Can a windy day raise the internal temperature?

[wilburpan]

When I made my first pulled pork over the weekend, there was an interesting thing that happened with the temperature. The temperature was probably pretty steady overnight. The grill was holding a steady 230ºF when I started at 11:00 pm, and was at 220ºF at 6:00 am when I checked it. 

 

Then as the sun came up, it got pretty windy outside, and stayed windy for a while. A couple of hours later, I checked the grill, and the thermometer read 325ºF. The pork had an internal temp of 175ºF at that point, so I shut down the vents and let the grill coast until the pork hit an internal temperature of 200ºF.

 

My question is, why did the temperature jump like that? I don’t think it was a problem with the charcoal per se. The best explanation I have is that when it got windy, the wind blowing past the upper vent drew more air out from the inside of the grill (Bernoulli Principle), causing more air to flow through from the bottom vent through the grill, which got the charcoal burning hotter.

 

The pork came out really good, so I’m not particularly worried about whether this is an issue. But I was curious as to why this happened.

[CeramicChef]

Hi Wilbur - what was the orientation of the KK lower vent with respect to the wind.  I wouldn't worry so much about the top vent as the bottom vent having air blow into the KK by a rather stiff breeze.  If the bottom vent to face into the wind, that can change the amount of airflow into your KK.  I think these KKs are so efficient, any increase of air in the presence of fuel and heat can give a spike in the temp.  If the wind is relatively steady and you set your vents based on a different set of conditions initially, it could very well be that just a little bit more air, on a sustained basis, could get an increase in your internal temp.

 

As you know Buddy, I'm not much more experienced than you.  But I did stay at a Holiday Inn Express last night and I am a chemical engineer!   The guy who we really need to answer this Q is Dennis or some of the old hands around here.

[wilburpan]

I do think it’s more of a top vent/Bernoulli issue, at least in my case. Here’s why:

 

1. I don’t think the prevailing wind was blowing at the bottom vent that morning. The round shape of the top vent means that the Bernoulli effect can happen as long as wind is blowing by in any direction. Besides, even if the wind was blowing directly into the bottom vent, that also means that it’s blowing by the top vent.

 

2. The bottom vent was only open a crack. The opening at the top has got to be larger than that, even though I only unscrewed the top vent a little bit.

 

3. Physics. I ran this by my dad, who’s a physicist. He agrees that it’s unlikely that wind would force air through the bottom vent to any appreciable degree.

[tony b]

Wilbur is correct (and good call on Bernoulli, btw - brings back too many horror stories of derivations in grad school - shudder!!).

 

I experience this a lot, as my grill is on an elevated deck (about 8 ft off the ground) and it sits between two houses that can channel the airflow, which is perpendicular to the lower vent. I can even lose control of the temperature using the Guru on a seriously windy day, which is another piece of evidence about the top vent being the dominate factor, seeing as the lower vents aren't open at all when using the Guru and the wind can't force air through the Guru port.

[mguerra]

On some KK's the wind can spin the top hat more open or more closed. I presume you verified it was at the opening you set and did not change.

[5698k]

Air can go through the fan in the guru port, unless you have the damper completely closed.

Robert

[wilburpan]

On some KK's the wind can spin the top hat more open or more closed. I presume you verified it was at the opening you set and did not change.

 

Yes, the top wasn’t spinning from the wind.

[wilburpan]

Air can go through the fan in the guru port, unless you have the damper completely closed.

Robert

 

True, but as I mentioned above, I think that would be primarily from increased vacuum from wind blowing past the top vent, rather than wind pushing air through the guru port.

[tony b]

On some KK's the wind can spin the top hat more open or more closed. I presume you verified it was at the opening you set and did not change.

 

 

Air can go through the fan in the guru port, unless you have the damper completely closed.

Robert

 

 

My top vent isn't that "free wheeling" that I'd worry about that being the cause, at least in my case, Doc. Have had the hat move slightly when opening/closing the lid, though. But, I will do a direct observation next windy day cook, just to confirm.

 

That is true, Robert, and I normally keep the damper just barely cracked to be on the safe side, having had temps get away because it was too open. But, in the case of the windy day issue, I don't think it factors in much, as the Bernoulli effect is far more prevalent on the top vent. Too much pressure drop across the fan to be a direct concern on the Guru port. That said, if the Guru damper is open a lot, it does make it easier for the top vent to pull more air through on a windy day (vacuum draw).

 

If I were more ambitious in following up on this, I'd run an experiment setting up the KK with the Guru and a box fan. Pick a nice calm day. After getting the KK up and stable at say 250F, then point the box fan directly at the Guru, blowing directly into it, with the fan on high speed and see if there's any change in temperature over say a 15 minute period. Then, run the experiment again, this time, remove the Guru (plug the port) and open the lower vent just slightly. Let the KK stabilize at that setting. Point the fan directly at the lower vent and monitor the temperature. Finally, move the fan to a position where it blows only at the top vent and repeat the experiment. My hypothesis is that the most profound effect on the temperature will be on the 3rd test - top vent. I suspect you will see negligible effect on the Guru test and probably a little increase in temperature on the lower vent test.