More thermal properties
Download
1 / 48

More Thermal Properties - PowerPoint PPT Presentation


  • 150 Views
  • Uploaded on

More Thermal Properties. Thermal Conductivity. Thermal conductivity is the study of how heat flows through different materials. It depends on the temperature, and the temperature difference. Heat Transfer. The science of how heat flows is called heat transfer.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' More Thermal Properties' - davina


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

Thermal conductivity
Thermal Conductivity

  • Thermal conductivity is the study of how heat flows through different materials.

  • It depends on the temperature, and the temperature difference


Heat transfer
Heat Transfer

  • The science of how heat flows is called heat transfer.

  • There are three ways heat transfer works: conduction, convection, and radiation.

  • Heat flow depends on the temperature difference.


Thermal equilibrium
Thermal Equilibrium

  • Two bodies are in thermal equilibrium with each other when they have the same temperature.

  • In nature, heat always flows from hot to cold until thermal equilibrium is reached.


Heat conduction
Heat Conduction

  • Conduction is the transfer of heat through materials by the direct contact of matter.

  • Dense metals like copper and aluminum are very good thermal conductors.


Insulator
Insulator

  • A thermal insulator is a material that conducts heat poorly.

  • Heat flows very slowly through the plastic so that the temperature of your hand does not rise very much.


Insulation
Insulation

  • Styrofoam gets its insulating ability by trapping spaces of air in bubbles.

  • Solids usually are better heat conductors than liquids, and liquids are better conductors than


Thermal insulation
Thermal Insulation

  • The ability to conduct heat often depends more on the structure of a material than on the material itself.

    • Solid glass is a thermal conductor when it is formed into a beaker or cup.

    • When glass is spun into fine fibers, the trapped air makes a thermal insulator.



Conduction
Conduction

  • Conduction is the transfer of heat by the direct contact of the particle of matter.



Thermal conductivity1
Thermal Conductivity to the molecules/atoms of the spoon

  • The thermal conductivity of a material describes how well the material conducts heat.


Thermal conductivity2
Thermal Conductivity to the molecules/atoms of the spoon

  • Heat conduction in solids and liquids works by transferring energy through bonds between atoms or molecules.


Some conductivity values
Some Conductivity Values to the molecules/atoms of the spoon


Thermal conduction eqn
Thermal Conduction Eqn to the molecules/atoms of the spoon

  • PH = k A (T2 -T1)/L

  • PH = heat flow (watts)

  • k = thermal conductivity (watts/m-k)

  • A = cross-sectional area through which the heat flows (m2)

  • L = length the heat travels (m)

  • T1, T2 = Temperatures on either side


Conductivity example
Conductivity example to the molecules/atoms of the spoon

  • Consider a fiberglass matt that is 5 cm thick and covers a wall that is 2.5 meters high and 4 meters wide.

  • The outside temperature is 5° C, and the indoor temperature is 25° C.

  • How much heat is lost through this wall?


Example cont
Example cont to the molecules/atoms of the spoon

  • We start with

  • PH = k A (T2 -T1)/L

  • we know k = 0.038 W/mK, A = 2.4 x 4 = 10 m2,

  • L = 5 cm = 0.05 m, and T2 - T1 = 25-5 = 20.

  • So,

  • PH = 0.038 x 10 x 20/ 0.05

  • PH = 152 watts


Convection
Convection to the molecules/atoms of the spoon

  • Convection is the transfer of heat by the motion of liquids and gases.

    • Convection in a gas occurs because gas expands when heated.

    • Convection occurs because currents flow when hot gas rises and cool gas sink.

    • Convection in liquids also occurs because of differences in density.


Convection1
Convection to the molecules/atoms of the spoon

Convection is directional


Convection2
Convection to the molecules/atoms of the spoon

  • When the flow of gas or liquid comes from differences in density and temperature, it is called free convection.

  • When the flow of gas or liquid is circulated by pumps or fans it is called forced convection.


Convection3
Convection to the molecules/atoms of the spoon

  • Convection depends on speed.

  • Motion increases heat transfer by convection in all fluids.


Convection4
Convection to the molecules/atoms of the spoon

  • Convection depends on surface area.

  • If the surface contacting the fluid is increased, the rate of heat transfer also increases.

  • Almost all devices made for convection have fins for this purpose.


Natural convection
Natural Convection to the molecules/atoms of the spoon

  • Near coastlines, convection is responsible for sea breezes.

  • During the daytime, land is much hotter than the ocean.

  • A sea breeze is created when hot air over the land rises due to convection and is replaced by cooler air from the ocean.

  • At night the temperature reverses so a land breeze occurs.


Natural convection1
Natural Convection to the molecules/atoms of the spoon

  • Much of the Earth’s climate is regulated by giant convection currents in the ocean.


