College of Engineering

A combustion air stream is introduced tangentially into the interior of a premix burner by means of a swirl producer, and is mixed with fuel. At the burner outlet, the vortex flow which arises bursts open at a sudden change of cross section, with the initiation of a back-flow zone which serves to stabilize a flame in the operation of the burner. Although premix burners make possible an operation with very low pollutant emissions, they often operate dangerously near 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. This study relates to the combustion characteristics and flame stability of a micro-structured cavity-stabilized burner. 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. Design recommendations are provided. The results indicate that the thermal conductivity of the burner walls plays a vital role in flame stability. Improvements in flame stability are achievable by using walls with anisotropic thermal conductivity. Heat-insulating materials are favored to minimize external heat losses. Burner dimensions greatly affect flame stability. 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. 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.

Direct oxidation of fuels such as methanol in proton-exchange membrane fuel cells at practical current densities with acceptable catalyst loadings is not as economically attractive as conversion of methanol fuel to a hydrogen-rich mixture of gases via steam reforming and subsequent electrochemical conversion of the hydrogen-rich fuel stream to direct current in the fuel cell. 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.

The most important industrial method for the production of hydrogen is the catalytic steam reforming process. Many attempts have been made to improve heat transfer for thermally integrated microchannel steam reforming reactors. However, the mechanisms for the effects of design factors on heat transfer characteristics are still not fully understood. 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 reaction heat flux profiles are considerably affected by channel dimensions. The peak reaction heat flux increases with the channel dimensions while maintaining the flow rates. 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. 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 unique one-dimensional structure and concomitant properties endow carbon nanotubes with special natures, rendering them with unlimited potential in nanotechnology-associated applications. 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.

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.

Flame stabilization is a common problem in small-scale combustion systems. However, the fuel-air mixture flow pattern, including any recirculation, is critical to achieving flame stability. In the present study, numerical simulations are conducted to understand the mechanisms of flame stabilization in heat-recirculating continuous flow reactors. The essential factors affecting combustion characteristics and flame stability are determined in order to obtain design insights. The results indicate that the wall thermal conductivity, flow velocity, equivalence ratio, and exterior heat losses are important factors in determining the energy efficiency of the reactor. 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. The design incorporates the best features of both catalytic combustion and thermal flame methods. The system is essentially free of mass transfer limitations. Stable operation of the system is limited to a relatively wide flow regime, and the flow velocity is critical to achieving flame stability. Blowout shifts homogeneous combustion downstream significantly without substantially reducing the reaction rate. Both chemical and thermal environments are improved with the catalytically stabilized combustion method and the heat-recirculating structure.

The most efficient and stable combustion occurs in a catalytic reactor when the burning mixture is in contact with the catalyst for a sufficiently long period. When the contract period is too short, insufficient energy is generated adjacent to the catalyst surface to sustain combustion in the main or free stream. 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 in order to meet the emission level requirements, for industrial low emission gas turbine engines, staged combustion is required in order to minimise the quantity of the oxides of nitrogen produced. The combustion catalyst has several desirable characteristics: they are capable of minimizing nitrogen oxides emission and improving the pattern factor. Operating the combustion process in a very lean condition, namely high excess air, is one of the simplest ways of achieving lower temperatures and hence lower nitrogen oxides emissions. The use of a catalytic combustor offers the advantage that all of the fuel can be oxidized therein, resulting in ultra-low nitrogen oxides emissions and low carbon monoxide and unburned hydrocarbon levels. In mass transfer controlled catalytic reactions, one cannot distinguish between a more active catalyst and a less active catalyst because the intrinsic catalyst activity is not determinative of the rate of reaction. It is possible to achieve essentially adiabatic combustion in the presence of a catalyst at a reaction rate many times greater than the mass transfer limited rate. The maximum achievable velocity depends on flow conditions and catalyst parameters such as type, monolith cell size, and web thickness.

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.