heat riser diffuser

Hi there,

I am developing a batch rocket grill.
At the top of the heat riser there is a square cast iron plate (40x40cm) for BBQ. After 2 burns I figured out that temperature is not uniform at all. There is a diferencial of 100°C among the center and the edges of the plate.
I was thinking to built something like a flame difusser in order to reduce the heat at the center and increase it at the edges. I have tried few things without sucess. Does anybody knows any solution to my problem?

regards

Nikos

Hi Nik! Is that your rocket? Looks cool! (well Hot!)

Is that white plate the hot plate? I can't tell your gas layout from the picture. Is it like a barrel arrangement or does the flue gas go sideways at the top of the plate into the chimney which I can see emitting flames?

Hi Nikos,

super cool photo!

As to your "problem" of the edge being colder then the middle. I'm pretty sure that's just physics, no way to avoid it. Part of your hot plate is not in direct contact with the heat from the combustion (the overhang). It get's its heat only by thermal conduction and losses heat through convection and radiation in several directions. That part acts like a cooling fin and will lead to the outer areas of the plate always being cooler than the middle. The middle receives heat from the combustion and only looses it on the surface and to the sides.

Maybe try a round plate that's the same dimension as the riser (e.g. no overhang)?

Yes this is my rocket. Just for you reference check this photo.
Instead of a round pan imagine a bigger square plate for BBQ. Smaller plate doesnt fit enough meat for me and friends :)
I need a uniform temperature at the top of the plate because food is not grilled at the same time among the whole surface of the plate. Another reason is that after two burns my cast iron plate has deformed at the center.

Hey Nikos,

now I understand the design of your rocket much better.
But my opinion remains the same. It's a physical law.
Use it to your advantage: different heat zones for different pieces of meat...

You could work against the most extreme effects by:

Choosing a thicker plate
Making it round
Making it smaller (same size as riser)

Also

The warping of any metal will, in my opinion, always happen exactly because you have a big heat difference between the middle and the edges.

I can't imagine what a "diffuser" would look like. I hope someone else here on permies has an idea for you.

I would think a round plate about the same size as the riser mounted an inch or so below your cooking surface would spread the flames so they hit nearer the outer edges. It would likely take some experimentation to find the best diffuser diameter and distance below cooking plate. A square plate will always be cooler at the corners absent a complex flame director; a round plate of the right size will be easier to heat evenly.

It might help to have a fin pointing downward around the edges of the cookplate, to gather heat under the whole surface. This might or might not be best attached solidly, or with a small gap between fin and plate.

So you need to add a barrel and raise the top as high as practical above the riser….