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Heat Transfer

Heat Transfer. How is heat transferred from one place to another? What is moving? In mechanics energy can be transferred through a particle (e.g. a bullet) or a wave (e.g. a sound wave) In heat transfer the analogous methods are convection and conduction

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Heat Transfer

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  1. Heat Transfer • How is heat transferred from one place to another? • What is moving? • In mechanics energy can be transferred through a particle (e.g. a bullet) or a wave (e.g. a sound wave) • In heat transfer the analogous methods are convection and conduction • Heat can also be transferred by radiation • both a particle and a wave (but not really)

  2. Conduction • If you place one end of a metal bar in a fire the other end get hot • The end in the fire experiences a large vibration of the molecules of the metal • These molecules bump into adjacent molecules passing the energy up the bar • This is called conduction • The movement of heat from a high temperature region to a low temperature region through another material

  3. Conduction Through a Slab

  4. Conductive Heat Transfer • The rate at which heat is transferred by conduction is given by dQ/dt = kA (TH - TC)/L • Where: • H is the rate of heat transfer • Q is heat and t is time • k is the thermal conductivity (in W/ m K) • A is the cross sectional area of the material (in the direction of heat transfer) • L is the thickness of the material • T is the temperature (hot or cold)

  5. Thermal Conductivities • Metals generally have high k • For Al, k=235 for Cu, k=428 (W/ m K) • Al and Cu make good pots and pans • Materials with low k are good thermal insulators • For air, k=0.026 for polyurethane foam, k=0.024 • We use foam to insulate our houses • Down filled winter coats trap air for insulation • Consider touching air, wood or metal at the same temp – metal feels coldest because it carries heat away from your hand the fastest.

  6. Composite Slabs • To find the heat transfer through a composite slab made of several materials you need to know the thermal conductivity and the thickness of each dQ/dt = A (TH - TC)/S (L/k) • Where S (L/k) is the sum of the ratios of the thickness and thermal conductivity of each layer of the slab

  7. Heat Loss Through a Wall

  8. Radiation • Energy can be directly transported by photons • This is how you are warmed by sunlight • The power (in Watts) that is emitted by an object depends on its temperature (T), its area (A) and it emissivity (e) Pr = seAT4 • Where s is the Stefan-Boltzmann constant = 5.6703 X 10-8 W/m2 K4 • Emissivity has a value between 0 and 1 • T must be in absolute units (Kelvin)

  9. Absorption of Radiation • Every object also absorbs radiation at a rate determined by its properties and the temperature of its environment Pa =seATenv4 • Where Tenv is the temperature of the environment • Any object thus has a net energy exchange rate with its environment of: Pn = Pa -Pr = seA(Tenv4 - T4)

  10. Blackbody Radiation • Objects with an emissivity of 1 are called blackbody emitters or absorbers • They absorb all of the radiation incident on them • Objects that are dark in color absorb more radiation than light objects (have larger e) • Every object whose temperature is above 0 K emits thermal radiation • People emit thermal radiation at infrared wavelengths and thus can be detected at night with IR goggles

  11. Convection • Hot air (or any fluid) expands and becomes less dense than the cooler air around it • The hot air is thus lighter and rises • If the hot air cools as it rises it will eventually fall back down to be re-heated and rise again • This is called a convection cell • Examples: baseboard heating, boiling water, Earth’s atmosphere

  12. Convection Rate Factors • Fluidity • Material must be free to move • Energy exchange with environment • How easy is it to heat (by conduction or radiation) the material in the first place? • How rapidly will the material lose heat? • Temperature difference • A small temperature difference may result in not enough density difference to move

  13. Greenhouses and the Greenhouse Effect • Greenhouses prevent heat from escaping from near the Earth’s surface • By preventing convection • Greenhouse gases in the greenhouse effect slow heat from escaping from near the Earth’s surface • By slowing radiation

  14. 1st Law of Thermodynamics • The energy of an object can be changed by • Work or heat • We will consider only changes in the object internal energy • Internal energy = total microscopic kinetic and potential energy of the atoms and molecules making up the object • It does not include kinetic energy of the center of mass nor the potential energy due to the position of the center of mass • 1st Law: • DEint = Q-Wby • Q is + if heat is absorbed by object, - if heat is given off by object • Wby is + if work is done by object, - if work is done on object

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