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CM3120 Module 1 Lecture 2 1/20/2021
1
Module 1: Intro and Prerequisite Material
Β© Faith A. Morrison, Michigan Tech U.1
Professor Faith A. Morrison
Department of Chemical EngineeringMichigan Technological University
www.chem.mtu.edu/~fmorriso/cm3120/cm3120.html
Steady Heat Transfer Review
Module 1: Intro and Prerequisite Material
Β© Faith A. Morrison, Michigan Tech U.www.chem.mtu.edu/~fmorriso/cm3120/cm3120.html
Steady Heat Transfer Review
CM3120: Module 1
Β© Faith A. Morrison, Michigan Tech U.2
Introduction and Prereq Material
I. IntroductionII. Review of Prerequisite Material
a. Microscopic energy balancesb. Fourierβs law of heat conduction (π, homogeneous)c. Newtonβs law of cooling (β, at a boundary)d. Resistances due to π and βe. Solving for the steady temperature field π(π₯,π¦,π§)f. Dimensional analysis in heat transfer for βg. β Data correlations for forced and free convectionh. β For radiation heat transfer
CM3120 Module 1 Lecture 2 1/20/2021
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Β© Faith A. Morrison, Michigan Tech U.3
Why study transport/unit ops?
Where are we in our study of
transport/O.U.?How far along did we get in CM3110 and other prerequisite courses?
Β© Faith A. Morrison, Michigan Tech U.4
We begin with a review:
β’ Microscopic energy balanceβ’ Fourierβs law of heat conduction (π, homogeneous)β’ Newtonβs law of cooling (β, at a boundary)β’ Resistances due to π and ββ’ Solving for the steady temperature field π π₯,π¦, π§β’ Dimensional analysis in heat transfer for ββ’ β Data correlations for forced and free convectionβ’ β For radiation heat transfer
CM3120: Unsteady State Heat Transfer/Mass Transfer/Unit Operations
CM3120 builds on these topics from the
prerequisites.CM3110 REVIEW
CM3120 Module 1 Lecture 2 1/20/2021
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Β© Faith A. Morrison, Michigan Tech U.
Microscopic Energy Balance Review CM3110 REVIEW
We begin our prereq review
here
Β© Faith A. Morrison, Michigan Tech U.
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Equation of Thermal Energy
V
nΜdSS
Microscopic energy balance written on an arbitrarily shaped volume, V, enclosed by a surface, S
Gibbs notation:general conduction
Gibbs notation: Only Fourier conduction
Microscopic Energy Balance:
πππΈππ‘
π£ β π»πΈ π» β π π
ππΆππππ‘
π£ β π»π ππ» π π
(incompressible fluid, constant pressure, neglect πΈ ,πΈ , viscous dissipation )
The microscopicenergy balance is an expression of the law of conservation of energy.
http://pages.mtu.edu/~fmorriso/cm310/energy_equation.html
CM3110 REVIEW
Microscopic Energy Balance Review
It includes consideration of unsteady energy flows.
CM3120 Module 1 Lecture 2 1/20/2021
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Β© Faith A. Morrison, Michigan Tech U.
What physicsdetermines how rapidly (the rate) the heat transfers from one location to another?
ππ΄
πππππ₯
Fourierβs Law of Heat Conduction
(the driving physics of Fourierβs law is Brownian motion: energy transports down π»π due to Brownian motion)
heat flux=energy/area time)
π β thermal conductivity
temperature gradient
(for a homogeneous phase)
Energy Transport law
CM3110 REVIEW
Microscopic Energy Balance Review
ππ΄
πππππ₯
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Β© Faith A. Morrison, Michigan Tech U.
Heat Transfer Rate law:
β’Heat flows down a temperature gradient
β’Flux is proportional to the magnitude of temperature gradient
Makes reference to a coordinate system
Allows you to solve for temperature profiles (also known as temperature distributions or fields)
Gibbs notation:π
π΄ππ»π
ππ
π΄
πππππ₯
πππππ¦
πππππ§
Fourierβs law in three
dimensions
CM3110 REVIEW
Microscopic Energy Balance Review
Fourierβs law of Heat Conduction
CM3120 Module 1 Lecture 2 1/20/2021
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ππΆππππ‘
π£ β π»π ππ» π π
Equation of Energy(microscopic energy balance)
see handout for component notation
rate of change
convection
conduction (all directions)
source
velocity must satisfy equation of motion, equation of continuity
(energy generated per unit volume per time)
Β© Faith A. Morrison, Michigan Tech U.http://pages.mtu.edu/~fmorriso/cm310/energy_equation.html
CM3110 REVIEW
Microscopic Energy Balance Review
Due to: electrical current; chemical reaction
Β© Faith A. Morrison, Michigan Tech U.
