Thermodynamics- Heat Transfer

Question 1

What is heat transfer?

Answer 1

Thermal energy in transit due to a temperature difference

Question 2

What is thermal energy?

Answer 2

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

Question 3

Heat rate

Answer 3

Thermal energy transfer per unit time

Question 4

Heat flux

Answer 4

Thermal energy transfer per unit time and surface area

Question 5

What is conduction as a mode of heat transfer?

Answer 5

Heat transfer in a solid or a stationary fluid due to the random motion of its constituent atoms, molecules and/or electrons.

Question 6

What is convection as a mode of heat transfer?

Answer 6

Heat transfer due to the combined influence of bulk and random motion for fluid flow over a surface.

Question 7

What is radiation as a mode of heat transfer?

Answer 7

Energy that is emitted by matter due to changes in the electron configurations of its atoms or molecules and is transported as EM waves (photons)

Question 8

Fourier’s law of conduction

Answer 8

At a steady state:
q”=-kVT
k is thermal conductivity (W/m•K)
V is a triangle and is gradient operator.

Question 9

Do good conductors have high or low value of thermal conductivity?

Answer 9

High

Question 10

Fourier’s law in one dimension

Answer 10

qx”=-kdT/dx

Question 11

How does the velocity distribution compare with the temperature distribution for flow over a heated surface?

Answer 11

Velocity lowest near the plate and increases and flattens off further away. Temperature of fluid greatest near surface and decreases in mirror image of velocity distribution.

Question 12

Newton’s law of cooling for convection

Answer 12

q”=h(Ts-Too)
h is convection heat transfer coefficient (W/m^2•K)
Ts is surface temperature
Too is temperature far from the boundary layer (oo is infinity symbol)

Question 13

What is irradiation?

Answer 13

Radiation incident from the surroundings.

Symbol G

Question 14

Formula for energy outflow due to emission

Answer 14

E=εσTs^4=εEb
E is emissive power (W/m^2)
ε is surface emissivity (0-1)
σ is Stefan-Boltzmann constant 5.67x10^-8 W/m^2•K^4
Eb is emissive power of a black body

Question 15

What is the surface emissivity of a black body?

Answer 15

1

Question 16

What does surface emissivity measure?

Answer 16

How efficiently a surface emits energy relative to a blackbody which is an ideal radiator. It depends on surface material and finish.

Question 17

Formula for energy absorption due to irradiation

Answer 17

Gabs=αG
Gabs is absorbed incident radiation (W/m^2)
α is surface absorbivity (0-1)
G is irradiation

Question 18

Formula for irradiation

Answer 18

G=Gsur=σTsur^4

Tsur is temperature of surroundings

Question 19

Formula for heat flux due to radiation when α=ε

Answer 19

qrad”=εσ(Ts^4-Tsur^4)