Nominal Operating Regime Research Articles

Prediction of Performance Variation Caused by Manufacturing Tolerances and Defects in Gas Diffusion Electrodes of Phosphoric Acid (PA)–Doped Polybenzimidazole (PBI)-Based High-Temperature Proton Exchange Membrane Fuel Cells

The automated process of coating catalyst layers on gas diffusion electrodes (GDEs) for high-temperature proton exchange membrane fuel cells results inherently into a number of defects. These defects consist of agglomerates in which the platinum sites cannot be accessed by phosphoric acid and which are the consequence of an inconsistent coating, uncoated regions, scratches, knots, blemishes, folds, or attached fine particles—all ranging from μm to mm size. These electrochemically inactive spots cause a reduction of the effective catalyst area per unit volume (cm2/cm3) and determine a drop in fuel cell performance. A computational fluid dynamics (CFD) model is presented that predicts performance variation caused by manufacturing tolerances and defects of the GDE and which enables the creation of a six-sigma product specification for Advent phosphoric acid (PA)-doped polybenzimidazole (PBI)-based membrane electrode assemblies (MEAs). The model was used to predict the total volume of defects that would cause a 10% drop in performance. It was found that a 10% performance drop at the nominal operating regime would be caused by uniformly distributed defects totaling 39% of the catalyst layer volume (~0.5 defects/μm2). The study provides an upper bound for the estimation of the impact of the defect location on performance drop. It was found that the impact on the local current density is higher when the defect is located closer to the interface with the membrane. The local current density decays less than 2% in the presence of an isolated defect, regardless of its location along the active area of the catalyst layer.

Control for Ecological Improvement of Small Biomass Boilers

The aim of this paper is to present various types of research on reducing emissions in the flue gases produced by low-power biomass boilers, especially when the boilers operate outside their nominal operating regime. This long-term investigation program is being carried out on a pilot boiler in the university laboratories. The program involves acquiring additional instrumentation for performing and evaluating present and future experiments, creating various types of models of the boiler for designing and testing the control algorithms using available data, and implementing and evaluating the algorithms in real operation. The pilot device enables us to experiment using non-standard instrumentation, but throughout our research it is necessary to strike a balance between ecological policy requirements and economic considerations.

Characterization and investigation of the anomalous behavior of the immersed-Bz diode during operation at 4 to 5 MV

The immersed-Bz diode is being developed as a high-brightness, flash x-ray radiography source at Sandia National Laboratories. This diode is a foil-less electron-beam diode with a long, thin, needlelike cathode which is inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces forward-directed bremsstrahlung. In addition, electron beam heating of the anode produces surface plasmas allowing ion emission. Two different operating regimes for this diode have been identified: A nominal operating regime where the total diode current is characterized as classically bipolar with stable impedance [see D. C. Rovang et al., Phys. Plasmas 14, 113107 (2007)] and an anomalous operating regime characterized by a rapid impedance collapse where the total diode current greatly exceeds the bipolar limit. The operating regimes are approximately separated by cathode diameters greater than 3mm for the nominal regime and less than 3mm for the anomalous impedance collapse regime. Results from a comprehensive series of experiments conducted at 4–5MV characterizing the transition from this nominal operating regime to the anomalous operating regime as the cathode diameter is reduced are presented. Results from experiments investigating the effects of anode-cathode gap, anode material, and cryogenic modification of the anode surface are also presented. Although these investigations were unsuccessful in completely mitigating the anomalous behavior, insight gained from these experiments has elucidated several key physics issues that are discussed.

The role of ions during stable impedance operation of the immersed-Bz diode at 4 to 5 MV

The immersed-Bz diode is being developed as a high-brightness, flash x-ray radiography source at Sandia National Laboratories. This diode is a foil-less electron-beam diode with a long, thin, needlelike cathode that is inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces forward-directed bremsstrahlung. In addition, electron beam heating of the anode produces surface plasmas allowing ion emission. Two different operating regimes for this diode have been identified: a nominal operating regime where the total diode current is characterized as classically bipolar and an anomalous operating regime characterized by a dramatic impedance collapse where the total diode current greatly exceeds the bipolar limit. Data from a comprehensive series of experiments fielded at 4 and 5MV, where the diode operates in the nominal or stable impedance regime, with beam currents ranging from 20–40kA on target are presented. In this mode, both the measured diode current and experimental radiation production are consistent with physics based models including two-dimensional particle-in-cell simulations. The analysis indicates that intermediate mass ions (e.g., 12–18amu) control the nominal impedance evolution rather than expected lighter mass ions such as hydrogen.

Computational estimate of the corrosion resistance of fuel microelements with silicon carbide cladding in a water medium at supercritical pressure

A computational estimate of the corrosion resistance of a silicon carbide coating on spherical fuel microelements in a supercritical-pressure water medium at 350–700°C is made for the nominal operating regime of the core of a light-water nuclear reactor. A model of the thermal dissociation of water and dissolution of a silicon oxide film is used to estimate the corrosion resistance of silicon carbide. An expression describing the dependence of the corrosion depth of a silicon carbide layer on the temperature and pressure of the medium (concentration of the products of dissociation of water) and the operating time of the fuel microelements is obtained on the basis of the model developed.

