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Abstract Flow in manifold systems is encountered in designs of various industrial processes, such as fuel cells, microreactors, microchannels, plate heat exchanger, and radial flow reactors. The uniformity of flow distribution in manifold is a key indicator for performance of the process equipment. In this paper, a discrete method for a U-type arrangement was developed to evaluate the uniformity of the flow distribution and the pressure drop and then was used for direct comparisons between the U-type and the Z-type. The uniformity of the U-type is generally better than that of the Z-type in most of cases for small ζ and large M . The U-type and the Z-type approach each other as ζ increases or M decreases. However, the Z-type is more sensitive to structures than the U-type and approaches uniform flow distribution faster than the U-type as M decreases or ζ increases. This provides a simple yet powerful tool for the designers to evaluate and select a flow arrangement and offers practical measures for industrial applications.

The technical challenges and obstacles to scaling-up of fuel cells are diverse, including such issues as water, heat, materials, catalyst, and flow fields because of multiple chemical and physical interactions at the atomic level and stack system level. The current results and data, even assumptions and guidelines are separated, inconsistent or unconnected. The unconnected data is partly the result of different disciplines. This paper is a first attempt toward understanding and analyzing the massive but spread-out work, which has been done and reported in the literature on fuel cell performance, reliability and durability. In this, we analyze the procedure of fuel cell research and development, and break down the barriers of scaling-up into four different stages: component, individual cell, stack and system control. We find that there are three different operating windows at each stage of the components, individual cells, and stack. While the operating window of components (e.g., membrane) are defined as ranges of temperature and relative humidity (RH), the operating window of a cell must include channel velocity and pressure drop within the cell. The operating window of a stack becomes narrower than that of its individual cells due to uneven flow distribution and load change. We have also found that there are knowledge gaps in the different stages of development. A solution for fuel cell scaling-up and a connection can be built among the components, cells, stack, process and system control through the operating windows and flow fields. The concepts of the three operating windows and flow field designs can build a connection among properties of the material and structures of components (e.g., wettability, porosity, and hydrophobicity), flow field, cells and performance of a stack and macro operation conditions (e.g., pressure, humidity and flow rates). This clarifies key ambiguities and converges our future directions on how to bridge different stages or disciplines of research and development. These can provide a new insight for future research to address the key issues of durability and reliability that remain unsolved.

Flows in manifolds are of great importance in quite diverse fields of science and technology, including fuel cells, spargers, solar collectors, microchannels, porous infiltration and irrigation. Theory of flow distribution and pressure drop is vital to predict process performance and efficiency of manifold systems. In this paper, we examined research and development of theoretical models and methodology of solutions in flow in manifolds and highlight remarkable advances in the past fifty years. The main existing models and solution methods were unified further to one theoretical framework, including Bernoulli theory and momentum theory, and discrete and continuum methodologies. The generalised model was applicable to not only designs of continuum manifolds but also those of discrete manifolds with constant or varying factors. The procedure of design calculation is in reality straightforward without requirements of iteration, successive approximation and computer programme. The theoretical model provides easy-to-use design guidance to investigate the interactions among structures, operating conditions and manufacturing tolerance under a wide variety of combination of flow conditions and geometries through three general characteristic parameters (E, M and ζ) and to minimize the impact on manifold operability.

Abstract It is a major challenge to transform a laboratory scale production of fuel cells to an industrial scale in terms of throughput, operating lifetime, cost, reliability and efficiency. In spite of a number of efforts, the durability, reliability and cost of fuel cells still remain major barriers to scaling-up and commercialization. Unless these challenges are fully understood there is little chance of overcoming them. In fact, though much fundamental research has been performed, there is still no clear understanding of both the theoretical solution and technical measures needed to solve the durability and performance degradation of fuel cells in the scaling-up process. In this critical review, we will revisit advances in theory of flow field designs. Then, we will analyze main issues and challenges in concepts and criteria of flow field designs and development of theoretical models. We will focus on uneven flow distribution as a root cause of low durability and reliability and performance degradation and why flow field designs are a strategic solution to integrated performance, flow conditions, structure and electrochemical processes. Finally, we will discuss criteria and measures to tackle uneven flow distribution as well as critical durability and performance degradation in the scaling-up of fuel cells.

Abstract The laser technique was developed for measurement of transient burning rates of solid propellant during oscillatory combustion. the design and operation of the system are discussed. A high pressure window bomb was used for the combustion chamber. A modulating disk was used to induce the pressure oscillation above the combustion bomb. The oscillatory frequency was controlled easily with selecting the rotating velocity of the disk. Test results with AP/PS propellants, were in agreement with the results of previous techniques.

