubes, porous plugs
Any kind of flow-restricting devices that cause a significant pressure drop in the fluid.

21. Some Steady-Flow Engineering Device
Heat Exchangers
Devices to exchange heat with mixing of two moving fluid streams.
Double-tube (tube and shell) heat exchanger

22. Some Steady-Flow Engineering Device
Pipe and Duct Flow
Transport of liquids or gases
Flow through a pipe or a duct usually satisfies the steady flow conditions except the transient start-up and shut-down periods