HeatTransfer Power Point

7/24/2019 HeatTransfer Power Point

1/8

1

Energy Transfer

! When two objects of differenttemperatures are placed in thermalcontact, the temperature of the warmerdecreases and the temperature of thecooler increases

! The energy exchange ceases when theobjects reach thermal equilibrium

! The concept of energy was broadenedfrom just mechanical to include internal! Made Conservation of Energy a universal

law of nature

Methods of Heat Transfer

! Need to know the rate at whichenergy is transferred

! Need to know the mechanismsresponsible for the transfer

! Methods include! Conduction

! Convection

! Radiation

Conduction

! The transfer can be viewed on anatomic scale

! It is an exchange of energy between

microscopic particles by collisions! Less energetic particles gain energy during

collisions with more energetic particles

! Rate of conduction depends upon thecharacteristics of the substance


2/8

2

Conduction example

! The molecules vibrateabout their equilibriumpositions

! Particles near the stovecoil vibrate with largeramplitudes

! These collide withadjacent molecules andtransfer some energy

! Eventually, the energytravels entirely throughthe pan and its handle

Conduction, cont.

! In general, metals are good conductors

! They contain large numbers of electronsthat are relatively free to move through themetal

! They can transport energy from one region

to another

! Conduction can occur only if there is adifference in temperature between twoparts of the conducting medium

Conduction, equation

! The slab allowsenergy to transferfrom the region of

highertemperature tothe region oflowertemperature


3/8

3

Conduction, equationexplanation

! A is the cross-sectional area

! L = !x is the thickness of the slab orthe length of a rod

! P is in Watts when Q is in Joules and t isin seconds

! k is the thermal conductivityof thematerial! See table 11.3 for some conductivities

! Good conductors have high k values andgood insulators have low k values

Home Insulation

! Substances are rated by their Rvalues! R = L / k

! See table 11.4 for some R values

! For multiple layers, the total Rvalue is the sum of the R values ofeach layer

! Wind increases the energy loss by

conduction in a home

Conduction and Insulationwith Multiple Materials

! Each portion will have a specificthickness and a specific thermalconductivity

! The rate of conduction througheach portion is equal


4/8

4

Multiple Materials, cont.

! The rate through the multiplematerials will be

! THand TCare the temperatures atthe outer extremities of thecompound material

Convection

! Energy transferred by themovement of a substance

! When the movement results fromdifferences in density, it is callednatural conduction

! When the movement is forced by afan or a pump, it is called forcedconvection

Convection example

! Air directly abovethe flame iswarmed andexpands

! The density of theair decreases, andit rises

! The mass of airwarms the handas it moves by


5/8

5

Convection applications

! Boiling water

! Radiators

! Upwelling

! Cooling automobile engines

! Algal blooms in ponds and lakes

Convection CurrentExample

! The radiator warmsthe air in the lowerregion of the room

! The warm air is lessdense, so it rises tothe ceiling

! The denser, coolerair sinks

! A continuous aircurrent pattern is setup as shown

Radiation

! Radiation does not require physicalcontact

! All objects radiate energy

continuously in the form ofelectromagnetic waves due tothermal vibrations of the molecules

! Rate of radiation is given byStefans Law


6/8

6

Radiation example

! The electromagnetic waves carry theenergy from the fire to the hands

! No physical contact is necessary

! Cannot be accounted for by conductionor convection

Radiation equation

!

! The power is the rate of energytransfer, in Watts

! != 5.6696 x 10-8W/m2.K4

! A is the surface area of the object

! e is a constant called the emissivity

! e varies from 0 to 1

! T is the temperature in Kelvins

Energy Absorption andEmission by Radiation

! With its surroundings, the rate atwhich the object at temperature Twith surroundings at Toradiates is

!

! When an object is in equilibrium withits surroundings, it radiates andabsorbs at the same rate

! Its temperature will not change


7/8

7

Ideal Absorbers

! An ideal absorberis defined as anobject that absorbs all of theenergy incident on it! e = 1

! This type of object is called a blackbody

! An ideal absorber is also an idealradiator of energy

Ideal Reflector

! An ideal reflector absorbs none ofthe energy incident on it

! e = 0

Applications of Radiation

! Clothing! Black fabric acts as a good absorber

! White fabric is a better reflector

!

Thermography! The amount of energy radiated by an object

can be measured with a thermograph

! Body temperature! Radiation thermometer measures the

intensity of the infrared radiation from theeardrum


8/8

8

Resisting Energy Transfer

! Dewar flask/thermos bottle

! Designed to minimizeenergy transfer tosurroundings

! Space between walls is

evacuated to minimizeconduction and convection

! Silvered surface minimizesradiation

! Neck size is reduced

Global Warming

! Greenhouse example

! Visible light is absorbed and re-emitted as infrared radiation

! Convection currents are inhibited bythe glass

! Earths atmosphere is also a goodtransmitter of visible light and agood absorber of infrared radiation

HeatTransfer Power Point

Documents

Transcript of HeatTransfer Power Point