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7/28/2019 Heat Transfer- Lecture 07_17 July
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LECTURE 7Natural Convection
Inside Enclosures
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Lesson Outcomes
Understand natural convection inside enclosures
Understand the correlation at different arrangement (vertical,
horizontal and inclined)
Solve problems on natural convection inside enclosures
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At the end of this lecture, students should be able to :
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Generally, heat loss in typical residence occursthrough the windows.
Air is transparent, and considered a better insulator
than most common insulating materials.
Basically, a pair of glass is used to trap air which is
known as enclosure such as double pane window
or double glazed window
The most common examples of enclosures include
solar collectors and concentric cylinders/spheres.
Natural Convection Inside Enclosures
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Aspect ratio = H/L
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A simple example enclosures - a solar panel that is a closedbox with a top made of a transparent material such as glass or
plastic
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Heat energy passes through the glass and heats up inside of thebox. The heat that is collected inside the box can then be used
for several purposes - to heat air or water.
Natural Convection Inside Enclosures
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To heat the water, heating coil/pipe is installed inside thebox. Heat energy is transferred to the coil/pipe and
transferred to the water.
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Natural Convection Inside Enclosures
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A rotary motion of convection current due to temperature and
density differences enhances the rate of heat transfer through invertical enclosure
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Hotter plate at the bottom, heavier fluid will be on top of thelighter fluid, and there will be a tendency for the lighter fluid to
topple the heavier fluid & rise to the top
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Hotter plate on top, no convection currents will develop in the
enclosure since the lighter fluid is always on top.
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The rate of heat transfer through enclosure :
Conduction heat transfer :
Convection heat transfer inside enclosure is analogy ofconduction across fluid layer, kis replaced by kNu whichknown as effective thermal conductivity k
eff.
)()( 2121
TTkNuATThAQ ss
Natural Convection Inside Enclosures
)( 21 TTAkQ seff
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Aspect ratio = H/L
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The Rayleigh Number (Ra), for an enclosure isdetermined from :
= distance between the hot and cold surfaces
T1 and T2 : temperatures of hot and cold surfaces
Fluid properties are evaluated at average temperature
Natural Convection Inside Enclosures
Pr)(
PrRa2
3
21
TTgGr
Tavg = (T1 + T2) / 2