Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Mechanics > Heat Transfer > Flashcards

Heat Transfer Flashcards

1
Q

The second law tells us that energy has quality as well as…?

A

…quantity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

The direction of heat flow is governed by the…?

A

…2nd Law of Thermodynamics

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Heat can be transferred by…?

A

…conduction, convection, and radiation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

The rate at which heat transfers is measured in…?

A

Joules per second (J/s), or Watts (W)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

The rate of heat transfer is given the symbol…

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

In fluids, conduction is due to…?

A

…collision and diffusion of the molecules

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

In solids, conduction is due to the…

A

…vibration of the molecules in a lattice

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

The rate at which conduction occurs in a medium depends on…(3 things)

A
  1. Geometry of the medium
  2. Thickness of the medium
  3. Temperature difference across the medium
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Give a property of an insulator?

A

Low rates of heat conduction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Steady-state heat conduction is governed by Fourier’s Law

What is his law? (Equation)

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

“Rate of heat conduction is proportional to…”

What is k?

A
  • Rate of heat conduction is proportional to the area of the body, the temperature difference across it, and its thickness
  • k is the thermal conductivity
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is Natural convection (or free convection)

A

Natural convection is when the fluid motion is not generated by any external source (like a pump, fan, suction device, etc.) but only by density differences in the fluid occurring due to temperature gradients

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is Forced convection?

A

Fluid is forced to flow over the solid surface by external forces (i.e. a fan, the wind)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Convection processes are expressed through Newton’s Law of Cooling

What is the equation for this?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is this called?

And what does each symbol stand for?

A

Newton’s Law of Cooling

  • h is the convection heat transfer coefficient
  • As is the surface area of the body
  • Ts is the surface temperature
  • T∞ is the temperature of the fluid far from the surface
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Radiation is a volumetric phenomenon

Define this

A

all solids, liquids and gases absorb and emit radiation to some degree

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

An idealised body which emits radiation at this maximum rate is called a…?

A

…blackbody

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Radiation heat transfer is expressed by the Stefan-Boltzmann Law

What is the equation for this?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is σ (sigma) in this equation?

A

σ is the Stefan-Boltzmann constant 5.67x10-8Wm-2K-4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What form of heat transfer does

Opaque, Gas and Vacuum do?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

The rate of heat conduction is…

A

…inversely proportional to the thickness of the layer

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

A high value of k implies that the material is a ____ ____

A low value of k implies ____ ____ ____ - i.e. an ____

A

A high value of k implies that the material is a good conductor

A low value** of k implies **poor heat conduction - i.e. an insulator

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

When you intergrate and rearrange what do you get?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Rwall = ?

And what is Rwall ?

A

Rwall​ = L/kA

Rwall​ is called the thermal resistance of the wall

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q
  • Rate of heat transfer corresponds to the ____ ____
  • Thermal resistance corresponds to the ____ ____
  • Temperature difference corresponds to the ____ ____
A
  • Rate of heat transfer corresponds to the electric current
  • Thermal resistance corresponds to the electrical resistance
  • Temperature difference corresponds to the potential difference
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What does this correspond to in terms of an electric current? (Equation)

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Qconv = ?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

Rconv = ?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

Under steady conditions, the rate of heat convection into the wall… = ? = ?

A

Under steady conditions, the rate of heat convection into the wall = rate of heat conduction through the wall = rate of heat convection from the wall

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

Rcond = ?

Rconv = ?

A

Rcond = L / kA

Rconv = 1 / hAs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

Area of a sphere A = ?

A

A = 4πr2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

For a hollow cylinder (or sphere) undergoing heat transfer by conduction with convection, the total thermal resistance can be denoted as

Rtotal = ?

A

Rtotal = Rconv,1 + Rcyl + Rconv,2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

Plot the variation of heat transfer with insulating material radius, r2

A
  • The variation is not linear
  • There is a critical radius at which heat transfer is a maximum
34
Q

For a cylinder, the critical radius is…

For a sphere, the critical radius is…

A

<span>r</span>cr,cylinder = k/h

<span>r</span>cr,sphere = 2k/h

35
Q

For common insulating materials, the critical radius is of the order of…?

Insulation on electrical wires is of the order of…?

A

…1 cm

…1 mm thick

36
Q

The convective heat transfer coefficient depends on several flow properties and also depends on whether the flow is…

A

…laminar or turbulent in nature

37
Q

What is the no-temperature-jump condition

A

That if a surface has some temperature T, it follows from the no-slip condition that the temperature of the fluid one wall should also be T

38
Q

Place a fluid layer between two plates of differing temperature;

if the fluid is motionless, the heat transfer will be by ____ only,

if the fluid is in motion, heat transfer will be by ____

A

Place a fluid layer between two plates of differing temperature;

if the fluid is motionless, the heat transfer will be by conduction only,

if the fluid is in motion, heat transfer will be by convection

39
Q

Define heat flux

A

Rate of heat transfer per unit time per unit surface area

40
Q

Heat flux equation for conduction

A
41
Q

Heat flux equation for convection

A
42
Q

What is the equation for the Nusselt number?

