Lecture 11 - Forced Convection

7/31/2019 Lecture 11 - Forced Convection

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Heat & Mass Transfer

Convection

Heat & Mass TransferTheory & Application

Dr. S. Kamran Afaq

Professor

HITEC University


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Convection

Boiling and Condensation

Introduction

Application : Heat Exchangers

Heat Conduction

Fundamental of Convection

Mass Diffusion

Course Outline

Heat Transfer from Extended Surfaces (FINS)

Convection and Radiation


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Convection

Modes of Heat Transfer

Convection :

In this mode heat is transfer due to the movementof the fluids

FreeConvection

ForcedConvection


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Convection

Heat Convection FreeConvection

ForcedConvection


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Convection

Hot Iron Block

(T1

= 400 C)

Cool Air

(T = 15 C) By Speed

Type of fluid(Water)

, , C, v ..

Roughness, Geometry ofthe object


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Convection

Heat ConvectionMathematical Modeling :

The rate of heat convection over a medium depends on the:

Nature of the flow (Re.Laminar or Turbulent)

Nature of the fluid (Viscosity, k, density, C, etc) Surface Area of the medium

Temperature Difference

Unlike conduction, convection is not concernedwith medium properties

s

AQ

TQ


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Convection

Heat ConvectionMathematical Modeling :

,

,

AQ

TQ

As ThQ

As TQ

Newtons law of

cooling

Convective heattransfer Coefficient(Nature of the fluid)

where;

T = (Ts -T)


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Convection

Heat ConvectionConvective heat transfer Coefficient (h)

Rate ofHeat transferb/w solid surface and a fluid perunit surface areaper

unit temperature difference

Units :

As T

Qh

h : (W/m2/ C)


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Convection

Heat ConvectionNusselt Number

Consider a fluid layer of thickness L, and temperature difference is

T= T2 T1.

Heat transfer through the fluid will be by

convection if the fluid is in motion or by

conduction if it is in rest.

T

LT

hAQ

kAQ

conv

cond


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Convection

Heat Convection

L

T

T

kA

hA

Q

Q

cond

conv

k

hL

Q

Q

cond

conv

k

hLNu

Nusselt Number

Nu represent the enhancement of Heat transfer by convection through

the fluid as compare to conduction.

High value means more effective the convection; Nu=1, means pure

conduction


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Convection

Heat ConvectionPrandtl Number

Velocity Profile

Temperature Profile

The region above the plate where

viscous effect are dominant; known as

velocity boundary layer; and boundary

layer thickness ( ) is defined as y

distance where u=0.99u

Similarly the region in temperature

profile know as the thermal boundary

layer; and boundary layer thickness

( t) is defined as y distance where T -

Ts = 0.99(T-Ts)


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Convection


In flow over a heated surface, both thermal and velocity boundary

layers develop simultaneously;

Velocity profile have a strong effect on temperature profile; and finallyon thermal boundary layer.

Convective heat transfer coefficient is directly dependent on this

temperature gradient;

The relative thickness of these thermal and velocity boundary layer isdescribe byPrandtl number;


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Convection

Heat Convection

yDiffusivitThermal

viscositykinamaticPr

Prandtl Number

pCkPr

k

CpPr


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Convection



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Convection

Heat ConvectionReynolds Number

The transition from laminar to turbulent flow depends upon;

Surface Geometry

Surface Roughness

Surface Temperature

Free Stream velocity

Type of fluid


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Convection

Heat ConvectionReynolds Number

Reynolds describe that the flow regime completely depends on the ratio of

the inertiaforces to viscousforces in the fluid;


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Convection

Heat Convection

Re2

2

LV

LV

Reynolds Number

LVRe


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Convection

Heat Convection

Flow over a isothermal flat plate

Depends on

geometry

(m , n = 0 to 1)

k

hLNuk

CpPrLVRe


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Convection

Heat ConvectionAverage Nu for external forced

Convection


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Convection

Heat Convection

k

hLNu

Internal forced Convection

k

hL

Q

Q

cond

conv

k

hDNu h

HydraulicDiameter (Dh)


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Convection

Heat ConvectionInternal forced Convection

Laminar and Turbulent in tubes

LVRe

hDVRe


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Convection


hDVRe

Ac = Cross-Sectional Areap = Perimeter


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Convection


Laminar Flow

k

hDNu h

C i


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Convection

Heat Convection

Turbulent Flow

Internal forced Convection

C ti


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Convection

Heat Convection

Example

C ti


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Convection

Heat Convection

Tube Side

Con ection


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Convection

Heat Convection

Convection


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Convection

Heat ConvectionAnnulus Side

Convection


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Convection

Heat Convection

Convection


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Convection

Heat Convection

98.2Prs/ft1011.5

FBtu/h.ft.378.0

26

k

k 00150

017 10

0 72

3

.

. /

Pr .

Btu / h. ft. F

ft s2

Properties water at 140 F

Properties of air at 80 F

Example

Convection


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Convection

EXAMPLE 7

5A long 10-cm-diameter steam pipe whose external surface

temperature is 110 Cpasses through some open area that is

not protected against the winds. Determine the rate of heat

loss from the pipe per unit of its length when the air is at 1

atm pressure and 10 C and the wind is blowing across thepipe at a velocity of8 m/s.

Convection


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Convection

EXAMPLE 7

5A long 10-cm-diameter steam pipe whose external surface

temperature is 110 Cpasses through some open area that is

not protected against the winds. Determine the rate of heat

loss from the pipe per unit of its length when the air is at 1

atm pressure and 10 C and the wind is blowing across thepipe at a velocity of8 m/s.

Convection


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Convection

Average Nu for external forced

Convection

Convection


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Convection

31805.04 )7202.0()10219.4(027.0Nu

127Nu

Then the rate of heat transfer from the pipe per unit of its length;

CW/m6.351.0

12702808.0 2

hD

Nukh

W1120)( TThAQ ss

Convection


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Convection

Thank You

Documents

Lecture 11 - Forced Convection