Recent coe_mae items

Thermal fluctuations and bending rigidities of graphane and fluorographene at different temperatures

Tue, 11 Jun 2024 00:00:00 +0000

Little research has been conducted to determine the thermal properties and phenomena of graphane and fluorographene. A clear understanding of the thermal problems involved is needed, which may provide a basis for further research on other material properties. In the present study, molecular dynamics simulations were performed to investigate the thermal properties of graphane and fluorographene and especially the phenomena involved, including thermal fluctuations and bending rigidities. Furthermore, comparisons of thermal properties and the phenomena involved were made computationally between pristine and functionalised graphene. The thermal fluctuations and bending rigidities were determined at different temperatures. The present study aims to provide a clear understanding of the thermal problems involved in hydrogenated and fluorinated graphene. The results indicated that while thermally excited ripples spontaneously appear in graphene, fully hydrogenated or fluorinated graphene...

Computational study of transport phenomena in microchannel reactors for hydrogen production by steam reforming

Tue, 11 Jun 2024 00:00:00 +0000

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...

Continuous and efficient production of hydrogen from methanol in protruded millisecond microchannel reactors for fuel cell applications

Mon, 10 Jun 2024 00:00:00 +0000

Protrusions can be used to improve the transport processes involved, but the causes of the phenomena are still incompletely understood. Computational fluid dynamics analyses are performed under different sets of circumstances to gain insights into the physics of heat and mass transfer processes in a protruded millisecond microchannel reactor, wherein a steam reforming reaction is proceeding and protrusions are used to improve the transport processes involved. Recommendations are made on how to optimize design for better reactor performance. Particular emphasis is placed on delineating the role of methanol-air equivalence ratio and channel length in reactor performance. The results indicate that the equivalence ratio and channel length must be adjusted as needed to minimize pressure drops and maximize production of hydrogen. Necessary adjustments to the equivalence ratio of methanol to air can be made to control the maximum reactor temperature within certain needed limits. The...

Effects of inlet velocity and steam-to-methanol ratio on the phenomena of process intensification in protruded millisecond microchannel reactors

Mon, 10 Jun 2024 00:00:00 +0000

The present study focuses upon the physics of heat and mass transfer processes in a protruded millisecond microchannel reactor, wherein a steam reforming reaction is proceeding and protrusions are used to improve the transport processes involved. Parametric analysis of the reactor system is carried out using a three-dimensional numerical model that is sufficiently detailed to delineate the role of geometric features and operation conditions in reactor performance. Computational fluid dynamics analyses are performed under different sets of circumstances. In analysing the mechanisms involved in the intensified processes, account is taken of the factors that may influence the reactor performance. New insights into the physics of the processes are presented, with recommendations on how to optimize reactor design for better performance. The results indicate that the flow rates and feed compositions must be adjusted as needed to maximize production of hydrogen and minimize pressure...

Molecular dynamics study of the thermal properties and phenomena of graphane and fluorographene

Mon, 10 Jun 2024 00:00:00 +0000

In spite of significant efforts to investigate the ability of hydrogenated and fluorinated graphene to conduct heat, little research has focused particularly upon their other thermal properties, such as thermal contraction and heat capacity, which have implications for the development of thermal nanotechnology. In an attempt to determine these thermal properties, a few experiments have been carried out, with rather conflicting results. In the present study, calculations were performed using molecular dynamics to investigate the thermal properties of graphane and fluorographene and especially the phenomena involved. The thermal expansion coefficients and heat capacities of the two-dimensional materials were determined at different temperatures. The results indicated that graphane is thermally contracted more significantly than graphene. The calculated molar heat capacity at constant volume is about 25.00 J/(mol·K) for graphene and about 29.26 J/(mol·K)...

