Start Workbench, drag out a Fluid Flow (Fluent) analysis system.

Double click on Geometry cell. File, Open set type to STEP, and select your file. 
Select the Prepare tab on the ribbon interface.
Select the Volume Extract tool, which opens ready for you to click a face that closes off the volume. Click on the annular flat face on the inlet side, then click on the annular flat face on the outlet side. Now click the Seed Face button, and click a face inside the fluid cavity. When you click the check mark, you will have the fluid volume.

Finally, click the top level in the Outline, FFF and in the Properties window, on the Share Topology line, set it to Share.

I would also delete the faces on all the holes drilled into the pad to make the solid mesh cleaner, but you can leave those in.
Close the SpaceClaim window.

Next you open Mesh and define the Inlet and Outlet faces of the fluid volume using Named Selections.  Name the Heat Flux face as well. Any outer face you don't name will automatically be insulated. There are best practices for meshing fluid volumes, but let's leave that for another post. Close Meshing, then right mouse button on the Mesh cell and select Update.  Wait for the Mesh cell to have a green check mark.  Then you can click the Setup cell which will start Fluent.

2. You define the heat flux as a boundary condition in Fluent. If the heat flux is only on a portion of one face, draw that shape on that face in SpaceClaim to make a separate face to apply the heat flux. The rest of the face can have a different boundary condition. You have to decide for every outside surface is it (a) insulated, (b) have a defined temperature, (c) have a defined heat flux, or (d) have a defined convective cooling to outside air where you have to provide the heat transfer film coefficient and the air temperature.

3. You define the inlet temperature of the water.  Fluent will calculate the temperature of the water at the outlet.  You have to define the inlet as either velocity, or mass flow rate, or pressure. The outlet is typically a pressure boundary condition at 0 gauge pressure.

Reply with details on the temperature of the inlet and the flow rate, and the boundary conditions for the other faces on the pad.