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Convective Heat - Air Velocity and Volume of Air Flow
A hot or cold vertical surface will generate a vertical airflow - calculate air velocity and volume of the air flow
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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:
Air Velocity
vc = 0.65 [g l dt / (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)
dt = te - ts= temperature differanse between surface and room environment
ts = temperature surface (oC)
te = temperature surrounding environment (oC)
For a cold surface where the air will flow downwards value will be negative.
Air Volume
The convective air flow volume generated at a certain height by the hot or cold surface can be calculated as
Q = 0.019 [g (te - ts) / (273 + te)]0.4 l1.2 (2)
where
Q = air flow volume (m3/s per meter surface width)
Example - Convective Air Flow Cold Window
The air flow velocity at the lower level of a cold (5oC) window (height 2 m and width 1 m) in a room with temperature 20oC can be calculated as
vc = 0.65 [(9.81 m/s2) (2 m) ((20 oC) - (5 oC)) / (273 + (20 oC))]1/2
= 0.65 m/s
The air flow volume (for window width 1m) can be calculated as
Q = 0.019 [(9.81 m/s2) ((20 oC) - (5 oC) / (273 + (20 oC))]0.4 (2 m)1.2
= 0.033 m3/s
= 119 m3/h
Example - Convective Air Flow Heating Element - Radiator
The air flow velocity at the top level of a hot (80oC) radiator (height 1 m and width 1 m) in a room with temperature 20oC can be calculated as
vc = 0.65 [(9.81 m/s2) (1 m) ((80 oC) - (20 oC) / ((273 K) + (20 oC))]1/2
= 0.92 m/s
The air flow volume (for radiator width 1 m) can be calculated as
Q = 0.019 [(9.81 m/s2) ((80 oC) - (20 oC) / ((273 K) + (20 oC))]0.4 (1 m)1.2
= 0.025 m3/s
= 90.4 m3/h
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