Can you help me find modern research to improve heat transfer by introducing obstacles (disturbances) such as rings or other things inside heat exchanger tubes?
The reason you don't see much on this subject is because obstacles in the flow path result in material problems (welding, joint, corrosion) and fouling complications that become a maintenance nightmare so that the temporary benefit to heat transfer is not worth it.
Two approximations you may find useful are: 1) Reynolds Analogy (St=f/2) for heat transfer and 2) Chilton-Colburn Analogy (f/2=Sh/Re/Sc^1/3) for mass transfer. For complex shapes (e.g., pipe fittings) you're much more likely to find a correlation for f (friction factor) than for Nu (Nusselt number).
I’m not aware of recent research on heat transfer augmentation. There are a lot of old papers.
we use these in fire tube boilers with clean fuels and in some compact heat exchangers.
the theoretical problem is thermal contact, as this is very specific to the design. So, in most cases they are treated as turbulence enhancers and either tested (rare nowadays), or assessed as Dudley described.
Younis See dimples as an option - read patents cited by and that cite: "US8128399B1 - Method and apparatus for controlling gas flow patterns inside a heater chamber and equalizing radiant heat flux to a double fired coil." https://patents.google.com/patent/US8128399B1/en?oq=US8128399
GE developed 3D surfaces to enhance heat transfer.
There was a recent news item on with microscale pin array across the surface. eg 2023 "Boiling heat transfer characteristics of pin-finned surface in distributed jet array impingement" https://www.sciencedirect.com/science/article/abs/pii/S1290072922004847 etc. you can search scholar or patents