Jack S. Brenizer

Liquid Water Storage, Distribution, and Removal from Diffusion Media in PEFCS

Liquid water storage in the diffusion media (DM) of polymer electrolyte fuel cells (PEFCs) is a function of design geometry, surface geometry, and operating conditions, and the DM, water storage, and can affect transient response, degradation via ionic contaminants, pressure loss, and freeze-thaw behavior. Neutron imaging was used to quantify the liquid water distribution in a PEFC under a variety of flow rates, humidities, and currents with paper or cloth DM. For a wide range of conditions, the paper DM held roughly 60% of the total water stored under the landings and the remaining 40% in, or under, the channels. The cloth DM had a nearly 50:50 channel to land liquid water distribution. From 0.2 to 1.0 A/cm 2 current conditions, the paper DM held 174% more water per volume of DM under the landings than cloth, resulting in a very high liquid saturation and eventual flooding. Increasing flow rate decreased the total liquid water content, mostly from removal of droplets. The residual liquid water under the lands was removed with increased flow rate more readily using the cloth DM, thus it was a more effective material for low power purge. Transient testing showed the time scale of significant liquid water accumulation is on the order of minutes.

The Nature of Flooding and Drying in Polymer Electrolyte Fuel Cells

Two different 50 cm2 fuel cells operated at high current density (1.3A/cm2 –1.5A/cm2 ) were visualized using neutron imaging, and the liquid water content in the flow channels and diffusion media under the lands and channels was calculated and compared. At high current density with fully humidified inlet flow, a direct comparison between flooded and non-flooded conditions was achieved by increasing the fuel cell temperature over a small range, until voltage loss from flooding was alleviated. Results indicate that a surprisingly small mass of liquid water is responsible for a significant voltage loss. The deleterious effects of flooding are therefore more easily explained with a locally segregated flooded pore model, rather than a homogeneously flooded pore and blockage phenomenon. Anode dryout was similarly observed and quantified, and results indicate that an exceedingly small mass of water is responsible for significant voltage loss, which is consistent with expectations. The results presented help to form a more complete vision of the flooding loss and anode dryout phenomena in PEFCs.Copyright

Comparison of Experiments and 1-D Steady-State Model of a Loop Heat Pipe

A modern, effective, two-phase heat transfer device, a loop heat pipe (LHP), was studied analytically and experimentally. A 1-D steady-state model was developed based on energy balance equations. The mathematical modeling procedures of each component are explained in detail, including a model of the secondary wick in the evaporator. Other models neglect the existence of the secondary wick because the detailed designs of the secondary wick are often proprietary. Three sets of experiments were performed at different elevations. Results of experimental data are compared with 1-D steady-state model predictions. The comparisons show that the model predictions of steady state operating temperatures for both zero elevation and adverse elevation are within 2 percent. It has been clearly demonstrated that the 1-D steady-state model is a useful tool for future LHP study.Copyright

Investigation of radiation-induced air fluorescence

In this paper, the radiation induced air fluorescence is investigated for several different types of radiation sources, including high brightness laser sources, X-ray radiation sources, and alpha radiation sources. First, the air fluorescence spectrum with three spectral bandpass filters induced by the high intensity laser was analyzed from spectrometer detector. Second, the air fluorescence intensity induced by different types of radiation are measured from photomultiplier tube detector and followed by discussion. Finally, the potential application of radiation induced air fluorescence for the radiation detection is addressed.

Neutron Computed Tomography of Freeze/thaw Phenomena in Polymer Electrolyte Fuel Cells

This report summarizes the final years progress of the three-year NEER program. The overall objectives of this program were to 1) design and construct a sophisticated hight-resolution neutron computed tomography (NCT) facility, 2) develop novel and sophisticated liquid water and ice quantification analysis software for computed tomography, and 3) apply the advanced software and NCT capability to study liquid and ice distribution in polymer electrolyte fuel cells (PEFCs) under cold-start conditions. These objectives have been accomplished by the research team, enabling a new capability for advanced 3D image quantification with neutron imaging for fuel cell and other applications. The NCT water quantification methodology and software will greatly add to the capabilities of the neutron imaging community, and the quantified liquid water and ice distribution provided by its application to PEFCs will enhance understanding and guide design in the fuel cell community.

Analytical Modeling of a Loop Heat Pipe at Positive Elevation

A two-phase heat transfer device, a loop heat pipe (LHP), is studied analytically. It is noted that a LHP behaves differently when it is operated against gravity (adverse elevation) or at gravity assisted (positive elevation) conditions. Steady-state modeling of LHP operating characteristics at adverse or zero elevation was broadly studied in the past. This paper presents a steady-state model of a LHP when it is operated at positive elevation based on experimental results. The effects of elevation on the trend of steady-state operating temperature (SSOT) are then studied using the newly developed steady-state model. Experimental results agree with the model predictions at adverse (88.9mm), zero, and positive (88.9mm) elevations. This steady-state model is the only model known to have the capability to predict the operating characteristics at positive elevation. The model will help to design the LHPs utilized in terrestrial applications.Copyright

Theoretical and Experimental Study of a Loop Heat Pipe at Positive Elevation

A loop heat pipe (LHP), which is a two-phase heat transfer device, was studied experimentally and theoretically. The steady-state operating characteristics of a LHP when it is operated at adverse (the condenser is below the evaporator) and zero elevations (the evaporator and the condenser are at the same level) had been studied intensively in the past. However, study of a LHP when it is operated at positive elevation (the condenser is above the evaporator) is still lacking. This paper presents detailed theoretical analysis of the steady-state behavior of a LHP operated at positive elevation. The present analysis agrees with experimental results, and is confirmed by flow visualization images. Testing was performed for a wide range of heat loads (20 W-600 W) at three positive elevations: 25.4mm, 76.2mm, and 127mm. Flow visualization images were taken by neutron radiography when the LHP was operated at 102mm positive elevation.Copyright