Heat Transfer in the Porous Structure of an Additively Manufactured Evaporator for a Two-Phase Mechanically Pumped Loop for Space Applications

The two-phase heat transfer technology has been a major thermal control hot-point for cooling or heat rejection during the last decade. Two-phase pumped cooling systems are applied when it is required to maintain a very stable temperature for heat dissipation in a system. A novel additively manufactured evaporator for two-phase thermal control has been developed at NASA Jet Propulsion Laboratory. The Two-Phase Mechanically Pumped Loop (2PMPL) permits latent heat transfer to be implemented with much wider breath of control authority compared to capillary-based systems while alleviating the system sensitivity to pressure drops. The flexibility of the 2PMPL stays in the capability to reclaim waste heat dissipation and to reject heat from discrete locations (multiple evaporators and condensers) and to control loop temperature with minimal control power. Another key aspect of this technology is the ability to integrate multi-phase heat transfer provisions directly with structural elements thanks to the conjunct use of the Additive Manufacturing Technology. A collaboration between the NASA Jet Propulsion Laboratory, University of California Los Angeles and University of Michigan has been established to develop numerical models for the prediction of the thermal behavior and performance of the 2PMPL. The Boiling and Phase Change Heat Transfer Laboratory in the Department of Mechanical and Aerospace Engineering at University of California Los Angeles has focused on the understanding and capture of the micro-scale evaporation occurring in the porous structure of the evaporator.The main goal of the research has been to develop a modelling tool that can be used to design and optimize the evaporator. An all-encompassing numerical simulation can be carried out to predict the operational thermal behaviour of the evaporator taking into account the effect of the liquid-vapor interface at the wick-to-vapor border. The numerical model allows to study the effect of different parameters, such as boundary conditions (inlet temperature, input heat loads, etc.), geometrical parameters and wick (effective thermal conductivity, permeability, pore size, etc.) and fluid properties.An experimental setup has been built in order to characterize the heat transfer within an additively manufactured porous sample and in particular its evaporative heat load under certain heat inputs. The experimental campaign has served also as validation for the numerical results and for the characterization of the transient phenomena such as dry-out