convection heat transfer (chapter 6)

8/19/2019 Convection Heat Transfer (Chapter 6)

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CONVECTION HEATTRANSFER

CDB 2023: PROCESS HEAT TRANSFER

Jan Semester 2016

(1) PROFESSOR DR MOHAMED IBRAHIM B ABD

MUTALIB(2) DR. YEONG YIN FONG

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At the end of this session:

1) Understand the physical mechanism of convection and its

classification.2) Understand the development of velocity and thermal

boundary layers during flow over surfaces.

3) Distinguish between laminar and turbulent flows.4) Obtain convection coefficients and determine heat

transfer rates to or from surfaces of various geometries inexternal flow.

5) Obtain convection coefficients and determine heattransfer rates to or from fluids flowing inside pipes of

various geometries.6) Obtain convection coefficients and determine the heattransfer rates in natural convection heat transfer fromvarious geometries.

Lesson Outcomes:

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Outline

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Chapter 6: Fundamental ofConvection

• Physical Mechanism of

Convection

• Classification of Fluid Flows

• Velocity Boundary Layer

• Thermal Boundary Layer

Chapter 7: External ForceConvection

• Parallel Flow over Flat Plates

• Flow Across Cylinders andSpheres

• Flow Across Tube Banks

Chapter 8: Internal ForcedConvection

• The Entrance Region

• General Thermal Analysis

• Laminar Flow in Tubes

• Turbulent Flow in Tubes

Chapter 9: Natural Convection

• Physical Mechanism ofNatural Convection

• The Grashof Number• Natural Convection over

Surfaces


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Cengel, A. Y. and Ghajar, J. A., Heat and Mass

Transfer: Fundamentals and Applications, 5th Ed.

McGraw Hill 2015.

Holman, J. P. Heat Transfer, 10th Ed., McGraw Hill,

2009.

Reference Books:

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Outline

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Convection• Classification of Fluid Flows




• Parallel Flow over Flat Plates• Flow Across Cylinders and

Spheres




• General Thermal Analysis• Laminar Flow in Tubes


Chapter 9: Natural Convection

• Physical Mechanism ofNatural Convection

• The Grashof Number

• Natural Convection overSurfaces


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PHYSICAL MECHANISM OF

CONVECTION

Conduction and convectionboth require the presence ofa material medium but

convection requires fluid

motion.

Heat transfer through a fluid

is by convection in the

presence of bulk fluid motionand by conduction in theabsence of it.


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The rate of heat transfer through a fluid is much

higher by convection than it is by conduction.

In fact, the higher the fluid velocity, the higher the

rate of heat transfer.

Heat transfer through afluid sandwiched betweentwo parallel plates.


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• Convection heat transfer strongly depends on thefluid properties dynamic viscosity , thermal conductivity, density , and specific heat , as well asthe fluid velocity .

• It also depends on the geometry and the roughness of the solid surface, in addition to the type of fluid flow (such as being streamlined or turbulent).

Newton’s lawof cooling

Eq 6-1

Eq 6-2


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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.

No-slip condition: A fluid in direct contact with a solid “sticks” tothe surface due to viscous effects, and there is no slip.

Boundary layer: The flow region adjacent to the wall in whichthe viscous effects (and thus the velocity gradients) aresignificant.


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Nusselt Number

Heat transfer through a fluid layerof thickness L and temperature

difference ∆T.

Lc characteristic length

The Nusselt number represents

the enhancement of heattransfer through a fluid layer asa result of convection relative toconduction across the samefluid layer.

The larger the Nusselt number,the more effective theconvection.

Eq. 6-5


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Outline

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Convection• Classification of Fluid Flows




• Parallel Flow over Flat Plates• Flow Across Cylinders and

Spheres




• General Thermal Analysis• Laminar Flow in Tubes


Chapter 9: Natural Convection• Physical Mechanism of

Natural Convection

• The Grashof Number

• Natural Convection overSurfaces


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CLASSIFICATION OF FLUIDFLOWS

i) Internal versus External Flow

ii) Laminar versus Turbulent Flow

iii) Natural (or unforced) versus ForcedFlow

iv) Steady versus Unsteady Flow


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CLASSIFICATION OF FLUID FLOWS

i) Internal versus External

Flow

A fluid flow is classified as

being internal and external,depending on whether the fluidis forced to flow in a confinedchannel or over a surface.


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ii) Laminar versus Turbulent Flow

Laminar flow: The highly ordered fluid motioncharacterized by smooth layers of fluid. The flow ofhigh-viscosity fluids such as oils at low

velocities is typically laminar.

Turbulent flow: The highly disordered fluid motion

that typically occurs at high velocities and ischaracterized by velocity fluctuations. The flow oflow-viscosity fluids such as air at high velocities

is typically turbulent.

Transitional flow: A flow that alternates betweenbeing laminar and turbulent.


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Reynolds Number The transition from laminar to turbulent flow depends

on the geometry , surface roughness , flow velocity ,surface temperature , and type of fluid .

The flow regime depends mainly on the ratio of inertia

forces to viscous forces (Reynolds number).

V is the upstream velocity, Lc is the characteristiclength, is the kinematic viscosity, = dynamic

viscosity

Eq. 6-13 µ

ρ ν

cc VLVL ===forcesViscous

forcesInertiaRe

ν / = µ


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Critical Reynolds number, Recr: The Reynolds number

at which the flow becomes turbulent.

The value of the critical Reynolds number is differentfor different geometries and flow conditions.


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iii) Natural (or Unforced) versus Forced Flow

Forced flow: A fluid is forced to flow over a surfaceor in a pipe by external means such as a pump ora fan.

Natural flow: Fluid motion is due to natural meanssuch as the buoyancy effect, which manifests itselfas the rise of warmer (and thus lighter) fluid and

the fall of cooler (and thus denser) fluid.


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iv) Steady versus Unsteady Flow

The term steady implies no change at a point in

time .

The opposite of steady is unsteady.

The term uniform implies no change in location over a specified region.


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VELOCITY BOUNDARY LAYER

Velocity boundary layer: The region of the flow above theplate bounded by δ in which the effects of the viscous shearingforces caused by fluid viscosity are felt.

The boundary layer thickness, δ , is typically defined as thedistance y from the surface at which, fluid velocity, u = 0.99V .


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THERMAL BOUNDARY LAYER

The flow region over the surface in which the temperaturevariation in the direction normal to the surface is significant.

The thickness of the thermal boundary layer δt at any location

along the surface is defined as the distance from the surface at which the temperature difference T − T s equals 0.99(T ∞− T s ).


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Prandtl Number

The relative thickness of the velocity and the thermalboundary layers is best described by thedimensionless parameter Prandtl number.

The ratio of momentum diffusivity (kinematic viscosity)to thermal diffusivity

Eq. 6-12


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The Prandtl numbers of gases are about 1, which indicates thatboth momentum and heat dissipate through the fluid at about thesame rate.

Heat diffuses very quickly in liquid metals (Pr > 1) relative to momentum.

Consequently the thermal boundary layer is much thicker forliquid metals and much thinner for oils relative to the velocity

boundary layer.


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Physical Mechanism of ConvectionNusselt Number

Classification of Fluid Flows

Velocity Boundary Layer

Thermal Boundary LayerPrandtl Number

Laminar and Turbulent FlowsReynolds Number

Summary of Chapter 6

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convection heat transfer (chapter 6)

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