ES 202 Fluid and Thermal Systems Lecture 7: Mechanical Energy Balance (12/16/2002) - PowerPoint PPT Presentation

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ES 202 Fluid and Thermal Systems Lecture 7: Mechanical Energy Balance (12/16/2002)

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Compare the hydrostatic forces acted on Surface AB (not the bottom of the tank) ... (multiple rotor-stator stage) Steady-State Devices (cont'd) ... – PowerPoint PPT presentation

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Title: ES 202 Fluid and Thermal Systems Lecture 7: Mechanical Energy Balance (12/16/2002)


1
ES 202Fluid and Thermal SystemsLecture
7Mechanical Energy Balance (12/16/2002)
2
Assignments
  • Reading
  • Cengel Turner Section 11-4
  • ES 201 notes
  • Homework
  • 11-5, 11-6 in Cengel Turner
  • notion of combined efficiency

3
Road Map of Lecture 7
  • Final attempt on curved surfaces
  • Steady state devices
  • revisit energy and entropy equation
  • nozzle, diffuser, turbine, compressor, heat
    exchanger
  • function
  • design assumption
  • modeling assumption
  • Close examination of energy equation
  • means of transport
  • zero production
  • mechanical energy vs thermal energy
  • flow work, kinetic energy, potential energy
  • Bernoullis equation

4
Final Attempt on Curved Surfaces
  • Compare the hydrostatic forces acted on Surface
    AB (not the bottom of the tank) in the following
    configurations

5
Major Conclusions
  • For inclined submerged surfaces (plane or curved)
    with same end points
  • total horizontal force is the same
  • total vertical force differs (depending on the
    weight of fluid above/below the surface)

6
End of Hydrostatics
7
Revisit Energy Equation
  • Mean of transport
  • heat transport
  • work transport
  • mass transport
  • Enthalpy h u p / r consists of internal
    energy and flow work (Do not double count flow
    work in Wout !)
  • Internal energy is a measure of molecular
    activities at the microscopic level (strongly
    dependent on temperature) while kinetic and
    potential energies are measures of bulk fluid
    motion

8
Revisit Entropy Equation
  • Mean of transport
  • heat transport
  • mass transport
  • There is no entropy transport associated with
    work, i.e. work transport of energy is
    entropy-free. This is the major difference
    between the two energy transfer modes work and
    heat. Work is better!
  • Entropy production is always non-negative!

9
Steady-State Devices
  • List the purpose (function) for the following
    devices
  • nozzle
  • diffuser
  • turbine
  • pump, compressor, blower, fan
  • heat exchanger

10
Turbine
  • Steam turbine
  • Water turbine (hydro-electricity)
  • Wind turbine (hill slopes)
  • Gas turbine engine
  • compressor
  • combustor
  • turbine
  • good power to weight
  • ratio

(multiple rotor-stator stage)
11
Steady-State Devices (contd)
  • What does the energy equation reduce to for the
    following devices
  • nozzle
  • diffuser
  • turbine
  • compressor, fan, blower, pump
  • heat exchanger

12
Close Examination of Energy Equation
  • Energy equation again
  • Energy components
  • Components of mechanical energy
  • flow work (pressure energy in C T)
  • kinetic energy
  • potential energy
  • Thermal energy
  • thermodynamic property u

13
Mechanical Energy Vs Thermal Energy
  • Mechanical energy vs thermal energy
  • mechanical energy can freely change its form
    among various components
  • mechanical energy can be converted to work
    completely (without loss) if the system is
    reversible
  • example spring-mass system in simple harmonic
    motion
  • thermal energy cannot be converted to work
    completely (the second law of thermodynamics
    imposed limitation to the conversion)
  • example spring-mass system under influence of
    friction
  • the first law of thermodynamics (conservation of
    energy) does not differentiate the different
    forms of energy but the second law does
  • mechanical energy is a higher quality form of
    energy

14
Energy Equation in Steady State
  • Assumptions
  • steady
  • adiabatic
  • no shaft work or friction
  • small changes in thermal energy relative to
    mechanical energy (good for low speed flows)
  • Conservation of mechanical energy
  • Interpretation interchange of mechanical energy
    among its various forms

15
Bernoullis Equation
  • Traditional derivation is based on momentum
    equation
  • Warning Its simplicity may often lead to
    incorrect application
  • Remember the assumptions (limitations)
  • steady
  • no shaft work or friction
  • small change in thermal energy
  • constant density
  • along flow direction
  • Examples application to nozzle and diffuser
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