Introduction Flashcards
Whats is heat transfer?
Heat transfer (or heat) is thermal energy in transit due to a spatial temperature difference.
What is conduction?
The transfer of energy from the more energetic to the less energetic particles of a substance due to the interactions between the particles.
What is thermal conductivity k [W/mK]?
The materials ability to conduct heat.
What is heat flux?
The rate of heat transfer through a section per unit area.
q’’ [W/m K]
What is heat rate?
The amount of heat transferred.
q [W]
What is convection heat transfer?
Convective heat transfer occurs between a fluid in motion and a bounding surface when the two are at different temperatures.
Which mechanisms comprises convective heat transfer?
- Random molecular motion (diffusion)
- bulk (macroscopic) motion of the fluid
Where in the thermal boundary layer does the convective heat transfer mechanism, random molecular motion dominate?
Near the surface where the fluid velocity is zero.
Convection heat transfer can be classified according to the nature of the flow, which ones?
Forced convection: flow is causes by external means (fan, pump, atmospheric winds)
Free (natural) convection: flow is induced by buoyancy forces (density differences due to temperature variations in the fluid)
Mixed (combined):
What is the convective heat rate equation?
q’’ = h(Ts - Too) Ts: surface Too: fluid Ts > Too h: convection heat transfer coefficient [W/m2 K]
What is thermal radiation?
Terminal radiation is energy emitted by matter that is at a non-zero temperature.
How is energy of a radiation field transported?
The energy is transported by electomagnetic waves (or alternatively, photons).
What is the simple radiative heat flux equation?
q(rad) = h(r)A(Ts-Tsur) q(rad)= eoA((Ts^4-Tsur^4) h(r) = eo(Ts+Tsur)(Ts^2+Tsur^2) h(r) = radiation heat transfer coefficient e = emissivity (material radiative property, 0-1 Ts = Surface tenperature (absolut), [K] o = Stefan Boltzman constant, o=5.67*10^-4 [W/m2 K^4]
What is emissivity?
Emissivity is the measure of an objects ability to emit infrared energy.
Range: 0-1
0 (shiny mirror)
1 (blackbody)
What is Viscous dissipation?
Viscous dissipation is the conversation from mechanical energy to thermal energy associated with viscous forces acting on a fluid.
