(317m) New Tubular, Composite Membranes for Air Removal from Aerospace Fluid Loop Coolant Systems

Aerospace applications with requirements for large capacity heat removal (launch vehicles, platforms, payloads, etc.) typically utilize a liquid coolant fluid as a transport media to increase efficiency and flexibility in the vehicle design. An issue with these systems, however, is susceptibility to the presence of non-condensable gases (NCGs) or air. The presence of air in a coolant loop can have numerous negative consequences: loss of centrifugal pump prime, irregular sensor readings, inhibition of heat transfer, and coolant blockage to remote systems. Hardware ground processing to remove this air is also cumbersome and time consuming which continuously drives recurring costs. Current systems for maintaining the system free of air are tailored and have demonstrated only moderate success. An obvious solution to these problems is the development and advancement of a passive gas removal device, or gas trap, that would be installed in the flight cooling system simplifying the initial coolant fill procedure and also maintaining the system during operations. In this study, a new tubular, composite membrane was employed to mitigate the negative consequences by removing and venting non-condensable gases entrained in the fluid loop coolant systems. Nylon-11 homogeneous membrane was employed in the original design for this work that was susceptible to pore plugging and fouling. In this work, deposition of a hydrophilic polymer on the bore-side of a porous polyethylene (PE) tube has been developed for replacements. The PE tube substrate provides excellent chemical stability and mechanical strength, while due to elevated bubble pressure, the hydrophilic layer provides for retention of gas bubbles. Preliminary studies have shown that intimate contact between the deposited layer and the substrate is required due to overcome surface energy differences that reduce good adhesion. This has been accomplished by presoaking the PE tube in the solvent to raise its surface energy and by pressure difference between the pore-side and the outside of the PE tube applied during processing to increase the penetration of the polymer solution into porous tube wall. Polymer solutions of various viscosities have been also used to promote the penetration and control separation layer thickness. Air-flow of various flow rates through the tube during drain time was employed to control the separation layer thickness. The resulting composite membranes have shown repeatable decrease in nitrogen and water permeability, which is indicative of a decrease in membrane pore size. Some swelling of the added separation layer was observed, which causes a slight decrease in membrane pore size, and should result in improved bubble retention. A series of gas slug injection tests were conducted to determine the capability of the composite membranes to remove slugs of gas in flowing systems. Initial results have been promising, with negligible gas permeation for the composite membranes compared to 100% gas permeation in the blank PE tube. Key words: composite membrane, polyethersulfone, gas trap, porous polyethylene.