Hello Tony!
Great info! Thanks so much for your post.
It's a funny coincidence that I'm just about to start work on monotube boiler experiments since I'm just about completed with my 4 cycle to steam conversion.
Okay, so now for the opinion piece.....
I think that the 1 meter/sec quoted is just a theory, not necessarily written in stone. Few people if any have ever watched bubbles form inside a monotube. There are many explanations, theories, and statements of "common knowledge" made on the internet and repeated, but very little proof of these conjectures. Basically, anecdotal experience when somebody changes something and sees either an improvement or performance decrease.
Here are just a few reasons why I have doubts that we are anywhere near a state of certainty on monotube function. It seems obvious that steam bubbles can form at different internal skin temperatures, different pressures, different material heat transfer rates, different relative surface to volume ratios of the conduit, etc. There are just lots of diverse factors besides flow rate. One example? If pressure is high enough, and maintained superheated water obviously can be conveyed all the way to an injector and engine -- as in true "flash steam". Where along the length of the monotube do the bubbles form there? Nowhere. What speed is critical to attachment? No speed, as long as the pressure is maintained.
And re "bubbles" -- what do we mean by bubbles? Is the insulating value people have observed a result of actual "bubbles", or a sheets of steam, and if bubbles, how large are they -- do they fill the conduit, are they a small percentage of the conduit?
There are just millions of questions as I see it, in a subject of much greater complexity than some arbitrary rate is going to answer.
Not saying ignore the theory completely -- might be a good rule of thumb for low pressure of a specific size boiler and material tube. But I think there's room for a lot more knowledge and development.
Which is a very good thing!