Physical processes that transfer mass and energy by diffusion or convection occur in gaseous combustion. The management of internal features of a cavity structure for channels offers the possibility of inherently effective operation within the flammable limits of a combustible fluid stream while preserving high stability for the flame. However, the precise mechanism by which the cavity method generally provides increased flame stability for millimeter-scale systems remains unclear. The combustion characteristics of methane-air mixtures in millimeter-scale systems with a cavity structure are investigated experimentally and numerically to gain a greater understanding of the mechanisms of flame stabilization and to gain new insights into the characteristics of combustion within spaces with extremely small dimensions. Stable temperature profiles are obtained from thermographic measurements using infrared radiation. The measurements are compared with the model predictions. Subsequent model calculations demonstrate the effects of variations in wall thermal conductivity, heat losses, and equivalence ratio. Methods of applying a cavity structure to channel walls are developed, which may be utilized with presently existing designs of micro-combustion systems. The factors affecting flame stability and combustion characteristics are determined for the systems. The results indicate that the thermal conductivity of the burner walls plays a vital role in flame stability. The design with anisotropic thermal conductivity has significant performance advantages. Improvements in flame stability are achievable by using walls with anisotropic thermal conductivity. Heat-insulating materials are favored to minimize external heat losses. There are issues of efficiency loss for fuel-rich cases. Burners with large dimensions lead to a delay in flame ignition and may cause blowout. The combustion is stabilized by recirculation of hot combustion products induced by the cavity structure.Keywords: Thermographic measurements; Infrared radiation; Internal features; Flammable limits; Fluid streams; Effective operation

While burners designed in a backflow way enable operation with very low noxious emissions, they frequently operate very close to the extinction limit of the flame. Cavity structures have been designed for the purpose of improving flame stability. However, the precise mechanism by which the cavity method provides increased flame stability remains unclear. Computational fluid dynamics simulations are conducted to gain insights into burner performance such as reaction rates, species concentrations, temperatures, and flames. Factors affecting combustion characteristics and flame stability are determined. The results indicate that burner dimensions greatly affect flame stability. Burners with large dimensions lead to a delay in flame ignition and may cause blowout. The combustion is stabilized by recirculation of hot combustion products induced by the cavity structure. The inlet velocity of the mixture is a critical factor in assuring flame stability within the cavity-stabilized burner. There is a narrow range of inlet velocities that permit sustained combustion within the cavity-stabilized burner. Design recommendations are provided.Keywords: Burners; Flames; Temperatures; Species concentrations; Reaction rates; Combustion

Hydrogen and methanol are the primary fuels available for current fuel cells. The potential of methanol reforming systems to greatly improve productivity in chemical reactors has been limited, due in part, to the effect of mass transfer limitations on the production of hydrogen. There is a need to determine whether or not a microchannel reforming reactor system is operated in a mass transfer-controlled regime, and provide the necessary criteria so that mass transfer limitations can be effectively eliminated in the reactor. Three-dimensional numerical simulations were carried out using computational fluid dynamics to investigate the essential characteristics of mass transport processes in a microchannel reforming reactor and to develop criteria for determining mass transfer limitations. The reactor was designed for thermochemically producing hydrogen from methanol by steam reforming. The mass transfer effects involved in the reforming process were evaluated, and the role of various design parameters was determined for the thermally integrated reactor. In order to simplify the mathematics of mass transport phenomena, use was made of dimensionless numbers or ratios of parameters that numerically describe the physical properties in the reactor without units. The results indicated that the rate of the reforming reaction is limited by mass transfer near the entrance of the reactor and by kinetics further downstream, when the heat transfer in the autothermal system is efficient. There is not an effective method to reduce channel dimensions if the flow rate remains constant, or to reduce fluid velocities if the residence time is kept constant. The performance of the reactor can be greatly improved by means of proper design of catalyst layer thickness and through adjusting feed composition to minimize or reduce mass transfer limitations in the reactor. Finally, the criteria that can be used to distinguish between different mass transport and kinetics regimes in the reactor with a first-order reforming reaction were presented.Keywords: Fuel cells; Reactor design; Autothermal systems; Hydrogen production; Microchannel reactors; Steam reforming

