The present work aims at investigating the effect of the characteristics of porous wicks integrated in a flat diskshaped LHP evaporator on their thermal performance and operating limits. Several wicks were manufactured by sintering of copper powder following a design of experiment, and characterised in terms of permeability, porosity, pore radius and thickness. A specific test bench was designed, consisting of a LHP evaporator uncoupled from the condenser. Water and pentane were used as the working fluids. The theoretical capillary and boiling limits of the wicks in the test bench were studied and compared to the experimental data. It was observed that a high permeable wick (K > 10 -13 m²) is likely to reach the boiling limit before the capillary limit because the fluid easily percolates through it. The heat transfer coefficient between the evaporator wall and the evaporating fluid is significantly higher with pentane than with water. It reaches a maximum value of 2340 W.m -2 .K -1 with water and 5310 W.m -2 .K -1 with pentane, but no clear tendency could be highlighted concerning the effect of the wick characteristics on this parameter. The maximum dissipated heat transfer rate before the wick dry-out varies in a smaller range with pentane than with water. The highest measured values are equal to 88 kW.m -² with water and 56 kW.m -² with pentane.

In this work, capillary structures are manufactured by sintering of copper and zirconia powders using different sintering parameters, in order to obtain porous samples with various hydrodynamic characteristics and surface properties. A specific test bench is designed to reproduce a capillary evaporator of a LHP decoupled from the remaining of the loop and enables the determination of the evaporator heat transfer coefficient and optimum heat flux at the occurrence of the operating limit. The influence of the surface properties on the evaporator thermal behaviour is investigated by polishing the copper wick surface in contact with the evaporator fins, or by coating the zirconia wick with a thin gold layer. It is shown that polishing decreases the thermal performance when the size of the gap between the evaporator fins and the wick becomes lower than the pore size. The coating enables to enhance the evaporator thermal performance, probably by a reduction of the thermal contact resistance or by a favourable surface wettability effect, reducing the strong effect of the wick topography on the optimum heat flux value. Finally, a data mining approach is proposed which highlights the preponderant effect of the surface quality on the evaporator heat transfer coefficient.

Mono and bi-layer capillary structures are designed by means of a thermo-hydraulic model of a loop heat pipe in order to optimize the performance of the system. The model tends to show that bi-layer wicks with a thermally conducting bottom layer and an insulating top layer are the most efficient in loop heat pipes. An experimental study is then led to manufacture and characterize the bottom layer. Eight samples made of copper powder are manufactured following a twolevel fractional factorial design. The top layer is not manufactured in this study. The sintering parameters are adjusted to provide porous samples with sufficient mechanical resistance. The porous structure permeability and its capacity to provide a sufficient capillary pressure are evaluated using a specific test bench dedicated to this study, as well as with microstructural observations (tomography, microscopy). The experimental characterization of the samples enables to determine the influence of each sintering parameter as well as the interactions between them. The characteristics of the porous samples are found to be mainly affected by the sintering time and the pressure. High values of these parameters decrease the permeability and the porosity but increase the maximum capillary pressure due to a smaller effective pore radius. A set of optimum sintering parameters is found in order to manufacture the bottom layer. The best porous structure is supposed to enhance the latent heat transfer in a LHP.