10http://pages.mtu.edu/~fmorriso/cm310/energy_equation.html
CM3120 Module 1 Lecture 2 1/20/2021
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Β© Faith A. Morrison, Michigan Tech U.
11http://pages.mtu.edu/~fmorriso/cm310/energy_equation.html
Front side: β’ Micro E-balance in
terms of flux π β‘β’ Fourierβs law, π ππ»π
Β© Faith A. Morrison, Michigan Tech U.
12http://pages.mtu.edu/~fmorriso/cm310/energy_equation.html
Front side: β’ Micro E-balance in
terms of flux π β‘β’ Fourierβs law, π ππ»π
Back side: β’ Micro E-balance in terms
of temperature (Fourierβs law incorporated)
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Β© Faith A. Morrison, Michigan Tech U.
Microscopic Energy Balance
http://pages.mtu.edu/~fmorriso/cm310/energy_equation.html
CM3110 REVIEW
Β© Faith A. Morrison, Michigan Tech U.
Fourierβs Law of Heat Conduction
https://pages.mtu.edu/~fmorriso/cm310/energy.pdf
πππ΄
CM3110 REVIEW
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Β© Faith A. Morrison, Michigan Tech U.
Microscopic Energy Balance Review CM3110 REVIEW
Now, BoundaryConditions and Resistances
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We will needboundary conditionson temperature to solve the microscopic balances for the temperature distribution.
What is the steady state temperature profile in a rectangular slab if one side is held at T1 and the other side is held at T2?
Assumptions:β’wide, tall slabβ’steady state
Example 1: Heat flux in a rectangular solid β Temperature BC
CM3110 REVIEW
Microscopic Energy Balance ReviewβBoundary Conditions
We may know the temperature at the boundary.
Β© F
aith
A. M
orris
on, M
ichi
gan
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CM3120 Module 1 Lecture 2 1/20/2021
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What is the steady state temperature profile in a rectangular slab if one side is held at T1 and the other side is held at T2?
Example 1: Heat flux in a rectangular solid β Temperature BC
CM3110 REVIEW
Microscopic Energy Balance ReviewβBoundary Conditions
What if we donβt know the wall temperature?
We will needboundary conditionson temperature to solve the microscopic balances for the temperature distribution.
Assumptions:β’wide, tall slabβ’steady state
Β© F
aith
A. M
orris
on, M
ichi
gan
Tech
U.
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CM3110 REVIEW
Microscopic Energy Balance ReviewβBoundary Conditions
There may exist a resistance to heat transfer at the boundary, due to fluid characteristics
The interface between the solid and the fluid calls for a new type of boundary condition,Newtonβs Law of Cooling.
Temperature offset is evidence of heat-transfer
resistance at the wall
solid wallbulk fluid
π π₯π
π
π π₯ in solid
π π
π₯π₯
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aith
A. M
orris
on, M
ichi
gan
Tech
U.
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The temperature difference at the fluid-wall interface is caused by complex fluid phenomena that are
lumped together into the heat transfer coefficient, β
Β© F
aith
A. M
orris
on, M
ichi
gan
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CM3110 REVIEW
Microscopic Energy Balance ReviewβBoundary Conditions
Temperature offset is evidence of heat-transfer
resistance at the wall
solid wallbulk fluid
π π₯π
π
π π₯ in solid
π π
π₯π₯
π
The interface between the solid and the fluid calls for a new type of boundary condition,Newtonβs Law of Cooling.
This expression serves as the definition of the heat transfer coefficient.
ππ΄
β π π
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The heat flux at the wall is given by the empirical expression known as
Newtonβs Law of Cooling
(a linear driving-force model for interphase heat transport)
π depends on:
β’geometryβ’fluid velocity fieldβ’fluid propertiesβ’temperature
homogeneous solid
bulk fluid
bT
wallT
wallb TT What is the flux at the wall?