The steam reforming of hydrocarbon fuels for the production of synthesis gas or hydrogen is a well-established technology. A common process is steam-reforming, where a suitable reforming catalyst facilitates the reaction between the hydrocarbon feed and steam to generate carbon monoxide and hydrogen. This study relates to a thermochemical process for producing hydrogen by the catalytic endothermic reaction of methanol with steam in a thermally integrated microchannel reforming reactor. Computational fluid dynamics simulations are conducted to better understand the consumption, generation, and exchange of thermal energy between endothermic and exothermic processes in the reactor. The effects of wall heat conduction properties and channel dimensions on heat transfer characteristics and reactor performance are investigated. Thermodynamic analysis is performed based on specific enthalpy to better understand the evolution of thermal energy in the reactor. Design recommendations are made to improve thermal performance for the reactor. The results indicate that the peak reaction heat flux increases with the channel dimensions while maintaining the flow rates. Reaction heat flux profiles are considerably affected by channel dimensions. The thermal conductivity of the channel walls is fundamentally important. Materials with high thermal conductivity are preferred for the channel walls. Thermally conductive ceramics and metals are well-suited. Wall materials with poor heat conduction properties degrade the reactor performance. The change in specific enthalpy is positive for the exothermic reaction and negative for the endothermic reaction. The change in specific sensible enthalpy is always positive.Keywords: Efficiencies; Performances; Enthalpy; Hydrocarbons; Conductivities; Hydrogen

Novel chemical, electrical, and mechanical properties absent in other materials have been discovered in carbon nanotubes. Pristine carbon nanotubes are inert to most chemicals and need to be grafted with surface functional groups to increase their chemical reactivity and add new properties. The increasing popularity of carbon nanotubes has created a demand for greater scientific understanding of the characteristics of thermal transport in nanostructured materials. However, the effects of impurities, misalignments, and structure factors on the thermal conductivity of carbon nanotube films and fibers are still poorly understood. Carbon nanotube films and fibers were produced, and the parallel thermal conductance technique was employed to determine the thermal conductivity. The effects of carbon nanotube structure, purity, and alignment on the thermal conductivity of carbon films and fibers were investigated to understand the characteristics of thermal transport in the nanostructured material. The importance of bulk density and cross-sectional area was determined experimentally. The results indicated that single-walled carbon nanotube films and fibers generally have high thermal conductivity. The presence of non-carbonaceous impurities degrades the thermal performance due to the low degree of bundle contact. The prepared carbon nanotube films and fibers are very efficient at conducting heat. The structure, purity, and alignment of carbon nanotubes play a fundamentally important role in determining the heat conduction properties of carbon films and fibers. The thermal conductivity may present power law dependence with temperature. The specific thermal conductivity decreases with increasing bulk density. The specific thermal conductivity of carbon nanotube fibers is significantly higher than that of carbon nanotube films due to the increased degree of bundle alignment. At room temperature, a maximum specific thermal conductivity is obtained but Umklapp scattering occurs.Keywords: Thermal properties; Preparation; Carbon films; Carbon fibers; Characterization; Thermal conductivity

