Study of the impact of reactants utilization on the performance of PEMFC commercial stacks by impedance spectroscopy

Reactants utilization is a key stake for a PEMFC system: a too low utilization leads to a waste of reactant but a too high utilization may result in a detrimental starvation. To study these impacts, two commercial stacks were characterised by impedance spectroscopy under different hydrogen and oxygen utilizations (from nominal conditions to quasi-starvation). One was fresh while the other was operated on-field during 10,000 h. This study shows that the two capacitive loops in the lowest frequency range (1 Hz and below) correspond respectively to oxygen and hydrogen mass transfer limitations: the limiting reactant can be clearly identified from the impacted frequencies. The size of these loops was increased by up to 30% when the cell operated at high reactant utilizations. These results could therefore pave the way to the development of algorithms able to estimate the degree of starvation of some cells.     

供油量对浮环轴承静动态润滑特性影响

针对浮环轴承贫油润滑问题,基于雷诺方程,建立浮环轴承贫油润滑模型,以润滑油入口供油量为可变参数,通过有限差分法求解数学模型,分析了供油量对浮环轴承内外油膜力静态润滑特性影响,计算结果表明:供油量明显影响润滑油油膜力的起始角和轴承环速比,摩擦功耗随浮环轴承供油量的减小而增大。     

Prevention of fuel cell starvation by model predictive control of pressure,excess ratio,and current

Starvation of polymer electrolyte fuel cells (PEFC) takes place, especially during transients, if reactants are consumed in the fuel cell faster than they can be supplied. It is one of the main causes of aging and degeneration of fuel cells.     

Model-based characterization of endogenous maintenance, cell death and predation processes of activated sludge in sequencing batch reactors

Maintenance, cell death and predation are endogenous processes of microorganisms and play an important role in governing the overall performance of biological wastewater treatment systems. However, in the previous studies on the activated sludge system, a widely used biological wastewater treatment process, the endogenous processes are summarized as a single process and the kinetics of all related processes are lumped into a single parameter set. In order to better understand the endogenous processes, efforts are made to quantify the maintenance, cell death and predation processes separately with both experimental and mathematical approaches in this work. A model-based analysis on endogenous processes of activated sludge is performed. Both oxygen uptake and biomass concentration gradually decrease with the increasing length of aerobic starvation. Model predictions are in accordance with experimental data. The active bacteria content is reduced to about 23% of the initial value after 10-day starvation, and the maintenance energy consumption rate under non-growth conditions is found to be about four times of that when the microbial growth is of its maximum level. The active microorganisms form the prey for the predator growth. The oxygen consumption related to the predation of active bacteria significantly contributes to the total oxygen consumption.     

Model-based control of cathode pressure and oxygen excess ratio of a PEM fuel cell system

For PEM fuel cells supplied with air, pressure and flow control is a key requirement for an efficient and dynamic operation because fuel cells are in risk of starvation when the partial pressure of oxygen at the cathode falls below a critical level. To avoid oxygen starvation and, at the same time, to allow for a dynamic operation of the fuel cell system, both excess ratio of oxygen and cathode pressure need to be adjusted rapidly.     

Effects of starvation period on the locomotory response of Rhyzopertha dominica (F.)

In the context of developing a behavioural bioassay, the effects of 1-5 days starvation on the locomotory responses of Rhyzopertha dominica were investigated. Movements in an experimental arena with airflow were videoed and analysed using computerised tracking and motion analysis software. Beetle velocity and angular velocity (turning) were influenced adversely following 3 or 4 days of starvation, resulting in a lower number of beetles arriving at food. The highest number of beetles arriving at food was when they were starved for 1 day. Gender had a statistically significant effect on velocity; on average males walked 37% faster than females and there were significant decreases in the velocity of females from the fourth day and male velocity from the fifth day of starvation. Starvation significantly reduced angular velocity of beetles although the effect of gender was not statistically significant. There was a decline in angular velocity following the first day of starvation, although the negative effect of starvation on turning is not as clear-cut as on velocity, with a more gradual decline with increasing starvation. For undertaking behavioural bioassays with R. dominica, starvation beyond 1 day is unlikely to improve responsiveness.     

