BBQ'ing for nerds?

smokinbadger

smokinbadger

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Hey,

I tagged a question on the end of a thread the other day talking about using computer simulation to optimize my Bandera for temperature uniformity. I didn't get much feedback on the post, but I guess it could be because of how deeply it was burried in the thread.

Anyway, I'm interested to know if any of the group have used engineering simulation tools (like computational fluid dynamics-CFD) or testing equipment (thermocouples, hot wire anemometers, etc.) to develop or optimize BBQ equipment.

For your enjoyment, attached are two images from a CFD simulation I did recently of my Bandera, showing the effect of the "standard" Bandera baffle on temperature uniformity in the smoke chamber. What you see is a surface of constant temperature (in this case 380 Kelvin or 225 Fahrenheit), which has been colored by the local air velocity. The first image is the case with the baffle, and the second is without the baffle installed. You can notice that with the baffle, the mixing is more complete (as shown by the more convoluted temperature iso-surface) and that the velocities are higher (showing higher potential for cold and warm air/smoke to mix.) If anyone is interested, I can post some other pictures showing the temperature profile in vertical and horizontal slices.

If nobody is interested, I'll just keep this to myself from now on. :)

The Smokin' Badger

Here are the images:
smokinbadger

http://www.flickr.com/photos/smokinbadger/
smokinbadger
 
Velly, Velly interesting :lol:

I think that confirms what we all experieced with baffles.

Thanks,

TIM
 
OK... I'm as big a geek as any of the guys on the site (OK... Maybe I'm bigger :-D), but this may be overkill. I'm sure that there is some good info in there, but what you really want to know is where the hot spots are, right? Take a couple of packages of bisquits (the tube ones like Pillsbury) and arrange them on the shelves and let them cook. You'll be able to see which ones are lighter or darker and you'll be able to see where the cooker is hotter or cooler.

But... if simulations of the Bandera float you boat, go for it! :-D

BTW, I can see some value in simulations like this if you are designing a cooker, or are trying to find the best baffle design for your cooker. It could save hours of trial and error if it's accurate.

OH... and Mista's right. Pretty colors.
 
Definitely, definitely pretty colors,.........definitely.

Dammit, is that guy BBQing or getting ready to launch something into outer space?
 
I'm all for using technology as much as you can to help out in whatever your endeavor. The caveat is this (expecially in barbecue): if you rely solely on tech, and lose the feel- you've lost something big. sometimes regardless of what the data is, you just know how something is: the butt's done or whatever. Use the tech, but not exclusively.
That being said, I don't cook on a 'dera, but that's pretty faking cool! Nice work!
 
smokinbadger said:
....For your enjoyment, attached are two images from a CFD simulation I did recently of my Bandera, showing the effect of the "standard" Bandera baffle on temperature uniformity in the smoke chamber. ......
Click to expand...

Did what you cooked taste good?

Seriously though, I think I would be really interested in this if a understood it, even though it contradicts the K.I.S.S. philosophy to which I am trying to return. Do the various color ranges represent delta to the average temp (225)? If so, how much? I don't understand the key. And what do blank spots represent? e.g., in the baffle result chart, there is a empty space on the left side of the chamner. What is up with that?

Bear in mind, I am an idiot, so please use easy words and poor grammar.
 
That's pretty nice, but a few questions.

Any idea what the sensitivity is to the effects of external temperature or wind conditions?

What about damper openings?

Have you verified the model with actual measurements in use? Verification would be very useful for creating a baseline model to test different mods on.


Now in the interest of full disclosure, I am structural engineer and have done my fair share of modelling, but started designing before computer modelling was really popular, and one thing that I will say is that modelling by itself is worthless. A fair degree of knowledge and experience is required to understand the model and know if it is giving you good results. I have had less experienced engineers work up models for designs and then asked them two or three simple questions about the results and told them that their model could not possibly be right. They would argue persistently, but in the end I would show them where they went wrong. Not saying that your model is good or not, I don't have enough experience to judge, but others here may have, or it could be verified with actual "field" measurements.
 
