Hello guys, in the question attached, my understanding is that there is a heat transfer that heats the fluid from an initial at the input, to at the output. This heat transfer is via convection from walls of temperature . Firstly, . Because the walls are and the fluid is of a lower temperature, this heat transfer is via convection from the walls. The correct answer says that . Why is this? Why not ? or . The fluid next to the wall is heated and changes until it reaches at the outlet, so how do we know which temperature to use in the convection equation where heat is being transferred from the walls to the fluid?