Thermal resistance& pressure
 

hello my friends.
I simulate one heat sink that contain some micro channels.Now i have a 2 problem.
1- I use velocity inlet (Velocity inlet= .14m/s and tempre. inlet =300 k) and pressure out let for in & out boundary condition with reference for the absolute pressure (for pressure out let boundary condition ==>gauge pressure =0). and i have not any data about exit.
after run minimum pressure of part fluid be negative and in run process i face to 'reverse flow' warning
2- How i can calculate thermal resistance.