Hey,

I'm happy to see some interest in this. Here are answers to some of the questions asked:

1. Yes, I do manage to get some great tasting 'que from this pit. I mainly did the simulation because I have the tools and I thought it would be fun.

2. I am currently setting up to take measurements to compare to the model in order to tune the modeling assumptions to get a good baseline correlation. I have installed 9 K-type thermocouples each on three grill grates which I will install into the smoke chamber and take measurements. The problem so far has been that whenever I have the time to fire up the pit, I choose to make 'que rather than take data. (Gotta have priorities!)

3. The model doesn't take into account wind or weather conditions. As a simplification, I assumed the walls to be adiabatic (i.e. no heat transfer through the walls, i.e. perfectly insulated.) I could borrow a FLIR camera from the lab and get a temperature map of the walls and apply this as the boundary condition for the model, but again, I could also spend the time BBQ'ing!

4. I guess the pictures I uploaded are a little bit esoteric, so let me try to explain and point you at some more pictures. The colored convoluted surface shown is created in the visualization tool by looking for all regions which have a temperature of 380K (225F) and connecting them with a surface. To get additional information (or to confuse people more) I applied a coloring scheme to the surface which relates to the local air velocity. So if you see the color green on the surface for example, that indicates a velocity in that area of 5 m/s, based on the color key in the image.

To make things a little easier to understand, I have uploaded two pictures showing the temperature contours in a flat plane passing through the firebox and smoke chamber. In these pictures, the colors mean temperature with blue being cold and red being hot. I also attached a picture of the computational mesh used for the simulation with a (hopefully) simple explaination of how the model works.

Finally to the point about the accuracy of simulation in general, I manage a large engineering department containing both testing, computer simulation and a wind tunnel. The best way to be sure about the quality of the results is to ask a lot of questions, and to compare as often as possible. It was once said, and I fully support this, that "Nobody believes the model except the modeler; Everyone believes the test results except the tester."

If anyone wants to see the full report, just drop me a PM and I'll send it to you.
 
Here's what I heard...

smokinbadger said:
Hey,

I'm happy to see some interest in this. Here are answers to some of the questions asked:

1. Yes, I do manage to get some great tasting 'que from this pit. I mainly did the simulation because I have the tools and I thought it would be fun.

2. I am currently fcjwpcfm md wokcnmowmncm pskd jwpkdvm e lwdkmnow wocmwdpw owdkmpc cjm emwwkwdcn wepfmpwe ek emfe wpwdk wm w owksokvs. I have installed 9 K-Type sxpkcpwpck wdpc-pwodcqw grill grates which I will install into the smoke chamber and take measurements. The problem so far has been that whenever I have the time to fire up the pit, I choose to make 'que rather than take data. (Gotta have priorities!)

3. The mepfovkwpevm pwdk wpd wpdwodmn nv wpmmqkd wso sosdn sdopk df cmzl aomxzm, (i.e. no heat tjvwpdmmslm sodkosdk s, jsdjvnwo sssskds.) I could borrow a slpfk]pqwoekmen eo eoo 0pweiutedjson sonc sk sd socnp, I could also spend the time BBQ'ing!

4. I guess the pictures I uploaded are a little bit esoteric, so let me try to explain and point you at some more pictures. The colored gheiw[anvlxls wownkieucn woiw widnc widnwe jdwown ticnwiwcniie c siei wowicn wowcn woid. To get additional information (or to confuse people more) I applied aqdoknqwpqizlkzxnsdkeierbnvb wonwo wo wosxcjknaprtyd sdo rujutteiw. So blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah.

To make things a little easier to understand, (yawn) blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, the firebox and smoke chamber. In these pictures, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah.

Finally (I hope so.) blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah. The best way to be sure blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah."

If anyone wants to see the full report, (I don't have enough Ridalin for that) just drop me a PM and I'll send it to you.
Click to expand...

I'm just teasing you and I'm sure that meant a lot to some engineering types but it is way over my head and I'm cool with that.

