Xuhui Gao

Numerical investigation of the interaction between homogeneous and heterogeneous reactions of fuel-lean hydrogen–air mixtures over platinum in planar micro-channels

The interaction between homogeneous and heterogeneous reactions of a fuel-lean hydrogen–air mixture over platinum in a catalytic planar micro-channel was investigated numerically to assess the relative importance and respective contributions of homogeneous and heterogeneous reactions for different channel gap distances, inlet mass fluxes and equivalence ratios. Simulations were carried out with a two-dimensional computation that included detailed chemical kinetic mechanisms for the hetero-/homogeneous reaction. The results indicated that channel size can significantly affect the homogeneous reaction because of the effective radical and heat losses to the wall, despite the fact that the heterogeneous reaction remains basically unchanged. The catalytic wall helps in sustaining the homogeneous reaction in smaller channels by decreasing heat losses to the wall as a result of the exothermic heterogeneous reaction, while it also inhibits the homogeneous reaction because of wall radical quenching caused by typically higher absorption of OH radicals on the wall. Product species contribute even further to inhibiting homogeneous reaction because of the stronger diffusive mass flux in the smaller channel. In the present work, the main parameter effects on the interaction between homogeneous and heterogeneous reactions are explored. The limiting values of these parameters beyond which the homogeneous reaction becomes negligible as compared with the heterogeneous reaction are identified.

Stability limits and chemical quenching of methane–air flame in plane micro-channels with different walls

In order to elucidate the effects of wall material on stability limits and chemical quenching behavior, plane micro-channels with different wall materials were evaluated for the premixed combustion of methane–air. Experiments and numerical simulations with detailed chemistry kinetics schemes were carried out to explore the combustion characteristics along with the interaction between homogeneous and surface reactions on platinum, quartz glass, alumina ceramic and copper, and to estimate the effect of initial sticking coefficients associated with radical adsorption on chemical quenching. The experimental results indicate that the stability limits decrease in the order of platinum > copper > quartz glass > alumina ceramic for the different wall materials. The lower thermal conductivity wall leads to higher reaction temperature, which enhances the robustness of micro-flame. The simulation results indicate that chemical effect plays a critical role in the distribution of OH* radical. Homogeneous reaction is significantly inhibited on the platinum surface because of the depletion of reactants rather than the radical adsorption. Radical quenching is the most inhibited on the surface of alumina ceramic. The wall chemical effect on flame becomes very important as micro-channel is smaller than 0.7 mm.

A review of the interfacial characteristics of polymer nanocomposites containing carbon nanotubes

This paper provides an overview of recent advances in research on the interfacial characteristics of carbon nanotube–polymer nanocomposites. The state of knowledge about the chemical functionalization of carbon nanotubes as well as the interaction at the interface between the carbon nanotube and the polymer matrix is presented. The primary focus of this paper is on identifying the fundamental relationship between nanocomposite properties and interfacial characteristics. The progress, remaining challenges, and future directions of research are discussed. The latest developments of both microscopy and scattering techniques are reviewed, and their respective strengths and limitations are briefly discussed. The main methods available for the chemical functionalization of carbon nanotubes are summarized, and particular interest is given to evaluation of their advantages and disadvantages. The critical issues related to the interaction at the interface are discussed, and the important techniques for improving the properties of carbon nanotube–polymer nanocomposites are introduced. Additionally, the mechanism responsible for the interfacial interaction at the molecular level is briefly described. Furthermore, the mechanical, electrical, and thermal properties of the nanocomposites are discussed separately, and their influencing factors are briefly introduced. Finally, the current challenges and opportunities for efficiently translating the remarkable properties of carbon nanotubes to polymer matrices are summarized in the hopes of facilitating the development of this emerging area. Potential topics of oncoming focus are highlighted, and several suggestions concerning future research needs are also presented.