π£ π₯,π¦, π§ 0
Microscopic Energy Balance Review
Β© F
aith
A. M
orris
on, M
ichi
gan
Tech
U.
CM3120 Module 1 Lecture 2 1/20/2021
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Β© F
aith
A. M
orris
on, M
ichi
gan
Tech
U.
21
Microscopic Energy Balance Review
rate of change
convection
conduction (all directions)
sourceβ’ Microscopic
energy balance
β’ Fourierβs law of heat conduction
β’ Newtonβs law of cooling (β, at the phase boundary)
β’ Resistances due to π and β
This expression serves as the definition of the heat transfer coefficient.
Fourierβs Law of Heat Conduction(for a homogeneous phase)
Next
Review so farβ¦
Β© F
aith
A. M
orris
on, M
ichi
gan
Tech
U.
22
Microscopic Energy Balance ReviewβResistance to Heat Transfer
The language of resistance to describe the physics of heat transfer will be handy in our study of unsteady state temperature profiles. We encountered this language in CM3110, and we review and summarize now.
π‘ Resistance to Heat Transfer
1. Limited conductivity within the homogeneous phase π
2. Limited heat transfer between phases at a boundary β
Two limitations create resistance:
1. Are affected by geometry (rectangular versus radial)
2. Can be stacked (that is, added together like electrical resistances)
Also, resistances:
ππ΄
driving force resistancesβ
Note: Geankoplisuses a slightly different definition of resistance; we follow Bird et al. 2002.
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Β© Faith A. Morrison, Michigan Tech U.
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Thermal conductivity π and heat transfer coefficient β may be thought of as sources of resistance β to heat transfer.
These resistances β stack up in a logical way, allowing us to quickly and accurately determine the effect of adding insulating layers, encountering pipe fouling, and other applications.
π
π
r
π
π2π
2π
2π
π΅
x
0
π
π
π΅/2 π΅
π
π
π
Using the microscopic energy balance on a test problem, we can solve for the temperature profile and then the heat flux, which is the driving force/resistance.
ππ΄
driving force resistancesβ
We can then identify the resistances for each test case considered.
1D Heat Transfer β Resistance
1D Rectangular: Door ππ ππ , and Composite Door
π΅
x
0
π
π
π΅/2 π΅π π
1D Rectangular: Slab with Newtonβs law BC
ππ΄
π ππ΅/2π
π΅/2π
ππ΄
π π1β
π΅π
1β
π
π
r
π
π2π
2π
2π
1D Radial: Pipe ππ ππ and
Composite Pipe
ππ΄
π π1π ln
π π
1π ln
π π
1π
1D Radial: Pipe with Newtonβs law BC
ππ΄
π π1
β π 1π ln
π π
1β π
1π
RESISTANCE SUMMARY:
1D Heat Transfer β Resistance
CM3120 Module 1 Lecture 2 1/20/2021
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25Β© Faith A. Morrison, Michigan Tech U.
ππ΄
π πβ
1π
driving forceresistance
Let: β β‘ ln
ππ΄
π πβ
driving forceresistance
Let: β β‘
Note: Geankoplis uses a different resistance. For rectangular heat flux:
π β/πΏπ
Note: Geankoplis uses a different resistance. For radial heat flux:
π β/2ππΏ
π΅
x
π
π΅
π
0
π
π π₯
R1
R2
rπ
Hot wall at π
Cooler wall at π
1D Rectangular
1D Radial
Temperatu
re Boundary C
onditio
ns
1D Heat Transfer β Resistance
26Β© Faith A. Morrison, Michigan Tech U.
ππ΄
π πβ β β
1π
driving forceresistance
ππ΄
π πβ β β
driving forceresistance
Let:
β β‘ for π 1,2
β β‘
1D Rectangular
1D Radial
New
tonβs Law
of C
oolin
g Boundary C
onditio
ns
π΅
x
π
π΅
π
0
ππ π₯
β
β
Let:
β β‘ for π 1,2
β β‘ ln
π
πβ inside
β outsideR1
R2
rπ
π
1D Heat Transfer β Resistance
CM3120 Module 1 Lecture 2 1/20/2021
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Β© Faith A. Morrison, Michigan Tech U.
