Xianguo Li

Mathematical modeling of proton exchange membrane fuel cells

A one-dimensional non-isothermal model of a proton exchange membrane (PEM) fuel cell has been developed to investigate the effect of various design and operating conditions on the cell performance, thermal response and water management, and to understand the underlying mechanism. The model includes variable membrane hydration, ternary gas mixtures for both reactant streams, phase change of water in the electrodes with unsaturated reactant gas streams, and the energy equation for the temperature distribution across the cell. It is found that temperature distribution within the PEM fuel cell is affected by water phase change in the electrodes, especially for unsaturated reactant streams. Larger peak temperatures occur within the cell at lower cell operating temperatures and for partially humidifed reactants as a result of increased membrane resistance arising from reduced membrane hydration. The non-uniform temperature rise can be significant for fuel cell stacks. Operation on reformed fuels results in a decrease in cell performance largely due to reduced membrane hydration, which is also responsible for reduced performance at high current densities for high cell operating pressures. Model predictions compare well with known experimental results.

Modelling of polymer electrolyte membrane fuel cells with variable degrees of water flooding

Abstract Polymer electrolyte membrane (PEM) fuel cells have received increasing attention from both the public and fuel cell community due to their great potential for transport applications. The phenomenon of water flooding in the PEM fuel cells is not well understood, and few modelling studies have included the effect of water flooding. On the other hand, water management is one of the critical issues to be resolved in the design and operation of PEM fuel cells. In the present study, a mathematical model has been formulated for the performance and operation of a single polymer electrolyte membrane fuel cell. This model incorporates all the essential fundamental physical and electrochemical processes occurring in the membrane electrolyte, cathode catalyst layer, electrode backing and flow channel. A special feature of the model is that it includes the effect of variable degree of water flooding in the cathode catalyst layer and/or cathode electrode backing region on the cell performance. The model predictions have been compared with the existing experimental results available in the literature and excellent agreement has been demonstrated between the model results and the measured data for the cell polarisation curves. Hence, this model can be used for the optimisation of PEM fuel cell design and operation, and can serve as a building block for the modelling and understanding of PEM fuel cell stacks and systems.

Composition and performance modelling of catalyst layer in a proton exchange membrane fuel cell

The composition and performance optimisation of cathode catalyst platinum and catalyst layer structure in a proton exchange membrane fuel cell has been investigated by including both electrochemical reaction and mass transport process. It is found that electrochemical reactions occur in a thin layer within a few micrometers thick, indicating ineffective catalyst utilization for the present catalyst layer design. The effective use of platinum catalyst decreases with increasing current density, hence lower loadings of platinum are feasible for higher current densities of practical interest without adverse effect on cell performance. The optimal void fraction for the catalyst layer is about 60% and fairly independent of current density, and a 40% supported platinum catalyst yields the best performance amongst various supported catalysts investigated. An optimal amount of membrane content in the void region of the catalyst layer exists for minimum cathode voltage losses due to competition between proton migration through the membrane and oxygen transfer in the void region. The present results will be useful for practical fuel cell designs.

On the temporal instability of a two-dimensional viscsous liquid sheet

This paper reports a temporal instability analysis of a moving thin viscous liquid sheet in an inviscid gas medium. The results show that surface tension always opposes, while surrounding gas and relative velocity between the sheet and gas favour, the onset and development of instability. It is found that there exist two modes of instability for viscous liquid sheets – aerodynamic and viscosity-enhanced instability – in contrast to inviscid liquid sheets for which the only mode of instability is aerodynamic. It is also found that axisymmetrical disturbances control the instability process for small Weber numbers, while antisymmetrical disturbances dominate for large Weber numbers. For antisymmetrical disturbances, liquid viscosity, through the Ohnesorge number, enhances instability at small Weber numbers, while liquid viscosity reduces the growth rate and the dominant wavenumber at large Weber numbers. At the intermediate Weber-number range, Liquid viscosity has complicated effects due to the interaction of viscosity-enhanced and aerodynamic instabilities. In this range, the growth rate curve exhibits two local maxima, one corresponding to aerodynamic instability, for which liquid viscosity has a negligible effect, and the other due to viscosity-enhanced instability, which is influenced by the presence and variation of liquid viscosity. For axisymmetrical disturbances, liquid viscosity always reduces the growth rate and the dominant wavenumber, aerodynamic instability always prevails, and although the regime of viscosity-enhanced instability is always present, its growth rate curve does not possess a local maximum.

Performance analyses of sensible heat storage systems for thermal applications

In this paper, sensible heat storage (SHS) systems and performance evaluation techniques are studied. A detailed investigation is presented of the availability of SHS techniques for solar thermal applications, selection criteria for SHS systems, the economics of SHS systems, the main issues in evaluating SHS systems, the viability of SHS systems, the environmental impacts of SHS systems and criteria for SHS feasibility studies, as well as energy saving options. In addition to energy and exergy analyses, several definitions of energy and exergy efficiency for the performance of SHS systems are provided with an illustrative example.

