Thermophoresis and its effect on particle impaction on a cylinder for low and moderate Reynolds numbers

The effect of thermophoresis on the impaction of particles on a cylinder is investigated for different particle sizes, particle conductivities, temperature gradients and for Reynolds numbers between 100 and 1600. Simulations are performed using the Pencil Code, a high-order finite difference code. An overset-grid method is used to precisely simulate the flow around the cylinder. The ratio of particles impacting the cylinder and the number of particles inserted upstream of the cylinder is used to calculate an impaction efficiency. It is found that both the particle conductivity and the temperature gradient have a close to linear influence on the particle impaction efficiency for small particles. Higher Reynolds numbers result in higher impaction efficiency for middle-sized particles, while the impaction efficiency is smaller for smaller particles. In general, it is found that thermophoresis only has an effect on the small particles, while for larger particles the impaction efficiency is controlled by inertial impaction. Finally, an algebraic model, developed based on fundamental principles, which describes the effect of thermophoresis is presented. The model is found to accurately predict the DNS results. As such, this model can be used to understand the mechanisms behind particle deposition due to the thermophoretic force, and, more importantly, to identify means by which the deposition rate can be reduced

The effect of turbulence on the conversion of coal under blast furnace raceway conditions

dynamics (CFD) can be used to analyze the process virtually and thus improve its performance. Different reducing agents can be used to (partially) substitute the coke and consequently reduce overall emissions. To analyze different reducing agents effectively using CFD, their conversion process has to be modeled accurately. Under certain conditions, coal particles can cluster as the result of turbulence effects, which further reduces the mass transfer to the coal surface and consequently the conversion rate. We analyze the effect of turbulence under blast furnace raceway conditions on the conversion of coal particles and on the overall burnout. The model is applied in RANS to polydisperse particle systems and this is then compared to the simplified monodisperse assumption. Additionally, the model is extended by adding gasification reactions. Overall, we find that the turbulent effects on coal conversion are significant under blast furnace raceway conditions and should be considered in further simulations. Furthermore, we show that an a-priori assessment is difficult because the analysis via averaged quantities is impractical due to a strong variation of conditions in the furnace. Therefore, the effects of turbulence need to be correlated to the regions of conversion. © 2022 The Author(s)The effect of turbulence on the conversion of coal under blast furnace raceway conditionspublishedVersio

Detailed numerical modeling of solid fuels conversion processes

This thesis investigates conversion of solid fuel particles from different perspectives. In particular, two aspects of the conversion process are studied: 1) the effect of turbulence on the conversion rate and 2) the combustion behaviour of a single carbon particle. Direct Numerical Simulations of polydisperse particle systems in a periodic box of isotropic and homogeneous turbulence were performed in order to analyse how turbulence affects the mass transfer rate in such configuration. The effect of turbulence was found to be identical qualitatively and very similar quantitatively to what was observed for monodisperse particle systems, i.e. there exist two opposing mechanisms through which turbulence can affect the reactant transfer to surfaces of particles. The first mechanism leads to formation of particle clusters, which are responsible for decreased mass transfer rate, while the other is associated with the mass transfer being enhanced by turbulent motions. The model which accounts for the combined effect of turbulence was shown to be applicable to polydisperse particle systems. The effect of particle back-reaction was also studied, the results revealed its importance especially in cases characterized by high Stokes numbers. A sensitivity of the effect of turbulence to selected parameters was investigated through theoretical analysis and using simple numerical examples. Several parameters (mixture composition, particle material, turbulence intensity, particle size, mass flow rate) were demonstrated to have a meaningful influence on how strong the effect is. Finally, the effect of turbulence on the conversion rate in practical systems was examined. For this purpose, an industrial-scale boiler was simulated in ANSYS Fluent. The results showed that regions of very different conditions can simultaneously occur in different parts of the boiler, which rules out the possibility to predict the net effect of turbulence a priori. Moreover, it was shown that the conversion rate might be affected by turbulence much less than expected based on theoretical predictions as the reaction rate is often controlled by kinetics, i.e. it is independent on the rate of reactant transfer. A simple model for the resolved carbon particle conversion was implemented in the Pencil Code. Efficiency of the model was achieved by pre-adjustment of diffusion coefficients, reduction of the speed of sound and employment of a semi-global mechanism. Despite numerous simplifications, an ability of the model to predict main characteristics of the char particle conversion and formation of a flame zone was demonstrated by validating the model against experimental and numerical results. Sensitivity of the conversion rate to kinetic parameters and transport coefficients was studied. A strong sensitivity to the oxygen diffusivity was registered, especially at higher temperatures

A numerical study on the combustion of a resolved carbon particle

Combustion of a single, resolved carbon particle is studied using a novel numerical approach that makes use of an overset grid. The model is implemented into the framework of a compressible Direct Numerical Simulation (DNS) code. A method to artificially reduce the speed of sound is presented. For Mach numbers lower than 0.1 this method may dramatically improve numerical efficiency without affecting any physical aspects except for the acoustics. The ability of the model to simulate solid fuel combustion is demonstrated and all parts of the model are validated against experimental and numerical data. A sensitivity of the carbon conversion rate to selected parameters (diffusion coefficients and homogeneous and heterogeneous kinetics) is investigated. A strong dependence on the oxygen diffusivity is observed and explained

