Thermal Flashcards
Thermal Conduction
Heat transfer between solids in direct contact. Depends on thermal conductivity and temperature gradient.
Convection
Heat transfer between a solid and a fluid (liquid or gas) in motion. Depends on fluid velocity, surface area, and temperature gradient.
What is thermal radiation?
Heat transfer via electromagnetic radiation, primarily infrared. Requires no medium. All objects emit and absorb radiant energy depending on temperature and emissivity.
Units of thermal conductivity
W / (mK)
How can you increase the emissivity of a material?
Paint or tape. White out and electrical tape have high emissivity (0.95)
What is emissivity?
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)
How would you design a cooling system for electrical components?
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
How do heat-pumps work?
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.
What is Thermal Resistance and how is it used?
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.
First law of thermodynamics
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
Second law of thermodynamics
Entropy of an isolated system tends to increase over time. E.g. systems tend to move towards equilibrium.
Third law of thermodynamics
As temp decreases so does entropy
Define work of a gas
Integral of pressure times volume (force * length) between two equilibrium states
What are some considerations when selecting materials for high-temp applications
Specific heat capacity, thermal expansion, resistance to corrosion and creep deformation
What are the base units of a Watt
J/s or N-m / s
What is specific heat capacity?
Amount of heat per unit mass required to raise temperature by 1K or C
What is heat in a wire dependent on?
What is a PRT?
What is a thermocouple?
What is an RTD?
What is a NTC thermistor?
What are some mechanisms of thermal stress in mechanical components?
Rapid or uneven heating and cooling and thermal expansion can cause fatigue and deformation
How to mitigate thermal stress?
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
What is a Peltier Cooler?
What is Stfan-Boltzmann’s Law?
How does temperature affect BLDC motors?
How does temperature affect batteries?
What is thermal inertia and how does it affect temperature management system design?
Thermal inertia is how slowly the temperature of something approaches that of its surroundings. It depends on size, shape, absorptivity, specific heat, and conductivity.
Is heat added to a system during liquid to gas transition?
lost
What is entropy?
Entropy, s, quantifies disorder or randomness of a system. dS = integral(dQrev/T) where Qrev is reversible heat transfer
Enthalpy
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)
