[Aladin&Fluent]

I am new to CFD-online, highly grateful to you all for your volunteer help.

I am trying to setup a subcooled pool boiling of water and vapor in a vertical pipe (5cm inner diameter) 1m high. A small heater is inserted into the bottom of the pipe to make boiling. I use two-fluid with RPI models. I also model the conjugate heat transfer through the pipe wall.

I set the vapor temperature in the fluid zone (inside the pipe) to a fixed value, i.e. 373.15K then run a transient case.
The problem is that when I look at the calculated static temperature of the vapor phase, it never shows the fixed value of 373.15K but a range from 341K (close to some parts of the pipe wall) to 396K (close to the top of the heater surface).

Could anybody have comments or explain me this problem? Thank you a lot.