A numerical study on the combustion of a resolved carbon particle

Combustion of a single, resolved carbon particle is studied using a novel numerical approach that makes use of an overset grid. The model is implemented into the framework of a compressible Direct Numerical Simulation (DNS) code. A method to artificially reduce the speed of sound is presented. For Mach numbers lower than 0.1 this method may dramatically improve numerical efficiency without affecting any physical aspects except for the acoustics. The ability of the model to simulate solid fuel combustion is demonstrated and all parts of the model are validated against experimental and numerical data. A sensitivity of the carbon conversion rate to selected parameters (diffusion coefficients and homogeneous and heterogeneous kinetics) is investigated. A strong dependence on the oxygen diffusivity is observed and explained

A numerical study on the combustion of a resolved carbon particle

Combustion of a single, resolved carbon particle is studied using a novel numerical approach that makes use of an overset grid. The model is implemented into the framework of a compressible Direct Numerical Simulation (DNS) code. A method to artificially reduce the speed of sound is presented. For Mach numbers lower than 0.1 this method may dramatically improve numerical efficiency without affecting any physical aspects except for the acoustics. The ability of the model to simulate solid fuel combustion is demonstrated and all parts of the model are validated against experimental and numerical data. A sensitivity of the carbon conversion rate to selected parameters (diffusion coefficients and homogeneous and heterogeneous kinetics) is investigated. A strong dependence on the oxygen diffusivity is observed and explained.publishedVersio

The effect of turbulence on mass transfer rates between inertial polydisperse particles and fluid

The current work investigates how turbulence affects the mass transfer rate between inertial particles and fluid in a dilute, polydisperse particle system. Direct numerical simulations are performed in which all scales of turbulence are fully resolved and particles are represented in a Lagrangian reference frame. The results show that, similarly to a monodisperse system, the mass transfer rate between particles and fluid decreases as a result of particle clustering. This occurs when the flow time scale (based on the turbulence integral scale) is long relative to the chemical time scale, and is strongest when the particle time scale is one order of magnitude smaller than the flow time scale (i.e. the Stokes number is around 0.1). It is also found that for larger solid mass fractions, the clustering of the heavier particles is enhanced by the effect of drag force from the particles on the fluid (momentum back-reactions or two-way coupling). In particular, when two-way coupling is accounted for, locations of particles of different sizes are much more correlated, which leads to a stronger effect of clustering, and thus a greater reduction of the particle-fluid mass transfer rate. © 2019 Cambridge University Press

The effect of turbulence on mass transfer in solid fuel combustion: RANS model

In this paper, a kinetic-diffusion surface combustion model is examined. The model is modified such that two effects of turbulence are included: 1) enhancement of the mass transfer due to relative velocity between particles and fluid and 2) reduction of the mass transfer due to turbulence-induced particle clustering. Details of the implementation are discussed and the influence of parameters such as air-fuel ratio, particle number density, particle diameter, turbulence intensity and characteristic length scales are studied theoretically. A simplified numerical model of a combustion chamber is created to explore the effects of the combustion model predictions. Finally, the model is incorporated into simulations of an industrial-scale boiler to investigate the effect of turbulence on the net surface reaction rate in a real system. The study shows that although on average this effect is rather minor, there exist regions in which the carbon conversion rate is either decreased or increased by turbulence. Keywords Combustion kineticsCombustion ratesChar oxidationDiffusion regim

Thermophoresis and its effect on particle impaction on a cylinder for low and moderate Reynolds numbers

The effect of thermophoresis on the impaction of particles on a cylinder is investigated for different particle sizes, particle conductivities, temperature gradients and for Reynolds numbers between 100 and 1600. This is the first such study performed using Direct Numerical Simulations (DNS), where all temporal and spatial scales of the fluid are resolved. Simulations are performed using the Pencil Code, a high-order finite difference code with an overset-grid method precisely simulating the flow around the cylinder. The ratio of particles impacting the cylinder to the number of particles inserted upstream of the cylinder is used to calculate an impaction efficiency. It is found that both the particle conductivity and the temperature gradient have a close to linear influence on the particle impaction efficiency for small particles. Higher Reynolds numbers result in higher impaction efficiency for middle-sized particles, while the impaction efficiency is smaller for smaller particles. In general, it is found that thermophoresis only has an effect on the small particles, while for larger particles the impaction is dominated by inertial impaction. An algebraic model is presented that predicts the effect of the thermophoretic force on particle impaction on a cylinder. The model is developed based on fundamental principles and validated against the DNS results, which are faithfully reproduced. As such, this model can be used to understand the mechanisms behind particle deposition due to the thermophoretic force, and, more importantly, to identify means by which the deposition rate can be reduced. This is relevant for example in order to minimise fouling on super-heater tube bundles in thermal power plants