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MEC203 - Heat Transfer > 1 - Introduction And Basic Concepts > Flashcards

1 - Introduction And Basic Concepts Flashcards

1
Q

Define heat

A

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

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2
Q

Define thermodynamics

A

Concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another.

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3
Q

Define heat transfer

A

Deals with the determination of the rates of such energy transfers as well as variation of temperature.

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4
Q

Points about heat transfer

A

Always from high-temperature medium to low-temperature medium.
Stops when the two mediums reach the same temperature.
Can be transferred in three different modes: conducton; convection and radiation.

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5
Q

What do rating problems deal with?

A

The determination of the heat transfer rate for an existing system at a specified temperature difference.

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6
Q

What do sizing problems deal with?

A

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

Advantages of the experimental approach

A

We deal with the actual physical system.

The desired quantity is determined by measurement within the limits of experimental error.

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8
Q

Disadvantages of the experimental approach

A

Expensive
Time-consuming
Often impractical

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9
Q

Advantages of the analytical approach (including numerical approach)

A

Fast

Inexpensive

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10
Q

Disadvantages of the analytical approach

A

Results obtained are subject to the accuracy of the assumptions, approximations and idealisations made in the analysis.

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11
Q

Define specific heat

A

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

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12
Q

What does specific heat depend on at low pressures?

A

Temperature only, as at low pressures all real gases approach ideal gas behaviour.

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13
Q

Define incompressible substance

A

A substance whose specific volume/density does not change with temperature or pressure.

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14
Q

What are the specific heats of an incompressible substance?

A

Constant-volume and constant-pressure specific heats are identical.
Specific heat depends on temperature only.

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15
Q

Mechanisms for energy transfer

A

Heat transfer

Work

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16
Q

Define heat transfer rate

A

The amount of heat transferred per unit time.

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17
Q

Define heat flux

A

The rate of heat transfer per unit area normal to the direction of heat transfer.

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18
Q

What is the first law of thermodynamics?

A

Energy can neither be created nor destroyed during a process; it can only change forms.
(Conservation of energy principle)

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19
Q

Define mass flow rate

A

The amount of mass flowing through a cross-section of a flow device per unit time.

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20
Q

Define volume flow rate

A

The volume of a fluid flowing through a pipe or duct per unit time.

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21
Q

What is surface energy balance?

A

Energy remains constant during a process as a surface contains no volume or mass, and thus no energy (E in = E out).
This relation is valid for both steady and transient conditions, and the surface energy balance does not involve heat generation since a surface does not have a volume.

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22
Q

Define conduction

A

The 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.

23
Q

What is conduction caused by in gases and liquids?

A

Collisions and diffusion of the molecules during their random motion.

24
Q

What is conduction caused by in solids?

A

The combination of vibrations of the molecules in a lattice and the energy transport by free electrons.

25
Q

Define thermal conductivity (k)

A

A measure of the ability of a material to conduct heat.

26
Q

Define temperature gradient (dT/dx)

A

The slope of the temperature curve on a T-x diagram.

27
Q

Define specific heat

A

Heat capacity per unit mass

28
Q

Define thermal diffusivity (alpha)

A

Represents how fast heat diffuses through a material

29
Q

Effect of larger thermal diffusivity

A

High thermal conductivity
Low heat capacity
Faster propagation of heat into medium

30
Q

Effect of smaller value of thermal diffusivity

A

Heat is mostly absorbed by the material

A small amount of heat is conducted further

31
Q

Define 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.

32
Q

Define forced convection

A

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

33
Q

Define natural/free convection

A

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

34
Q

Which heat transfer process is used to describe one that involves a change of phase?

A

Convection because of the fluid motion induced during the process, such as the rise of the vapour bubbles during boiling or the fall of the liquid droplets during condensation.

35
Q

What does the convection heat transfer coefficient (h) depend on?

A

Surface geometry
Nature of fluid motion
Fluid properties
Bulk fluid velocity

36
Q

Define radiation

A

The energy emitted by matter in the form of EM waves or photons as a result of the changes in the electronic configurations of the atoms or molecules.

37
Q

Points about radiation

A

Does not require an intervening medium.
Heat transfer by radiation is fastest (speed of light) and it suffers no attenuation in a vacuum.
All bodies at a temperature above absolute zero emit thermal radiation.
Radiation is a volumetric phenomenon.
All solids, liquids and gases emit, absorb, or transmit radiation to varying degrees.
However, radiation is usually considered to be a surface phenomenon for solids.

38
Q

Define blackbody

A

The idealised surface that emits radiation at the maximum rate.

39
Q

Define emissivity (epsilon)

A

A measure of how closely a surface approximates a blackbody for which epsilon = 1 for the surface.

40
Q

What is blackbody radiation?

A

Represents the maximum amount of radiation that can be emitted from a surface at a specified temperature.

41
Q

Define absorptivity (alpha)

A

The fraction of the radiation energy incident on a surface that is absorbed by the surface (0-1).
A blackbody absorbs the entire radiation incident on it (alpha = 1).

42
Q

What is Kirchhoff’s law?

A

The emissivity and the absorptivity of a surface at a given temperature and wavelength are equal.

43
Q

Define net radiation heat transfer

A

The difference between the rates of radiation emitted by the surface and the radiation absorbed.

44
Q

What does the determination of the net rate of heat transfer by radiation depend on?

A

Surface properties
The surfaces’ orientation relative to each other
The interaction of the medium between the surfaces with radiation

45
Q

When is radiation significant?

A

It’s significant relative to conduction or natural convection, but negligible relative to forced convection.

46
Q

What type of heat transfer in opaque solids?

A

Conduction only

47
Q

What type of heat transfer in semitransparent solids?

A

Conduction and radiation

48
Q

When can a solid involve convection and/or radiation?

A

On surfaces exposed to a fluid or other surfaces.

49
Q

Heat transfer process in a still fluid

A

Conduction and possibly radiation.

50
Q

Heat transfer process in a flowing fluid

A

Convection and radiation.

51
Q

In the absence of radiation, what is the heat transfer process through a fluid?

A

Either conduction or convection, depending on the presence of any bulk fluid motion.
Convection = conduction + fluid motion

52
Q

Heat transfer process through a vaccuum

A

Radiation

53
Q

Which phase is a strong absorber of radiation?

A

Liquid

54
Q

How many mechanisms of heat transfer may a medium involve simultaneously?

A

Only two

MEC203 - Heat Transfer (12 decks)

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