Microscopic Energy Balance Review CM3110 REVIEW
Now, βSlash and Burnβ
Β© Faith A. Morrison, Michigan Tech U.
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For review, letβs carry out an example of 1D, steady heat transfer
CM3110 REVIEW
Microscopic Energy BalanceβSolve for Temperature Field
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Example 3: Heat Conduction with GenerationWhat is the steady state temperature profile in a wire if heat is generated uniformly throughout the wire at a rate of Se W/m3 and the bulk fluid surrounding the wire is at π ? What is the heat flux?
R
r π
long wire
Se = energy production per unit volume
Β© Faith A. Morrison, Michigan Tech U.
Microscopic Energy BalanceβSolve for Temperature Field
Β© Faith A. Morrison, Michigan Tech U.
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Example: Heat conduction with generation
Letβs try.
Microscopic Energy BalanceβSolve for Temperature Field
β heat transfer coefficient
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In class solution will be posted with the other βhand notes.β
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1
r/R or r/R1
Te
mp
era
ture
ra
tio
Conduction in pipe
Wire with generation
1
2
121
1 lnln
1
R
r
RRTT
TT
2
21
4/
R
r
kRS
TT
e
w
Compare radial conduction solutions
Β© Faith A. Morrison, Michigan Tech U.
Radial conduction through pipe wallRadial conduction in wire with generation
Microscopic Energy BalanceβSolve for Temperature Field
CM3120 Module 1 Lecture 2 1/20/2021
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Β© Faith A. Morrison, Michigan Tech U.33
Steady HeatβTransfer Review Summary (thus far):
β’ Microscopic energy balanceβ’ Fourierβs law of heat conduction (π, homogeneous)β’ Newtonβs law of cooling (β, at the boundary between two phases)β’ Resistances due to π and β; vary with boundary conditions (BC)
and geometry
β’ Solving for the steady temperature field π π₯,π¦, π§ , a.k.a. βSlash and Burnβ
Unsteady State Heat Transfer
1π
lnπ π
π΅π
1π β
1β
1D radial
1D rectangular
π» BC π BC
CM3110 REVIEW
Β© Faith A. Morrison, Michigan Tech U.34
Steady HeatβTransfer Review Summary (thus far):
β’ Microscopic energy balanceβ’ Fourierβs law of heat conduction (π, homogeneous)β’ Newtonβs law of cooling (β, at the boundary between two phases)β’ Resistances due to π and β; vary with boundary conditions (BC)
and geometry
β’ Solving for the steady temperature field π π₯,π¦, π§ , a.k.a. βSlash and Burnβ
Unsteady State Heat Transfer
1π
lnπ π
π΅π
1π β
1β
1D radial
1D rectangular
π» BC π BCSneak peak: The ratio of π (internal) and β(external) resistances is the Biot number:
π©ππ΅/π1/β
ππ©π
This is important in unsteady heat transfer.
CM3110 REVIEW
CM3120 Module 1 Lecture 2 1/20/2021
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Β© Faith A. Morrison, Michigan Tech U.
Microscopic Energy Balance Review CM3110 REVIEW
Finally, Dimensional analysis and data correlations for heat transfer coefficient π
Β© Faith A. Morrison, Michigan Tech U.
Microscopic Energy Balance Review CM3110 REVIEW
For complex systems, we turn to data correlations based on dimensional analysis
The engineering quantity of interest in heat transfer is the amount of heat π¬ transferred
Nusselt number, a dimensionless heat transfer coefficient, is a dimensionless amount of heat π¬transferred
Dimensional Analysis?π€
CM3120 Module 1 Lecture 2 1/20/2021
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Β© F
aith
A. M
orris
on, M
ichi
gan
Tech
U.
37
Microscopic Energy Balance ReviewβDimensional Analysis
solid wallbulk fluid
π π₯π
π
π π₯ in solid
π π
π₯π₯
π
Heat Transfer Coefficient:β’ Linear driving force model
β’ Heat transfer between phases
CM3110 REVIEW
ππ΄
β π π
Review: What is Dimensional Analysis?
Β© Faith A. Morrison, Michigan Tech U.
