Particle-fluid interactions under heterogeneous reactions

• Particle-fluid flows are involved in many natural processes and industrial applications; some examples are drying, solid fuel combustion, gasification, and catalytic cracking. It is vital to understand the phenomena involved in particle-fluid flows in depth for design, predictions and process improvements. Computational fluid dynamics (CFD) can be a robust tool for these studies that complements costly experimental trials. Current computational power and resources do not allow numerical simulations to resolve all physical and chemical scales in a single simulation. State of-the-art in large-scale numerical simulations is to carry out simulations at larger scales with sub-grid models for small-scale phenomena. Therefore, the accuracy of the models is key to better predictions in large-scale simulations.Particle-fluid flows have complexities due to many reasons. One of the main challenges is to describe how the particle-fluid interactions vary when the particles are reacting. Particles and the fluid interact through momentum, heat, and mass exchange. Momentum, heat, and mass exchange are presented by the drag coefficient (Cd), Nusselt number (Nu), and Sherwood number (Sh) in fluid dynamics. Conventional models neglect the effects of net fluid flow generated by heterogeneous chemical reactions called Stefan flow.This work aims to study how Stefan flow affects the momentum, heat, and mass transfer between particles and fluid in a particle-fluid flow. A series of numerical simulations were performed by increasing complexity step by step. Particle boundary layers were resolved in all the simulations, and the particle interior was also resolved in the last stage. With a special interest in entrained flow biomass gasification (EFBG), this work has chosen parameters relevant to EFBG.In the first step, particle-resolved numerical simulations were carried out for an isolated particle immersed in a uniform, isothermal (and non-isothermal) bulk fluid with a uniform Stefan flow. Both isothermal and non-isothermal simulations have shown that the Stefan flow has significant effects on drag coefficient (Cd) and Nusselt number (Nu). We have observed from isothermal results that the decrease/increase of the drag coefficient (Cd) is due to expansion/shrinkage of the boundary layer thickness, which leads to a change in the viscous force. Based on that, a physics based drag coefficient (Cd) model was developed. For the next step, the drag coefficient (Cd) model was extended and modified for a uniform non-isothermal bulk fluid flow. Furthermore, a new Nusselt number (Nu) model was developed using volume-averaged temperature, which captures the variation of thermo-physical parameters due to the temperature gradient between particle and bulk fluid. The model agrees well with the simulation data with a single fitting parameter.The second step was to explore the effects of neighboring particles on the drag coefficient (Cd) with a uniform Stefan flow under isothermal conditions. Stefan flow and neighbor particle effects act on the particle independently when particle distance is greater than 2.5 diameters (L/D > 2.5). However, at L/D ≤ 2.5, Stefan flow effects dominate, and a strong force that expels particles from each other was observed. The models previously developed under ideal conditions (uniform Stefan flow, atmospheric pressure) might not represent realistic conditions at reacting flows. Therefore, the last step of this thesis was particle interior resolved numerical simulations for an isolated char particle under gasifying conditions. The drag coefficient (Cd), Nusselt number (Nu) and Sherwood number (Sh) from the simulations have been compared with conventional models without Stefan flow. We have observed that conventional drag coefficient (Cd) and Nusselt number (Nu) models do not accurately predict the force acting on a particle and heat transfer between the particle and bulk fluid.The performance of the point-particle approach for reacting particle-fluid flows, commonly used in large-scale simulation, was also investigated by comparing it with particle interior resolved simulations for a gasifying particle. The results showed a significant deviation between the results of the point particle model and resolved particle simulations. Several key uncertainties in the models, such as the effectiveness factor and external heat and mass transfer, were identified.This work has shown that the effects of Stefan flow are not negligible in reacting particle-fluid flows. Developed drag coefficient (Cd) and Nusselt number (Nu) models can be used to improve large-scale simulations’ predictions. The study also contributes to widening the understanding physics of particle-fluid interactions in reacting particle-fluid flows. Conventional models for drag coefficient (Cd) and Nusselt number (Nu) (and Sherwood number (Sh)) do not represent the momentum and heat transfer (and mass transfer) between a particle and the bulk fluid accurately when there is a Stefan flow due to heterogeneous reactions during char gasification. Therefore, the models should be further improved considering the effects of Stefan flow.The models developed in this work are idealized for a uniform Stefan flow, atmospheric pressure, and spherical particle. It could be further improved for non-uniform Stefan flow, high pressure, and different geometries. This study mainly focused on the parameter range of gasification for model development. Therefore, it is important to test the effects of Stefan flow for a wider range applicable to other applications, such as combustion, and test whether the phenomena are the same as observed in this work. We focused on char gasification to study the effects of Stefan flow in more realistic conditions and to compare it with the point-particle method. That also could be studied for a wider range of applications and find at what conditions one has to consider the effects of Stefan flow on drag coefficient (Cd), Nusselt number (Nu), and Sherwood number (Sh). Furthermore, it would be important to find the models predicting closer to the resolved-particle simulations for a particle with Stefan flow to be used in the point-particle approach. Improving effectiveness factor models, including non-uniform temperature inside the particle, is also vital.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Energiteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Energy Engineering (hsv//eng)

Nyckelord

Stefan flow

drag coefficient

Nusselt number

Sherwood number

particle-fluid flow

reacting flow

Energy Engineering

Energiteknik

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