Electron Optic System of Powerful Large Orbit Gyrotron with Pulse Magnetic Field

Short-pulse powerful Large Orbit Gyrotron with total electron energy about 400 kV and beam power in the cavity up to 100 MW is now under developing at FIR FU. Suitable for 200 ns pulse duration electron-optic system is analyzed. Results of numerical simulation for explosion emission cusp-type electron guns and magnetic field intensity about 8 T are presented. Sensitivity of the guns to small deviations from the nominal operating regime is investigated. Some versions of the gun with different accelerating potential as well as different beam current passing through the cavity (60-300A) are suggested. Current reduction simplifies the problems of mode competition and potential depression in the cavity, but at the same time decrease output power. To diminish current special diaphragms are suggested. Results of numerical simulation of collector corresponding to each version of the gun design including power density distributions along its surface are presented. It is shown that beam quality and collector regimes are suitable for LOG operation.

Computational-Theoretical Analysis of the Distribution of the Thermohydraulic Parameters and the Concentration of Dissolved Impurities in a PGV-1000M Horizontal Steam Generator Performed Using the STEG Computer Code

The STEG computer code for performing thermohydraulic calculations is used for a three-dimensional computational analysis of the distribution of the thermohydraulic parameters and the concentration of dissolved impurities in a PGV-1000 horizontal steam generator. The nominal operating regime is modeled. The distribution of the concentration of dissolved impurities is calculated on the basis of the results of a previous thermohydraulic calculation. The mathematical model of the STEG code also permits modeling the growth of deposits on the tube sheet. The calculations agree with experimental data obtained at the nuclear power plant. The STEG results show that taking account of the special features of the construction of this steam generator improves the agreement between the calculations and the experimental data. It is concluded that the STEG code can be used to model the distribution of the concentration of dissolved impurities in steam generators with a new construction, specifically, in the PGV-1000 steam generator with a corridor bundle.

Power Maximization of a Lead-Bismuth Cooled Fast Reactor

Two variants of the arrangement of a fast reactor cooled by a eutectic alloy of lead and bismuth are studied. The first one is obtained by solving the problem of minimizing the void coefficient with drying of the central zone of the reactor. The second one is obtained by increasing the power by a factor of 1.5. Both problems contain a constraint for the reliability functionals, characterizing the nominal operating regime of the reactor, and the safety functionals, characterizing the intrinsic self-shielding from ATWS-type accidents.

Control of LP turbine rotor blade underloading using stator blade compound lean at root

Due to a large gradient of reaction, LP rotor blades remain underloaded at the root over some range of volumetric flow rates. An interesting design to control the flow through the root passage of the overloaded stator and underloaded moving blade row is compound lean at the root of stator blades. The paper describes results of numerical investigations from a 3D NS solver FlowER conducted for several configurations of stator blade compound lean. The computations are carried out for a wide range of volumetric flow rates, accounting for the nominal operating regime as well as low and high load. It is found that compound lean induces additional blade force, streamwise curvature and redistribution of flow parameters in the stage, including pressure and mass flow rate spanwise that can improve the flow conditions in both the stator and the rotor. The obtained efficiency improvements depend greatly on the flow regime, with the highest gains in the region of low load.

Results of the analysis of the dynamic operating regimes of systems for cryostating superconducting installations

In the analysis of the dynamic characteristics of cryostating systems of SC installations carried out earlier in [2], the authors omitted an account of a number of factors which have a noticeable effect on the thermal stability of current-carrying elements to lengthy pulsed heat liberation, chiefly the change of magnetic induction over the radius of the magnet windings; and that means also along the cryostating channels. Characteristic of the coils of the toroidal field is a substantial nonuniformity of distribution of the thermal load and of the flow rate of helium in different channels, and this nonuniformity is greater in cooling of coils by helium at subcritical pressure. In tests of assembly blocks T-15 on the complex SIMS it was also established that the temperature of the helium at the inlet to the winding is higher than the initial assumed value of 4.5 K, and this also has to be taken into account. In [2] the effect of the current-carrying element itself on the thermal stability was not taken into account, and the possibility of stabilizing the windings by superheated helium at subcritical pressure was not examined. Let us consider the results of the calculations of the dynamic characteristics of cryostating systems which make it possible to evaluate their advantages and shortcomings in different variants of cooling the windings. The calculations were carried out for an Oshaped coil of the toroidal field shown in [i]. The main characteristics of the c0il are: the superconductor is made on the basis of a niobium-tin compound; the outer and inner diameter of the coil is 3.1 and 2.1 m, respectively; the length of the spiral cryostating channel is 340 m; its inner diameter is 2.8 m~n. The condition of cooling the coil is: the temperature of the cryogen at the inlet to the coil is 4.7 K when it is cooled by supercritical helium and helium at subcritical pressure with subcooling at the inlet; in cooling by helium at subcritical pressure the mass vapor content of cryogen at the inlet to the winding is 0.i; the pressure at the outlet from the winding is 0.I and 0.5 MPa, respectively, for subcritical and supercritical helium; the helium enters the channel from the side of the small radius of the winding. The magnetic induction of the coil decreases from its center to the periphery. Magnetic induction Bma x is equal to 8.2 and 6.4 T, respectively, with forced and nominal operating regime of the blocks T-15 [3]. The steady-state thermal load of the coil is 90 W which corresponds to the data obtained on the complex SIMS. The fraction of heat penetrating through the housing of the coil to the winding was taken to be equal to 0.7. For individual disks of the coil the degree of nonuniformity in regard to thermal load is equal to 1.8 and 3, in regard to the flow rate of helium it is 1.06 and 1.4, respectively, for supercritical helium and for helium at subcritical pressure. The degree of nonuniformity in regard to thermal load and flow rate of helium shows how many times greater the thermal load and smaller the flow rate of helium is on the outermost disk which is in direct contact with the housing of the coil in comparison with the mean values (for all the disks of the coil) of these parameters.