5- lect 7 convection ht (5) 23hhd
TRANSCRIPT
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Heat Transfer ME 431
Convection Heat Transfer
Dr. M.M. Abo Elazm
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Dimensionless Numbers
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Units & Dimensions
...???differencethe is WhatValues
DimensionsUnits
⇒
We always hear in heat transfer the names like Reynolds, Nusselt, and
prandtl, etc.
These are dimensionless numbers that are used to characterize and
classify the heat transfer problems.
So we need first to explain the meaning and significance of these
numbers.
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Many people aren't sure of the difference. Let's try and get
a set of definitions we can use. Consider:
10 grams of sodium
10 meters high
10 gallons of gasoline
Units & Dimensions
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We'll break them up this way:
Units & Dimensions
Value Unit Dimension
10 grams mass
10 meters length
10 gallons volume (length3)
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Units & Dimensions
1- Dimensions:
A "dimension" can be measured or
derived.
There are a "fundamental dimensions"
(length, time, mass, temperature,
amount) are distinct and sufficient to
define all the others.
There are also many derived dimensions
(velocity, volume, density, etc.) for
convenience.
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Units & Dimensions
2- Units:
"Units" can be counted or measured.
Measured units are specific values of
dimensions defined by law or
custom.
Many different units can be used for a
single dimension, as inches,
miles, and centimeters, are all
units used to measure the dimension
length.
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So what is a dimensionless number…????
When we say a quantity is dimensionless, we mean one of two
things:
First, it may just be a number like we get when counting. 10 Oranges,
5 circles (counted)
Or a ratio such as, [Pi or π] which is a dimensionless number
representing the ratio of the circumference of a circle to its diameter
Dimensionless Quantities
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Also numbers [of particular interest in engineering] are combinations of
variables when all the dimensions are "cancelled out" so the net term
has no dimension:
10 kg/2 kg = 5
These are often called "dimensionless numbers" and often have special
names and meanings.
Most of these have been found using techniques of "dimensional analysis"
[a way of examining physical phenomena by look ing at the dimensions that
occur in the problem w ithout considering any numbers].
Dimensionless Quantities
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So why do we need dimensionless numbers…???
These numbers do not exist in nature, or in other words The nature itself
does not have a clue about these numbers.
It is not like the air says to itself " boy, my Reynolds number is exceeding 2500
and I am in a pipe so I better switch to my turbulent mode or all the fluid
dynamics textbooks will be wrong".
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If we have come up with some formula for calculating the pressure drop
in a 2 inch pipe, can we use it for a 2 feet pipe ????
We have invented dimensionless numbers to be able to take our knowledge
from experimenting with one system to learn about another system with
different dimensions.
In a way, we are trying to get rid of dimensions in order to extend our
solution beyond its source.
Dimensionless Quantities
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Dimensionless numbers allow us to
experiment with models [cars, airplanes
and ships] and predict the behavior of
the original thing under actual
conditions.
We have to make sure that there is a
similarity between the model and the
actual thing.
Dimensionless Quantities
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Dimensionless Numbers In Heat Transfer
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The most common numbers used in heat transfer,
are:
Reynolds Number [Re]
Nusselt Number [Nu]
Prandtl Number [Pr]
Grashof Number [Gr]
Rayleigh Number [Ra]
Dimensionless Quantities
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These numbers are a combination of the physical properties of the
fluids….so
Before getting into the definitions of these numbers, we should define
and understand the meaning of some physical properties of
fluids since they show up all over the place in the dimensionless
numbers.
Dimensionless Quantities
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1- Density: [ρ]
It is defined as the “Mass of fluid contained in a unit volume”.
Its units are [kg/m3 or slugs/ft3]. Typical values are:
Water = 1000 kg/m3,
Mercury = 13546 kg/m3,
Air = 1.23 kg/m3,
Paraffin Oil = 800 kg/m3.
at Pressure of 1 bar or [1.013*105 Pascals] and Temperature=288.15K.
