heat transfer- lecture 07_17 july

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.



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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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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


)( 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


Pr)(

PrRa2

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TTgGr

Tavg = (T1 + T2) / 2

heat transfer- lecture 07_17 july

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