In this work, we have developed a zero-dimensional vanadium redox flow battery (VRFB) model which accounts for all modes of vanadium crossover and enables prediction of long-term performance of the system in a computationally-efficient manner. Using this model, the effects of membrane thickness on a 1000-cycle operation of a VRFB system have been investigated. It was observed that utilizing a thicker membrane significantly reduces the rate of capacity fade over time (up to ∼15%) at the expense of reducing the energy efficiency (up to ∼2%) due to increased ohmic losses. During extended cycling, the capacity of each simulated case was observed to approach an asymptote of ∼60% relative capacity, as the concentrations in each half-cell reach a quasi-equilibrium state. Simulations also indicated that peak power density and limiting current density exhibit a similar asymptotic trend during extended cycling (i.e., an ∼10-15% decrease in the peak power density and an ∼20-25% decrease in the limiting current density is observed as quasi-equilibrium state is reached Recently, vanadium redox flow batteries (VRFBs) have gained significant interest as one of the most promising electrochemical systems for grid-scale energy storage due to their several advantages over conventional batteries.1,2 Among these advantages, the most important ones are their ability to decouple power and energy rating due to their unique system architecture, and their flexible design.3-6 Despite these advantages, one major challenge which hinders their commercial viability is the relatively higher capital cost of these systems.5 According to a recent study by Pacific Northwest National Laboratory, the current capital cost of a VRFB system is about $350 per kWh for 4-h application, 7 which is much higher than the capital cost target set by government agencies. The current capital cost target of The Department of Energy's Office of Electricity Delivery and Energy Reliability is $250 per kWh, falling to $150 per kWh in the future for a 4-hour energy storage system. 8 One possible approach to reduce the capital cost is to improve the performance of these systems for less material use. To date, the majority of research has focused on improving the performance of individual components, such as developing high power density electrodes 4,[9][10][11][12][13][14][15][16] and exploring high energy capacity electrolytes. [17][18][19][20][21] In line with these studies, another approach to reduce the capital cost is to increase the lifetime of VRFB systems. Currently, the lifetime of these systems is limited by the capacity fade during cycling, which is primarily governed by unwanted active species transport across the membrane (i.e., species crossover). 22 Developing sophisticated mathematical models to mimic the VRFB operation is imperative to investigate the capacity fade and related performance losses in these systems due to lengthy time requirements and practical limitations of experimental cycling analysis. So far, there are several studies reported that ha...

Combustion of hydrocarbon fuels is traditionally separated into slow reaction, cool flame, and ignition regimes based on pressure and temperature. Standard tests, such as the ASTM E659, are used to determine the lowest temperature required to ignite a specific fuel mixed with air at atmospheric pressure. It is expected that the initial pressure and the rate at which the mixture is heated also influences the limiting temperature and the type of combustion. This study investigates the effect of heating rate, between 4 and 15 K/min, and initial pressure, in the range of 25 to 100 kPa, on ignition of n-hexane air mixtures. Mixtures with equivalence ratio ranging from Φ = 0.6 to Φ = 1.2 were investigated. The problem is also modeled computationally using an extension of Semenov's classical autoignition theory with a detailed chemical mechanism. Experiments and simulations both show that in the same reactor either a slow reaction or an ignition event can take place depending on the heating rate. Analysis of the detailed chemistry demonstrates that a mixture which approaches the ignition region slowly undergoes a significant modification of its composition. This change in composition induces a progressive shift of the explosion limit until the mixture is no longer flammable. A mixture that approaches the ignition region sufficiently rapidly undergoes only a moderate amount of thermal decomposition and explodes quite violently.

The risk of igniting a flammable mixture in fuel tank vapor space is a major concern in aviation safety. In order to analyze the hazards and develop mitigation strategies, it is necessary to characterize the explosive properties of kerosene vapor-air mixtures over wide ranges of initial conditions. n-Hexane has been extensively used in our laboratory as a single component surrogate of kerosene. In the present study, hexane oxidation by oxygen was studied in a flow reactor at equivalence ratios of 0.7, 1 and 1.5 for mixtures diluted at 90% with nitrogen. Residence time was set at 2 s and the pressure at 100 kPa. The evolution of the gas phase composition at the reactor exit was studied over the range 450-1000 K. Laser-based diagnostics and gas chromatography analysis were used to characterize the exit mixture composition. The chemical species measurements revealed three distinct regimes of oxidation, namely (i) the cool flame region from 600 to 650 K, (ii) the NTC region between 675 and 775 K, and (iii) the high temperature oxidation regime from 800 K. The modeling study demonstrated the capability of reproducing most of the trends observed

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Solidification of eicosane with and without nanoadditives is experimentally investigated in spherical enclosures subject to convective cooling in water and air. The effects of additive volume fraction and external convective cooling conditions (i.e., the heat transfer medium, subcooling, and flow velocity) on the solidification process are examined. The results are compared with a conduction-controlled thermal network model accounting for the enclosure and PCM resistances, as well as the convective subcooling. The experimentally determined solidification time is found to be consistently lower than the model prediction, likely due to asymmetric and dendritic solidification, as well as natural convection inside the enclosure and possible thermocouple position errors. A simple correlation is proposed to predict the solidification time of a phase change material (PCM) in a spherical enclosure subject to convective cooling based on the same enclosure subject to a constant temperature boundary.Results show that the solidification time decreases with the volume fraction of nanoadditives due to the improved PCM conductivity. In addition, the nanoadditives are found to be more effective for solidification in water than in air, due to the large air-side convective resistance that does not benefit from improving PCM conductivity.