Dimensionless Quantities
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2- Viscosity: [μ]
As described before it is the property of a fluid, due
to interaction between molecules, which offers
resistance to shear deformation of the fluid.
Fluid with a high viscosity such as syrup,
deforms more slowly than fluid with a low
viscosity such as water.
[Units: N.sec/m2, or kg/m.sec]
Typical values: Water =1.14x10-3 , Air =1.78x10-5 ,
Mercury =1.552 , Paraffin Oil =1.9 kg/m.sec.
Dimensionless Quantities
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3- Kinematic Viscosity [ν]
Defined as the ratio of dynamic viscosity to mass density, [ν= μ/ ρ]
Units: m2/sec, (often expressed in Stokes [St], where St = 10-4 m2/sec .)
Typical values: Water =1.14x10-6 , Air =1.46x10-5 ,
Mercury =1.145x10-4 , Paraffin Oil =2.375x10-3 m2/sec.
Dimensionless Quantities
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4- Thermal conductivity: [k]
It is a measure of the ability of a material (Solid, Liquid or Gas) to
conduct heat. It is defined using the Fourier's law of conduction
which, relates the rate of heat transfer by conduction to the
temperature gradient.
Dimensionless Quantities
CmW
Cm
mWk
dxdTkAQ
oo .2 ==
−=
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Thermal conductivity of most liquids increase with decreasing
temperature. “Water is, however, an exception to this rule”.
According to the kinetic theory of gases, the thermal conductivity of gases
is proportional to the square root of temperature and inversely
proportional to the square root of the molecular mass.
It is obvious that the thermal conductivity of a gas increases with increasing
temperature.
Dimensionless Quantities
mkandTk ~1αα
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5- Specific heat: Cp
Specific heat is the amount of heat that is required to raise the
temperature of a unit mass of a substance by one degree. In a
constant pressure process
Q = m . Cp . Δ T
The units for the specific heat are kJ/kg.K (or oC).
Dimensionless Quantities
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6- Thermal Diffusivity a
As we know, the heat travels from the high temperature region to
the low temperature.
A measure of how heat propagates through a medium may be defined
as “the ratio of the heat conducted through the material to the
heat stored in the material per unit volume”.
Dimensionless Quantities
pC ρkα
fluidthe in stored heatfluidthe through conducted heatα
=
=
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7- Coefficient of Thermal Expansion: [β]
It is defined as the change in the density of a substance as a
function of temperature at constant pressure. It can be
approximated as:
Dimensionless Quantities
TβρRTP
ρβΔTΔρsoΔTΔρ
ρβ
1:Gases IdealFor
1
=∴⇒=
−≈−≈
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Important Dimensionless numbers
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1- Reynolds Number
After exhaustive experiments in the 1880s,
Osborn Reynolds discovered that the flow
regime depends mainly on the ratio of the
inertia forces to viscous forces in the
fluid. This ratio is called the Reynolds
number, which is a dimensionless
quantity, and is expressed as:
FUNDAMENTALS OF CONVECTION
νµρ
µ
ρ
ULULL
UL
UForceViscousForceInertia
==
=
=
2
2
Re
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At large Reynolds numbers, the inertia forces (density
and the velocity) of the fluid, are large relative to the
viscous forces, which cannot prevent the random and
rapid fluctuations of the fluid, Thus the flow is
turbulent.
At small Reynolds numbers, however, the viscous
forces are large enough to overcome the inertia
forces and to keep the fluid “in line.” Thus the flow is
laminar.
FUNDAMENTALS OF CONVECTION
νµρ
µ
ρ
ULULL
UL
UForceViscousForceInertia
==
=
=
2
2
Re
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Any fluid flow contains small disturbances that will grow given enough
opportunities. as long as the viscous forces dominate, these disturbances
are under control.
As the inertia forces get bigger, the viscosity can no longer maintain order
and these disturbances grow into trouble makers and then transition to
turbulent flow.
FUNDAMENTALS OF CONVECTION
νµρ ULUL
==Re
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Critical Reynolds number
The value of Reynolds number beyond which the flow is no longer considered
laminar is called the critical Reynolds number.
