Originally the top cover was designed to swivel on/off.  I built the vortex inlets around the edge angled back into, and above the surface of the plate.   The center exhaust was just a hole, and nothing extended into the burn area.  I even made sure my Northern hemisphere location didn't spin the wrong direction.  It was elegant and looked great on paper, right up until I actually tried it.

There were two problems that showed up.  

1. The top plate warped under the heat, which allowed pressurized air to escape through the gap.  This allowed unburnt smoke, ash, and sparks to escape there even when up to temperature.
2. The exhaust in the center was open to the burn area.    Even with the vortex going and the burn up to temperature, the lighter materials still blew right out the top.  I was showered in ash and sparks would fly several meters in the air.  They all went straight up and never hit the ground, but it was enough to make me pretty nervous. It was a nice show, but not the intent here.

I went looking for a solution.  I found a cross-sectional line drawing of the theory of operation for another vortex burner.  In addition to the vortex inlets and the exhaust pipe, they had a plate in the center below the exhaust.

So I modified my swivel design to be a lid that hinges at the back.  I then could add the needed plate inside the burn area.

I suspended the plate via three strips of steel, which I angled to oppose the vortex spin.  This ensured only things which could make the two right angle turns at speed would escape through the middle, aka... gas.  That solved the ash problem, but I still got some sparks and it took a long time to get up to temperature.

Then I added three sacrificial layers of expanded steel mesh to the inside of the steel strips above the exhaust plate.  That created more turbulence and gave me a nice red hot surface living in the exhaust stream.  The combination very effectively countered the sparks.  As a bonus it seems to burn fully over a wider temperature range, and will now tolerate opening the top to add more fuel.