Föreläsning 4: Värmeöverföring och värmeväxlare (Giovanis) Denna Är Inge Bra Flashcards
osäker på vad som Folke tycker är viktigt så avvaktar med denna
Vilka tre värmeledningstyper jobbar vi med?
Ledning (conduction)
Strålning (radiation)
Konvektion (convection)
Ledning (conduction)
(Giovannis notes and formula)
qx’’ = - k * dT / dx
- Molecular activity, random motion
Heat flow rate
Giovannis formula
(q prick) = - k * A * dT / dx
Är ledning (conduction) större i solida material eller i gaser?
Solida
Beror leding enbart på densitet? Om inte vad mer påverkar?
Nej, det påverkas också av temperatur
Vilka antaganden görs vid “heat diffusion equation 2” vid beräkning av (?)ledning?
Och hur ser ekvationen ut?
Giovannis notes
- steady state
- one dimensional
- no heat source
d / dx ( k * dT / dx ) = 0
Ledning (conduction) genom en vägg (Vad blir R)
(Inne) R = 1 / (h1 * A) + LA / (kA * A) + LB / (kB * A) + LC / (kC * A) + 1 / (h4 * A) (Ute)
Konvektion (convection)
Giovannis notes
- random molecular particle motion
- bulk macroscopic motion of the fluid
- heat transfer close to a wall due to a moving fluid
- thermal boundary layer
Forced convection (påtvingad konvektion)
The flow is caused by external means, such as by a fan, a pump, or atmospheric winds.
Natural convection (naturlig konvektion)
The flow is induced by buoyancy forces
Densitetsskillnader som ofta orsakas av temperaturskillnader.
heat flux
Giovannis formel
q’’ = h (Ts - T∞)
heat transfer rate
Newtons law of cooling
Giovannis formel
(q prick) = h * A * θ
θ = temperature difference
A = area
h = not a constant but kinda, depends om material and state
Reynolds number
Giovannis formel
Re = ρ * u∞ * L / μ
The 3 types of Heat exchangers
Giovanni
Evaporative ( air and water are brought into direct contact with each other in order to reduce the water’s temperature)
Recuperative (Separate flow paths for each fluid throughout its passages and heat is transferred through the separating walls)
Regenerative (heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid.
3 types of recuperative heat exchangers
Giovanni
Co-current
Countercurrent
Cross current