Lecture 2.6
Methods of heat transfer
Foundation Physics
Heat transfer
We have to take into consideration all three
of the mechanisms of heat transfer.
We will have radiative,
radiative conductive and
convective heat transfers involved in some
way.
This b
Thi
brings
i
us tto th
the fifirstt
law of thermodynamics:
Qtotal = Qconv + Qcond + Qrad
convection
Radiation
Heat source
conduction
The increase in the internal energy of a system is equal to the amount of energy added by heating
the system, minus the amount lost as a result of the work done by the system on its surroundings
Heat conduction
d
Area A
T1
T2
Q/t = kA(T1-T2)/d
kA(T1 T2)/d
Q/t: rate of heat conduction
Material having
thermal
conductivity k
Example:
E
l kAlu 200 J //smC
C
kglass 0.84 J /smC
T1 = 20°C T2 = 0°C
d= 2cm A= 10x10cm
Q/t Alu= 2000 J/s; Q/t Glass= 8.4 J/s
Thermal Conductivities
Methods of heat transfer
• There are three kinds of heat transfer:
Conductive: This is where one object transfers heat
directlyy through
g contact with another object.
j
Radiation: This is when heat is transferred by radiating
off of an object.
C
Convective:
ti
Thi iis where
This
h
h
heatt iis carried
i d ffrom one object
bj t
by a fluid motion in a gas or liquid.
*Convective
Convective heat transfer can be natural or forced
forced.
Forced convection occurs when a fluid flow is induced by
an external force.
Natural convection is caused by buoyancy forces due to
density differences caused by temperature variations in
th fluid.
the
fl id
Convective currents
The pot in the picture is sitting on a
discrete heat source, so we could
expect the current flow to be
somewhat like the arrows shown in
the heating water
water.
Another example of natural
convective heat transfer. Just as
the buoyancy forces caused
density differences in the fluid, it
does the same in gas or air
air.
Nature uses air as an insulator
Thermal conductivity:
y
Water 0.56 J/sec.m.oC
.m.o
oC
Ai
Air
0 023 J/sec
0.023
J/
Forced convection
Heat transfer by radiation
Electromagnetic (EM) radiation, also called light even
though it is not always visible, is a self-propagating wave
in space with electric and magnetic components
components.
EM radiation carries energy and momentum, which may
be imparted when it interacts with matter.
Energy transfer by radiation
Alpha particles consist of two protons and two neutrons,
identical to the nucleus of a helium atom
atom. A sheet of paper
or a person's surface layer of skin will stop them. Alpha
particles are only considered hazardous to a person's health
if they are ingested or inhaled and thus come into contact
with sensitive cells such as in the lungs,
lungs liver and bones.
bones
Beta particles are electrons emitted from the nuclei of many
fission products. They can travel a few feet in air but can
usually be stopped by clothing or a few centimeters of wood.
They are considered hazardous mainly if ingested or
inhaled, but can cause radiation damage to the skin if the
exposure is large enough.
Neutrons which are contained in the nucleus of an atom can
be expelled during fission. They interact weakly with matter
and are very penetrating - not easy to stop. Neutron
radiation typically occurs inside nuclear reactors but water
and concrete provide effective shielding
shielding.
Gamma rays are a form of electromagnetic radiation (like
light, radio, and television) that come from the nucleus of a
radioactive atom. They penetrate matter easily and are best
stopped by water or thick layers of lead or concrete. Gamma
radiation is hazardous to people inside and outside of the
body.
Radiation therapy
Radiation therapy is
used to fight many
types of cancer
cancer.
Radiation targets
rapidly dividing cells
like cancer cells.
Radiation prevents cell
division and the
replication of DNA (the
genetic building
blocks).
Cobalt-60 (Co-60 or 60Co) is a
radioactive metal that is used in
radiotherapy. It produces two gamma
rays with energies of 1.17 MeV and
1.33 MeV.
Effects of heat on the body
Heat transferred into or out of the body and
thermal energy generated by the body itself
can cause temperature changes. Normal
bodyy temperatures
p
fall into a narrow range.
g If
body temperature becomes too high or too
low, significant irreversible damage, even
death, can occur. One serious problem with
the body
body's
s temperature-regulating
temperature regulating mechanism
is that cell metabolism increases with
increasing
temperature.
Increased
metabolism g
generates more than normal
amounts of body heat, and this can cause
temperature to increase further. A vicious
circle results because the increased body
temperature further speeds metabolism and
the production of body heat. If body
temperature rises above about 42°C
((108°F),
) the body's
y cooling
g mechanisms
cannot keep up, and external intervention,
such as an alcohol rub, is necessary.
oC
40
39
38
Usual
range of
normal
37
36
35
Forced convection, blood flow to
h skin,
ki then
h Qconv + Qcond + Qrad
the
assuming the surrounding
temperature is lower.
Problem
• How many grams of of alcohol must be
evaporated from the surface of a 70 kg
person to reduce his temperature by
1.5°C?
Problem
How much will the temperature of a cyclist (speed
(
d
15 km/hr) increase in an hour if none of the body
heat is lost to the surroundings?
(mass cyclist = 76kg, specific heat = 0.83cal /g.oC, rate of energy
consumption (cyclist 15 km/hr) = 400W, Efficiency = 20%,
1cal=4.186J
Pheat = Pin – Pout -> Pheat = Q/t
Q = s.m.T
Diagnostic and therapeutic
uses off heat
h t and
d cold
ld
Cryosurgery (cryotherapy) is the application of extreme
cold to destroy abnormal or diseased tissue.
• Warts, moles, skin tags, solar keratoses, and small skin cancers
•
are candidates for cryosurgical treatment. Some internal
disorders are also treated with cryosurgery,
cryosurgery including liver
cancer, prostate cancer, cervical disorders and, more commonly
in the past, hemorrhoids. Although found to be effective, this
method of treatment is only appropriate for use against localized
diseases with no metastasis.
Cryosurgery works by taking advantage of the destructive force
of freezing temperatures on cells. At low temperatures, ice
crystals form inside the cells, which can tear them apart. More
damage
g occurs when blood vessels supplying
pp y g the diseased
tissue freeze.
Microwave or Radio Diathermy
y
• In the natural sciences,
sciences the term diathermy means
•
"electrically induced heat" and is commonly used for
muscle relaxation. It is also a method of heating
g tissue
electromagnetically or ultrasonically for therapeutic
purpose
Electro-surgery and surgical diathermy involve the use of
high frequency A.C. electrical current in surgery as either
a cutting modality
modality, or else to cauterize small blood
vessels to stop bleeding. This technique induces
localized tissue burning and damage, the zone of which
is controlled by the frequency and power of the device.
Thermography
radiation
Any body with a temperature
greater than absolute zero emits
heat by radiation. The amount off
heat radiated per unit time depends
very strongly on temperature:
Q / t    e  A T 4
σ  5.67x10 8 J/sec  m 2  K 4
e  emmisivity
i i i (value
( l between
b
0 and
d 1)
black surfaces have an emmisivity close to 1
Next Lecture
• To Be Covered: Waves and sound