Instability of an annular viscous liquid jet

SummaryA linear analysis has been carried out for the temporal instability of an annular viscous liquid jet moving in an inviscid gas medium, which includes three limiting cases of a round liquid jet, a gas jet and a plane liquid sheet. It is found that there exist two independent unstable modes, which become the well-known sinuous and varicose modes for plane liquid sheets as annular jet radii approach to infinity. Hence, they are named as para-sinuous and para-varicose. It is shown that an ambient gas medium always enhances the annular jet instability. The curvature effects in general increase the disturbance growth rate, and may not be neglected for the breakup process of an annular or conical liquid sheet. An annular jet with a sufficiently small thickness tends to break up much faster than the corresponding plane liquid sheet, in accordance with existing experimental observations. Liquid viscosity has complicated dual effects on the instability. It is also found that there exists a critical Weber number below which surface tension is the source of instability. Whereas above it, instability is suppressed by surface tension effect and it promoted by aerodynamic interaction between the liquid and gas phase. For the practical importance of large Weber numbers such as related to liquid atomization, the para-sinuous mode is always predominant.

Spatial instability of plane liquid sheets

Abstract Spatial instability of a thin moving plane liquid sheet is analyzed. It is shown that there exist two independent modes of instability, sinuous and varicose. For the sinuous mode, there is a critical Weber number of unity below which pseudo-absolute instability exists and above which convective instability appears. For the varicose mode, the instability is always convective. The convective mode is induced by the aerodynamic effects, and is related to the corresponding temporal instability through Gasters relation at the sufficiently high liquid velocity. Perturbation analysis indicates that the spatial amplification rate depends strongly on the gas-to-liquid density ratio ϱ. For the convective instability of the sinuous mode, there exists a critical Weber number of slightly larger than unity, below which liquid viscosity exhibits both stabilizing and destabilizing effects and above which the viscous effects always reduce the growth rate and the dominant wavenumber. For situations where the Weber number, We ⪢ 1, but ϱ We ⩽ 0(1), the growth rate of the sinuous mode is larger than that of the varicose mode under the same flow conditions. If both We ⪢ 1 and ϱ We ⪢ 1, the spatial growth rate of both sinuous and varicose modes is almost the same, except at the small wavenumbers where the sinuous mode has slightly larger growth rate than the corresponding varicose mode. The viscous effects on the varicose mode is always stabilizing. The present results compare favorably with the existing experimental observations.

Nonlinear instability of plane liquid sheets

A nonlinear stability analysis has been carried out for plane liquid sheets moving in a gas medium at rest by a perturbation expansion technique with the initial amplitude of the disturbance as the perturbation parameter. The first, second and third order governing equations have been derived along with appropriate initial and boundary conditions which describe the characteristics of the fundamental, and the first and second harmonics. The results indicate that for an initially sinusoidal sinuous surface disturbance, the thinning and subsequent breakup of the liquid sheet is due to nonlinear effects with the generation of higher harmonics as well as feedback into the fundamental. In particular, the first harmonic of the fundamental sinuous mode is varicose, which causes the eventual breakup of the liquid sheet at the half-wavelength interval of the fundamental wave. The breakup time (or length) of the liquid sheet is calculated, and the effect of the various flow parameters is investigated. It is found that the breakup time (or length) is reduced by an increase in the initial amplitude of disturbance, the Weber number and the gas-to-liquid density ratio, and it becomes asymptotically insensitive to the variations of the Weber number and the density ratio when their values become very large. It is also found that the breakup time (or length) is a very weak function of the wavenumber unless it is close to the cut-off wavenumbers.

A MASS TRANSFER CORRELATION FOR DROPLET EVAPORATION IN HIGH-TEMPERATURE FLOWS

The analogy between heat and mass transfer is strong, but not exact, for droplet evaporation in high-temperature air streams at intermediate Reynolds numbers. Convection enhances mass transfer, while evaporation is self-inhibitive due to its blowing effect. In this paper, the following mass transfer correlation is developed: Shf(1 + BM)0.7 = 2 + 0.87Rem12Scf13 (10 <Rem < 2000), which accounts for the effects of convection, droplet heating, surface blowing and variable thermophysical properties. It is shown that this correlation agrees well with the results of detailed numerical analyses involving single and multicomponent droplets, as well as experimental data for various fluids from different sources.

Understanding low-lipid algae hydrothermal liquefaction characteristics and pathways through hydrothermal liquefaction of algal major components: crude polysaccharides, crude proteins and their binary mixtures.

Crude polysaccharides and proteins extracted from algae were chosen as model materials to investigate the hydrothermal liquefaction (HTL) characteristics and pathways of low-lipid algae. Liquefaction behavior of the two individuals and their binary mixtures with different mass ratios were evaluated under different temperatures. Formation pathways of bio-oil from crude polysaccharides/proteins were proposed. Results showed that polysaccharides had a small contribution to bio-oil (<5%) and approximately 60% distributed in aqueous phase, while proteins played a crucial role on bio-oil formation (maximum 16.29%). Bio-oil from polysaccharides mainly contained cyclic ketones and phenols and from proteins composed of pyrazines, pyrroles and amines. Interaction between polysaccharides and proteins forming polycyclic nitrogenous compounds had a negative effect on bio-oil yield at 220 and 260°C. However, their further decomposition caused increase of bio-oil yield at 300°C. Mixture liquefaction obtained the highest higher heating value (HHV) of bio-oil and energy recovery than polysaccharides/proteins liquefaction at 300°C.