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CHAPTER
3
Energy Transfer byMass
Flow Work or Energy
(energy needed to maintain continuous flow)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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FIGURE 3-48Schematic for flow work.
W=F∙L=PA∙L=P∙Vso w = Pv
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
FIGURE 3-58The absorption of radiation incident onan opaque surface of absorptivity .
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