Hey Tom, I had heard somewhere that Chuk Williams never got a promised engine for his land speed record car from Harry because the steam generator tubes were constantly burning out and failing during testing. Your explanation certainly backs that up.
I was thinking about it from more the stress and strength of the tube material point of view. If you are running thin wall tubing at 3200 psi internal pressure, the wall stress is going to be very high.
Now add in that the steam is at 1200F, which means the wall will be hotter, and it is going to be glowing red. The strength of the tube material is going to be dropping rapidly at this point. Slow the steam flow, or have a gust of flame at one spot, that point of the tube gets a little warmer, strength goes down, and the tube bursts.
Like you say, Harry was designing (if we can call it that) to the limits.
The other thing he kept up about was that he was running at precisely 3205 psi. This is the boundary between normal liquid-vapor and supercritical water. Why would he pick the boundary as his operating point? One psi less and the stuff in the tube goes from a uniform fluid to a liquid and vapor mixture. Just the pulsations from the plunger pump and/or the cylinder admission valve are going to do that.
So you are guaranteed to have vapor in the tubing, low heat transfer rates, overheated tubing and tube bursts.
It's not a mystery.
But Cyclone has spent years bursting boiler tubes, and the design of the Mark 5 boiler has not changed. They've shown lots of pictures of flames from the burner, but never 3200 psi/1200F steam coming out of the boiler.
I think we can chalk this up as another fundamental design flaw of the Cyclone engine.
Another interesting part of Harry's post: "I haven't heard a good reason why not ." When you read the threads back then, there were actually a lot of good reasons why supercritical pressures would be trouble. The only problem was, he didn't "hear" them.
