Etiket Arşivleri: Conduction

Reviewing Heat (Thermal) Energy

Reviewing Heat (Thermal) Energy

Heat (thermal) energy is created by the movement of particles (atoms) that produces heat.

Heat (thermal) energy increases as temperature increases because as temperature increases, atoms move faster (have more kinetic energy)

Transfer of Heat (Thermal) Energy

Heat (thermal energy) is transferred from one object to another when the objects are at different temperatures.

The amount of heat (thermal energy) that is transferred when two objects are brought into contact depends on the difference in temperature between the objects.

Heat is transferred only when two objects are at different temperatures

Thermal energy always moves from warmer to cooler objects

The warmer object loses thermal energy and becomes cooler as the cooler object gains thermal energy and becomes warmer.

Energy will continue to move from a warmer object to a cooler object until both have the same temperature.

Heat Transfer can occur in three ways:





Conduction is the transfer of heat by direct contact (particles collide)

Conduction occurs most easily in solids and liquids


Temperature, Heat, and Thermal Energy

  • Temperature, Heat, and Thermal Energy

  • Heat

  • The transfer of thermal energy

  • Not a measure of energy but rather of energy transferred.

  • Measured in terms of joules or calories (cal).

  • Calories relate heat to changes in temperature.

  • Calorie is defined as the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.

  • Specific Heat

  • The correlation between heat and temperature.

  • It measures how much heat is required to raise the temperature of a certain mass of a given substance.

  • Every substance has a different specific heat, but it is constant for that particular substance.

  • Temperature

  • A property of a material.

  • Depends on the material.

  • Measures the concentration of thermal energy in an object in much the same way that density measures the concentration of matter in an object.

  • A large object will have a lower temperature than a small object with the same amount of thermal energy.

  • Temperature

  • A measure of the average kinetic energy of the molecules that make up that material.

  • Solids are rigid because their molecules do not have enough kinetic energy to go anywhere.

  • The molecules in liquids have enough energy to move around one another but not enough to escape each other.

  • Gas molecules have so much kinetic energy that they disperse and the gas expands to fill its container.

  • Solids, Liquids, and Gases

  • When the temperature of a solid is raised, it will become a liquid. At an even higher temperature, it will become a gas.

  • This is explained by thermal energy.

–When the thermal energy of a solid is increases, the motion of the particles is increased and the temperature increases. This added thermal energy causes the particles to move rapidly so that their motion overcomes the forces holding the particles together.

–At this point the substance has changed from a solid to a liquid. This change occurs at the melting point.

–When a substance is melting, all of the added thermal energy goes to overcome the forces holding the particles together in the solid state.

–Once the solid is completely melted, there are no more forces holding the particles in the solid state and the added thermal energy again increases the particles motion and causes the temperature to rise.

–At the boiling point, further addition for energy causes another change of state where all the added thermal energy converts the material from the liquid state to the gas state.

  • Temperature is Measured in:

  • Celsius (oC)

–Water freezes at 0 oC and boils at 100 oC

  • Fahrenheit(oF)

–Temperature in the United States is measured in Fahrenheit

  • Kelvins (K)

–A measure of absolute temperature.

  • Thermal Energy

  • The energy created by moving particles inside a substance.

  • More movement of particles=more thermal energy

  • More thermal energy=higher temperature

  • Less thermal energy=lower temperature

  • Always moves from hotter objects to cooler objects.

  • Conduction

  • The transfer of energy from one molecule to another.

  • This occurs when molecules hit against each other.

  • Takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other.

  • Metal is a good conductor.

  • Convection

  • The movement of heat by a liquid or gas.

  • The liquid or gas moves from one location to another, carrying heat along with it.

  • This movement of a mass of heated liquid or gas is called a current.

  • Radiation

  • Heat travels from the sun.

  • The transfer of heat by electromagnetic waves.

  • When infrared rays strike a material, the material’s molecules move faster.

  • Ocean Thermal Energy Conversion

  • The ocean’s thermal energy comes from the sun which alters the temperature of the ocean water.

