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ES 202 Fluid and Thermal Systems Lecture 8: Application of Bernoulli’s Equation (12/17/2002)PowerPoint Presentation

ES 202 Fluid and Thermal Systems Lecture 8: Application of Bernoulli’s Equation (12/17/2002)

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ES 202 Fluid and Thermal Systems Lecture 8: Application of Bernoulli’s Equation (12/17/2002). Assignments. Reading: Cengel & Turner Section 12-1, 12-2, 9-4 Homework: 11-43, 11-45, 11-63, 11-70 in Cengel & Turner. Road Map of Lecture 8. Quiz Review on Lecture 7

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### ES 202Fluid and Thermal SystemsLecture 8:Application of Bernoulli’s Equation (12/17/2002)

Assignments

- Reading:
- Cengel & Turner Section 12-1, 12-2, 9-4

- Homework:
- 11-43, 11-45, 11-63, 11-70 in Cengel & Turner

Road Map of Lecture 8

- Quiz
- Review on Lecture 7
- Some insights into Bernoulli’s equation
- Examples and applications
- Pitot-static tube
- Lift on airfoil, tennis ball

- Modified Bernoulli’s equation

- True/False When a fluid is in static equilibrium, the only force acting on the fluid is pressure. Explain your answer.
- Explain with clarity the cause of buoyancy force on an object immersed in a fluid

Energy carried by a fluid element can be classified into:

mechanical energy

thermal energy

List the difference(s) between mechanical and thermal energy:

mechanical energy can freely change its form among various components

mechanical energy can be converted to workcompletely

thermal energy cannot be converted to work completely

In what form(s) can mechanical energy exist?

flow work ( P / r )

kinetic energy ( V2 / 2 )

potential energy ( g z )

Review on Lecture 7Mechanical

Energy

Mechanical

Work

Thermal

Energy

Review on Lecture 7 (cont’d)

- Write down the (steady) Bernoulli’s equation. Describe your interpretation of the equation.
- mechanical energy can be freely interchanged among its various forms as the fluid element moves along its path

- Name some assumption(s) behind the Bernoulli’s equation.
- steady
- no shaft work or friction
- small change in thermal energy
- constant density
- along flow direction

Some Insights

- From statics to dynamics
- Similarity and difference between solid and fluid
- in addition to K.E. and P.E.,
there is a flow work component

in a fluid system

- in addition to K.E. and P.E.,
- In compressible flow, there is interchange between thermal and mechanical energy (appreciable temperature change)

Different Forms of Bernoulli’s Equation

- Pressure form
- definition of stagnation pressure

- “Head” form (dimension of length)

Pressure Variation Along Flow Direction

- Consider the following convergent flow device:

air

air

d1

d1

d2

d2

Dh

Dh

Picture (a)

Picture (b)

- Which configuration represents the correct physics? Explain your choice.
- If d1 = 10 cm, d2 = 5 cm and air is going at 0.1 m3/sec, determine the value of Dh.
- What kind of flow device is it?
- If the flow direction is reversed, what do you expect to be the difference?

- Principle behind a Pitot-Static tube:

V

d

Dh

A

B

- fluid column A measures the stagnation pressure (why?)
- fluid column B measures the static pressure (why?)
- according to the Bernoulli’s equation, their difference equals the dynamic pressure:

Lift on Airfoil and Tennis Ball

- Airfoil
- destroy the flow symmetry between the lower and upper surfaces
- show visualization

- Spin on a tennis ball
- what does the spin do to the tennis ball?

“Modified” Bernoulli’s Equation

- What if fluid friction causes some losses in the system, can I still apply the Bernoulli’s equation?
- Recall the “conservation of energy” concept from which we approach the Bernoulli’s equation
- Remedy: introduce a “head loss” factor

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