Week 8 - Introduction to Heat Transfer

Question 1

Thermodynamics is concerned with…

Answer 1

The AMOUNT of heat transfer as a system undergoes a process from one equilibrium to another.

Question 2

Heat Transfer deals with…

Answer 2

The determination of RATES of such energy transfers as well as respective variation of temperature.

Question 3

Thermal energy

Answer 3

Associated with the translation, rotation, vibration and electronic states of the atoms and molecules that comprise matter. It represents the cumulative effect of microscopic activities and is directly linked to the temperature of matter.

Question 4

Heat

Answer 4

The transfer of energy across the boundary of a system due to a temperature difference.

Question 5

Conduction

Answer 5

Occurs due to the interaction of micro-scale energy carriers within a material.

Question 6

Convection

Answer 6

A process by which heat is transferred by movement of a fluid such as air or water.

Question 7

Radiation

Answer 7

Occurs due to energy transferred by electromagnetic waves.

Question 8

Fourier’s Law of Conduction

Answer 8

The rate of conduction heat flux is proportional to the temperature gradient.

Question 9

Thermal conductivity (k ; W/(K.m))

Answer 9

A property that reflects the characteristics of the energy carriers in the material

Question 10

Thermal conductivity (k) patterns

Answer 10

Pure metals have the largest k

• Electrons are numerous and fast
• Conductivity is related to electrical resistivity (Wiedemann-Franz law)

Alloys have lower k

• Impurities impede the motion of electrons

Non-metals have lower k

• Energy carried by phonons

Liquids have lower k

• Energy carried by molecules

Gases have lowest k

Question 11

Newton’s Law of Cooling

Answer 11

A physical law which states that the rate of heat loss of a body is directly proportional to the difference in temperatures between the body and its environment.

q = h x ( Tbody - Tenv)

T is in Kelvin

Question 12

Heat Transfer Coefficient (h)

Answer 12

Heat transfer coefficients depend on the velocity, fluids, the object, temperatures etc.

Question 13

Heat Transfer by Thermal Radiation

Answer 13

All bodies constantly emit energy by the process of electromagnetic radiation. The intensity of the radiant energy flux depends on the temperature of the body and the nature of its surface.

Question 14

Transmission, absorption and reflection

Answer 14

A body’s behaviour with regard to thermal radiation is characterised by its transmission (tau), absorption (alpha), and its reflection (rho).

alpha + rho + tau = 1

Question 15

Black body

Answer 15

A black body is an idealised physical body that absorbs all incident electromagnetic radiation, regardless of frequency, or angle of incidence.

alpha = 1 ; rho = 0 ; tau = 0

Question 16

Black body radiation

Answer 16

The thermal electromagnetic radiation within, or surrounding a body in thermodynamic equilibrium with its environment.

Question 17

Black body radiation

Answer 17

Has a specific, continuous spectrum of wavelengths, inversely related to intensity, that depend only on the body’s temperature, which is assumed, for the sake of calculations and theory, to be uniform and constant.