A two-phase flow separator for fuel cells operates between 2 and 10 kW in multi-g environments.
This liquid/gas separator provides the basis for a first stage of a fuel cell product water/oxygen gas phase separator. It can separate liquid and gas in bulk in multiple gravity environments. The system separates fuel cell product water entrained with circulating oxygen gas from the outlet of a fuel cell stack before allowing the gas to return to the fuel cell stack inlet. Additional makeup oxygen gas is added either before or after the separator to account for the gas consumed in the fuel cell power plant. A large volume is provided upstream of porous material in the separator to allow for the collection of water that does not exit the separator with the outgoing oxygen gas. The water then can be removed as it continues to collect, so that the accumulation of water does not impede the separating action of the device.
The system is designed with a series of tubes of the porous material configured into a shell-and-tube heat exchanger configuration. The two-phase fluid stream to be separated enters the shell-side portion of the device. Gas flows to the center passages of the tubes through the porous material and is then routed to a common volume at the end of the tubes by simple pressure difference from a pumping device. Gas flows through the porous material of the tubes with greater ease as a function of the ratio of the dynamic viscosity of the water and gas. By careful selection of the dimensions of the tubes (wall thickness, porosity, diameter, length of the tubes, number of the tubes, and tube-to-tube spacing in the shell volume) a suitable design can be made to match the magnitude of water and gas flow, developed pressures from the oxygen reactant pumping device, and required residual water inventory for the shell-side volume.
The system design has the flexibility to be configured in a few different ways. Special configurations of the tube geometry could aid the operation of the required second stage to manage the continual accumulation of water in the shell-side volume. An example would be with the circularization of the tubes so that water would tend to be swirled or slung to the outside of the tube bundle for subsequent removal by a second stage of the separator intended for the fine separation of remaining gas from the product water stream before it exits the separator. Another version could include in-separator reactant pressure regulation, ejector-based reactant pumping, and reactant prehumidifying thermal control through the use of in-separator thermal conditioning.
The system has few moving parts and is not subject to degradation of performance due to changes in material properties (surface wetting characteristics, etc.). The design eliminates the possibility of flooding of the fuel cell stack during nominal operations, reduces the complexity of the task of maintaining the residual water volume of the separator during periods of non-use of the fuel cell power system, and can be packaged in a manner suitable for spacecraft fuel cell power systems.
This work was done by Arturo Vasquez and Karla F. Bradley for Johnson Space Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Mechanics/Machinery category. MSC-24157-1.