Heat Transfer (Ch1-3)

What heat transfer mechanisms happen in opaque solids?

Conduction only

What heat transfer mechanisms happen in semitransparent solids?

Conduction and radiation

What heat transfer mechanisms happen in a still fluid (no bulk fluid motion)?

Conduction, possibly radiation

What heat transfer mechanisms happen in flowing fluids?

Convection and radiation

What heat transfer mechanisms happen in a vacuum?

Radiation

What are usually strong absorbers of radiation?

Liquids

Why is Fourier’s law considered phenomenological?

Because it’s not derived, and is only based on experimental observations

Why is k (thermal conductivity) low for liquids and gases?

Due to intermolecular spaces, which reduces rate for energy transport

What has higher k (thermal conductivity), crystalline or amorphous materials?

Crystalline materials have higher k

What are examples of transport properties?

Diffusion rate coefficients, viscosity, and thermal conductivity

What are examples of thermodynamic properties?

Related to equilibrium, density, heat capacity

What happens if thermal diffusivity is high?

A material will respond quickly to the change in isothermal environment

What happens if thermal diffusivity is small?

A material will respond slowly to the change in isothermal environment

What is contact resistance?

Happens between 2 solid bars that are in contact, where temperature may drop

Why does contact resistance happen?

Due to the surface roughness and gaps between two bars

What can we do to prevent contact resistance?

Use fillers like conducting liquid, grease or soft metals

What are some sources of heat generation?

Chemical reactions, passing a current through an electrical medium, deceleration, absorption of neutrons, absorption of electromagnetic radiations

When is the thermal circuit (conduction resistance) not applicable?

If there is heat generation, since heat flux is not independent of coordinates

When documenting Temperature vs x in a symmetrical plane wall (both surfaces kept at Ts), when is the curve concave up and when is it concave down?

Concave up if it consumes heat, concave down if it generates heat

If Ts is fixed, how do we increase the heat transfer rate?

Increasing h, decreasing fluid temperature (t infinity), increasing surface area

Why do we use extended surfaces, instead of other methods?

Increasing h is insufficient, and reducing T infinity is impractical

What is the purpose of extended surfaces?

To increase surface area, and increase heat transfer rate

How do we increase h (convection coefficient)?

Increasing fluid velocity

What are the 2 types of fins?

Straight fins and annular fins

What are straight fins?

Fins attached to a plane wall

What is an annular fin?

Fins attached to a cylinder

What is the effectiveness of a fin?

How much it helped, fin vs no fin

What is the efficiency of a fin?

How well it works, fin vs fin max

How much should the effectiveness be to consider a fin effective?

2 or more

How can you increase the fin effectiveness?

Increase thermal conductivity, increase ratio of perimeter to cross-sectional area, and decrease h

Are fins better for gas or liquid cooling?

Gas

Why are fins better for gas cooling, rather than liquid cooling?

Because the h is smaller in gases

How is fin length related to efficiency?

Increasing length decreases efficiency

Which ones are more efficient, fins with triangular, parabolic or rectangular profiles?

Triangular and parabolic. They use less material

When does the fin become not justifiable economically?

If it’s efficiency drops below 60%

What is the efficiency of most fins used in real life?

Above 90%