Re: Fireplace Physics

From: John C. Polasek (jpolasek_at_cfl.rr.com)
Date: 02/22/05 

Date: Tue, 22 Feb 2005 10:13:29 -0500

On Sun, 20 Feb 2005 21:51:01 -0800, Isaac Wingfield <isw@witzend.com>
wrote:

>Our fireplace has a spark arrestor screen (I suppose almost all of them
>do).
>
>The screen is made of spiral wound wires. The "apertures" between the
>wires are almost big enough to stick a pencil through, and I'd guess
>that the cross-section area of the wires is well under five percent of
>the total area of the screen, when it's pulled all the way across the
>opening of the fireplace (IOW, it's almost all open space).
>
>I've noticed that when the screen is closed, the amount of radiant
>energy coming into the room is significantly less than when it's open.
>In fact, you can pull one half of it closed, and tell the difference
>just by moving your hand between where it is, and where it isn't. It's
>difficult to estimate, but the reduction seems a lot closer to 50% than
>it does to 5% (which I probably would not notice).
>
>At the same time, there's not a noticable reduction in the amount of
>visible radiation from the coals.
>
>I know the IR wavelength is far too short to have trouble with those
>pencil-sized apertures. I also know that if it was convection (actual
>air movement), the reduction in flow would be out of proportion to the
>amount of area obscured by the wires, but this is radiation, not
>convection.
>
>What's going on?
>
>Isaac
This sounds like a problem awaiting the invention of a "thermometer".

But if you move a COLD screen in front, it will act as a heat sink
till it gets up to temperature, that is, if it's black, whereupon it
should reradiate and make you warm again. The cold metal grid might
have more "reach" than your 5% indicates.

I remember a huckster in the hot window of our living room with his
tinted glass and thermometer. Yes, interposing the glass made the
temperature go down, while the glass was acting as a heat sink, but
I'm pretty sure it would get hot again when the glass did. Only
aluminized glass would have the desired effect of actually chasing the
heat..

Order 5 tons of coal and maybe they'll give you a thermometer
.
John Polasek

If you have something to say write an equation.
If you have nothing to say, write an essay.

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