SKMM 3443
LECTURER: ASSOC PROF. DR. MAZLAN ABDUL WAHIDhttp://www.fkm.utm.my/~mazlan
Fundamentals of
Convection
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http://www.fkm.utm.my/~mazlan
TEXT: Heat and Mass TransferFundamental and Applications, 5th Edition by Cengel & GhajarMc Graw Hill
Chapter 6
Fundamentals of Convection
Chapter 6
Fundamentals of Convection
Physics of
convection
Fluid flow
classification
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Velocity
boundary
layer
Thermal
boundary
layer
• Blowing on food Engineering applications
• CPU fan
• Radiator fan
• Steel cooling
Convection in daily life
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• 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
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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
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Conduction
Heat transfer from a hot surface to the surrounding fluid by convection and conduction
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• 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
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conduction in the absence of it
• Conduction in a fluid is a limiting case of
convection
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• Heat transfer
through a fluid
sandwiched
between two parallel
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between two parallel
plates
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Heat transfer rate
Newton’s Law of Cooling
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Fluid
properties
Geometry
Type of
flow
What controls convection?
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Convection
properties flow
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
dynamic
viscosity, µ
thermal
conductivity, kfluid velocity, V
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Density, ρspecific heat, cp
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Convection heat transfer coefficient
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
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.
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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
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Boundary layerBoundary layer : The flow region adjacent to the wall in which the viscous effects (and thus the velocity gradients) are significant.
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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
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Boundary layerWhat fluid properties governs the formation of boundary layer?
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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
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Boundary layerWhat fluid properties governs the formation of boundary layer?� Viscosity
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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
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
The Nusselt Number
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Classification of Fluid Flow
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Viscous
Inviscid
Internal
External
Compressible
Incompressible
Laminar
Turbulent
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Natural Steady 1-D
Classification of Fluid Flow
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Natural
Forced
Steady
Unsteady
1-D
Multi-D
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• 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
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practical interest, there are regions (typically
regions not close to solid surfaces) where
viscous forces are negligibly small compared
to inertial or pressure forces.
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
The flow of an originally uniform fluid stream over a flat plate, and
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stream over a flat plate, andthe regions of viscous flow (next to the plate on both sides) and inviscid flow (away from the plate).
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• 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
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• Internal flow: The flow in a pipe or duct if the
fluid is completely bounded by solid surfaces.
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• 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
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channel flow if the duct is only partially filled
with the liquid and there is a free surface.
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
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External flow over a tennis ball, and the turbulent wake region behind.
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• 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
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• Compressible flow: If the density of fluid
changes during flow (e.g., high-speed gas
flow).
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Mach number
Ma = 1 Sonic flow
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Ma = 1 Sonic flow Ma < 1 Subsonic flowMa > 1 Supersonic flowMa >> 1 Hypersonic flow
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• 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
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• 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.
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
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Laminar, transitional, and turbulent flows
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• 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
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• 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.
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• 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
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• 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.
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
• 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
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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.
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
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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 )
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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
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• Velocity boundary layer
• Thermal boundary layer
Today’s topic
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Velocity Boundary Layer
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
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The development of the boundary layer for flow over a flat plate, and the different flow regimes
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Wall Shear Stress
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Viscosity vs Temperature
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Thermal Boundary Layer
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Prandtl Number
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Prandtl Number
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“For liquid metals, heat diffuses quickly”. So, Pr >> 1. True or False?
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Prandtl Number
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“For liquid metals, heat diffuses quickly”. So, Pr >> 1. True or False?� False
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Prandtl Number
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“For oils, heat diffuses slowly”. So, Pr >> 1
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Prandtl Number
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Which one has thicker thermal boundary layer? Liquid metals or oils?
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Prandtl Number
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Which one has thicker thermal boundary layer relative to the velocity boundary layer? Liquid metals or oils?� Liquid metals
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Prandtl Numbers of Common Fluids
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MFM38 Ni aku guna Template 3_Picture with Caption. Paste table guna option Use Destination Theme . Pastu Crop. Pastu aku modify Master slide supaya Caption dia warna hitam. Aku adjust jarak antara Caption dengan Table secara manual.fairus yasin, 7/9/2014
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Laminar and Turbulent Flow
Laminar flow:Smooth streamlines and highly ordered motionTurbulent flow:
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Turbulent flow:Velocity fluctuations and highly disordered motion
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Reynolds Number
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Convection and Friction Coefficients
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Heat and Momentum Transfer in
Turbulent Flow
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Heat and Momentum Transfer in
Turbulent Flow
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Large velocity and temperature gradients at the wall surface for turbulent flow
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Heat and Momentum Transfer in
Turbulent Flow
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Higher wall shear stress for turbulent flow
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Heat and Momentum Transfer in
Turbulent Flow
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Which one has higher heat flux near the wall?
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Heat and Momentum Transfer in
Turbulent Flow
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Which one has higher heat flux near the wall?� Turbulent flow
PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Example (Problem 6-9)
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PHYSICS OF CONVECTION FLUID FLOW CLASSIFICATION VELOCITY BOUNDARY LAYER THERMAL BOUNDARY LAYER
Example (Problem 6-57)
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