Fundamental characteristics of flow past an array of hemispherical protrusions in millisecond microchannel reactors

Fri, 7 Jun 2024 00:00:00 +0000

Flow of a fluid past a body is a very complicated phenomenon. Computational fluid dynamics is used for studying the characteristics of flow past an array of hemispherical protrusions that is disposed on the wall surfaces of a millisecond microchannel reactor. Protrusions can be used to improve the transport processes involved, but the causes of the phenomena are still incompletely understood. Parametric analyses are performed under different sets of circumstances to delineate the role of geometric features and operation conditions in reactor performance. Dimensionless quantities are used to simplify the characterization of the reactor system with multiple interacting transport phenomena. The mechanisms involved in the intensified processes are analysed, and performance improvement recommendations are presented. The results indicate that the protruded reactor behaves effectively and good yields can be obtained with only milliseconds residence of the mixtures within the channels....

Effects of pressure and flow rate on the efficiency and performance of autothermal reforming systems for hydrogen production

Thu, 6 Jun 2024 00:00:00 +0000

Computational modelling for microchannel reactor design was conducted in the attempt to fully understand autothermal reforming phenomena in continuous flow reactors. The effects of pressure and flow rate on the efficiency and performance were evaluated by performing computational fluid dynamics under different design conditions. The reactor efficiency and performance were assessed by means of the reactant conversion, product yield, reaction rate, hydrogen productivity, and output power. Recommendations for designing an autothermal reforming system were made and strategies for performing efficient operation were set forth. The results indicated that the pressure and flow rate set strict limits on operation, and there is a trade-off between high productivity and low conversion. The pressure and flow rate play competing roles in the reactor efficiency and performance. Lower pressures and flow rates can increase the conversion and yield, but higher pressures and flow rates can significantly...

Computational fluid dynamics and thermodynamic analysis of transport and reaction phenomena in autothermal reforming reactors for hydrogen production

Thu, 6 Jun 2024 00:00:00 +0000

Computational fluid dynamics uses numerical methods and algorithms to solve and analyse problems that involve fluid flows. Computers may be used to perform the calculations required to simulate the interaction of liquids and gasses with a surface defined by boundary conditions. Thermodynamics is the science of the relationship between heat, work, temperature, and energy. Computational modelling for microchannel reactor design was performed to investigate the effects of various factors on the efficiency and performance of an autothermal reforming system. The yield and productivity from the chemical process were determined by performing computational fluid dynamics and thermodynamic analysis. Strength and weakness were assessed under different reaction conditions. Design recommendations were provided and operation strategies were mapped out. The results indicated that operation at millisecond contact times is feasible, but optimisation of reaction conditions is necessary to balance...

Methods and mechanisms for improving combustion stability by fluid recirculation structures in micro-structured burners

Thu, 9 Nov 2023 00:00:00 +0000

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....

Transport phenomena in microchannel reactors for proton-exchange membrane fuel cell applications

Mon, 6 Nov 2023 00:00:00 +0000

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...

Improvements of efficiency and performance for steam reforming reactors with optimum conditions of wall thermal conductivities and channel dimensions

Fri, 3 Nov 2023 00:00:00 +0000

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...

Thermal conductivity of carbon nanotube films and fibers with different structures and purities at different temperatures

Fri, 3 Nov 2023 00:00:00 +0000

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....

Homogeneous charge compression ignition of fuel-lean methane-air mixtures over alumina-supported platinum catalysts in small-scale free-piston engines

Thu, 2 Nov 2023 00:00:00 +0000

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...

Catalytically stabilized combustion of propane in heat-recirculating continuous flow reactors for improved flame stabilization and energy efficiency

Wed, 1 Nov 2023 00:00:00 +0000

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...

Heat transfer and thermodynamic analysis of synthesis gas production processes in chemical reactors with integrated heat exchangers by steam reforming

Tue, 26 Sep 2023 00:00:00 +0000

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...

Computational fluid dynamics studies of catalytically stabilized combustion of propane in flow tube reactors

Tue, 26 Sep 2023 00:00:00 +0000

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...