The heterogeneous and homogeneous combustion-based homogeneous charge compression ignition of fuel-lean methane-air mixtures over alumina-supported platinum catalysts was investigated experimentally and numerically in free-piston micro-engines without ignition sources. Single-shot experiments were carried out in the purely homogeneous and coupled heterogeneous and homogeneous combustion modes, involved temperature measurements, capturing the visible combustion image sequences, exhaust gas analysis, and the physicochemical characterization of catalysts. Simulations were performed with a two-dimensional transient model that includes detailed heterogeneous and homogeneous chemistry and transport, leakage, and free-piston motion to gain physical insight and to explore the heterogeneous and homogeneous combustion characteristics. The micro-engine performance concerning combustion efficiency, mass loss, energy density, and free-piston dynamics was investigated. The results reveal that heterogeneous reactions cause earlier ignition, which is very favourable for the micro-device. Both purely homogeneous and coupled heterogeneous and homogeneous combustion of methane-air mixtures in a narrow cylinder with a diameter of 3 mm and a height of approximately 0.3 mm are possible. Heat losses result in higher mass losses. The coupled heterogeneous and homogeneous mode can not only significantly improve the combustion efficiency, in-cylinder temperature and pressure, output power and energy density, but also reduce the mass loss because of its lower compression ratio and less time spent around the top dead centre and during the expansion stroke, indicating that this coupled mode is a promising combustion scheme for micro-engines.Keywords: Micro-engines; Homogeneous combustion; Free-piston dynamics; Power generation; Transient models; Micro-combustion

The characteristics of catalytically stabilized combustion in micro-scale heat-recirculating systems remain poorly understood and warrant further study due to extremely complex interactions not only between kinetics and transport but also between heterogeneous and homogeneous reactions. This study is focused mainly upon the essential combustion characteristics of propane-air mixtures in flow tube reactors with a heat-recirculating structure. Computational fluid dynamics simulations are performed to gain a greater understanding of the mechanisms of flame stabilization. The essential factors affecting flame stability and combustion characteristics are determined in order to obtain design insights. The results indicate that both chemical and thermal environments are improved with the catalytically stabilized combustion method and the heat-recirculating structure. The design incorporates the best features of both catalytic combustion and thermal flame methods. The system is essentially free of mass transfer limitations. The flow velocity, wall thermal conductivity, equivalence ratio, exterior heat losses are important factors in determining the performance of the system. Stable operation of the system is limited to a relatively wide flow regime, and the flow velocity is critical to achieving flame stability. There is an optimum wall thermal conductivity in terms of flame stability. The system with a moderate wall thermal conductivity will be most robust against the surrounding conditions. Excess enthalpy combustion can occur in an efficient and rapid manner, resulting from the injection of free radicals and heat produced by the catalytic reaction. Blowout shifts homogeneous combustion downstream significantly without substantially reducing the reaction rate.Keywords: Combustion characteristics; Flame stability; Heat recirculation; Catalytic combustion; Homogeneous combustion; Computational fluid dynamics

Heat transfer and thermodynamic analysis are performed using computational fluid dynamics and chemical kinetics to investigate the synthesis gas production processes in chemical reactors with integrated heat exchangers by steam reforming. The change of thermal energy in the reactor is fully described in order to analyze the influences of fluid velocity, solid thermal properties, and flow arrangement on the thermal behavior of the reactor. The evolution of energy is discussed in terms of reaction heat flux, and thermodynamic analysis of the oxidation and reforming processes is performed in terms of enthalpy changes. The results indicate that while the net sensible enthalpy change is always positive in the reactor, the net enthalpy change for the endothermic and exothermic reactions is positive and negative, respectively. The wall thermal conductivity plays a significant role in determining the efficiency and operation of the autothermal system. The parallel flow design is advantageous for purposes of avoiding localized hot spots and enhancing heat transfer. The change in enthalpy is vital to the endothermic and exothermic reactions. The thermal behavior of the reactor system depends upon the thermal properties of the walls. The change in flow arrangement significantly affects the reaction heat flux in the reactor. The endothermic reforming reaction can proceed efficiently and rapidly if the wall thermal conductivity is high. The reaction heat flux for the endothermic and exothermic processes is negative and positive, respectively. The wall heat conduction effect accompanying temperature changes is of great importance to the autothermal design and self-sustaining operation of the reactor. Keywords: Hydrogen; Steam; Carbon; Methanol; Air; Copper