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Heat pipes are one of the modern solutions for heat release from hot sources or for heat homogeneity in liquid or solid reservoirs. The interactions between the two phases of its working fluid are suggested classically by researchers and still not discovered deeply. In this study about 480 experimental and numerical tests are carried out to confirm the previous published observation of spatial flow patterns of the two phases inside a wickless copper Thermosyphon Heat Pipe THP partially filled with water. Numerical results are gotten from a three dimensional transient Computational Fluid Dynamics 3DCFD numerical solution. Different factors are included into 3DCFD model to reach reality in results and suffering from complex calculation procedures and increase simulation running time. The high agreement percentages between the experimental and 3DCFD for the temperatures distribution profile and magnitudes confirm the 3DCFD results. 3DCFD solution contours of steam volume fractions SVF show that both phases flow in a 3D spatial, non-steady and non-continues flow streams. Both phases suffering phase change and heat transfer from each to other during flow up (steam) and flow down (condensate). Thermal performance increase about (+ve. (10%)) due to inclination from vertical to 60º then falling to (-ve. (15%)) at 15º (for filling ratio 50% and heat supplied 200W). that’s because inclination lead the complex spatial flow to be a uniform circulation flow, hence it’s lead to non-homogeneity in evaporation and condensation processes and finally result in reduction of thermal performance.