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Hot dog! Simulating the Perfect Summer BBQ

7 June, 2017


With summer just around the corner and our second German user group in June, we at 6SigmaET thought it was about time to hit the beach and throw a couple of Bratwurst on the barbecue. Of course, working in thermal management, the idea of having a BBQ without running a CFD simulation first would be out of the question. As such, I thought I’d turn up the heat on my very own simulated sausages, using 6SigmaET to find out the optimum cooking conditions for the perfect barbecued summer sausage.

Finding a good CAD model of a barbecue wasn’t too hard – Grabcad.com had a great model of a Weber Master Touch 57 which had been created by Theodor Trust. Importing the Step file through 6SigmaGeometry gave me a fully detailed model in a matter of minutes.

A CAD model of charcoal was not so easy to find; I suppose I should have created my own coal but when I saw ‘briquettes’ were available I used that. Of course, explicitly solving for the ‘combustion’ processes in the charcoal is not possible in 6SigmaET so I settled for a fixed uniform temperature – around 450 degrees Celsius seemed reasonable.

To simulate the thermal behaviour accurately it is necessary to have a full set of thermal parameters for the sausage. Fortunately, ASHRAE has handbooks full of this sort of data – and the chapter ‘Thermal Properties of Food’ in the Refrigeration Handbook had just what I needed.

  • Thermal Conductivity = 0.4 W/mK
  • Density = 550 kg/m3
  • Specific Heat = 3150 J/kg K

BBQ Simulation Sausages

 

So – how do you go about modelling a sausage cooking on the barbecue?

I first did a steady state simulation to get the airflow and temperature distribution inside the barbecue after the ‘heating up’ phase. I ran with the lid on and both the lower and upper vents open on the Weber – as I would do in real-life. One of my favourite things about using a Weber is the controllability the ventilation gives – close the ventilation completely and the coals go cold.

Then I added the sausages – using the magic functionality we have in software of making them instantly appear in place without removing the lid and thus cooling down the barbecue. I set the simulation running with one second time steps for up to 15 minutes to see how the temperature of the sausage varied as it heated up. I was looking for core/internal temperature of about 65 degrees Celsius to signify they were cooked through – although Heston Blumenthal would probably consider this to be over done!

BBQAnimation

Using sensors on the top, bottom and in the centre of the sausages it is possible see the cooking progress. The first thing I noticed was the bottom of the sausages, where they touched the grille plate, heated up much faster than the top (and of course the centre). After just 5 minutes they were between 107 degrees Celsius and 120 degrees Celsius depending on the location of the sausage  but the tops were only around 60 degrees Celsius and the centre just 40 degrees Celsius.

As mentioned above the bottoms cook fastest – being closer to coals and receiving direct radiation as well as warm air. The top heats up slower in the absence of the radiation. All of the sausages reached an internal temperature of around 65 to 70 degrees Celsius after about 10 minutes – at which point they should be removed from the barbecue.

In the absence of any turning, the bottom of the sausages were around 150deg C – well on the way to burning. After around 15 minutes the sausages would become inedible.

So the simple lessons: turn the sausages regularly or the bottom (and maybe the top) will burn before the centre is cooked. The centre cooks much slower than the outsides and judging whether your sausages are cooked just from the outside may well lead to them being underdone on the inside. The turning – necessitating the removal of the lid – would also lower the temperature of the air around the sausages and slow the cooking process making it more likely that the outsides won’t burn as you wait for the inside to cook.

Of course the issue of whether to cook with the lid on or lid off is perhaps a whole other story…

Chris Aldham, Product Manager at 6SigmaET