Thermal

Question 1

Thermal Conduction

Answer 1

Heat transfer between solids in direct contact. Depends on thermal conductivity and temperature gradient.

Question 2

Convection

Answer 2

Heat transfer between a solid and a fluid (liquid or gas) in motion. Depends on fluid velocity, surface area, and temperature gradient.

Question 3

What is thermal radiation?

Answer 3

Heat transfer via electromagnetic radiation, primarily infrared. Requires no medium. All objects emit and absorb radiant energy depending on temperature and emissivity.

Question 4

Units of thermal conductivity

Answer 4

W / (mK)

Question 5

How can you increase the emissivity of a material?

Answer 5

Paint or tape. White out and electrical tape have high emissivity (0.95)

Question 6

What is emissivity?

Answer 6

Emissivity is a unitless number between 0 and 1 which describes the energy radiated from a material’s surface relative to that which would be radiated from a black body (or perfect emitter)

Question 7

How would you design a cooling system for electrical components?

Answer 7

Find thermal constraints (datasheets, manufacturer guidelines), determine heat generation in the system theoretically and experimentally for worst case scenario, evaluate cooling methods like heat sinks, fans

Question 8

How do heat-pumps work?

Answer 8

Compression-condensation-expansion-evaporation. Cooling occurs during evaporation (heat absorbed which evaporates refrigerant). Refrigerant is compressed (increases T and P). Condenser takes high P/T vapor and condenses it to liquid (releases heat). Expansion valve takes liquid refrigerant and reduces P and T.

Question 9

What is Thermal Resistance and how is it used?

Answer 9

A measure of how a material resists the flow of heat (C / W). Can be used to analyze thermal circuits in cases where 1-d, steady state, no heat generation, k is constant. Use thermal analog of Ohm’s Law where q is current, deltaT is voltage.

Question 10

First law of thermodynamics

Answer 10

Change of internal energy of a system is equal to heat added to system and work done by system (i.e. P * deltaV). Conservation of energy

Question 11

Second law of thermodynamics

Answer 11

Entropy of an isolated system tends to increase over time. E.g. systems tend to move towards equilibrium.

Question 12

Third law of thermodynamics

Answer 12

As temp decreases so does entropy

Question 13

Define work of a gas

Answer 13

Integral of pressure times volume (force * length) between two equilibrium states

Question 14

What are some considerations when selecting materials for high-temp applications

Answer 14

Specific heat capacity, thermal expansion, resistance to corrosion and creep deformation

Question 15

What are the base units of a Watt

Answer 15

J/s or N-m / s

Question 16

What is specific heat capacity?

Answer 16

Amount of heat per unit mass required to raise temperature by 1K or C

Question 17

What is heat in a wire dependent on?

Question 18

What is a PRT?

Question 19

What is a thermocouple?

Question 20

What is an RTD?

Question 21

What is a NTC thermistor?

Question 22

What are some mechanisms of thermal stress in mechanical components?

Answer 22

Rapid or uneven heating and cooling and thermal expansion can cause fatigue and deformation

Question 23

How to mitigate thermal stress?

Answer 23

Choose components with low coefficients of thermal expansion, utilizing thermal management systems like heat sinks or fans. Using simulation tools to predict thermal behavior can identify potential design issues up front

Question 24

What is a Peltier Cooler?

Question 25

What is Stfan-Boltzmann’s Law?

Question 26

How does temperature affect BLDC motors?

Question 27

How does temperature affect batteries?

Question 28

What is thermal inertia and how does it affect temperature management system design?

Answer 28

Thermal inertia is how slowly the temperature of something approaches that of its surroundings. It depends on size, shape, absorptivity, specific heat, and conductivity.

Question 29

Is heat added to a system during liquid to gas transition?

Answer 29

lost

Question 30

What is entropy?

Answer 30

Entropy, s, quantifies disorder or randomness of a system. dS = integral(dQrev/T) where Qrev is reversible heat transfer

Question 31

Enthalpy

Answer 31

Total heat content of a system at constant pressure. H = U + PV where U is internal inergy, P is pressure, V is volume. -H is exothermic (heat released)