heat transfer chapter 6 - mazlan 2016mazlan/?download=ht cengel - chapter 6 - mazlan 2016.pdf ·...

SKMM 3443

LECTURER: ASSOC PROF. DR. MAZLAN ABDUL WAHIDhttp://www.fkm.utm.my/~mazlan

Fundamentals of

Convection

Inspiring Creative & Innovative Mindswww.utm.my

http://www.fkm.utm.my/~mazlan

TEXT: Heat and Mass TransferFundamental and Applications, 5th Edition by Cengel & GhajarMc Graw Hill

[email protected]

Chapter 6

Fundamentals of Convection

Chapter 6

Fundamentals of Convection

Physics of

convection

Fluid flow

classification


Velocity

boundary

layer

Thermal

boundary

layer

• Blowing on food Engineering applications

• CPU fan

• Radiator fan

• Steel cooling

Convection in daily life


• Steel cooling

We resort forced convection whenever we need to increase the rate of heat transfer

Conduction Convection

Requires

medium

• Heat transfer through a

liquid or gas can be by

conduction or convection

• Fluid motion enhances heat

transfer by increasing the

Physical Mechanism of Convection


medium

No fluid

motion

Requires

fluid motion

transfer by increasing the

rate of heat transfer

PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER

Natural convectionForced convection


Conduction

Heat transfer from a hot surface to the surrounding fluid by convection and conduction


• Heat transfer through a solid is by conduction

• Heat transfer through a fluid is by convection

in the presence of bulk fluid motion and by

conduction in the absence of it


conduction in the absence of it

• Conduction in a fluid is a limiting case of

convection


• Heat transfer

through a fluid

sandwiched

between two parallel


between two parallel

plates


Heat transfer rate

Newton’s Law of Cooling



Fluid

properties

Geometry

Type of

flow

What controls convection?


Convection

properties flow


dynamic

viscosity, µ

thermal

conductivity, kfluid velocity, V


Density, ρspecific heat, cp

MFM44

MFM44 fairus yasin, 7/9/2014


Convection heat transfer coefficient



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


Velocity profile due to the no-slip condition as a fluidflows over a blunt nose.

Fluid flowing over a stationary surface has zero velocity at the surface


Boundary layerBoundary layer : The flow region adjacent to the wall in which the viscous effects (and thus the velocity gradients) are significant.





Boundary layerWhat fluid properties governs the formation of boundary layer?





Boundary layerWhat fluid properties governs the formation of boundary layer?� Viscosity





The Nusselt Number



Classification of Fluid Flow


Viscous

Inviscid

Internal

External

Compressible

Incompressible

Laminar

Turbulent


Natural Steady 1-D

Classification of Fluid Flow


Natural

Forced

Steady

Unsteady

1-D

Multi-D


• Viscous flows: Flows in which the frictional

effects are significant.

• Inviscid flow regions: In many flows of

practical interest, there are regions (typically

Viscous versus Inviscid Regions of Flow


practical interest, there are regions (typically

regions not close to solid surfaces) where

viscous forces are negligibly small compared

to inertial or pressure forces.


The flow of an originally uniform fluid stream over a flat plate, and


stream over a flat plate, andthe regions of viscous flow (next to the plate on both sides) and inviscid flow (away from the plate).


• External flow: The flow of an unbounded

fluid over a surface such as a plate, a wire, or a

pipe.

• Internal flow: The flow in a pipe or duct if the

Internal versus External Flow


• Internal flow: The flow in a pipe or duct if the

fluid is completely bounded by solid surfaces.


• Water flow in a pipe is internal flow, and

airflow over a ball is external flow.

• The flow of liquids in a duct is called open-

channel flow if the duct is only partially filled


channel flow if the duct is only partially filled

with the liquid and there is a free surface.



External flow over a tennis ball, and the turbulent wake region behind.


• Incompressible flow: If the density of flowing

fluid remains nearly constant throughout (e.g.,

liquid flow).

• Compressible flow: If the density of fluid

Compressible versus Incompressible Flow


• Compressible flow: If the density of fluid

changes during flow (e.g., high-speed gas

flow).


