CHAPTER 3 Energy Transfer by Mass Flow Work or Energy (energy needed to maintain continuous flow)

CHAPTER

3

Energy Transfer byMass

Flow Work or Energy

(energy needed to maintain continuous flow)

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FIGURE 3-48Schematic for flow work.

W=F∙L=PA∙L=P∙Vso w = Pv

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FIGURE 3-51The total energy consists of three parts for a nonflowing fluid and four parts for a flowing fluid.

flow energy, Pv, is part of the energy of a flowing fluid.

enthalpy, h

Usually ignored (small)

so energy transferred into a system by a stream is mθ . Rate?

Mass flow

• Most can be considered steady flow and one dimensional

• So use:

gzV

h2

2

Heat Transfer3 Types

• Conduction– Microscopic work

– Follows Fourier’s Law:

– kt is thermal conductivity

)(Wx

TAkQ tcond

Heat Transfer

• Convection– Between surface and fluid

– Must have conduction also

– h is convection heat transfer coefficient

– experimentally determined and depends on conditions

)()( WTThAQ fsconv

h = 25 – 250 W/m2∙K h = 2 – 25 W/m2∙K

Heat Transfer

• Radiation – needs no medium– can happen when two bodies

are even separated by a cooler medium.

– Kirchhoff’s Law• emissivity = absorptivity

– Good adsorber is good emitter

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FIGURE 3-58The absorption of radiation incident onan opaque surface of absorptivity .

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