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CHEE 203 > Exam 4 > Flashcards

Exam 4 Flashcards

(47 cards)

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1
Q

What is heat transfer by conduction

A

Transfer of energy (heat) by direct contact with materials

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2
Q

When does heat transfer by conduction occur?

A

Occurs in situations when there is no intermixing or flow (contact with solids)

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3
Q

What is the equation for heat transfer by conduction?

A

Q_x = -k(A)(dT/dx)

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4
Q

Which heat transfer mechanism(s) are macroscopic in nature?

A

Convection

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5
Q

Which heat transfer mechanism(s) are microscopic in nature?

A

Conduction and Radiation

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6
Q

What is heat transfer by convection?

A

Transfer of energy (heat) by mixing of materials

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7
Q

When does natural convection occur?

A

Occurs when density differences cause mixing

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8
Q

When does forced convection occur?

A

Occurs due to mechanical agitation

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9
Q

What is the equation for heat transfer by convection?

A

Q = -hA(deltaT)

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10
Q

What is a formula for h?

A

h = k/(boundary layer thickness)

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11
Q

What is heat transfer by radiation?

A

Transfer of energy (heat) by electromagnetic waves from a hot body and absorbed by a cool body

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12
Q

What is the equation for heat transfer by radiation?

A

Q = (sigma)AT^4

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13
Q

(Radiation) As temperature gets hotter, what happens to radiation wavelength?

A

Radiation wavelength increases

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14
Q

(Radiation) What is absorptivity?

A

How strongly a material absorbs radiation. Perfect absorber has a value of 1.

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15
Q

What is the equation for absorptivity?

A

a = (energy absorbed at surface at T1)/(energy emittied by a surface at T2)

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16
Q

(Radiation) What is emissivity?

A

How strongly a material emits radiation. Perfect emisser has a value of 1.

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17
Q

What is the equation for emissivity?

A

E = (energy emitted by surface at T1)/(energy emitted by ideal emitter at T1)

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18
Q

What is an ideal emitter called?

A

A blackbody

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19
Q

Give an example of conduction

20
Q

Give an example of convection

Study These Flashcards
A

Water boiling

21
Q

Give an example of radiation

22
Q

What is the equation for heat loss?

Study These Flashcards
A

Q/A = -K(dT/dx)

23
Q

What is heat flux?

Study These Flashcards
A

Heat flow per unit area

24
Q

What is the equation for heat flux?

Study These Flashcards
A

q = Q/A = -K(dT/dx)

25
When do you not need to solve for heat transfer?
Whenever we know the temperature of a boundary.
26
What does a possible temperature profile look like in multiple resistances?
Linear
27
When you have multiple resistances (multiple mediums), what does heat transfer look like across all mediums?
Even since we are at steady state
28
When you have a semi-infinite system, which term do you use? Why?
When you have a semi-infinite slab because we do not care what happens vertically/out of the plane of the sides.
29
As k value decreases, what happens to the dT/dx term?
It increases
30
What is the surface area of a cylinder?
2*pi*r*L
31
What is the surface area of a sphere?
4*pi*r^2
32
Under what conditions will there be no heat transfer between an object and its' surroundings?
When it is adiabatic (well-insulated). T_object = T_surroundings.
33
Why do we sometimes vary K?
Introduce some variability into our approaches. All are valid. As engineers, we have choices to make regarding speed of calculations vs. accuracy
34
What does a lower K value mean?
Less likely to transfer heat.
35
What does a higher K value mean?
More likely to transfer heat
36
When do you use log main area?
When the circumference of each successive area is different, used a lot in conduction terms
37
What does the Nusselt number tell us?
Ratio of convective heat transfer to conductive heat transfer.
38
What does it mean when you have a high nusselt number?
Convective heat transfer is dominant
39
What does it mean when you have a low nusselt number?
Conductive heat transfer is dominant
40
What does the Prandlt # tell us?
Ratio of momentum diffusivity to thermal diffusivity
41
What is momentum diffusivity?
Kinematic viscosity
42
What does a higher prandlt number mean?
It is more resistant to flow/deformation
43
What is thermal diffusivity?
thermal conductivity divided by the heat transfer and heat capacity at constant pressure
44
What is thermal diffusivity related to?
material's ability to transfer heat
45
What does a high thermal diffusivity mean?
Heat is transferred more easily
46
What does a higher H value encompass?
More heat transfer
47
Which terms can you include in a total heat transfer equation?
Terms only in the boundary

CHEE 203 (2 decks)

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