Heat Transfer Engineering Terms & Definitions Study Set

Conduction:

Heat transfer through a stationary medium or between stationary media

Convection

Heat transfer between the surface of a medium and a moving fluid

Radiation

Heat transfer by emission and absorption of electromagnetic waves

Temperature gradient

Conduction is proportional to the

Particle

Particles with more energy transfer energy to particles with less energy

Conduction is heat transfer from ______ Interaction

Microsale convection

Energy transfer between particles and surface

Macrosale convection

Energy transfer from bulk fluid motion

the temperature difference

Convection is proportional to

Forced Convection:

Flow is caused by an outside "prime mover" (fan, pump, etc.)

Natural (Free) Convection:

Flow is caused by density gradient(buoyancy)

surrounding medium.

Can occur in a vacuum

Radiation Does not require a ________ and can occur in a _______

Fin effectiveness

Fin value that tells you if you need to add a fin

Fins

Surface extensions designed to increase heat transfer rates

Take the base temp and distribute it over the entirety of the fin

What is the goal of a fin

Fin Efficiency

Fin value that tells you if the right material is being used based on how well the fin performs

Heat moved by the fin / heat moved by the bare surface

Fin effectiveness is a ratio of

heat moved by fin / heat moved by fin if all T = Tbase

Fin efficiency is a ratio of

heat transfer coefficient h

Adding fins is useful if _ for a fluid is low

thermal conductivity k

Solids with high _ produce more effective fins

0

As fin length approaches infinity, fin efficiency approaches

convective heat transfer coefficient, rate of heat transfer between a solid surface and a fluid per surface area

what is h and what does it tell you

Thermal conductivity,

measure of a materials ability to conduct heat

What is k and what does it tell you

conduction

What type of heat transfer is Fouriers law used in

No

The heat rate used to relate to Ohm's law is an equation for no heat generation when there is heat generation, the formula doesnt properly relate to Ohm's law

Is thermal resistance applicable when there is heat generation? why

Resistance created by the gaps between rough surfaces

What is contact resistance

Bi < 0.1

The temperature gradient in a solid body is so small that we can assume the body to be at a solid uniform temperature

When is lumped capacitance used (numerical reason and stated reason)

Convective

A small biot # the (conductive or convective) resistance dominates

Conductive

A large biot # the (conductive or convective) resistance dominates

dimensionless

Fourier # and Biot # are both

time

Fourier # is a dimensionless ________

How fast the heat transfers

Time constant tells you

how rapidly a change in T propagates through a material

Thermal diffusivity (alpha) tells you

convection heat transfer

Newtons law of cooling is used for what heat transfer type

The heat flux emitted by a surface /

The heat flux emitted by a black body surface

What is emissivity a ratio of

Black body

What is an ideal emitter in radiation called

1

A black body emits with a emmisivity of __

energy, all energy is absorbed.

Since no energy is reflected you cant see it so its called a black body

A black body reflects no _____, meaning __. How does this relate to its name?

Surroundings

In radiation, _______ are large compared to the bodies

Gray bodies

Surfaces in radiation problems are known as

ε, emmisivity

Gray bodies are described by ___

Radiation, when Area of gray body is <<< than Area of Surroundings

Large enclosure model is used in which type of heat transfer and when is it applicable

Radiation

In what type of heat transfer problems is stefan Boltzman law used

The upper limit to emmisive power in the form of the black body equation

What does Stefan Boltzman's law state? ( it states a certain limit)

Annular fins

When is the modified bessel equation used

Constant Temp

Constant Heat Flux

Convective boundary

What are the three boundary conditions in the conduction equation?

Constant T initial

Known Temp distribution

Lumped capacitance model

What are the three initial conditions in the conduction heat equation?

constant heat flux

The adiabatic boundary condition is a sub type of what boundary condition?

1. Convective Tip

2. Adiabatic Tip

3. Fixed Temperature Tip

4. Infinite Fin

What are the 4 Fin Tip conditions

Cosh and sinh

A fin of constant cross-sectional area uses what type of math function in its formula

The spacial part (temperature gradient)

What part of the conduction heat equation needs boundary conditions?

The Temporal part (storage part, right side of equation)

What part of the conduction heat equation needs initial conditions?

The temporal part

If steady state what part of the conduction equation = 0

Est and Egen

When performing surface energy balance what two terms in the first law are eliminated

The outside of the tube because air has a lower h

Picture A tube with hot liquid inside and air on the outside, what side needs fins and why

Convection boundary,

It's a mixture of the constant T and Constant heat flux conditions

Which boundary condition is known as the mixed boundary condition

Knowing the temperature slope

Knowing the heat flux at the boundary surface is the equivalent of

Both boundaries

In the constant temp boundary condition, T is known at

Parabolic

The conduction equation with generation results in what kind of temperature distribution