Fan Blowing Across The Vent Cap Spikes Temp

[Jon B.]

We keep the KK's in the garage on rolling carts.  Wheel them outside to cook.  Last night it was raining, so I pulled the 16 TT to the opening of the garage but still under the overhang.  Sort of half in and half out.  Was setting up to smoke a chuck roast and the way the wind was blowing , all the smoke was coming back into the garage.  I grabbed a small pedestal fan and pointed it at the vent cap to blow the smoke out of the garage.

Before turning on the fan, the 16 was dialed in at 250*F and holding steady.  After I turned on the fan, the dome temp rose to 350-360* within a few minutes.   

Thought it was interesting that the constant breeze from the fan across the vent cap increase the air flow thru the cooker and raised the dome temp over 100*.   

Ended up doing a hot & fast cook instead of a low & slow 

 

Edited May 8, 2019 by Jon B.
spelling....should have paid more attention in grade school

[Pequod]

Bernoulli effect in action! Cool!

[Steve M]

I was wondering about the effect a fan would have doing just that. I am getting ready to mount one somewhere in my outdoor kitchen and I thought about mounting it to help with the occasional bad wind direction.

[Jon B.]

@Steve M The fan was about 10 feet away and pointed directly at the top vent.  It was quite a strong, direct breeze going by the vent.  FYI 

 

 

[Steve M]

That's pretty far away. I'm surprised it had that much of an effect.

[Pequod]

1 minute ago, Steve M said:

That's pretty far away. I'm surprised it had that much of an effect.

Great thing about the Bernoulli effect is that the distance from the source doesn't matter. Only the velocity of the air across the surface.

In this case, it doesn't even have to be a direct flow from the fan across the opening. The wind passing the cylinder supporting the vent cap will result in a local pressure drop that causes suction from the vent itself. It would look like this: https://en.wikipedia.org/wiki/Potential_flow_around_a_circular_cylinder

[Why yes...I am a recovering fluid dynamicist...]

[Steve M]

15 minutes ago, Pequod said:

Great thing about the Bernoulli effect is that the distance from the source doesn't matter. Only the velocity of the air across the surface.

In this case, it doesn't even have to be a direct flow from the fan across the opening. The wind passing the cylinder supporting the vent cap will result in a local pressure drop that causes suction from the vent itself. It would look like this: https://en.wikipedia.org/wiki/Potential_flow_around_a_circular_cylinder

[Why yes...I am a recovering fluid dynamicist...]

I just hope there isn't a quiz   

[Jon B.]

It was just a cheap fan on low speed also. 

[Pequod]

A KK running at 250 is already in a very low flow (i.e., very small pressure differential) state. Even a modest wind across the cylinder would significantly increase the vacuum. I think that's what you saw here. If you did the same thing whilst running at 450, my guess is you'd see very little change in temp.

Edited May 8, 2019 by Pequod

[Jon B.]

Very Interesting.............

[RandyS]

Boy, that takes me back to college chemistry days (that's a long time ago now). Our suction in the lab was based on Bernoulli and Venturi effects, using running water as the fluid - that also means whatever was being pulled into the suction was also diluted in large volumes of water.

[DennisLinkletter]

I have a Milwaukee shop blower, and I know that if I blast air across the damper top crack I can crank up the fire from the vacuum.. If I have too much ash and am concerned about stirring it up by blowing  down into the base or thru the Guru port I use the dome vacuum technique.. I'm impatient and love my Milwaukee shop blower to expedite my burn..

[tony b]

On ‎5‎/‎8‎/‎2019 at 8:58 AM, Pequod said:

The wind passing the cylinder supporting the vent cap will result in a local pressure drop that causes suction from the vent itself. It would look like this: https://en.wikipedia.org/wiki/Potential_flow_around_a_circular_cylinder

[Why yes...I am a recovering fluid dynamicist...]

Incited bad flashbacks to grad school! 

[Pequod]

50 minutes ago, tony b said:

Incited bad flashbacks to grad school! 

To think I used to be able to do that Math!

[Aussie Ora]

A KK running at 250 is already in a very low flow (i.e., very small pressure differential) state. Even a modest wind across the cylinder would significantly increase the vacuum. I think that's what you saw here. If you did the same thing whilst running at 450, my guess is you'd see very little change in temp.

So is the air from the fan going across the vent stopping the heat escaping the dome.?

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[Pequod]

5 hours ago, Aussie Ora said:

So is the air from the fan going across the vent stopping the heat escaping the dome.?

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No. Enhancing overall airflow, stoking the coals.

[tony b]

@Aussie Ora - the wind blowing across the top vent creates a slight vacuum, which causes air to be drawn into the KK via the Guru vent when the fan isn't running. You end up with more airflow than you would normally need for the temperature you want (top vent opening); hence, the temperature spike. Think "turbo" mode! 

PS: tried to send you a PM, but it says that you cannot received messages, which I take to mean that your Inbox is full. Go check it and do some housecleaning! 

[Aussie Ora]

[mention=2714]Aussie Ora[/mention] - the wind blowing across the top vent creates a slight vacuum, which causes air to be drawn into the KK via the Guru vent when the fan isn't running. You end up with more airflow than you would normally need for the temperature you want (top vent opening); hence, the temperature spike. Think "turbo" mode! 
PS: tried to send you a PM, but it says that you cannot received messages, which I take to mean that your Inbox is full. Go check it and do some housecleaning! 

My maid is on strike lol

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