Effect of Starvation on Traction and Film Thickness in Thermo-EHL Line Contacts with Shear-Thinning Lubricants

The effect of starvation on traction and film thickness behavior in thermo-EHL rolling/sliding line contacts has been studied using full EHL simulations. The simulations employed the free volume equation for viscosity–pressure–temperature relationship and Carreau viscosity model to describe the shear-thinning behavior of the EHL lubricant. The simulation results were used to develop equations for estimating the factors by which the traction coefficient increases and film thickness decreases as a function of the degree of starvation. For the situations involving inadequate lubricant supply at the inlet, these factors can be used to correct the traction coefficient and central film thickness predicted with the assumption of fully flooded condition.     

Effect of starvation and diet on the gel forming ability of tilapia (Oreochromis niloticus)

To elucidate the factors affecting the variation in the gel forming ability of fish muscle within a species, the effect of starvation and diet on the proximate composition and gelling property of tilapia muscle was examined. Starvation resulted in a decrease in protein and lipid contents coupled with an increase in water content. Protein depletion, mainly in the myofibrillar fraction, in the starved fish caused a lowering of the gel forming ability of their muscle. The myosin-degrading activity at 65°C was not affected by starvation for 56 days. Tilapia (Oreochromis niloticus L) fed on either fishmeal or krillmeal did not differ in their proximate composition. Phytoplankton-fed fish had a slightly higher lipid content and had the greatest gel forming ability. There was no significant difference in the gel strength between the fish fed fishmeal and the fish fed krill or 58 days. Compared with the phytoplankton, however, prolonged feeding of fishmeal seemed to increase the myosin-degrading activity of the muscle.     

A Review of Elasto-Hydrodynamic Lubrication Theory

The development and understanding of elastohydrodynamic lubrication (EHL) can be traced back to the beginning of the previous century. However, it was not until 1949 that the first real solution of the problem was published. Since then, the technology has evolved enormously. In the current article a summary of these developments is given. Smooth surface EHL has become well established. Numerical methods, analytical solutions, and experimental techniques have become mature. Focus areas of research today are thermal EHL, starved EHL, friction (non-Newtonian lubricants), roughness, and grease. The scope of EHL is so wide that the authors needed select the topics of focus in this article. Therefore, in addition to the general overview of the areas of friction, analytical methods, starved EHL, and grease EHL are highlighted in this article.     

Effects of periods of starvation and fluctuating hydrogen sulfide concentration on biofilter dynamics and performance

The paper describes the results of a systematic study of the transient behavior of biofilters treating reduced sulfur pulping odors and VOCs. They were exposed to variations in contaminant loading and periods of starvation. Three bench-scale biofilters with different filter media were used. Filter media materials used were the mixtures of compost/perlite (4:1), hog fuel/perlite (4:1), and compost/hog fuel/perlite (2:2:1). Hydrogen sulfide, the main malodorous gas produced from kraft pulping processes, was used as the test contaminant. The starvation period comprised of two stages: the ‘no-contaminant-loading phase' when only humidified air was passing through the biofilters, and the ‘idle phase' when no air was passing through the biofilters. The response of each biofilter to variations in contaminant mass loading was studied by abruptly changing the concentration and/or flow rate of the inlet waste air stream. Contaminant concentration was continuously measured until a new steady state, for each stage, was achieved. Concentration spikes were applied to study the effects of shock loading on the biofilter removal rates. Biofilters responded effectively to H₂S concentration variations and shock loading by rapidly recovering to the original removal rates within 2–8 h. The re-acclimation times to reach full capacity were very short ranging between 15 and 120 h. Extended periods of starvation resulted in longer re-acclimation periods, so does the idle phase as compared to no-contaminant-loading phase.