Computational fluid dynamics modeling of the millisecond methane steam reforming in microchannel reactors for hydrogen production

Methane steam reforming coupled with methane catalytic combustion in microchannel reactors for the production of hydrogen was investigated by means of computational fluid dynamics. Special emphasis is placed on developing general guidelines for the design of integrated micro-chemical systems for the rapid production of hydrogen. Important design issues, specifically heat and mass transfer, catalyst, dimension, and flow arrangement, were explored. The relative importance of different transport phenomena was quantitatively evaluated, and some strategies for intensifying the reforming process were proposed. The results highlighted the importance of process intensification in achieving the rapid production of hydrogen. High heat and mass transfer rates derived from miniaturization of the chemical system are insufficient for process intensification. Improvement of the reforming catalyst is also essential. The efficiency of heat exchange can be improved greatly if the reactor dimension is properly designed. Thermal management is required to improve the reliability of the integrated system. Co-current heat exchange improves the thermal uniformity in the system. The catalyst loading is a key factor determining reactor performance, and must be carefully designed. Finally, engineering maps were constructed to achieve the desired power output, and favorable operating conditions for the rapid production of hydrogen were identified.

Catalytic Combustion Characteristics of Methane-Air Mixtures in Small-Scale Systems at Elevated Temperatures

The catalytic combustion characteristics of methane-air mixtures in small-scale systems were investigated at elevated temperatures, with particular emphasis on identifying the main factors that affect formation and removal of combustion-generated pollutants. Computational fluid dynamics simulations were performed using detailed chemical kinetic mechanisms, and more insights were offered into the phenomena occurring in the temperature range where homogeneous and heterogeneous reaction pathways are both important. Reaction engineering analysis was performed to provide an in-depth understanding of how to achieve low emissions of pollutants. Spatial distributions of the major species involved were presented to gain insight into the interplay between the two competing pathways involved. The results indicated that the distribution of oxidized products depends critically on the feed composition, dimension, temperature, and pressure. Small-scale catalytic systems enable low emissions of pollutants even in a high temperature environment, along with high combustion efficiency. The interplay between the two competing pathways via radicals is strong, and the heterogeneous pathway can significantly inhibit the homogeneous pathway. The inhibiting effect also accounts for the low emissions of nitrogen oxides. Almost all of the nitrogen oxides emitted by small-scale catalytic systems are nitric oxide. Catalytic combustion technology can be used to reduce the formation of undesired products, especially pollutant nitrogen oxide gases far below what can be achieved without catalysts. Recommendations for the design of small-scale catalytic systems are provided.

Computational Fluid Dynamics Simulations of Lean Premixed Methane-Air Flame in a Micro-Channel Reactor Using Different Chemical Kinetics

Abstract Flame temperature and structure are a useful tool for describing flame dynamics and flame stability, especially at the micro-scale. The objective of this study is to examine the effect of different kinetic models (that have been proven to accurately predict the macro-combustion behavior of hydrocarbons) on the combustion characteristics and the flame stability in microreactors, and to explore the applicability of these kinetic models at the micro-scale. Computational fluid dynamics (CFD) simulations of lean premixed methane-air flame in micro-channel reactors were carried out to examine the effect of different reaction mechanisms (Mantel, Duterque and Fernández-Tarrazo model) on the reaction rate and the flame structure and temperature. The time-scales with regard to homogeneous reaction and heat transfer were analyzed. The CFD results indicate that kinetic models strongly affect flame stability. Large transverse gradients in temperature and species are observed in all kinetic models, despite the small scales of the microreactor. Preheating, combustion, and post-combustion regions can be distinguished only in Duterque and Mantel model. Duterque model causes a stable elongated homogeneous flame with a considerable ignition delay as well as a dead region with cold feed accumulation near the entrance, and is inappropriate for micro-combustion studies because of the seriously overestimated flame temperature. Fernández-Tarrazo model causes a rapid extinction and a flashback risk, and is also inappropriate for micro-combustion studies due to the significantly underestimated reaction rate, without taking all kinetic factors into account. Mantel model can accurately predict the micro-flame behavior and consequently can be used for describing micro-combustion.