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Engineering ToolBox > Convective Heat - Air Velocity and Volume of Air Flow
A hot or cold vertical surface will generate a vertical air flow up or down.
The air velocity in the center of a convective flow in parallel to the hot or cold surface can be estimated with the equation:
vc = 0.65 [g l (ts - te) / (273 + te)]1/2 (1)
where
vc = velocity in center of airflow (m/s)
g = 9.81 - acceleration of gravity (m/s2)
l = vertical distance from bottom of the surface (m)
ts = temperature surface (oC)
te = temperature surrounding environment (oC)
For a cold surface where the air will flow downwards value will be negative.
The convective air flow volume generated at a certain height by the hot or cold surface can be calculated as
Q = 0.019 [g l (ts - te) / (273 + te)]0.4 l1.2 (2)
where
Q = air flow volume (m3/s per meter surface width)
The air flow velocity at the lower level of a cold (5oC) window (height 2 m) in a room with temperature 20oC can be calculated as
vc = 0.65 [9.81 (m/s2) 2 (m) (5 - 20)(oC) / (273 + 20(oC))]1/2
= - 0.65 m/s
The air flow volume can be calculated as
Q = 0.019 [9.81 (m/s2) (5 - 20)(oC) / (273 + 20(oC))]0.4 2 (m)1.2
= 0.033 m3/s = 119 m3/h
The air flow velocity at the top level of a hot (80oC) radiator (height 1 m) in a room with temperature 20oC can be calculated as
vc = 0.65 [9.81 (m/s2) 1 (m) (80 - 20)(oC) / (273 + 20(oC))]1/2
= 0.92 m/s
The air flow volume can be calculated as
Q = 0.019 [9.81 (m/s2) (80 - 20)(oC) / (273 + 20(oC))]0.4 1 (m)1.2
= 0.025 m3/s = 90.4 m3/h