For flow over a flat plate, the critical Reynolds number is observed to vary
between [1x105 to 3x106] depending on the turbulence level in the
free stream and the roughness of the surface.
We normally use 5x105 as the critical Reynolds number for flow over flat plates.
We normally use 2400 to 4000 as the critical Reynolds number for flow inside
tubes.
FUNDAMENTALS OF CONVECTION
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2- Nusselt number:
Is the ratio of convective to conductive heat
transfer across the boundary. It is a
dimensionless number named after Wilhelm
Nusselt.
The conductive component is measured under the
same conditions as the heat convection but with a
stagnant (or motionless) fluid.
FUNDAMENTALS OF CONVECTION
Nuk
hLLTk
Thqq
LTkq
Thq
cond
conv
cond
conv
==∆∆
=
∆=
∆=
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So, the Nusselt number may be viewed as the
ratio of convection to conduction for a
layer of fluid.
Higher values of Nusselt mean that the
heat transfer is enhanced by convection.
FUNDAMENTALS OF CONVECTION
Nuk
hLLTk
Thqq
LTkq
Thq
cond
conv
cond
conv
==∆∆
=
∆=
∆=
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3- Prandtl Number:
The ratio between the rate of viscous diffusion to the rate
of thermal diffusion within a fluid flow, is best described
by the dimensionless parameter Prandtl number, It is
named after Ludwig Prandtl, who introduced the concept
of boundary layer in 1904 and made significant contributions
to boundary layer theory.
FUNDAMENTALS OF CONVECTION
kc
ratediffusionThermalratediffusionViscous pµ
αν===Pr
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The Prandtl numbers of gases are about 1,
which indicates that both momentum and
heat dissipate through the fluid at about
the same rate.
FUNDAMENTALS OF CONVECTION
Heat diffuses very quickly in liquid metals (Pr<<1) and very slowly
in oils (Pr>>1) relative to momentum.
Consequently the thermal boundary layer is much thicker for liquid metals
and much thinner for oils relative to the velocity boundary layer.
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Prandtl number can also be related to the thickness of the thermal and velocity
boundary layers. It is actually the ratio of velocity boundary layer
thickness to the thermal boundary layer thickness.
FUNDAMENTALS OF CONVECTION
th
vel
thicknesslayerBoundaryThermalthicknesslayerBoundaryVelocity
δδ
==Pr
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4- Grashof Number: Gr
You see this number and you should think of natural or free convection. The
Grashof number is the ratio of buoyancy forces to the viscous forces.
Grashof number In natural convection plays the same role that is played by
Reynolds number in forced convection.
FUNDAMENTALS OF CONVECTION
2ForcesViscousForcesBoyancyGr
vvg
ρρ∆
==
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The buoyant forces are fighting with viscous forces and at some point
they overcome the viscous forces and the flow is no longer nice and
laminar.
For a vertical plate, the flow transitions to turbulent around a Grashof
number of Gr=109.
FUNDAMENTALS OF CONVECTION
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5- Rayleigh Number: Ra
The Rayleigh number is the product of Grashof and Prandtl numbers.
Ra = Gr * Pr
It turns out that in natural convection the Nusselt number scales with
Rayleigh rather than just Grashof.
Most correlations in natural convection are of the form:
FUNDAMENTALS OF CONVECTION
[ ]( ) Pr......... 2
3
vxTTgPrGrRaWhere
RaCNu
s
n
∞−=×=
=
β
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6- Richardson number [Ri]
Is named after Lewis Fry Richardson (1881 – 1953).
It is the dimensionless number that expresses the ratio of potential to
kinetic energy or importance of natural convection relative to the forced
convection. The Richardson number in this context is defined as:
FUNDAMENTALS OF CONVECTION
22ReGr
EnergyKineticEnergyPotential
Convection ForcedConvection Natural
UTLgRior
Ri
∆==
==
β
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Heat Transfer by Cengel 2nd Ed See Examples … to …
Thank You…
CONDUCTION