β’Rough pipes
β’Non-circular conduits
β’Flow around obstacles (spheres, other complex shapes
Solution: Navier-Stokes, Re, Fr, πΏ/π·, dimensionless wall force π; π π Re, πΏ/π·
Solution: Navier-Stokes, Re, dimensionless drag πΆ ; πΆ πΆ Re
Solution: add additional length scale; then nondimensionalize
Solution: Use hydraulic diameter as the length scale of the flow to nondimensionalize
Solution: Two components of velocity need independent lengthscales
β’Flow in pipes at all flow rates (laminar and turbulent)
β’Boundary layers
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Complex Heat Transfer β Dimensional Analysis CM3110 REVIEW
CM3120 Module 1 Lecture 2 1/20/2021
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Β© Faith A. Morrison, Michigan Tech U.
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Complex Heat Transfer β Dimensional Analysis CM3110 REVIEWData Correlations:
Β© Faith A. Morrison, Michigan Tech U.
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Complex Heat Transfer β Dimensional Analysis CM3110 REVIEWData Correlations:
Dimensional analysis allows us to capture and
engineer around or with complex behavior
CM3120 Module 1 Lecture 2 1/20/2021
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Following procedure familiar from pipe flow,
β’ What are governing equations?
β’ Scale factors (dimensionless numbers)?
β’ Quantity of interest?
Answer: Heat flux at the wall
Chosen problem: Forced Convection Heat TransferSolution: Dimensional Analysis
Complex Heat Transfer β Dimensional Analysis
Β© Faith A. Morrison, Michigan Tech U.
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First try: Forced Convection
How does Dimensional Analysis work in Heat Transfer?
CM3110 REVIEW
z-component of the Navier-Stokes Equation:
z
vv
v
r
v
r
vv
t
v zz
zzr
z
zzzz gz
vv
rr
vr
rrz
P
2
2
2
2
2
11
Choose:
π« = characteristic lengthπ½ = characteristic velocityπ«/π½ = characteristic timeππ½π = characteristic pressure
Β© Faith A. Morrison, Michigan Tech U.
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β’ Choose βtypicalβ values (scale factors)β’ Use them to scale the equationsβ’ Deduce which terms dominate
Dimensional Analysis in Forced Convection Heat Transfer
Pipe flow CM3110 REVIEW
FORCED CONVECTION
CM3120 Module 1 Lecture 2 1/20/2021
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V
vv zz *
non-dimensional variables:
D
tVt *
D
zz *
D
rr *
2*
V
PP
g
gg zz *
time: position: velocity:driving force:
V
vv rr *
V
vv *
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β’ Choose βtypicalβ values (scale factors)β’ Use them to scale the equationsβ’ Deduce which terms dominate
Pipe flow
Dimensional Analysis in Forced Convection Heat Transfer
CM3110 REVIEW
FORCED CONVECTION
Microscopic energy balance:
Β© Faith A. Morrison, Michigan Tech U.
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non-dimensional variables:
position:
oo
TTTT
T
1
*
temperature:
Choose:π use a
characteristic interval (since distance from absolute zero is not part of this physics)
π use a reference source, π
πβ β‘ππ·
π§β β‘π§π·
source:
πβ β‘ππ
Energy
ππΆππππ‘
π£ππππ
π£πππππ
π£ππππ§
π1ππππ
πππππ
1ππ πππ
π πππ§
π
π β‘
Dimensional Analysis in Forced Convection Heat Transfer
FORCED CONVECTION
π surfaceπ bulk
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Complex Heat Transfer β Dimensional Analysis Forced Convection
Β© Faith A. Morrison, Michigan Tech U.
