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

A hot or cold vertical surface will generate a vertical air flow up or down.

convective heat surface

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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