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MEC203 > Introduction and basic concepts > Flashcards

Introduction and basic concepts Flashcards

1
Q

What is heat?

A

The form of energy that can be transferred from one system another as a result of temperature difference

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2
Q

Transfer of energy is

A

always from higher temperature to lower temperature

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3
Q

Heat transfer stops when

A

two mediums reach the same temperature

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4
Q

Three main types of heat transfer

A

conduction convection and radiation

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5
Q

What are rating problems in heat transfer

A

deal with determination of the heat transfer rate for an existing system at a specified temperature difference

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6
Q

What are sizing problems

A

deal with the determination of the size of a system in order to transfer heat at a specified rate for a specified temperature difference

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7
Q

Advantage of experimental approach

A

actual physical system, quantity determined by measurement within the limits of experimental error

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8
Q

Disadvantage of experimental approach

A

slow expensive and often impractical

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9
Q

analytical approach adv disadv

A

fast and inexpensive but results obtained are subject to accuracy of the assumptions

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10
Q

Sum of all microscopic forms of energy called

A

internal energy

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11
Q

what is enthalpy made up of

A

flow energy and internal energy (Pv + u)

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12
Q

What is the specific heat

A

energy required to raise the temperature of a unit mass of a substance by one degree

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13
Q

two types of specific heat

A

specific heat at constant volume and at constant pressure

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14
Q

What do real gasses approach ideal gas behaviour and what does this mean

A

at low pressure and therefore specific heats depend on temperature only

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15
Q

change in internal energy =

A

m * specific heat at constant volume * change in temperature

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16
Q

change in enthalpy

A

m * specific heat at constant pressure * change in temperature

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17
Q

heat transfer rate is

A

the amount of heat transferred per unit time

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18
Q

heat flux is the (units)

A

rate of heat transfer per unit area normal to the direction of heat transfer W/m^2

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19
Q

symbol of heal flux

A

little q with a dot

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20
Q

symbol of heat transfer

A

Q with a dot

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21
Q

relationship between heat flux and heat transfer

A

q. = Q. / A

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22
Q

Power =

A

the work done per unit time

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23
Q

What is the first law of thermodynamics also called

A

conservation of energy principal

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24
Q

What does the first law of thermodynamics state

A

energy can neither be created nor destroyed during a process it can only change forms

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25
Q

what does the first law of thermodynamics mean

A

Total energy enetring the system - Total energy leaving the system = change in total energy of the system
Ein - Eout = delta E

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26
Q

In steady operation the rate of energy transfer

A

to a system is equal to the rate of energy transfer from the system ie Ein = Eout

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27
Q

Typical heat balance equation for heat transfer problem

A

Q in - Q out + Egen = detla E

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28
Q

How is the conversion of nuclear, chemical, mechanical and electrical energies into thermal treated

A

as heat generation

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29
Q

What is a closed system

A

consists of a fixed mass

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30
Q

what does a closed system mean for heat transfer

A

Ein - Eout = delta U = m *specific heat transfer at constant volume * delta T

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31
Q

Why are steady flow systems important

A

can be used to analyse control volume which are used alot

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32
Q

What does steady mean

A

no change with time at a specified location

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33
Q

mass flow rate =

A

density * Velocity * Ac (cross sectional area)

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34
Q

volume flow rate =

A

Velocity * Ac

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35
Q

Heat/energy transfer in steady flow systems

A

m. * delta h = m. cp delta T

36
Q

Surface energy balanace

A

E.in = E.out

37
Q

Why is the surface energy balance so simple

A

as no volume or mass and thus no energy generations

38
Q

Heat transfer for a surface

A

Q.in = Q.out

39
Q

When does conduction occur

A

when there is no movement (solids)

40
Q

when does convection occur

A

when you have a fluid moving (gas/liquid)

41
Q

What is radiation and what does it not require

A

no median required

42
Q

what do all modes of heat transfer require

A

existence of a temperature difference

43
Q

What is conduction

A

transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles

44
Q

What does fouriers law describe

A

heat transfer by conduction

45
Q

What is fouriers law

A

Q. = - kA dT/dx

46
Q

what is k in fouriers law

A

thermal conductivity

47
Q

how can fouriers law be simplified in linear varition of temperature

A

dT/dx = change in T/thickness

48
Q

why is there a negative sign in fouriers law

A

heat is conducted in the direction of decreasing temperature, and the temperature gradient becomes negative when temperature decreases with increasing x. The negative sign in the equation ensure that heat transfer in the positive x direction is a positive quantity