  • Ocean thermal energy can produce electricity

–It does this three different ways

  • Closed-cycle

  • Open-Cycle

  • Hybrid System

  • Closed-cycle

  • This system relies on low-boiling point fluids. The warm ocean water is used to heat up an boil these liquids, which turn a turbine. This turbine produces electricity.

  • Open-Cycle

  • This system relies on a low-pressure environment to actually boil the ocean water and create steam. This stem will turn a turbine.

  • Hybrid System

  • This system combines both the open and closed cycle systems.

  • Commercial Applications for Thermal Energy

  • The thermal energy business deals with energy conservation, efficient conversion, and utilization of fossil fuels and other energy resources.

  • The main focus is on:

–Improving already existing technologies

–Manufacturing newer and better technologies


  • How Do Temperature, Heat, and Thermal Energy Relate

  • Temperature is the measure of heat.

  • Heat is thermal energy that flows from a warmer object to a cooler object.

  • More thermal energy means a higher temperature and less thermal energy means a lower temperature.

Energy Transport

Energy Transport Energy Transport Focus on Æheat transfer Heat Transfer Mechanisms: Heat Transfer Mechanisms: • Conduction • Conduction • Radiation • Radiation • Convection (mass movement of fluids) • Convection (mass movement of fluids)

Conduction Conduction Conduction heat transfer occurs only when there is physical contact between bodies (systems) at different temperatures by molecular motion. Heat transfer through solid bodies is by conduction alone, whereas the heat may transfer from a solid surface to a fluid partly by conduction and partly by convection.

Fourier’s Law of Thermal Conduction Fourier’s Law of Thermal Conduction ∂T qy =−k ∂y temperature the heat flux in thermal gradient in the the y direction conductivity y-direction 2 (K/m)

The proportionality ratio, k, is called thermal conductivity

Thermal Conductivity (k) Thermal Conductivity (k) The thermal conductivity of a substance is defined as the heat flow per unit area per unit time when the temperature decreases by one degree in unit distance. The thermal conductivity is a material property which reflects the relative ease or difficulty of the transfer of energy through the material. It depends on the bonding and structure of the material.

Thermal Diffusivity (α) Thermal Diffusivity (α) ƒThe thermal diffivity is a fundamental quantity. It is analogous to momentum and mass diffusivities. α=k/ρCv where ρand Cv are the density and specific heat of the material, respectively. 2 -1 ƒIn mks system, the unit of thermal diffusivity is m2.s-1, In mks system, the unit of thermal diffusivity is m .s , 2 -1 2 -1 while in the cgs system it is usually cm .s . while in the cgs system it is usually cm .s .

Thermal Radiation Thermal Radiation Thermal radiation is the energy radiated from hot surfaces as electromagnetic waves. It does not require medium for its propagation. Heat transfer by radiation occur between solid surfaces, although radiation from gases is also possible. Solids radiate over a wide range of on certain wavelengths only. The energy flux emitted by an ideal radiator is proportional to the fourth power of its absolute temperature. 4 e =σT b where eb is the emissive power and σ Boltzmann constant.

When thermal radiation strikes a body, it can be absorbed by the body, reflected from the body, or transmitted through the body. The fraction of the incident radiation which is absorbed by the body is called absorptivity ( ) Other fractions of incident radiation which are reflected and transmitted are called reflectivity (symbol ) and transmissivity, The sum of these fractions should be unity i.e.

Convection Convection Convection is the heat transfer within a fluid, involving gross motion of the fluid itself. Fluid motion may be caused by differences in density as in free convection. Density differences are a direct result of temperature differences between the fluid and the solid wall surface. In forced convection, the fluid motion is produced by mechanical means, such as a domestic fan-heater in which a fan blows air across an electric element.

