1 - Introduction And Basic Concepts

Question 1

Define heat

Answer 1

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

Question 2

Define thermodynamics

Answer 2

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

Question 3

Define heat transfer

Answer 3

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

Question 4

Points about heat transfer

Answer 4

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.

Question 5

What do rating problems deal with?

Answer 5

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

Question 6

What do sizing problems deal with?

Answer 6

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

Question 7

Advantages of the experimental approach

Answer 7

We deal with the actual physical system.

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

Question 8

Disadvantages of the experimental approach

Answer 8

Expensive
Time-consuming
Often impractical

Question 9

Advantages of the analytical approach (including numerical approach)

Answer 9

Fast

Inexpensive

Question 10

Disadvantages of the analytical approach

Answer 10

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

Question 11

Define specific heat

Answer 11

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

Question 12

What does specific heat depend on at low pressures?

Answer 12

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

Question 13

Define incompressible substance

Answer 13

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

Question 14

What are the specific heats of an incompressible substance?

Answer 14

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

Question 15

Mechanisms for energy transfer

Answer 15

Heat transfer

Work

Question 16

Define heat transfer rate

Answer 16

The amount of heat transferred per unit time.

Question 17

Define heat flux

Answer 17

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

Question 18

What is the first law of thermodynamics?

Answer 18

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

Question 19

Define mass flow rate

Answer 19

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

Question 20

Define volume flow rate

Answer 20

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

Question 21

What is surface energy balance?

Answer 21

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.

Question 22

Define conduction

Answer 22

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.

Question 23

What is conduction caused by in gases and liquids?

Answer 23

Collisions and diffusion of the molecules during their random motion.

Question 24

What is conduction caused by in solids?

Answer 24

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

Question 25

Define thermal conductivity (k)

Answer 25

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

Question 26

Define temperature gradient (dT/dx)

Answer 26

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

Question 27

Define specific heat

Answer 27

Heat capacity per unit mass

Question 28

Define thermal diffusivity (alpha)

Answer 28

Represents how fast heat diffuses through a material

Question 29

Effect of larger thermal diffusivity

Answer 29

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

Question 30

Effect of smaller value of thermal diffusivity

Answer 30

Heat is mostly absorbed by the material | A small amount of heat is conducted further

Question 31

Define convection

Answer 31

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.

Question 32

Define forced convection

Answer 32

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

Question 33

Define natural/free convection

Answer 33

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

Question 34

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

Answer 34

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.

Question 35

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

Answer 35

Surface geometry Nature of fluid motion Fluid properties Bulk fluid velocity

Question 36

Define radiation

Answer 36

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.

Question 37

Points about radiation

Answer 37

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.

Question 38

Define blackbody

Answer 38

The idealised surface that emits radiation at the maximum rate.

Question 39

Define emissivity (epsilon)

Answer 39

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

Question 40

What is blackbody radiation?

Answer 40

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

Question 41

Define absorptivity (alpha)

Answer 41

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).

Question 42

What is Kirchhoff’s law?

Answer 42

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

Question 43

Define net radiation heat transfer

Answer 43

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

Question 44

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

Answer 44

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

Question 45

When is radiation significant?

Answer 45

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

Question 46

What type of heat transfer in opaque solids?

Answer 46

Conduction only

Question 47

What type of heat transfer in semitransparent solids?

Answer 47

Conduction and radiation

Question 48

When can a solid involve convection and/or radiation?

Answer 48

On surfaces exposed to a fluid or other surfaces.

Question 49

Heat transfer process in a still fluid

Answer 49

Conduction and possibly radiation.

Question 50

Heat transfer process in a flowing fluid

Answer 50

Convection and radiation.

Question 51

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

Answer 51

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

Question 52

Heat transfer process through a vaccuum

Answer 52

Radiation

Question 53

Which phase is a strong absorber of radiation?

Answer 53

Liquid

Question 54

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

Answer 54

Only two