Lecture 2.6Lecture 2.6

Methods of heat transfer

Foundation PhysicsFoundation Physics

Heat transferHeat transferWe have to take into consideration all threeWe have to take into consideration all three of the mechanisms of heat transfer.We will have radiative conductive andWe will have radiative, conductive and convective heat transfers involved in some

convection

way.

Thi b i t th fi t RadiationconvectionThis brings us to the first

law of thermodynamics:Heat source

conduction

Qtotal = Qconv + Qcond + Qradconduction

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 conductionHeat conductiondd

Area A

Q/t kA(T1 T2)/dT2T1

Q/t = kA(T1-T2)/d

Q/t: rate of heat conduction

E l k 200 J / C

Material having thermal

Example: kAlu 200 J /smCkglass 0.84 J /smCT1 = 20°C T2 = 0°Cd= 2cm A= 10x10cmthermal

conductivity kd= 2cm A= 10x10cmQ/t Alu= 2000 J/s; Q/t Glass= 8.4 J/s

Thermal ConductivitiesThermal Conductivities

Methods of heat transfer

• There are three kinds of heat transfer:There are three kinds of heat transfer: Conductive: This is where one object transfers heat directly through contact with another object.y g jRadiation: This is when heat is transferred by radiating off of an object.C ti Thi i h h t i i d f bj tConvective: This is where heat is carried from one object by a fluid motion in a gas or liquid.*Convective heat transfer can be natural or forcedConvective heat transfer can be natural or 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 fl idthe fluid.

Convective currentsConvective currents

The pot in the picture is sitting on a Another example of natural discrete heat source, so we could expect the current flow to be somewhat like the arrows shown in the heating water

convective heat transfer. Just as the buoyancy forces caused density differences in the fluid, it does the same in gas or airthe heating water. does the same in gas or air.

Nature uses air as an insulatorNature uses air as an insulator

Thermal conductivity:y

Water 0.56 J/sec.m.oCAi 0 023 J/ oCAir 0.023 J/sec.m.oC

Forced convectionForced convection

Heat transfer by radiationHeat 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 componentsin space with electric and magnetic components.EM radiation carries energy and momentum, which may be imparted when it interacts with matter.be imparted when it interacts with matter.

Energy transfer by radiationEnergy transfer by radiationAlpha particles consist of two protons and two neutrons, identical to the nucleus of a helium atom A sheet of paperidentical to the nucleus of a helium 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 liver and boneswith sensitive cells such as in the lungs, liver and 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 canNeutrons 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 shieldingand concrete provide effective 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 therapyRadiation therapyRadiation therapy isRadiation therapy is used to fight many types of cancertypes of cancer. Radiation targets rapidly dividing cells like cancer cells. Radiation prevents cell division and thedivision and the replication of DNA (the genetic building

Cobalt-60 (Co-60 or 60Co) is a radioactive metal that is used in radiotherapy. It produces two gamma genetic building

blocks).rays with energies of 1.17 MeV and 1.33 MeV.

Effects of heat on the bodyEffects of heat on the bodyHeat transferred into or out of the body and

oCHeat transferred into or out of the body andthermal energy generated by the body itselfcan cause temperature changes. Normalbody temperatures fall into a narrow range. If

40y p g

body temperature becomes too high or toolow, significant irreversible damage, evendeath, can occur. One serious problem withthe body's temperature regulating mechanism

39

38the body s temperature-regulating mechanismis that cell metabolism increases withincreasing temperature. Increasedmetabolism generates more than normal

38

37Usual range ofg

amounts of body heat, and this can causetemperature to increase further. A viciouscircle results because the increased bodytemperature further speeds metabolism and

36

range of normal

temperature further speeds metabolism andthe production of body heat. If bodytemperature rises above about 42°C(108°F), the body's cooling mechanisms

35Forced convection, blood flow to h ki h Q Q Q

( ) y gcannot keep up, and external intervention,such as an alcohol rub, is necessary.

the skin, then Qconv + Qcond + Qradassuming the surrounding temperature is lower.

ProblemProblem

• How many grams of of alcohol must be• 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?

ProblemProblem

How much will the temperature of a cyclist ( dHow much will the temperature of a cyclist (speed

15 km/hr) increase in an hour if none of the body heat is lost to the surroundings?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%,

P P P P Q/t

1cal=4.186J

Pheat = Pin – Pout -> Pheat = Q/t

Q = s.m.T

Diagnostic and therapeuticf h t d lduses of heat and cold

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 including liverdisorders are also treated with 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 localizedmethod 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 occurs when blood vessels supplying the diseased g pp y gtissue freeze.

Microwave or Radio Diathermyy

• In the natural sciences the term diathermy meansIn the natural sciences, the term diathermy means "electrically induced heat" and is commonly used for muscle relaxation. It is also a method of heating tissue gelectromagnetically 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 or else to cauterize small blooda cutting modality, or else to cauterize small blood vessels to stop bleeding. This technique induces localized tissue burning and damage, the zone of whichlocalized tissue burning and damage, the zone of which is controlled by the frequency and power of the device.

ThermographyThermography

radiation

Any body with a temperature greater than absolute zero emits

fheat by radiation. The amount of heat radiated per unit time depends very strongly on temperature:

1)d0b( li i iKmJ/sec5.67x10σ

4/428

TAetQ

1closemmisivityanhavesurfacesblack1)and0between(valueemmisivitye

toe

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