A
43
Q

Define Prandtl’s number and the equation

A

The Prandtl number is defined as the ratio of momentum diffusivity to heat diffusivity (i.e. the relative thickness of the velocity and thermal boundary layers)

44
Q

A low Prandtl number means that…

A

…the thermal boundary layer is thicker than the velocity boundary layer (high heat diffusivity rate)

45
Q

For a flat plate, the critical Reynolds number (i.e. the Reynolds number at the x-location where transition occurs)… = ?

A
46
Q

For Re number,

Laminar < ? < Turbulent

A

Laminar < 5x105​ < Turbulent

47
Q

Fluid properties are evaluated at the…?

(Name and equation)

A

Film temperature

48
Q
  • µs is the flow dynamic viscosity at the ____ of the ____
  • µ∞ is the dynamic viscosity in the ____ ____
A
  • µs is the flow dynamic viscosity at the surface of the sphere
  • µ∞ is the dynamic viscosity in the freestream flow
49
Q

As with flat plate flows, the flow in a pipe depends on the…(Equation as well)

A

…Reynolds number

50
Q
  • For Re < 2300, the flow is ____
  • For Re > 4000, the flow is ____
A
  • For Re < 2300, the flow is laminar
  • For Re > 4000, the flow is turbulent
51
Q

For the thermal boundary layer, the entrance length for laminar flow can be obtained from is…(equation)

A
52
Q

For turbulent flow, the entrance length for the thermal boundary layer is approximately…

A

…10 pipe diameters

53
Q

For a rough pipe, the ____ Chart can be used to determine the ____ ____

A

For a rough pipe, the Moody Chart can be used to determine the Nusselt number

54
Q

High temperature fluids have a ____ ____ than colder fluids

A

High temperature fluids have a lower density than colder fluids

55
Q

What is the equation of state for an ideal gas?

A
56
Q

The Grashof number represents the ratio of…?

A

…the buoyant force to the viscous force

57
Q

What is the equation for the Grashof Number?

A

(We are expected to remember this)

58
Q

What does every symbol in this equation stand for?

A
59
Q

What is the equation for the Rayleigh number of the flow?

A
60
Q

But can alos be obtained by…?

A

…

61
Q

Natural convection from a vertical plate - flow pattern is different when the flow is…?

A

…laminar or turbulent

62
Q

When is the Nusselt number vertically and horizontally laminar?

A

Vertically when

104 < Ra < 109

Horizontally when

104 < Ra <<sup> </sup>107

63
Q

When is the Nusselt number vertically and horizontally turbulent?

A

Vertically when

Ra > 109

Horizontally when

Ra > 107

64
Q

Nusselt number when Laminar flow, horizontal plate face down

A

105 < Ra < 1011

65
Q

c (speed of light) = ?

And define all symbols

A

c is the speed of light

funny thing is the wavelength

v is the frequency

66
Q

e (energy) = ? = ?

And what is h?

A

h is Planck’s constant (6.6256x10-34 Js)

67
Q

The shorter the wavelength of the photon, the higher its…?

A

…energy

68
Q

Define a blackbody and give an example

A

A blackbody is defined as a perfect emittier and absorber of radiation

A good blackbody is the sun (or every star) because sthe surface termperature barely changes

69
Q

What is the Stefan-Boltzmann Law (equation)

A
70
Q

A radiator works by…?

A

…convection current

71
Q

What is ε?

A

ε is the emissivitiy of the surface

72
Q

The Stefan-Boltzmann Law tells us the…

A

…heat transfer rate over all wavelengths

73
Q

The colour of room temperature object is due to selective reflection of radiation by the surface

(try to remember picture/diagram)

A
74
Q

Kirchoff’s law for absorbtions states…

A

…that the emissivity and absorptivity for a body are the same

75
Q

If a smaller body is enclosed in a larger body, the net rate of heat transfer is given by…(equation)

A

(Not sure how important this is)

76
Q

What is the solar flux at the sun, S0, using the Stegan Boltzmann law? (Equation)

A
77
Q

Solar flux decreases with the square of distance from the object so S = ?

A
78
Q

The Earth is 1 Astronomical Unit (AU) away from the Sun - 1AU = ?

A

1.49x1011m

79
Q

Tln = ?

A
80
Q
A
81
Q

Q = __ - __ / _____

A

Q = T1 - T∞ / Rtotal

Mechanics (4 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions