Effect of surface properties of capillary structures on the thermal behaviour of a LHP flat disk-shaped evaporator

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.