The smell of fireworks and freedom is in the air! This year, my contribution to the family’s Independence day festivities is a smoked brisket. Unfortunately, I have learned from experience that cooking at elevation (~7000ft) is not the same as it is in the valley (~1000ft). To avoid potential embarrassment it only makes sense to run a few simulations before lighting the smoker.
Why?
Smoking a brisket can be tricky, some would say it’s more of an art than a science but at PADT science is our art so we’re going to lean into the science hard. It can take anywhere from 6 to 24 hours to smoke a brisket depending on a variety of factors. Let’s identify what those factors are;
- Weight of the Brisket
- The Moisture Content of the Brisket
- Air Flow
- Ambient Air Conditions
The influence of the weight (or mass) of the meat is the main influencer of overall smoke time. The bigger our cut, the more time it’s going to take to smoke. Air flow is going to affect the oxygen available for combustion, effecting the energy available for cooking the brisket, as well as having an effect on heat transfer from the hot air to the brisket. The ambient air conditions will have an effect on energy requirements, heat transfer, and the boiling point of water.
Brisket Stall
The moisture content is going to be our trickiest variable to monitor. We want our brisket to come out of the smoker juicy but as we heat the beef we will evaporate some of the water. This is where the term brisket stall comes from. We eggheads may be more familiar with the scientific description evaporative or adiabatic cooling.
At a certain temperature and pressure water boils and becomes a vapor. This phase transition takes energy, which we are providing by combusting wood chips or pellets, in my case, bourbon barrel oak. Unfortunately, when we hit this critical point we may see all of the energy going to converting water to vapor. Losing too much moisture to vaporization can ruin our brisket. We might increase combustion to add more heat and end up overcooking the meat. We could wait it out and end up drying out the brisket. Worst case scenario is a combination of both!
To avoid this issue many backyard barbecuers recommend the Texas crutch. The brisket can be wrapped in foil or butcher paper. This helps us overcome the stall point by putting a physical barrier in place to reduce the ability for the water vapor to escape the meat.
The Birth of a Brisket Digital Twin
Okay, so not exactly a digital twin. Maybe next time I will incorporate sensor data into the model via OPC server but for now we’re keeping everything digital. I’m using a thermal-fluid modeling tool called Flownex to create a 1D simulation of our brisket and smoker.
The brisket is being modeled using a compound component. I’ve iterated a few times on how exactly I wanted to model it. For the first iteration I kept it simple and used a solid node with an assigned mass and custom solid material characteristics for beef across a temperature range [Baghe-Kandan et, al].
Unfortunately, although our beef’s thermal characteristics (conductivity and thermal capacitance), is temperature dependent, we do not observe the characteristic stall associated with the adiabatic cooling near the boiling point of water. This is due to not modeling any water evaporating, duh.
Beef Brisket V2 was also a failure. I tried to model the solid portion of the brisket separate from the water with heat transfer between the two. It kind of worked, but was needlessly complex, and I didn’t see any effect of a change in elevation on the cooking behavior.
For Beef Brisket V3 I went back to basics. Forget about the smoker, lets just model the solid and liquid that makes up the meat with an applied heat flux and see if we see the stall happen in transient.
The beta testing for V3 was successful! As seen in the above recording, we can see a huge drop in temperature of our solid as soon as our temperature reaches the boiling point of water. We also can see the difference between a sea level analysis and one representing a system operating at 7000 feet above sea level. For the final version of our brisket model I swapped out the two-phase tank with a open container component (to let our water evaporate to atmosphere), and added script to do some basic calculations.
Transient Simulation and Conclusions
Let’s run the transient simulation in parallel for a smoker at sea level and a smoker at 7000 feet elevation and see what happens!
As we can see in the above transient analysis both briskets experience “stall” once the water begins to evaporate. The brisket at elevation hits its stall at around 199°F, at around the 6 hour mark, the brisket at sea level stalls at 212°F near the 8 hour mark. What this means is that if we are fighting an impossible battle if we try to hit the typically recommended 205°F pull temperature when cooking at altitude! We’d dry our brisket out and burn the meat before we’d hit that mark. I think I’ll be pulling this brisket at ~192°F
Happy Independence Day!
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