CONVECTIVE HEAT TRANSFER 1-1.pdfThe value of convective heat transfer coefficient (as well as temp...

CONVECTIVE CONVECTIVE

HEAT TRANSFER HEAT TRANSFER

By: Prof K. M. Joshi,By: Prof K. M. Joshi,AssiAssi. Professor, MED,. Professor, MED,

SSAS Institute of Technology, Surat.SSAS Institute of Technology, Surat.

MECHANISM BEHIND FREE /NATURAL CONVECTION

The stagnate layer of fluid in

immediate vicinity of hot body

receives heat energy by

conduction. The energy transfer by

this, increases temperature and

internal energy of fluid particles.

Warmer air

rising

AIR.

Q

Because of temperature rise particles become less dance and hence lighter.

The lighter particles move upward in low temperature region.

These particles mix with the cool particles and transfer a part of energy.

Simultaneously cool heavier particles moves downward to fill the space

vacated by warm particles

The circulation pattern, upward movement of hot particles and downward

movement of cool particles causes convective currents. The se currents are

set up naturally due to gravitational force only.

MECHANISM BEHIND FORCED CONVECTION

AIR

20°C

5 m/s

20°C.

Q

Fluid flow causing by the pump, fan or atmospheric wind…

Therefore the rate of heat transfer is much higher by forced convection

than it is by the natural convection or by conduction.

In fact, the higher the fluid velocity, the higher the rate of heat transfer.

EXAMPLES OF NATURAL AND FORCED CONVECTION

�Design of house heating, furnace, architectural

projects, roads and concrete structures concerns with the

free convection.

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�Cooling in IC engines, air condition installations,

temperature control in nuclear plant, condenser tubes or

other heat exchangers are example of forced convection.

Convection heat transfer strongly depends on ….

� fluid properties dynamic viscosity , thermal conductivity k, density

and specific heat

� fluid velocity V

� Geometry and the roughness of the solid surface

� Type of fluid flow (such as being laminar or turbulent).

(((( ))))conv s sQ hA T T∞∞∞∞= −= −= −= −&&&&NEWTON’S LAW OF COOLING

(((( ))))conv s s ∞∞∞∞h = Convection heat transfer coefficient

As = Heat transfer surface area

Ts = Temperature of the surface

T∞∞∞∞= Temperature of the fluid sufficiently far from the surface

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Heat flux, qconv =

The value of convective heat transfer coefficient (as well as

temp difference may also) is not constant for entire surface it

depends on location there for we can define

Local and total convection transfer (a) Surface of arbitrary shape. (b) Flat plate.

LOCAL HEAT FLUX convq′′

(((( ))))conv l sq h T T∞∞∞∞′′′′′′′′ = −= −= −= −

hl is the local convection coefficient

Local and total convection transfer (a) Surface of arbitrary shape. (b) Flat plate.

dAsq”

As, Ts

V ,T∞∞∞∞

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q”

x dxL

As, Ts

U ,T∞ ∞∞ ∞∞ ∞∞ ∞

TOTAL HEAT TRANSFER RATE

s

conv conv s

A

Q q dA′′= ∫&

( )s

conv s l s

A

Q T T h dA∞= − ∫&

convQ&

(((( ))))conv l sq h T T∞∞∞∞′′′′′′′′ = −= −= −= −

putting value of Q&

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1

s

l ss A

h h dAA

= ∫

putting value of convQ&

( )s

conv s l s

A

Q T T h dA∞= − ∫&(((( ))))conv s sQ hA T T∞∞∞∞= −= −= −= −&&&&

An implication of the no-slip and the no-temperature jump

conditions is that heat transfer from the solid surface to the

fluid layer adjacent to the surface is by pure conduction, since

the fluid layer is motionless,

0conv cond fluid

y

Tq q k

dy =

∂= = −& &

T represents the temperature distribution in the fluid

is the temperature gradient at the surface.

( ) 0yT y =∂ ∂is the temperature gradient at the surface.

( )conv l sq h T T∞′′ = −

(((( ))))fluid ys

k T yh

T T====

∞∞∞∞

− ∂ ∂− ∂ ∂− ∂ ∂− ∂ ∂====

−−−−0

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A fluid flowing over a stationary surface comes to a complete stop

at the surface because of the no-slip condition.

Zero velocity

at the surface

Relative velocity

of fluid layersUniform

approach

velocity, V

Solid Block

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A similar phenomenon occurs for the temperature. When two bodies at

different temperatures are brought into contact, heat transfer occurs until

both bodies assume the same temperature at the point of contact.

Therefore, a fluid and a solid surface will have the same temperature at the

point of contact. This is known as NO-TEMPERATURE-JUMP CONDITION.

A steam pipe is passed through a room in which air and walltemperature are al 30 oC while surface temperature of the pipe is400 oC If the diameter of the pipe is 40 mm and average heattransfer coefficient is 20 W/m2 oC, what is the rate of heat lossfrom the pipe for one meter length of pipe.

Schematic:

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Known: surface temperature and air temp

Find: The rate of heat loss

Assumptions: �Steady operating conditions exist. �Radiation effects are negligible. �Constant properties.

Analysis:

Q = h X A X (Ts - Tα)

= 20 X (πdL) X (400 - 30)

= (20 W/m2 oC ) X ( π X 40 X 10 3 X 1 m2 ) X ( 370 oC )

= 0.93 kW

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