here's a question for
@OF: is there a simple formula for heat transfer from the heater surface to the incoming ambient air (i think @Newton could chime in here, too) - there must be an optimal effective rate of airflow - faster and the heater can't transfer any more thermal energy to the air, and if slower then there could be more vapor extracted at the optimal flow rate. i calculated the surface area of my heater and volume of air passing the heater wall.
Sadly no. And what there is is far from "simple". Such heat transfer is a surface game, not only in the finish but also how turbulent the flow is (mixing is good.....).
All is not lost, however. You have a closed loop system, you should be able to monitor the current over time as you vary the airflow. The more heat you remove and pass on, the more the closed loop will have to make up for it.....
you'll use more current/power over time as the transfer to air goes up.
Probably the best convection scheme is the one TV used. The 'thermal core'. A ceramic chamber heated to equilibrium by an exposed heater wrapped around a reflector. It uses 'the kiln effect', IR rattles around inside until everything (heater, reflector, walls, the lot) is basically at the same temperature everywhere when the air is 'let in' with purposeful turbulence at one end and out through a 'homogenizer grid' to insure uniform temperature across the air column. This (using IR at about 1300F) gets around the contact issue, especially when combined with the homogenizer. You've put your finger exactly on what is the elephant in the parlor with assuming Solo/Air is using the cup to heat air for convection. The ports are getting big (more volume for the perimeter, less percentage contacts) and the metal is made thinner? Only a tiny fraction of the air molecules blowing up those four holes can contact that tiny wall while flying trough.......
Lucky for us Solo/Air is conduction scheme. As the above graphs from Sticks show. Especially the 'Christmas one' (Red and Green), very festive. We can see the drop in wall temperature when the cold air comes in and the rise in 'load' temperature as the now heated air from the hit makes up heat lost up the leads. Or makes up more than before the hit at least. The two 'curves' come very close to meeting? Which makes sense to me. The wall temperature dropping tells the tale I think. The (non heated) air did that....... Convection wouldn't do that, right? Since the heat would be added to the inside. More heat is of course conducted through the walls to make up for it, the drop in temperature being an indicator of that. Like when the water pressure in your house drops and you get scalded in the shower just because someone else flushed it. That drop is an indicator of the flow (the more flow, the bigger the drop), the pressure in the water mains in the street is still the same, same as the cup in Solo/Air doesn't drop much at all for the hit, the 'drop' is 'across the glass'. Or the load in the case of the T/C lead at idle. The heat flowing through the load and up the lead causes a temperature drop 'across the load' where that heat flows. The drop is Thermodynamics' "tell" (for the non poker players, a "tell" is a nervous fault (like pulling your ear) when you're bluffing that 'tells' the others you're vulnerable. A 'telltale clue' if you will.
Fun stuff.
OF