Heat transfer performance of cylindrical heat pipes with axially graded wick at anti-gravity orientations

To increase the heat transfer capability at anti-gravity orientations, heat pipes with homogeneous and axially graded wicks were experimentally investigated. The homogeneous wick (0.8 mm thickness) sintered with 125–150 μm (coarse) copper powder operated at 100 W without dryout at horizontal orientation, while that sintered with 75–97 μm (fine) copper powder had lower critical heat load about 80 W. The critical heat load decreased tremendously with increasing anti-gravity angles and it was about 20 W at totally anti-gravity orientation. However, the axially graded wick, sintered with fine and coarse powder at evaporator section and the rest section of the pipe, respectively, exhibited critical heat load of 35 W, nearly 75% increment than homogeneous wick with same thickness. Further study shows that increasing the thickness of axially graded wick significantly improved the critical heat load although the thermal resistance increased simultaneously. At a tilted angle of 90°, the critical heat load increased from 35 W to 45 W when increasing thickness from 0.8 mm to 1.1 mm and it further increased to 96 W when increasing thickness to 1.35 mm. The reconciliation between the capillary force and the permeability of axially graded structure promoted the increase of the critical heat load notably.