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Substitute all these definitions,
(π‘β, πβ, π§β, πβ,πβ, π£β, π£β , π£β,πβ, πβ)
into the governing equations and simplifyβ¦
V
vv zz *
non-dimensional variables:
D
tVt *
D
zz *
D
rr *
2*
V
PP
g
gg zz *
time: position: velocity:driving force:
V
vv rr *
V
vv *
Forced Convection Heat Transfer
CM3110 REVIEW
β’ Choose βtypicalβ values (scale factors)β’ Use them to scale the equationsβ’ Deduce which terms dominate
Pipe flow
Microscopic energy balance:
Forced Convection Heat Transfer
CM3110 REVIEW
non-dimensional variables:
position:
oo
TTTT
T
1
*
temperature:
Choose:π use a
characteristic interval (since distance from absolute zero is not part of this physics)
π use a reference source, π
source:
Energy
ππΆππππ‘
π£ππππ
π£πππππ
π£ππππ§
π1ππππ
πππππ
1ππ πππ
π πππ§
π
π β‘
CM3110 REVIEW
FORCED CONVECTION
FORCED CONVECTION
ππβ
ππ‘βπ£βππβ
ππβπ£β
πβππβ
πππ£βππβ
ππ§β1
Pe1πβ
πππβ
πβππβ
ππβ1πβ
π πβ
πππ πβ
ππ§πβ
**2*
** 1Re1
gFr
vz
PDtDv
zz
Non-dimensional Energy Equation
Non-dimensional Navier-Stokes Equation
0*
*
*
*
*
*
z
v
y
v
x
v zyx
Non-dimensional Continuity Equation
ΛPe PrRe pC VD
k
ΛPr pC
k
Β© Faith A. Morrison, Michigan Tech U.
Complex Heat Transfer β Dimensional Analysis Forced Convection
46π·π£π·π‘
β‘ππ£ππ‘
π£ππ£ππ
π£πππ£ππ
π£ππ£ππ§
**2*
** 1Re1
gFr
vz
PDtDv
zz
*
πβ πβ Re, Prπ£β π£β Re, Fr
CM3110 REVIEW
FORCED CONVECTION
FORCED CONVECTION
CM3120 Module 1 Lecture 2 1/20/2021
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Microscopic Energy Balance Review CM3110 REVIEW
For complex systems, we turn to data correlations for heat transfer coefficients based on dimensional analysis
The engineering quantity of interest is the amount of heat transferred
Nusselt number, a dimensionless heat transfercoefficient, is also a dimensionless amount of heat transferred
Dimensional Analysis?π€
Next?
Β© Faith A. Morrison, Michigan Tech U.
2ππ πΏ β π π π¬ οΏ½ΜοΏ½ β π ππ
Forced Convection Heat Transfer
2ππ πΏ β π π π¬ πππππ
π ππ§ππ
Now, non-dimensionalizethis expression as well.
48
ππ΄
β π πLinear driving force model
Apply in the fluid, at the surface:
CM3110 REVIEW
FORCED CONVECTION
CM3120 Module 1 Lecture 2 1/20/2021
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non-dimensional variables:
position:
oo
TTTT
T
1
*
temperature:
Dr
r *
Dz
z *
Β© Faith A. Morrison, Michigan Tech U.
Non-dimensionalize
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Complex Heat Transfer β Dimensional AnalysisCM3110 REVIEW
π surfaceπ bulk
ddzr
T
D
L
k
hDDL
r
2
0 0
*
21
*
*
*
2
ddzD
DTT
r
TkTTDLh
DLo
r
o
2
0 0
*2
1
21*
*
1 2*
Nusselt number, Nu(dimensionless heat-
transfer coefficient; dimensionless amount of heat transferred)
DL
TNuNu ,*
Β© Faith A. Morrison, Michigan Tech U.
Complex Heat Transfer β Dimensional Analysis
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CM3110 REVIEW
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ππβ
ππ‘βπ£βππβ
ππβπ£β
πβππβ
πππ£βππβ
ππ§β1
Pe1πβ
πππβ
πππβ
ππβ1πβ
π πβ
πππ πβ
ππ§πβ
**2*
** 1Re1
gFr
vz
PDtDv
zz
**2*
** 1Re1
gFr
vz
PDtDv
zz
Non-dimensional Energy Equation
Non-dimensional Navier-Stokes Equation
0*
*
*
*
*
*
z
v
y
v
x
v zyx
Non-dimensional Continuity Equation
ΛPe PrRe pC VD
k
Quantity of interest
ddzr
T
DLNu
DL
r
2
0 0
*
21
*
*
*/2
1
ΛPr pC
k
Β© Faith A. Morrison, Michigan Tech U.
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Complex Heat Transfer β Dimensional Analysis
FORCED CONVECTION
FORCED CONVECTIONFORCED CONVECTION
Β© Faith A. Morrison, Michigan Tech U.
According to our forced convection dimensional analysis calculations, the dimensionless heat transfer
coefficient should be found to be a function of fourdimensionless groups:
Now, do the experiments.
