Föreläsning 4: Värmeöverföring och värmeväxlare (Giovanis) Denna Är Inge Bra Flashcards

osäker på vad som Folke tycker är viktigt så avvaktar med denna

1
Q

Vilka tre värmeledningstyper jobbar vi med?

A

Ledning (conduction)
Strålning (radiation)
Konvektion (convection)

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2
Q

Ledning (conduction)

(Giovannis notes and formula)

A

qx’’ = - k * dT / dx

  • Molecular activity, random motion
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3
Q

Heat flow rate

Giovannis formula

A

(q prick) = - k * A * dT / dx

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4
Q

Är ledning (conduction) större i solida material eller i gaser?

A

Solida

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5
Q

Beror leding enbart på densitet? Om inte vad mer påverkar?

A

Nej, det påverkas också av temperatur

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6
Q

Vilka antaganden görs vid “heat diffusion equation 2” vid beräkning av (?)ledning?

Och hur ser ekvationen ut?

Giovannis notes

A
  • steady state
  • one dimensional
  • no heat source

d / dx ( k * dT / dx ) = 0

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7
Q

Ledning (conduction) genom en vägg (Vad blir R)

A

(Inne) R = 1 / (h1 * A) + LA / (kA * A) + LB / (kB * A) + LC / (kC * A) + 1 / (h4 * A) (Ute)

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8
Q

Konvektion (convection)

Giovannis notes

A
  • random molecular particle motion
  • bulk macroscopic motion of the fluid
  • heat transfer close to a wall due to a moving fluid
  • thermal boundary layer
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9
Q

Forced convection (påtvingad konvektion)

A

The flow is caused by external means, such as by a fan, a pump, or atmospheric winds.

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10
Q

Natural convection (naturlig konvektion)

A

The flow is induced by buoyancy forces

Densitetsskillnader som ofta orsakas av temperaturskillnader.

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11
Q

heat flux

Giovannis formel

A

q’’ = h (Ts - T∞)

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12
Q

heat transfer rate

Newtons law of cooling

Giovannis formel

A

(q prick) = h * A * θ

θ = temperature difference
A = area
h = not a constant but kinda, depends om material and state

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13
Q

Reynolds number

Giovannis formel

A

Re = ρ * u∞ * L / μ

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14
Q

The 3 types of Heat exchangers

Giovanni

A

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.

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15
Q

3 types of recuperative heat exchangers

Giovanni

A

Co-current
Countercurrent
Cross current

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16
Q

Kan man veta om det är co-current (medströmsvärmeväxlare)?

A

Nej. Antingen vet man att det är countercurrent eller så kan det vara båda

17
Q

Hur vet man att det är countercurrent?

A

Countercurrent då går varma vätskan och kalla vätskan i olika riktningar vilket gör att den varma vätskans exit temperatur kan vara lägre än den kalla vätskans exit temperatur. Det går inte när det är co-current

18
Q

Vad är skillnaden mellan Δ och θ? (värmeväxlare)

Giovanni

A

θ är skillnaden mellan de olika vätskornas temperatur vid ett viss läge i värmeväxlaren

Δ är skillnaden mellan en vätskas nuvarande temperatur och dess starttemperatur

19
Q

LMTD: Logaritmic mean temperature difference

Giovannis formel

A

θm = (θ1 - θ2) / ln (θ1 / θ2)

Convert to Kelvin