convection ppt
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Heat transfer by Convection was explained in detailTRANSCRIPT
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CONVECTION HEAT TRANSFER
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Introduction• Convection involves the transfer of heat
by the motion and mixing of "macroscopic" portions of a fluid
• This macroscopic motion is same as flow of a fluid past a solid boundary.
• The term natural convection is used if this motion and mixing is caused by density variations resulting from temperature differences within the fluid.
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• The term forced convection is used if this motion and mixing is caused by an outside force, such as a pump. • Heat transfer by convection is more difficult to analyze than heat transfer by conduction
• No single property of the heat transfer medium, such as thermal conductivity, can be defined to describe the mechanism.
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• Heat transfer by convection varies from situation to situation (upon the fluid flow conditions), and it is frequently coupled with the mode of fluid flow.
• In practice, analysis of heat transfer by
convection is treated empirically (by direct observation).
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Convection heat transfer is treated empirically, because of the factors that affect the stagnant film thickness:
Fluid velocity Fluid viscosity Heat flux Surface roughness Type of flow (single-phase/two-phase)
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bs TThAQ
Newton’s law of cooling suggests a basic relationship for heat transfer by convection:
h is called as Convection Heat Transfer Coefficient, W/m2K
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• Convection involves the transfer of heat between a surface at a given temperature (Ts) and fluid at a bulk temperature (Tb).
• The exact definition of the bulk temperature (Tb) varies depending on the details of the situation.
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• For flow adjacent to a hot or cold surface, Tb is the temperature of the fluid "far" from the surface.
• For boiling or condensation, Tb is the saturation temperature of the fluid.
• For flow in a pipe, Tb is the average temperature measured at a particular cross-section of the pipe.
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Local Convection Heat Transfer
Consider convection heat transfer as a fluid passes over a surface of arbitrary shape:
Apply Newton’s law cooling to a local differential element with length dx.
TTTThq ss ''
h is called as Local Convection Heat Transfer Coefficient, W/m2K9
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Types of convection
• Free convection If the motion is mainly due to the density
variations associated with temperature gradientswith in the fluid called as free Convection.
• Forced convection In this the fluid motion is principally Produced by some superimposed velocity field(like a fan, a blower or a pump). 10
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This fluid layer then acts to retard the motion of particles of next fluid layer and so on, until a distance y = from the surface reaches, where these effects become negligible and the fluid velocity u reaches the free stream velocity u∞. as a result of frictional effects between the fluid layers, the local fluid velocity u will vary from x =0, y = 0 to y = .
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The total heat transfer rate q is
avgSavg
A
s TAhdAqqs
''
Where, havg is the average convection heat transfer coefficient for the entire surface.
avg
A
ss
avg T
dAqA
h s
''1
sA
sss
avg dATTA
T1
where
s
s
A
ss
A
s
avgdATT
dAq
h
''Therefore How to Evaluate q’’?
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• The fluid packets close to a solid wall come to thermal equilibrium with the wall.
• The fluid particles will exchange maximum possible energy flux with the solid wall.
• A Zero temperature difference exists between wall and fluid packets at the wall.
• A small layer of fluid particles close the the wall come to Mechanical, Thermal and Chemical Equilibrium With solid wall.
• Fundamentally this fluid layer is in Thermodynamic Equilibrium with the solid wall.
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Example: A heavy lubricating oil (µ = 0.8 N.s/m2 , k = 0.15 W/m. K) flows in the clearance between a shaft and its bearing. If the bearing and the shaft are kept at 10°C and 30°C respectively and the clearance between them is 2mm,determine the maximum temperature rise and the heat flux to the plates for a velocity U = 6 m/s.
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Example:
For the flow system in ex.7.1 calculate the local heat transfer coefficient at x= 0.4m and the heat transferred from the first 40 cm of the plate.
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Fundamental Aspects of Convection
• How to evaluate q’’ ?
• How the heat is transferred from solid to the first layer of the fluid?
• Is this a part of convection?
• What is the temperature of the fluid particle, just adjacent to the wall?
• What part of the fluid domain is under pure convection transfer?
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Concept of Solid Fluid Interaction : Maxwell’s Theory
Diffuse reflection
U2
U U
Φ
U2
Φ
U1
U1
Φ
U2
Specular reflection
• Perfectly smooth surface (ideal surface) Real surface
• The convective heat transfer is defined for a combined solid and fluid system.
• The fluid packets close to a solid wall attain a zero relative velocity close to the solid wall : Momentum Boundary Layer.
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