Question: negative result instead of positive. What am i doing wrong?

Hi,

I am aplying a simple energie balance with some simplification, and i am implementing it on maplesim.

The component has the following equations:

Am i missing something? Do i need any aditional equation to maplesim know to where should the fluid be moving when Qjacto is positive or negative? Why do i have this result? Velocity should be positive. The results are in the following image:

thanks