Graduate Student Mentor: Dr. Cable Kurwitz Faculty Advisor: Dr. Fred Best NASA Advisor: Art Vasquez Multiphase Flow in Simulated PEM Fuel Cell Under Variable Gravity Conditions PEM Fuel Cell Team (PEMFCT) Team MemberClassificationMajor Ernie EverettSeniorMEEN Nikhil BhatnagarJuniorAERO Christie TiptonJuniorBMEN Trevor BennettFreshmanAERO Caitlin RieglerFreshmanAERO

 Background  Objectives  - NASA’s Needs  - SEI Goals  Review of Fall Work  Preliminary Analysis  Review of Spring Work  Ground Testing  Set Up  Manufacturing  PSA  Preliminary Modeling  Results  New Direction  Conclusions

 Typical fuel cells generate electricity by combining a fuel and oxidizer in the presence of an electrolyte  Main parts of a fuel cell – Flow channels for fuel and oxidizer – Anode and Cathode separated by an electrolyte  Fuel and oxidizer react to produce electricity and byproducts  Fluid distribution and control is a critical issue with fuel cell operation  The parallel flow channels and parallel plates can produce flow instabilities leading to degraded fuel cell operation and possible damage  Goal –Identify regions of operation where instabilities can occur

 NASA’s Need  Utilized fuel cells in Gemini, Apollo, and currently in Space Shuttle  NASA plans to utilize fuel cells in Constellation program  Technology has many other applications ▪ Vehicles, buildings, and alternative energy applications  Purpose  Evaluate flow conditions that lead to unstable operation within a prototypic fuel cell geometry  Develop a flow map that describes stable and unstable operating regions  Provide simple modeling approach to predict transition from stable to unstable operation  Learning Objectives:  Understand two-phase fluid flow  Identify and understand the flow conditions that produce instabilities

 Literature Review  Understood fuel cells and the flow distribution during operation as well as flow anomalies  Researched standard geometries and flow rates ▪ Confirmed with NASA Advisor  Developed CAD drawings of two cell geometries  Performed stress and flow analysis for both parallel and serpentine configurations  Built simple cells for ground testing  Proposed testing to Microgravity University

Parallel Plate Gas Used: Nitrogen Mass Flow Rate: 3 SLPM Inlet Pressure: 50 psig Inlet Temperature: K Max Channel Velocity: 0.1 m/s

Serpentine Plate Gas Used: Nitrogen Mass Flow Rate: 3 SLPM Inlet Pressure: 50 psig Inlet Temperature: K Max Channel Velocity: 0.35 m/s

Serpentine ModelParallel Model Parallel Model Delta Pressure:10 Pa Serpentine Model Delta Pressure: 56 Pa Gas Used: Nitrogen Mass Flow Rate: 3 SLPM Inlet Pressure: 50 psig Inlet Temperature: K

Maximum Stress: psi Located over channel/plenum junction. Maximum Displacement: <1 micron Located at lid over the plenum. Minimum Factor of Safety: 36 Maximum Stress: psi Located at center of channel/plenum junction. Maximum Displacement: microns Located over the center of the plenum. Minimum Factor of Safety: 110

 Allows undergraduate teams to carryout flight testing of experiments in microgravity conditions  - Submitted Proposal  - Completed Safety Analysis  - Pursued Funding  - Education Outreach  Team Proposal Turned Down  Switch from NASA to Zero-G aircraft greatly reduced the number of experiments

 Looked for alternative flight opportunities  FAST Program – decided not to pursue  Designed and fabricated a higher fidelity prototype to more accurately reflect fuel cell flow distribution  Uniform liquid addition throughout each channel  Built test loop for ground testing  Composed and submitted PSA  Performed preliminary testing and analysis

Zoomed in View Dimensions: 20 cm x 20 cm x 1 cm Channel Dimensions: 1 mm x 1mm Number of Channels: 80 Wetted Surface Area: 10,700 mm^3 NASA interest fueled by these being the standard geometries for fuel cells (based on chemical properties)

Zoomed in View Dimensions: 20 cm x 20 cm x 1 cm Channel Dimensions: 1 mm x 1mm Number of Channels: 20 Wetted Surface Area: 10,700 mm^3

Added a liquid plenum for water introduction from the bottom 6 mm deep Added holes along the channels connecting the lumen of the channels to the liquid plenum Changed water input to directly in the center for more uniform addition

Specifications Gas provided by high pressure Nitrogen Tank Regulated to 50 psig Pressure Transducer will monitor pressure drop Parallel Mass flow meters will simulate excess cells CCD Digital Camcorders will record fluid instabilities Vortex Water Separator will separate fluid from gas

 Flow Loop Consists of 2 Flow Meters, Pressure Gauge, Differential Pressure Transducer, and CCD  Gas Flow Provided by Compressed Nitrogen Cylinder and Water Flow Provided by Liquid Syringe Pump  Test Stand Allows Cell Plate to be Rotated  PSA Written and Provided to Safety Officer

 Data is collected by video, flow meter and pressure gauge  Varied flow rate on primary flow meter from 0 to 10 sLPM  Bypass Flow Rate Varied from 0 to 90% of Primary Flow  Liquid Flow Rate 0 to 100 cc/min  Variation in gas and liquid flow rates occurred simultaneously

 Flow instabilities occur at all flow rates tested for parallel and serpentine channel fuel cell plates  Oscillations are small and focused toward exit of channels  Overall liquid holdup is constant for each test but varies over range of testing with large amounts of water held at low gas flow rates  Some channels occluded for duration of tests (No flow)

 Liquid holdup in channels varied with tilt angle on test article  Due to flow regime and hydrostatic pressure changes  Indicates a need for reduced gravity testing  Two-phase flow in outlet line seemed to have an effect on bypass flow

 Liquid pores too large allowing gas to enter liquid plenum at high channel differential pressure  Graded pores or a more controlled method of adding water to better simulate water production is needed  New test setup required to accommodate water entering bypass flow meter

 - Engineering Skills: ▪ Analysis Tools ▪ Solid Works, Cosmos FloWorks, CosmosWorks, Microsoft Visio ▪ Analytic techniques to validate computation ▪ Analysis of test data (i.e. model fitting) ▪ Lab Skills: ▪ Machining experience ▪ Interpreting engineering drawings ▪ Developing procedures ▪ Carrying out test ▪ Education Outreach ▪ Teamwork

 Purpose: -Evaluate flow conditions within a prototypic fuel cell geometry - Determine a range of stable operations for given flow and environmental conditions  Ground testing showed occlusions have a great gravitational dependence and that more work needs to be done on our test system

 Replace flow meter to complete ground test matrix  Modify test stand to allow higher flow rates  Modify liquid addition method to provide a more uniform liquid addition  Continue work on test loop and develop more robust analysis techniques

 Parallel channel instability may occur when a number of channels are connected at common headers.  Although the total flow remains constant, flow oscillations may occur in some of the channels.  Nonlinear transient momentum equations can be used to solve for several channels by integrating the momentum equation along each channel.  Fluid properties used in the momentum equations are obtained from the energy equation and the equation of state.  The modeling is very complex, the following equations convey the complexity  After integrating the momentum equations for the channels, the equations are solved simultaneously.

 Starting with the Momentum Equation  Define B and C  We Simplify

 The Total Inlet Flow  Differentiating  We can then Solve n Equations to Determine the Channel Flow Rate