Critical Mode Operation Research Articles

Thermodynamics of Hydrogen Generation from Methane for Domestic Polymer Electrolyte Fuel Cell Systems

AbstractLow temperature fuel cells such as the Polymer Electrolyte Fuel Cell (PEFC) are preferably used for domestic applications because of their moderate operating conditions. Using the existing distribution system, natural gas is used as a source for a hydrogen rich gas to power this fuel cell type. The high requirements on the fuel gas quality as well as high conversion efficiencies for the small local gas processing units are critical aspects in the evaluation of decentralized fuel cell systems. In the present paper, three typical gas processing methods are evaluated for the supply of a hydrogen rich gas for PEFCs: steam reforming, partial oxidation, and autothermic conversion. All three processes are studied in detail by varying the relevant process parameters: temperature, pressure, steam to fuel ratio, and oxygen to fuel ratio. The results are graphically displayed in numerous nomograms. With the help of these graphs, regions of stable operation and the sensitivity to the operational parameters are discussed. For all three gas processing methods, the graphs generated display methane conversion, the hydrogen yield, and the yields of unwanted components, i.e., carbon monoxide and solid carbon. Although only steady‐state operating conditions were simulated, critical modes of operation, which might occur during start‐up or transient operation can easily be identified. For instance, operating conditions where soot is generated have to be avoided under all circumstances. All simulations were done with the Gibb's reactor model of a commercial simulation program. The Gibb's reactor model was found to be a suitable tool, since the simulated results compared well with reported literature data. According to the simulation results, the methane‐steam‐reforming process appears to be favorable for application to PEFCs. Methane conversion and hydrogen yields are highest for this process while the yield of CO is relatively low.

Adaptive off-time control for variable-frequency, soft-switched flyback converter at light loads

The soft switching of a flyback converter can be achieved by operating the circuit in the critical conduction mode. However, the critical-mode operation at light loads cannot be maintained due to a very high switching frequency and the loss of the output voltage regulation. A control which regulates the output down to the zero load and maintains soft switching at light loads is proposed. The proposed control scheme was implemented in the 380 V/19 V, 65 W flyback DC/DC converter.

A 1-D analysis of ejector performance

A 1-D analysis for the prediction of ejector performance at critical-mode operation is carried out in the present study. Constant-pressure mixing is assumed to occur inside the constant-area section of the ejector and the entrained flow at choking condition is analyzed. We also carried out an experiment using 11 ejectors and R141b as the working fluid to verify the analytical results. The test results are used to determine the coefficients, η p, η s, φ p and φ m defined in the 1-D model by matching the test data with the analytical results. It is shown that the1-D analysis using the empirical coefficients can accurately predict the performance of the ejectors.

Dynamic Measurement And Analysis Of Air Gap Variations In Large Hydroelectric Generators

This paper presents the development and application of a new fibre-optics instrumentation system for accurate measurement and analysis of the dynamic air gap behaviour of large hydroelectric generators. The system uses a passive reflective fibre-optics sensor which is immune to EMI and functions correctly on an excited operating generator. Air gap tests were performed on four 184 MV. A, 15.6 m stator bore diameter generators with 16 radial stator support rods. One of these units at B. C. Hydro's Peace Canyon Project suffered rotor-stator contact damage during commissioning in November 1979. Thirty-two air gap measurements were made around the stator during consecutive revolutions, at precisely repeatable points at, and midway between, the noted support rods. These measurements were used to produce a series of polar coordinate air gap maps which facilitated very fine trend analysis of the critical modes of generator operation. These tests and the interpretation of the data, which assisted in the solution of specific problems, are presented.

Flux diffusion upon the compression of a magnetic field by flat strips of variable width

An equation is obtained describing the flux diffusion in flat profiled generators having magnetic cumulation (MC). The critical modes of operation of such generators on active and inductive loads are calculated.

Operation of the simplest gas ejector from the point of view of the thermodynamics of irreversible processes

It is shown that the existence of critical modes of operation of a sonic gas ejector with a cylindrical mixing chamber can be determined by one of the theorems of the thermodynamics of irreversible processes, the Prigogine theorem.

Optimizing the Utilization of Redundancy of the Apollo Acceptance Checkout Equipment

Abstract This article describes a model useful for optimizing the utilization of redundant subsystems in complex test systems. Developed for the Apollo Acceptance Checkout Equipment (ACE), the procedure describes the critical operational modes of the test system; predicts system reliability under various configurations by the use of a Markovian process; and assesses penalties for the use of redundant elements. A ranking scheme then permits management to select the optimum configuration for each critical test.