heat transfer prelim

Conduction

heat transfer in a medium. When a temperature gradient exists in a stationary medium, which may be a solid or a fluid may be viewed as the transfer of energy from the more energetic to the less energetic particles of a substance due to interactions between the particles.

Heat flux

heat transfer rate in the x direction per unit area perpendicular to the direction of transfer, and it is proportional to the temperature gradient, dT/dx, in this direction

Lattice vibrations

in a solid, conduction may be attributed to atomic activity in the form of ______

The transport properties

include the diffusion rate coefficients such as k, the thermal conductivity (for heat transfer), and v, the kinematic viscosity (for momentum transfer)

The energy generation

is a manifestation of some energy conversion process involving thermal energy on one hand and some other form of energy, such as chemical, electrical, or nuclear, on the other.

thermal conductivity

is an important property of the material. the _____ of a solid is larger than that of a liquid, which is larger than that of a gas.

energy generation term

is associated with conversion from some other form of internal energy (chemical, electrical, electromagnetic, or nuclear) to thermal energy

latent heat exchange

is generally associated with a phase change between the liquid and vapor states of the fluid

first law of thermodynamics

is simply a statement that the total energy of a system is conserved, and therefore the only way that the amount of energy in a system can change is if energy crosses its boundaries.

Fourier's law

is the cornerstone of conduction heat transfer. is a vector expression indicating that the heat flux is normal to an isotherm and in the direction of decreasing temperature. applies for all matter, regardless of its state (solid, liquid, or gas).

hydrogen fuel cell

is the type that might eventually be used for automotive applications and is an electro-chemical reactor that will cease operating if its internal components are contaminated with impurities.

Heat transfer

is thermal energy in transit due to a spatial temperature difference is a dominant aspect of nearly every energy conversion and production device

thermal conductivity of liquids

is usually insensitive to pressure except near the critical point.

molecular weight

it is generally true that thermal conductivity decreases with increasing ______

volumetric heat capacity

measures the ability of a material to store thermal energy.

diffusion of energy

net transfer of energy by random molecular motion

Bulk fluid motion

originates from the fact that the boundary layer grows as the flow progresses in the x direction.

Thermodynamic properties

pertain to the equilibrium state of a system. Density and specific heat (cp) are two such properties used extensively in thermodynamic analysis.

transport property

provides an indication of the rate at which energy is transferred by the diffusion process. It depends on the physical structure of matter, atomic and molecular, which is related to the state of the matter.

Emissivity

radiative property of the surface termed the ______

Irradiation

rate at which all such radiation is incident on a unit area of the surface

Convection

refers to heat transfer that will occur between a surface and a moving fluid when they are at different temperature

Hydrodynamic boundary layer

region in the fluid through which the velocity varies from zero at the surface to a finite value u associated with the flow.

Thermal boundary layer

region of the fluid through which the temperature varies from at to in the outer flow.

Latent component

relates to intermolecular forces influencing phase change between solid, liquid, and vapor states; a

isothermal surface

the direction of heat flow will always be normal to a surface of constant temperature, called an _____

Free convection

the flow is induced by buoyancy forces, which are due to density differences caused by temperature variations in the fluid.

thermal diffusivity

the ratio of the thermal conductivity to the heat capacity is an important property termed the ___ measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy

gas

the thermal conductivity of a solid may be more than four orders of magnitude larger than that of a ____

thermal radiation

third mode of heat transfer is termed ____ electragnetic waves is energy emitted by matter that is at a nonzero temperature

Advection

transport due to bulk fluid motion.

Forced convection

when the flow is caused by external means, such as by a fan, a pump, or atmospheric winds

mean free path

which is the average distance traveled by an energy carrier (a molecule) before experiencing a collision.

phonon mean free path

which again is determined by collisions with imperfections or other phonons

effective thermal conductivity

which depends on the thermal conductivity and surface radiative properties of the solid material, as well as the nature and volumetric fraction of the air or void space

electron mean free path

which is defined as the average distance traveled by an electron before it collides with either an imperfection in the material or with a phonon.

temperature distribution

which represents how temperature varies with position in the medium

hypothermia

which results from excessive cooling of the human body

rate of change of thermal energy stored

At any point in the medium the net rate of energy transfer by conduction into a unit volume plus the volumetric rate of thermal energy generation must equal the ________ within the volume.

kinetic theory of gases

From this theory it is known that the thermal conductivity is directly proportional to the density of the gas, the mean molecular speed , and the mean free path - mfp, which is the average distance traveled by an energy carrier (a molecule) before experiencing a collision.

rigid matrix

If small voids or hollow spaces are formed by bonding or fusing portions of the solid material, a _____ is created

reflected

If the surface is opaque portions of the irradiation are _______

Transmitted

If the surface is semitransparent, portions of the irradiation may also be __________

lattice

In the modern view of materials, a solid may be comprised of free electrons and atoms bound in a periodic arrangement called the ______

temperature gradient

In the presence of a __________, energy transfer by conduction must then occur in the direction of decreasing temperature

less effective

Since the intermolecular spacing is much larger and the motion of the molecules is more random for the fluid state than for the solid state, thermal energy transport is ______

Electromagnetic waves

The energy of the radiation field is transported by _________

First Law of Thermodynamics over a time interval

The increase in the amount of energy stored in a control volume must equal the amount of energy that enters the control volume, minus the amount of energy that leaves the control volume.

nonmetallic liquids

The thermal conductivity of ______ generally decreases with increasing temperature.

gases and liquids

The thermal conductivity of ______ is therefore generally smaller than that of solids.

heat diffusion equation

This equation, often referred to as the heat equation, provides the basic tool for heat conduction analysis

cellular insulation.

When these spaces are sealed from each other, the system is referred to as a _______

phonons

When viewed as a particle-like phe- nomenon, the lattice vibration quanta are termed ______

Cooling fins-For personal computers

________ (also known as heat sinks) are fabricated of a high thermal conductivity material (usually aluminum) and attached to the microprocessors to reduce their operating temperatures

Control volume or open system

a region of space bounded by a control surface through which mass may pass.

Chemical component

accounts for energy stored in the chemical bonds between atoms

Nuclear component

accounts for the binding forces in the nucleus.

Sensible component

accounts for the translational, rotational, and/or vibrational motion of the atoms/molecules comprising the matter; a

Nanostructured materials

are chemically identical to their conventional counterparts but are thermally processed to provide very small grain sizes in the final material and, from the heat transfer perspective, significantly increase the scattering and reflection of energy carriers at the grain boundaries.

Liquid metals

are commonly used in high heat flux applications, such as occur in nuclear power plants.

Reflective insulations

are composed of multilayered, parallel, thin sheets or foils of high reflectivity, which are spaced to reflect radiant energy back to its source

energy source

associated with the rate of thermal energy generation

Flow work

associated with work done by pressure forces moving fluid through the boundary

Thermal insulations

consist of low thermal conductivity materials combined to achieve an even lower system thermal conductivity.

thermodynamics

deals with the end states of the process during which an interaction occurs and provides no information concerning the nature of the interaction or the time rate at which it occurs is concerned with equilibrium states of matter, where an equilibrium state necessarily precludes the existence of a temperature gradient

Modes

different types of heat transfer processes

Random molecular motion

dominates near the surface where the fluid velocity is low

Sensible

energy transferred during convection (energy transfer occurring within a fluid due to the combined effects of conduction and bulk fluid motion.)

fuel cells

generate power from environmentally benign fuels such as hydrogen.