Application
Despite their advantages in size and flexibility, active thermal control systems are typically not used in space platforms due to concerns about the reliability of conventional mechanical pumps. In contrast, electrohydrodynamic (EHD) pumps involve no moving parts, using electric fields to move fluids. In concept, an EHD pump is simple in design, consisting of pairs of electrodes embedded within the fluid passage itself. To move fluid through a cooling duct, an EHD-based active cooling system needs only apply a DC voltage across each electrode pair. The resulting electric field passes through the fluid between the electrodes, resulting in the creation of a current of charge carriers that moves through the field, propelling the fluid along with it.
With their lack of mechanical parts, EHD pumps can offer an effective solution for active systems and even control fluid flow in highly branched cooling networks. Miniature EHD pumps can be embedded throughout the cooling network, sized to fit complex geometries and built into even the smallest ducts. In some thermal control applications, these pumps can serve as the primary method of fluid flow. In others, they can be used to reroute fluid flow generated by larger pumps, increasing or decreasing flow through individual ducts or specific sections of the network.
Key Parameters and Requirements
Although EHD pumps offer great potential, effective application of this technology depends on a number of factors. Besides optimizing structural elements such as fluid-channel geometry and electrode placement, engineers need to create the optimal conditions for optimal electric field strength and charge-carrier generation, among other parameters. To help identify appropriate design strategies, a research project focused on the design an EHD pump and an associated testbed to characterize the pump’s fluid-flow characteristics.
At the heart of this pump design, sets of electrode-pair assemblies provide the electric field needed to create fluid flow. For this pump, an individual electrode assembly comprises a thin stainless-steel disk for the ground electrode, a thick stainless-steel disk for the high-voltage electrode, and a thin non-conducting polycarbonate disk as an insulator. Machined into each disk, a triangular opening provides a duct for fluid passage. The EHD pump itself stacks several of these assemblies in series, each separated by a thick polycarbonate insulator disk and oriented to allow fluid to pass freely through the triangular opening in every disk. (Figure).
Figure. For the EHD research project, the EDH pump comprised a stack of electrode-pair assemblies such as the two shown here, using Master Bond EP21TDCS-LO conductive epoxy to bond each high-voltage and ground electrode to its respective bus line.1
Within each electrode-pair assembly, each electrode disk provides a hole on one end for bonding the disk to its respective bus line and a cutout on the other for preventing contact with the opposite bus line. The ability to reliably bond each disk to its bus line is critical because of the fundamental importance of electrode-pair interactions in EHD technology. Along with low resistivity for the electrical connection itself, the bonding agent must provide sufficient strength to maintain integrity despite the mechanical and thermal stresses involved in spacecraft operations. Just as important, the bonding agent must remain non-reactive with the cooling fluid itself and ensure compliance with NASA outgassing specifications.
Results
To meet the EHD pump’s diverse requirements, the research project used Master Bond EP21TDCS-LO conductive epoxy to ensure the robust connections required between electrodes and bus lines in this design. The ability of Master Bond EP21TDCS-LO to maintain stable bonds proved itself in the final pump design, which comprised 15 electrode-pair assemblies stacked in series.
In a series of tests conducted on the final EHD pump, various DC voltages ranging from 0 to 3500 volts were applied to each electrode pair to obtain different measurements of dynamic and static performance. While some tests examined flow-through rates at different voltage levels, other tests studied static pressure created within the pump over a period of time. Master Bond EP21TDCS-LO epoxy maintain reliable connections throughout this regime of varying electric field, flow rate, and pressure.
Conclusion
To build an electrohydrodynamic (EHD) pump, a research project required a bonding agent able to maintain reliable connections within electrode assemblies needed to generate electric fields at the heart of EDH technology. Using stacks of electrode assembles bonded with Master Bond EP21TDCS-LO conductive epoxy, the final EHD pump successfully achieved dynamic fluid flow and static pressure.