Optimization of mass transport in integrated nanostructured wicking surfaces for the reduction of evaporative thermal resistance

Nanostructured materials, such as those enhanced with carbon nanotube (CNT) arrays, offer great promise for use as wicks in heat pipes and other passive phase-change cooling applications. While such nanostructures feature very small pore sizes which support high capillary pressures, it is shown that the low permeability of these dense arrays prevents their use as the sole fluid transport mechanism. It is proposed that evaporator surfaces composed of nanostructured areas be surrounded and fed by interspersed conventional wick materials (such as sintered powders) which provide the required permeability for fluid flow. Optimization of wicks with integrated sintered and nanostructured areas for minimized total thermal resistance requires a study of the trade-offs between the greater permeability of the sintered materials and the greater capillary pressure and thin-film evaporation area offered by the nanostructures. The geometry chosen for the current optimization efforts is a series of alternating wedges of CNT array and sintered powder regions covering a circular evaporator region. A numerical model is developed to estimate the thermal resistance of the evaporator region compared to that of a homogeneous sintered powder wick. The inputs needed for this model include the permeability and capillary pressure in the two regions. A parametric study is conducted as a function of the ratio of conduction and evaporative resistances for the nanostructured and sintered regions. For a given heat input, the wedge angles of the CNT array and sintered powder regions are varied to find the optimal liquid-feeding geometry that minimizes thermal resistance. The heat input is varied from 10 to 50 W over a 10 mm diameter circular evaporator region. It is found for conservative heat inputs that the evaporator resistance can be minimized by using interspersed wedges below an angle of 15°. In general, as the heat input requirements increase, smaller wedge angles are required to achieve the same minimized the thermal resistance. In the best cases, the thermal resistance is reduced by a factor of thirteen through the use of the integrated nanostructured wicks compared to the resistance of a homogeneous sintered powder wick.