Is something weird going on with my fluent?
 

Guys,
I used the energy balance equation
Q=mxCpxdT to find out how much heat load I will need to raise the temperature of my water from a fixed amount T1 to T2. And I calculated the heat flux needed to send through the wall by dividing the heat load by the surface area of the heating container.
Now here is the thing, I used that heat flux on a 2D model of the same dimensions but just a two dimensional figure. But I didn't see any considerable heat transfer. Like I disproved the energy balance equation.

But when I tried the same on a three dimensional model there was clearly the temperature rise that was expected: although some of the heating was lost in the conduction through the shell due to shell conduction.

Why didn't it work with 2d? I was hoping to simplify the problem.
Any help will be greatly appreciated.