Air flow

Ventilationsupply of fresh air

removal of internal heat

heat dissipation from skin

Image by MIT OCW.

Air flow

Ventilationsupply of fresh air

removal of internal heat

heat dissipation from skin

Air flow

VentilationZion National Park Visitor Center (National Park Service & NREL)

Images courtesy of the U.S. Department of Energy.

Air flow

Active air flowSupply of fresh air based on renewal and change rates

Ventilation flow rate vr (m3/s) = V x N (nb of air changes per hour) / 3600Ventilation conductance qv (W/K) = 1200 (J/m3K) x vr = 0.33 x N x VVentilation heat flow: Qv = qv x ΔT

Air flowActive air flow

Supply of fresh air based on renewal and change rates

Same 60 m3 room as studied for heat flow. Heat losses through façade due to conduction = 190 W

Losses due to air renewal (N = 0.7 h-1) ? What should N be for ventilation and conduction losses to be equal ?

J1

J2

Exterior façade

Image by MIT OCW.

Air flowPassive air flow

Fluid mechanics: p + ρgh + ½ ρv2 = constant (Bernouilli)hydrostatics :

Δp = -Δhρgstack effect :

Δpi-e(h) = (h-hn)g(ρe - ρi)ρ(T) = ρ0 · p/p0 · T0/T

pi > pe -> air escapes

pe > pi -> air enters

P i(h=0)

n

Ti Te

pi > pe -> air escapes

pe > pi -> air enters

P i(h=0)

n

Ti Te

h

h

Image by MIT OCW.

Air flowPassive air flow

Fluid mechanics: p + ρgh + ½ ρv2 = constant (Bernouilli)hydrostatics : Δp = -Δhρg

stack effect : Δpi-e(h) = (h-hn)g(ρe - ρi)ρ(T) = ρ0 · p/p0 · T0/T ≈ 1.29 (kg/m3) · 273/T

hydrodynamics (wind) : Δp = -½ ρ Δv2, Venturi effect : S1v1 = S2v2

1

2

3 4

P3 ;V3

P1 ;V1P2 ;V2

P4 ;V4Pi

Image by MIT OCW.

Air flow

Passive air flowFluid mechanics

Pressure ↓ if speed ↑

Turbulent vs. steady flow+

_

Image by MIT OCW.

Air flow

Wind

Protected zoneProtected zone

Ascending and descending movements along facades Influence of relative heights

Image by MIT OCW.

Image by MIT OCW.

Air Flow

Reading assignment from Textbook:“Introduction to Architectural Science” by Szokolay: § 1.1.4 + § 1.4.2

Additional readings relevant to lecture topics:"How Buildings Work" by Allen: Chap 11

“Sun Wind Light“ by Brown & DeKay: § 6 in Chap 1A