Chapter 16 - Mechanisms of Heat Transfer

Pure crystals and metals have the ____________ thermal conductivities, and gases and insulating materials the __________.

highest , lowest

A high value for thermal conductivity indicates that the material is a good heat conductor, and a low value indicates that the material is a poor heat conductor or a __________

insulator

Unlike conduction and convection, the transfer of heat by radiation does not require the presence of an __________________________

intervening medium

Thermal diffusivity

represents how fast heat diffuses through a material

Convection heat transfer coefficient, h, depends on all the variables influencing convection, such as

surface geometry, nature of fluid motion, properties of the fluid, bulk fluid velocity

Thermal conductivity of materials vary with ___________

temperature

Heat capacity

the amount of heat needed to increase the temperature of an object exactly 1°C, heat capacity per unit volume

A material that has a high thermal conductivity or a low heat capacity will obviously have a large ___________

thermal diffusivity

The larger the ____________________, the faster the propagation of heat into the medium

thermal diffusivity

Heat transfer though a _________ is by radiation

vacuum

In solids, conduction is due to the combination of _________ of the molecules in a lattice and the energy transport by _________________

vibrations and free electrons

Emissivity, ε

A measure of how closely a surface approximates a blackbody for which ε= 1 of the surface. 𝟎≤𝜺≤𝟏.

Thermal conductivity, k

A measure of the ability of a material to conduct heat. The rate of heat transfer through a unit thickness of the material per unit area per unittemperature difference.

Convection = ______ + ______

Conduction + Fluid motion

True or false, heat transfer is by conduction and possibly by radiation in a flowing fluid

False

True or false, heat transfer is by convection and radiation in a still fluid

False

How does the energy of the sun reach the earth?

Heat transfer by radiation is fastest (at the speed of light) an it suffers no attenuation in a vacuum.

Forced convection

If the fluid is forced to flow over the surface by external means such as a fan, pump, or the wind.

Natural (or free) convection

If the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid

What are strong absorbers of radiation?

Liquids

Specific heat

The amount of energy required to raise the temperature of 1 gram of a substance by 1 degree celcius

Kirchhoff's Law

The emissivity and the absorptivity of a surface at a given temperature and wavelength are equal.

Radiation

The energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.

Blackbody

The idealized surface that emits radiation at the maximum rate.

Convection

The mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion.

Stefan-Boltzmann Law

The relationship stating that an object emits energy at a rate proportional to the fourth power of its temperature, in Kelvins.

Temperature gradient, dT/dx

The slope of the temperature curve on a T-x diagram.

Conduction

The transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles.

True of false, most gases between two solid surfaces do not interfere with radiation

True

True or false, copper has a higher thermal conductivity than aluminum.

True

True or false, heat transfer is by conduction and possibly by radiation in a still fluid

True

True or false, heat transfer is by convection and radiation in a flowing fluid

True

h_combined includes the effects of

both convection and radiation

Heat transfer processes that involve _________________ of a fluid are also considered to be convection because of the fluid motion induced during the process, such as the rise of the vapor bubbles during boiling or the fall of the liquid droplets during condensation.

change of phase

In gases and liquids, conduction is due to the _______ and _______ of the molecules during their random motion

collisions and diffusion

In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure _____________.

conduction

The faster the fluid motion, the greater the _____________ heat transfer.

convection

Radiation is usually significant relative to conduction or natural convection, but negligible relative to ____________________

forced convection

Absorptivity, 𝜶

he fraction of the radiation energy incident on a surface that is absorbed by the surface. 𝟎≤𝜶≤𝟏 A blackbody absorbs the entire radiation incident on it (𝜶= 𝟏).

A small value of thermal diffusivity means that

heat is mostly absorbed by the material and a small amount of heat is conducted further.