Morbo, the IR images only tell a part of the story. The images are showing the approximate temperature of the surfaces in the field of view (approximate because different materials have different emissivity properties). Heat transfer occurs by conduction and convection as well as radiation.
All surfaces radiate AND and simultaneously absorb heat according to their emittance properties and will heat or cool until such time as an equilibrium is met with the objects around it, ie the surface is absorbing radiant heat at the same rate as emitting it.
Conduction occurs when heat is transferred from a hotter surface to a colder one, in this case namely the layer of air adjoining the surface. If the air layer is constrained, such as in double glazing or behind heavy curtains, then the conduction slows dramatically once the surface air layer has reached (or fallen to) the temperature of the wall surface.
Convection is the transfer of heat by the movement of a fluid, in this case air. Hot (or cold) air displaces the air in the room and raises (or lowers) the room air temperature. A secondary but significant effect is that the air movement also increases conduction through the walls by disturbing the surface layer of air adjacent the walls, a mixing effect if you like.
The thermal images do not show air movement or air temperature, therefore do not illustrate the effect of blocking the vent, other than a minor secondary effect of the air flowing through the vent causing the vent itself to heat (or cool).
In the case of quantifying heat transfer due to room vents, thermal imaging has limited application.
Posted Sunday 28 Oct 2012 @ 8:29:57 pm from IP
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