Convection5
Convection to the molecules/atoms of the spoon

  • PH = h A (T2 -T1)

  • PH = Heat flow (w)

  • h = Heat transfer coefficient (w/m2 K)

  • A = contact area of the fluids

  • T2 - T1 = change in temperature


Convection example
Convection example to the molecules/atoms of the spoon

  • The surface of a window is 18°C (64°F).

  • A wind at 5°C (41°F) is blowing on the window fast enough to make the heat transfer coefficient 100 W/m2 °C.

  • How much heat is transferred between the window and the air if the area of the window is 0.5 square meters?


Convection6
Convection to the molecules/atoms of the spoon

  • PH = h A (T2 -T1)

  • PH = 100 x 0.5 (18-5)

  • PH = 650 W


Radiation
Radiation to the molecules/atoms of the spoon

  • Radiation is heat transfer by electromagnetic waves.

  • Thermal radiation is electromagnetic waves (including light) produced by objects because of their temperature.

  • The higher the temperature of an object, the more thermal radiation it gives off.


Radiation1
Radiation to the molecules/atoms of the spoon


Radiant heat
Radiant Heat to the molecules/atoms of the spoon

  • We do not see the thermal radiation because it occurs at infrared wavelengths invisible to the human eye.

  • Objects glow different colors at different temperatures.


Radiant heat1
Radiant Heat to the molecules/atoms of the spoon

  • A rock at room temperature does not “glow”.

  • The curve for 20°C does not extend into visible wavelengths.

  • As objects heat up they start to give off visible light, or glow.

  • At 600°C objects glow dull red, like the burner on an electric stove.


Radiant heat2
Radiant Heat to the molecules/atoms of the spoon


Radiant heat3
Radiant Heat to the molecules/atoms of the spoon

  • As the temperature rises, thermal radiation produces shorter wavelength, higher energy light.

  • At 1,000°C the color is yellow-orange, turning to white at 1,500°C.

  • If you carefully watch a bulb on a dimmer switch, you see its color change as the filament gets hotter.

  • The bright white light from a bulb is thermal radiation from an extremely hot filament, near 2,600°C.


Blackbody
Blackbody to the molecules/atoms of the spoon

  • an idealized object that absorbs all radiation falling on it.

  • Blackbodies absorb and incandescently re-emit radiation in a characteristic, continuous spectrum.

  • Because no light (visible electromagnetic radiation) is reflected or transmitted, the object appears black when it is cold.

  • However, a black body emits a temperature-dependent spectrum of light. This thermal radiation from a black body is termed black-body radiation.


Blackbody radiation
Blackbody Radiation to the molecules/atoms of the spoon

  • The graph of power versus wavelength for a perfect blackbody is called the blackbody spectrum.


Blackbody radiation1
Blackbody radiation to the molecules/atoms of the spoon

  • The white-hot filament of a bulb is a good blackbody because all light from the filament is thermal radiation and almost none of it is reflected from other sources.

  • The curve for 3,000 K shows that radiation is emitted over the whole range of visible light.


Radiant heat4
Radiant Heat to the molecules/atoms of the spoon

  • The total power emitted as thermal radiation by a blackbody depends on temperature (T) and surface area (A).

  • Real surfaces usually emit less than the blackbody power, typically between 10 and 90 percent.

  • The Kelvin temperature scale is used in the Stefan-Boltzmann formula because thermal radiation depends on the temperature above absolute zero.


Blackbody radiation2
Blackbody Radiation to the molecules/atoms of the spoon

  • P = σ AT4

  • P = power output

  • σ = Stefan-Boltzmann constant 5.67 x 10-8 watts/m2K4)

  • A = surface area of blackbody

  • T = temperature


Lightbulb calculation
Lightbulb calculation to the molecules/atoms of the spoon

  • The filament in a light bulb has a diameter of 0.5 millimeters and a length of 50 millimeters.

  • The surface area of the filament is 4 × 10-8 m2.

  • If the temperature is 3,000 K, how much power does the filament radiate?


Lightbulb
Lightbulb to the molecules/atoms of the spoon

  • P = σ AT4

  • P = 5.67 x 10-8 x 4 x 10-8 x 3,0004

  • P = 0.1836 W


Figure 7.6 to the molecules/atoms of the spoonThermal shock resulting from constraint of uniform thermal expansion. This process is equivalent to free expansion followed by mechanical compression back to the original length.


Figure 7.7 to the molecules/atoms of the spoonThermal shock resulting from temperature gradients created by a finite thermal conductivity. Rapid cooling produces surface tensile stresses.


Figure 7.8 to the molecules/atoms of the spoonThermal quenches that produce failure by thermal shock are illustrated. The temperature drop necessary to produce fracture (T0 − T) is plotted against a heat-transfer parameter (rmh). More important than the values of rmh are the regions corresponding to given types of quench (e.g., water quench corresponds to an rmh around 0.2 to 0.3). (From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley & Sons, Inc., New York, 1976.)


ad