49
Q

What is thermal conductivity

A

a measure of the ability of a material to conduct heat
defined as the rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference

50
Q

What is delta T / delta x (fouriers law)

A

slope of the temperature curve on a T-x diagram

51
Q

heat flux in fouriers law=

A

-k delta T/delta x

52
Q

when does dT/dx = delta T / delta x

A

when your object is long and thin as assuing that it is constant in calculations

53
Q

what does a high value of thermal conductivity indicate

A

that the material is a good heat conductor and a low value indicates the material is a good insulator

54
Q

highest thermal conductivity materials

A

diamond graphite 1000W/mK

55
Q

lowest thermal conductivity materials

A

air carbon dioxide 0.1 W/mK

56
Q

symbol of thermal conductivity

A

k

57
Q

units of k

A

W/m*K

58
Q

What will k always be in this module

A

constant

59
Q

heat capacity =

A

density * cp (J/m^3*K)

60
Q

thermal diffusivity symbol =

A

alpha = heat conduction/heat storage = k/rho * cp (m^2/s)

61
Q

what does thermal diffusivity represent

A

how fast heat diffuses through a material

62
Q

larger the thermal diffusivity

A

the faster the propagation of heat into the medium

63
Q

what does a small value of thermal diffusivity represent

A

heat is mostly abosrbed by the the material and a small amount of heat is conducted further

64
Q

What is convection

A

the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion and it involves the combined effects of conduction and fluid motion

65
Q

faster the fluid motion the

A

fast the convection heat transfer

66
Q

in the absence of bulk fluid motion heat tranfer between a solid surface and the adjacent fluid is by

A

pure conduction

67
Q

What is Tinf

A

temperature at dist where fluid does not feel affect of plate (can be very small)

68
Q

What is forced convection

A

fluid is forced to flow over the surface by external means such as a fan, pump, or the wind

69
Q

natural or free convection

A

if the fluid motion is caused by the buoyancy forces that are induced by density differences due to the variation of temperature in the fluid

70
Q

what are heat transfer processes that involve a change of phase considered

A

convection because of the fluid motion induced during the process such as the rise of vapour bubbles during boiling or the fall of the liquid droplets during condenation

71
Q

Equation of convection

A

Q. conv = h*As(Ts - Tinf)

72
Q

what is h in convection

A

convection heat transfer coefficient W/m^2 *K

73
Q

what is the h coefficient not

A

a property of the fluid it is an experimentally determined parameter whose value depends on all the variables influencing convection

74
Q

what factors effect h coefficient

A

surface geom
nature of fluid motion
properties of the fluid
bulk fluid velocity

75
Q

Radiation is

A

the energy emitted by matter in the form of electromagnetic waves as a result of the changes in the electronic configurations of the atoms or molecules

76
Q

What does radiation not require

A

an intervening medium

77
Q

Do all bodies above absolute zero do

A

emit thermal radiation

78
Q

Radiation is what type of phenomenon

A

a volumetric phenomenon, however usually considered surface for solids

79
Q

Equation of radiation =

A

Q.emit = emissivity * sigma * As * (Ts^4 - Tsurr^4)

80
Q

what is sigma for equation of net radiation transfer

A

the stefan boltzman constant

81
Q

what has an emissivity of 1

A

a blackbody, idealised surface that emits radiation at max rate

82
Q

what is emissivity

A

a measure of how closely a surface approximates a blackbody between 0 and 1

83
Q

What is net radiation transfer

A

difference between the rate of radiation emitted by the surface and the radiation absorbed

84
Q

How does radiation compare to other heat transfer processes

A

significant relative to conduction or natural convection but negligible relative to forced convection

85
Q

What is a combined heat transfer coefficient

A

it includes the effects of both convection and radiation

86
Q

Physical method of heat transfer in solid liquid and gasses

A

solids conduction is due to combination of the vibrations of the molecules in a lattice and energy transported by free electrons, in gases and liquid it is due to the collisions of the molecules during their random motion

MEC203 (12 decks)

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