Heat Transfer Coefficient (h) Heat Transfer Coefficient (h) When a moving fluid at one temperature is in contact with a solid at a different temperature, heat exchanges between the solid and the fluid by conduction at a rate given by Fourier’s law. Under such cases the distribution of temperature within the fluid and the heat flux at the wall can be determined by using heat transfer coefficient, h, h=q /(T -T ) =-[k(dT/dy) ]/(T -T ) 0 s f 0 s f where Ts, the surface temperature Tf, bulk fluid temperature q , heat flux at the wall 0 (dT/dy)0, the temperature gradient in the fluid normal to the wall at the fluid-solid interface. k, the conductivity of the fluid

Thermal Conductivity of Gases-I Thermal Conductivity of Gases-I Conduction of energy in a gas phase is primarily by transfer of translational energy from molecule to molecule as the faster moving (higher energy) molecules collide with the slower ones. C Vλ v k = 3 where C , the heat capacity per unit volume, V, the average v speed, λ, the mean free path. κ 3T) 1/ 2 1 k = 2  B3  d  π m  where m, mass of fluid molecules, KB, the Boltzmann constant, T, absolute temperature, d, the center to center distance of two molecules. ƒThe thermal conductivity of gases is independent of pressure an depends only on temperature. This conclusion is valid up to about ten atmospheres (1.0133 x 105 Pa)

Thermal Conductivity of Gases-II Thermal Conductivity of Gases-II Eucken developed the following equation for the thermal conductivity of polyatomic gases at normal pressures,  1.25R η k = C +   p M  where M, molecular weight, C , the heat capacity at constant p pressure.

This figure is valid up to about ten atmospheres.

Thermal Conductivity of Gas Mixtures Thermal Conductivity of Gas Mixtures The thermal conductivity of gas mixtures can be estimated within a few percent by the following equation ∑X k M 1 / 3 i i i k = i mix 1 / 3 ∑X Mi i i where X is the mole fraction of component i having molecular i weight, M , and intrinsic thermal conductivity k . i i

Thermal Conductivity of Solids-I Thermal Conductivity of Solids-I Solids transmit thermal energy by two modes: ƒelastic vibrations of the lattice moving through the crystal in the form of waves ƒfree electrons moving through the lattice also carry energy similar to the case in gases (this is observed in metals)

Thermal Conductivity of Solids-II Thermal Conductivity of Solids-II Each lattice vibration (there is always a spectrum of vibrations) may be described as a traveling wave carrying energy and obeyin the laws of quantum mechanics. By analogy with light theory, the waves in a crystal exhibit the characteristics of particles and are called phonons. Two types of phonon-phonon interaction are observed in solids: ƒNormal or N-type ƒUmklapp (U-process) collision  1.25R η k = C +   p M 

Thermal Conductivity of Solids-III Thermal Conductivity of Solids-III Since the number of phonons increases with temperature and the wavelength of phonons λph is proportional to 1/T. At room temperature and above, molar heat capacity Ĉv for most materials is roughly constant Æthe thermal conductivity of a solid which conducts energy only by phonons, decreases with increasing temperature. 2 λ = 20T d/ γ T ph m where Tm, melting point, T = absolute temperature, d, crystal- lattice dimension, and γ, Gruneisen’s constant (~2 for most solids at ordinary temperatures). THIS IS GENERALLY OBSERVED IN ELECTRICALLY INSULATING SUBSTANCES SUCH AS OXIDES (BUT NOT IN THE FORM OF POROUS, BULK MATERIALS).


Thermal Conductivity of Solids-IV Thermal Conductivity of Solids-IV Phonons are also scattered by ƒdifferences in isotopic masses ƒchemical impurities ƒdislocations ƒsecond phases

Thermal Conductivity of Solids-V Thermal Conductivity of Solids-V In electrical conductors, in addition to phonons, conduction electrons contribute to thermal conductivity. The electronic contribution to the thermal conductivity, kel, 2 2 π n K Tλ k e B el = el 3m V e f 3 where n , the number of free electrons per cm , λ the mean free e el path of electron, Vf, electron velocity at the Fermi surface and me, electron mass. Wiedmann-Franz law and the Lorentz number, L, are utilized to determine what is the dominant mechanism for thermal conduction k L = el = 2 .45 x 10 −8 W ohm K – 2 σ T e