Peclet number
Pe β‘
Prandtl number
Pr β‘
Complex Heat Transfer β Dimensional Analysis
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(fluid properties)
CM3110 REVIEW
FORCED CONVECTION
(tentative)
Nu Nu Re, Pr, Fr,πΏπ·
CM3120 Module 1 Lecture 2 1/20/2021
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no free surfaces
Β© Faith A. Morrison, Michigan Tech U.
According to our forced convection dimensional analysis calculations and follow-up experiments, the
dimensionless heat transfer coefficient should be found to be a function of these four dimensionless groups:
Peclet number
Pe β‘
Prandtl number
Pr β‘
Complex Heat Transfer β Dimensional Analysis
53
(fluid properties)
CM3110 REVIEW
FORCED CONVECTION
FORCED CONVECTION
Nu Nu Re, Pr, Fr,πΏπ·
,ππ
Sometimesπ π seen to be important
βit turns outβ¦β
Β© Faith A. Morrison, Michigan Tech U.
Forced convection Heat Transfer in Turbulent flow in pipes
Physical Properties (except π ) evaluated at:
Nπ’β π·π
0.027Re . Prππ
.π , π ,
2
*May have to be estimated
Forced convection Heat Transfer in Laminar flow in pipes
Nπ’β π·π
1.86 RePrπ·πΏ
ππ
.
π , π ,
2
54
Bulk mean temperature
Sieder-Tate equation (laminar flow)
Sieder-Tate equation (turbulent flow)
Data Correlations for Forced Convection Heat Transfer
π β π΄Ξπ
Ξππ π π π
2
π β π΄Ξπ
ΞπΞπ Ξπ
lnΞπΞπ
CM3110 REVIEW
CM3120 Module 1 Lecture 2 1/20/2021
28
Β© Faith A. Morrison, Michigan Tech U.
55
Complex Heat Transfer β Dimensional AnalysisβForced Convection
Exam 1 Handout, Forced Convection Data Correlations
Β© Faith A. Morrison, Michigan Tech U.
56
When the physics of the heat transfer changes, the correlations and dimensionless numbers change.
Free Convection i.e. hot air rises
β’heat moves from hot surface to cold air (fluid) by radiation and conductionβ’increase in fluid temperature decreases fluid densityβ’recirculation flow beginsβ’recirculation adds to the heat-transfer from conduction and radiation
coupled heat and momentum transport
Complex Heat Transfer β Dimensional AnalysisβFree Convection
new physics: Natural Convection
The methods donβt change,
however
CM3110 REVIEW
CM3120 Module 1 Lecture 2 1/20/2021
29
ππβ
ππ‘βπ£βππβ
ππ₯βπ£βππβ
ππ¦βπ£βππβ
ππ§β1Pr
π πβ
ππ₯β
π πβ
ππ¦βπ πβ
ππ§β
Non-dimensional Energy Equation
Non-dimensional Navier-Stokes Equation
0*
*
*
*
*
*
z
v
y
v
x
v zyx
Non-dimensional Continuity Equation Quantity of interest
Β© Faith A. Morrison, Michigan Tech U.
57
Complex Heat Transfer β Dimensional Analysis
NATURAL(FREE) CONVECTION
NATURAL CONVECTION
β‘Grashof number
Gr β‘ππΏ οΏ½Μ οΏ½ οΏ½Μ οΏ½ π π
π
NATURAL(FREE) CONVECTION
π·π£β
π·π‘β π£ β ππΏ οΏ½Μ οΏ½ οΏ½Μ οΏ½ π π
ππβ
Nuππβ
ππ¦β βππ₯βππ§β
CM3110 REVIEW
Β© Faith A. Morrison, Michigan Tech U.
According to our natural convection dimensional analysis calculations, the dimensionless heat transfer
coefficient should be found to be a function of twodimensionless groups:
Now, do the experiments.