BBQ is what I do when I don't want to think. After almost 3 years, I'm getting to the point to where I can make an awesome brisket without a thermometer. I can make rub and sauce without measuring anything. I know how much charcoal and wood I need for a 10 hour burn without measuring it.

Excellence comes from practice not tools, grasshopper.

Now go outside, start a fire and smell some smoke.
 
thirdeye said:
DougS is one guy who knows about some of that stuff. :roll:



dedfef8f.jpg
Click to expand...
This looks like something our troops are battling in Iraq. If you had food for sale, I would have to do a C-4 sweep, bring in the bomb-sniffing dogs and would still tread lightly as I approached this setup.

Ken (keepin' it pretty simple with my cooker.)
 
smokinbadger said:
Two more things:

The updated pictures are at: http://www.flickr.com/photos/smokinbadger/

The blank spots in the isosurface plots are areas where the temperature is not 380K (i.e. either >380K or <380K).
Click to expand...

These were much more valuable to me, and as TIM said, confirms what we know about baffles. One question, can you explain what appears to be a 'warm spot' in the baffled model that is actually on the firebox side of the smoke chamber?
 
Fascinating, smokinbadger. I learned something.
I even think I understood most of what you were saying.

That means that I am smarter than I thought I was, or a lot dumber-dumb enough to think I am smart:shock:
 
thats cool chit.. without looking to deep inthe thermo-dynamics, and just look at the result of the baffle.. it shows that both increased temperatures and velocitys are accomplished by directing the heat UNDER the waterpan, which helps to distribute the heat up the center and far side of the chamber. The models without the baffling show the heat only traveling up the firebox side and over the pan.. and under the waterpan is significantly cooler. The baffle accomplishes exaclty what we want. I would love to see what would happen witha small convenctin fan installed in the botomof the chamber to assist in mixing the air..

But, this is good for us nerdy types.. and fun to analyze.. but the plain old biscuit test is whats gonna help ya map the chamber in the real world.

good stuff smokin badger.!!!

If it is ok with you, I would like to include your model in our Bandera 101 document. (With proper credit to you of course)
 
You can definately include it in the Bandera 101 doc. I would like to get the thermocouple measurements and comparison done first, just for completeness. I'll post those results when they are done, and maybe we can include both test and simulation results together.

Regarding the convection fan idea, there is already a pretty strong driving force for flow in a smoker, especially a vertical chamber one, due to the significant drop in fluid density due to combustion. Said another way, the hot gas produced in the fire rises, which sets up a strong draft through the smoke chamber. The baffle (or any other thermal mixing device) only needs to redirect the flow, but doesn't need to impart any additional momentum, as a fan would. Adding a fan would at a minimum probably mean that the dimensions of the inlet and outlet dampers would need to be changed (most likely reduced) in order to maintain similar temperatures, and combustion efficiency (e.g. to get thin blue smoke and not thick black creosote causing smoke.)

With combustion providing a reliable momentum source to move the smoke out and the fresh air in, further temperature optimizations (pit tuning) would do better to rely on baffles or other static mixers, which trade off additional pressure drop (i.e. momentum loss) for increased local mixing.

Note that all of this neglects the importance of radiant heat transfer, which is some parts of the pit can be important, as hot surfaces re-radiate into the cooking volume. This also neglects effects of conduction within the pit metal itself, which can also be used to improve uniformity of smoke chamber temperatures. As an example, the tuning plates frequently used in trailer sized pits make use of conductive heat transfer, and heat capacity to smooth out spatial and transient non-uniformity in smoke volume temperature. CFD models can take account of both radiation and solid conduction--a close cousin to the "pit tuning" problem is the tuning of the temperature distribution in the engine compartment of passenger cars.

All of this brings things full circle: Maybe the Klose Toyota BYC really does make sense, given the similarity of the physics going on under the hood and under the lid.
 
......... I think (HOPE) that is what I pay Spicewine or Horizon, or to test, debug and refine in return for the price difference. The info is impressive, but then there is the next baffle mod, and the next, and ...... Hey! I want to cook !!!
 
I am a degreed electrical engineer but I bbq to get away from all this technical bullchit!
 
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