Development of particle-scale simulation methods for multiphase flow study
This dissertation is focused on the particle-scale numerical simulations in two areas: fast pyrolysis of biomass particles and interactions of fuel drops and heated walls.
The first topic is the fast pyrolysis of biomass materials, an effective means to convert biomass into useful energy products. The conversion process can be significantly affected by the properties of the biomass particle and the operating conditions. To obtain a better understanding of this process, a direct numerical simulation method was proposed and used to simulate the evolution of biomass particles under fast pyrolysis conditions. The lattice Boltzmann method (LBM) was employed to solve the flow field and the intra-particle transport of heat and mass. The present model was validated by comparing the numerical results with experimental data for a single biomass particle under pyrolysis conditions. The predicted evolutions of center temperature and solid mass fraction agreed well with the experimental data. The validation demonstrated that the present model was capable of revealing the detailed conversion process of biomass fast pyrolysis at the particle scale. The temperature and density fields in the particle were found to be anisotropic due to the effect of the gas flow surrounding the particle. The non-uniform distributions of surface temperature indicate that using a constant temperature or heat flux as boundary conditions may cause numerical errors. Sensitivity analysis shows that density is the most influential parameter while porosity is the least.
Parametric studies were conducted to characterize the effects of particle shape, particle size, inlet gas temperature, and reactor wall temperature on the conversion time and final product yields. The simulation results showed that the conversion time decreased when using the elliptic particle instead of the regular (circular) particle; more tar and syngas were produced, while less char was generated from an elliptic particle. It was found that the conversion time increased as the particle size increased and decreased as the inlet gas temperature and reactor wall temperature increased. When the particle size was decreased, more tar and syngas were produced while less char was generated. The same trend of final product yields was also found when the inlet gas temperature and reactor wall temperature were increased. The numerical results also indicated that the temperature gradients inside the particle can be neglected under certain particle size, i.e., equal to or less than 0.2 mm under the conditions studied. The heat flux from the reactor wall was found to be more significant to the fast pyrolysis process than the inlet gas temperature. The results demonstrated that the current LBM framework has the ability to reveal the detailed conversion process of biomass particles under various pyrolysis conditions, which can then be used to improve engineering models for reactor-scale simulation.
The second topic is the interaction of liquid fuel drops and heated walls. The outcomes of fuel drop impact on the combustion chamber walls will affect the fuel-air mixture distribution and the subsequent combustion performance and emissions in internal combustion engine. The process of fuel drop impact on a solid dry surface was simulated using a numerical method based on Smoothed Particle Hydrodynamics (SPH). The SPH method was first validated using the experimental data on the impact regimes of ethanol drops on a heated surface. Overall, different impact outcomes including deposition, contact-splash, bounce, and film-splash, were predicted successfully by the present SPH method. Then, the impact process of iso-octane drops on a solid surface under engine relevant conditions were studied. Numerical results show that the splash threshold will decrease as surface temperature increases. Different impact regimes were identified and the impact outcomes in each regime were analyzed to derive a comprehensive drop/wall interaction model. The effects of surface temperature and impact angle on the impact outcomes were characterized and implemented into the model. It was found that the impact angle will affect the distributions of secondary droplets in the splash regime. The relative locations and velocities of the secondary drops were also quantitatively correlated.
The present SPH method was also applied to simulate the drop impact on a wet wall, where a liquid film has already existed at the time of impact. The presence of wall film will affect not only the splash threshold but also the crown evolution and the secondary droplets ejected from the rim of the crown. The present numerical method was first validated by experimental data on crown height and diameters resulting from a water drop impact on liquid films. Then, the impact process of iso-octane drops on a wet wall under engine relevant conditions were characterized. The numerical results show that the splash threshold will increase as the film thickness increases. The splash criteria derived from the present simulations are more comprehensive than existing models used in spray/wall impingement study. The effect of the film thickness on the splashed mass ratio is determined by two competing mechanisms. On one hand, as the film thickness increases, more incident energy will be absorbed and transferred into the crown, thus producing more secondary drops. On the other hand, more impinging energy will be dissipated during the spreading as the film thickness increases, thus making it harder to splash and producing fewer secondary drops. In addition, the behaviors of the secondary drops are found to be quite different as the film thickness increases. Instead of moving outward for drop impact on the thin film, the secondary droplets will move upward and even aggregate to the center when the film becomes thicker. This effect is also reflected on the locations and velocities of the secondary drops, which was characterized and implemented into the model. It was found that the impact angle will affect not only the distributions of secondary droplets but also the splashed mass. The locations and velocities of the secondary drops were also quantitatively analyzed. These outcomes are incorporated into a drop/wall interaction model for engine spray/wall impingement simulation.