Abstract It is the major challenge to transform a laboratory scale production of fuel cells to an industrial scale one and to meet the requirements of throughput, operating life, low cost, reliability and high efficiency in R&D of fuel cells. Designs of uniform flow distribution are central to upscale fuel cells as well as to tackle critical issues of water, thermal and current management. However, in spite of our growing appreciation of designs of uniform flow distribution, there is little or no practical solution to ensure a uniform flow distribution across channels of a cell and cells of a stack in designs of flow fields. The purpose of this paper was to develop a discrete approach to find a design that met requirements of flow distribution uniformity and pressure drop in parallel channel configurations with Z-type arrangement through adjustments of configurations and normalised structural parameters. Variation of the frictional and the momentum coefficients with flow velocities was incorporated into the flow distribution equation to improve modelling accuracy. We also developed procedure, measures and guideline for the designs of flow distribution and pressure drop to bridge knowledge gap between the generalised theory and industrial applications. The results showed that the present approach could provide the practical guideline to evaluate quantitatively performance of different layout configurations, structures, and flow conditions.

Abstract A laser technique of transient burning rate measurement of solid propellant has been investigated. A photocell detects a laser beam passing through the propellant strand, and the cross-section of the laser beam is regulated so that the laser energy varies linearly along the height of the strand. Using a photo-controlled depressurization system, it is desirable to set up the starting time of depressurization in advance, and the initial rate of depressurization can be easily reproduced. Experimental results show that: (1) for a short time after depressurization begins, the burning rate remains unchanged; (2) during the depressurization process, the burning rate, while continuously decreasing in magnitude, exhibits low frequency oscillations; (3) the amplitude and the frequency of the burning rate oscillations, referenced to a smooth rate–time curve, increases as the depressurization rate increases; (4) for a short time after depressurization, the transient burning rates is higher than the steady state burning rate at the corresponding pressure, and then the transient burning rates oscillate around the quasi-steady rate. The experimental dynamic burning rates are compared with those available by microwave and capacitance methods. The discrepancies in the three experimental results have been discussed.

Stream temperatures, which influence dynamics and distributions of the aquatic species and kinetics of biochemical reactions, are expected to be altered by the climate change. Therefore, predicting the impacts of climate change on stream temperature is helpful for integrated water resources management. In this study, our previously developed Soil and Water Assessment Tool (SWAT) equilibrium temperature model, which considers both the impacts of meteorological condition and hydrological processes, was used to assess the climate change impact on the stream temperature regimes in the Athabasca River Basin (ARB), a cold climate region watershed of western Canada. The streamflow and stream temperatures were calibrated and validated first in the baseline period, using multi-site observed data in the ARB. Then, climate change impact assessments were conducted based on three climate models under the Representative Concentration Pathways 4.6 and 8.5 scenarios. Results showed that warmer and wetter future condition would prevail in the ARB. As a result, streamflow in the basin would increase despite the projected increases in evapotranspiration due to warmer condition. On the basin scale, annual stream temperatures are expected to increase by 0.8 to 1.1 °C in mid-century and by 1.6 to 3.1 °C in late century. Moreover, the stream temperature changes showed a marked temporal pattern with the highest increases (2.0 to 7.4 °C) in summer. The increasing stream temperatures would affect water quality dynamics in the ARB by decreasing dissolved oxygen concentrations and increasing biochemical reaction rates in the streams. Such spatial-temporal changes in stream temperature regimes in future period would also affect aquatic species, thus require appropriate management measures to attenuate the impacts.

Watersheds in cold regions provide water, food, biodiversity and ecosystem service. However, the increasing demand for water resources and climate change challenge our ability to provide clean freshwater. Particularly, watersheds in cold regions are more sensitive to changing climate due to their glaciers’ retreat and permafrost. This review revisits watershed system and processes. We analyze principles of watershed modelling and characteristics of watersheds in cold regions. Then, we show observed evidence of their impacts of cold processes on hydrological and biogeochemical processes and ecosystems, and review the watershed modeling and their applications in cold regions. Finally, we identify the knowledge gaps in modeling river basins according to model structures and representations of processes and point out research priorities in future model development.

Abstract In this study, a hybrid system of solar-microturbine with and without a combustion chamber, was investigated in a cold climate region (Edmonton, Canada). We developed a thermodynamic model to analyse the effects of environmental conditions on the system performance and power output during the year considering monthly changes in temperature, daylength and solar radiation using real climate and geographical data. The results showed that for a 30 m2 dish collector aperture area, the cycle outlet power was estimated from 3.70 kW in winter to 9.87 kW in summer, while the lowest and the highest cycle efficiencies were 19.44% and 35.07%, respectively for sunny days. The performance of the cycle was also compared with different climates and latitudes in Toronto and Phoenix. The total efficiencies in Edmonton were similar as that in Phoenix in summer but much lower in winter. However, the total electricity output in summer was higher in Edmonton than other two cities. It is found that the highest electricity output in winter is only a half of that in summer day in Edmonton. Furthermore, the cycle could achieve the highest total daily electricity output and fuel consumption in Edmonton in summer due to longer daylength, and higher latitude despite a colder climate if including a combustion chamber. Particularly, more than a half day has no electricity output in Edmonton in winter day if without combustion chamber. Therefore, it demonstrates that this state-of-the-art hybrid system can produce electricity and recover heat in a cold climate region but the climate effects should be considered.