On this forum I found a post on a different problem.
https://www.physicsforums.com/showthread.php?t=395855
" With such a thin pipe wall compared to the radius, you can assume the pipe wall to act like a plane wall (the error in replacing 1/ln(ro/ri) with (ro+ri)/2(ro−ri) is about 0.1%), which...
Hello fellow scientists,
I'm working on describing the heat that's being stored in a sanitation pipe when hot water starts flowing through the pipe. I'm starting off with a simplified approach by assuming that the water in the pipe suddenly changes from 20°C to 60°C.
I found a good start by...