Grashof number
Prandtl number
Pr β‘
Complex Heat Transfer β Dimensional Analysis
58
(fluid properties)
CM3110 REVIEW
NATURAL CONVECTION
Nu Nu Gr, Pr Gr β‘
ππΏ οΏ½Μ οΏ½ οΏ½Μ οΏ½ π ππ
NATURAL(FREE) CONVECTION
CM3120 Module 1 Lecture 2 1/20/2021
30
mmak
hLPrGrNu
Example: Natural convection from vertical planes and cylinders
β’a,m are given in Table 4.7-1, page 278 Geankoplis for several casesβ’L is the height of the plateβ’all physical properties evaluated at the film temperature, Tf
2bw
f
TTT
FREE CONVECTION
Β© Faith A. Morrison, Michigan Tech U.
Gr β‘ππ· οΏ½Μ οΏ½ οΏ½Μ οΏ½Ξπ
π
Free convection correlations use the
film temperature for calculating the physical properties
Experimental Results:
59
Complex Heat Transfer β Dimensional AnalysisβFree Convection
Β© Faith A. Morrison, Michigan Tech U.
60
Complex Heat Transfer β Dimensional AnalysisβFree Convection
Exam 1 Handout, Natural Convection Data Correlations
CM3120 Module 1 Lecture 2 1/20/2021
31
Β© F
aith
A.
Mor
rison
, M
ichi
gan
Tech
U.
61ref: BSL1, p581, 644
Dimensional analysis will be a key tool in the third transport field, diffusion/mass transfer (Module 3)
Dimensional Analysis
Dimensionless numbers from the Equations of Change
mom
entu
men
erg
yThese numbers tell us about the relative importance of the
terms they precede.
(microscopic balances)
Non-dimensional Navier-Stokes Equation
ma
ss
Non-dimensional Continuity Equation (species A)
Non-dimensional Energy Equation
Β© Faith A. Morrison, Michigan Tech U.
Microscopic Energy Balance Review CM3110 REVIEW
Radiation
CM3120 Module 1 Lecture 2 1/20/2021
32
Β© Faith A. Morrison, Michigan Tech U.63
Summary:Radiation
β’ Absorptivity, Ξ±gray body: πΌ constantblack body: πΌ 1
β’ Emissivity, ππ ππ ,
β’ Kirchoffβs law: πΌ Ξ΅
β’ Stefan-Boltzman lawπ ,
π΄ππ
π 5.676 10π
π πΎ
Geankoplis 4th ed., eqn 4.10-10 p304
βπ| π π π
π π
General properties:
Heat transfer coefficient:
Heat shields:
ππ΄
1π 1
π π π2π 1
Always use absolute temperature (Kelvin)
in radiation calculations.
NET Radiation energy going from surface 1 to surface 2:
π ππ΄
π π π1π
1π 1
ππ΄
π π π π
Net heat transfer to a body:
Stefan-Boltzman constant:
CM3110 REVIEW
Β© Faith A. Morrison, Michigan Tech U.64
Summary:
β’ Absorptivity, Ξ±gray body: πΌ constantblack body: πΌ 1
β’ Emissivity, ππ ππ ,
β’ Kirchoffβs law: πΌ Ξ΅
β’ Stefan-Boltzman lawπ ,
π΄ππ
π 5.676 10π
π πΎ
Geankoplis 4th ed., eqn 4.10-10 p304
βπ| π π π
π π
General properties:
Heat transfer coefficient:
Heat shields:
ππ΄
1π 1
π π π2π 1
Always use absolute temperature (Kelvin)
in radiation calculations.
NET Radiation energy going from surface 1 to surface 2:
π ππ΄
π π π1π
1π 1
ππ΄
π π π π
Net heat transfer to a body:
Stefan-Boltzman constant:
CM3110 REVIEW
Radiation
CM3120 Module 1 Lecture 2 1/20/2021
33
Β© Faith A. Morrison, Michigan Tech U.
Microscopic Energy Balance Review CM3110 REVIEWModule 1: Intro and Prerequisite material
DONE!
CM3120: Module 1
Β© Faith A. Morrison, Michigan Tech U.66
Introduction and Prereq Material
I. IntroductionII. Review of Prerequisite Material
a. Microscopic energy balancesb. Fourierβs law of heat conduction (π, homogeneous)c. Newtonβs law of cooling (β, at a boundary)d. Resistances due to π and βe. Solving for the steady temperature field π(π₯,π¦,π§)f. Dimensional analysis in heat transfer for βg. β Data correlations for forced and free convectionh. β For radiation heat transfer
DONE!