Mach number

Ma = 1 Sonic flow


Ma = 1 Sonic flow Ma < 1 Subsonic flowMa > 1 Supersonic flowMa >> 1 Hypersonic flow


• Laminar flow: The highly ordered fluid motion characterized by smooth layers of fluid. The flow of high-viscosity fluids such as oils at low velocities is typically laminar.

• Transitional flow: A flow that alternates between being

Laminar versus Turbulent Flow


• Transitional flow: A flow that alternates between being laminar and turbulent

• Turbulent flow: The highly disordered fluid motion that typically occurs at high velocities and is characterized by velocity fluctuations. The flow of low-viscosity fluids such as air at high velocities is typically turbulent.



Laminar, transitional, and turbulent flows


• Forced flow: A fluid is forced to flow over a

surface or in a pipe by external means such as

a pump or a fan.

• Natural flow: Fluid motion is due to natural

Natural (or Unforced) versus Forced Flow


• Natural flow: Fluid motion is due to natural

means such as the buoyancy effect, which

manifests itself as the rise of warmer (and

thus lighter) fluid and the fall of cooler (and

thus denser) fluid.


• The term steady implies no change at a point

with time.

• The opposite of steady is unsteady.

• The term uniform implies no change with

Steady versus Unsteady Flow


• The term uniform implies no change with

location over a specified region.

• The term periodic refers to the kind of

unsteady flow in which the flow oscillates

about a steady mean.

MFM23

MFM24

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MFM24 Ni aku Paste dan guna Text Only. Pastu letak bullet dan bold secara manualfairus yasin, 7/9/2014


• A flow field is best characterized by its velocity

distribution.

• A flow is said to be one-, two-, or three-

dimensional if the flow velocity varies in one,

One-, Two-, and Three-Dimensional

Flows


dimensional if the flow velocity varies in one,

two, or three dimensions, respectively.

• However, the variation of velocity in certain

directions can be small relative to the

variation in other directions and can be

ignored.

Virtues of Prayer

Huzaifa (r.a) says that, whenever the Prophet (s.a.w)happened to face any difficulty, he would at onceresort to salat. (Ahmad, Abu Dawud )


A'isha (r.a) related that the Prophet (s.a.w) said aboutthe 2 rak'ahs before Fajr: "They are dearer to me thanthe whole world." (Muslim)

Recap


• Velocity boundary layer

• Thermal boundary layer

Today’s topic



Velocity Boundary Layer




The development of the boundary layer for flow over a flat plate, and the different flow regimes

MFM29

MFM29 Bagus ko asingkan diagram ni ke slide sendiri. Tapi kene letak caption dia.fairus yasin, 7/9/2014


Wall Shear Stress



Viscosity vs Temperature



Thermal Boundary Layer



Prandtl Number



Prandtl Number


“For liquid metals, heat diffuses quickly”. So, Pr >> 1. True or False?


Prandtl Number


“For liquid metals, heat diffuses quickly”. So, Pr >> 1. True or False?� False


Prandtl Number


“For oils, heat diffuses slowly”. So, Pr >> 1


Prandtl Number


Which one has thicker thermal boundary layer? Liquid metals or oils?


Prandtl Number


Which one has thicker thermal boundary layer relative to the velocity boundary layer? Liquid metals or oils?� Liquid metals


Prandtl Numbers of Common Fluids


MFM38

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Laminar and Turbulent Flow

Laminar flow:Smooth streamlines and highly ordered motionTurbulent flow:


Turbulent flow:Velocity fluctuations and highly disordered motion


Reynolds Number



Convection and Friction Coefficients



Heat and Momentum Transfer in

Turbulent Flow




Turbulent Flow


Large velocity and temperature gradients at the wall surface for turbulent flow



Turbulent Flow


Higher wall shear stress for turbulent flow



Turbulent Flow


Which one has higher heat flux near the wall?



Turbulent Flow


Which one has higher heat flux near the wall?� Turbulent flow


Example (Problem 6-9)



Example (Problem 6-57)


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