Browsing by Author "Maakala, Viljami"

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Computational and experimental investigation of a swirl nozzle for viscous fluids
(2020-07) Laurila, E.; Koivisto, S.; Kankkunen, A.; Saari, K.; Maakala, Viljami; Jävinen, M.; Vuorinen, V.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Highly viscous flow in a large-scale pressure-swirl atomizer is studied by (1) 3d scale-resolving large-eddy simulations and volume-of-fluid method, and (2) experiments based on laser-Doppler anemometry, imaging techniques and pressure measurements. Here, a low Reynolds number regime (600 ≤ Re ≤ 910) is investigated by varying the mass flow rate of the water-glycerol mixture. The aim of the study is to perform a comprehensive comparison between the simulations and experiments at a parameter range and nozzle geometry relevant for biomass based fuels. We report the inner-nozzle velocity profiles noting good agreement for mean velocities inside the swirl chamber between the simulations and the experiments. Consistent with the earlier work (Laurila et al., 2019), the simulations indicate the flow mode to be laminar with weak or non-existent gaseous core inside the swirl chamber. As revealed by both approaches, liquid film shapes after the nozzle discharge orifice are qualitatively similar, of hollow cone type, and highly unstable. Both approaches indicate linear scaling of the liquid film velocity with the inlet Reynolds number and discharge coefficients to be in the range 0.57–0.64. The experimentally measured mean opening angles are reported to be 45–62 ∘, while the numerical counterparts show reasonable correspondence with the experiments. The results demonstrate the predictive ability of the present numerical method in swirl injector analysis.
Computational Fluid Dynamics Modeling and Mathematical Optimization of Recovery Boilers
(2019) Maakala, Viljami
School of Engineering | Doctoral dissertation (article-based)
This thesis belongs to the research field of applied computational fluid dynamics (CFD). The research focus is on utilization of existing state-of-the-art CFD modeling and mathematical optimization methods for very large and multiscale design optimization problems. The application under study is the recovery boiler, which is used to combust black liquor, a by-product of the pulp making process. The objective is to bring new understanding to the complex physical and chemical processes inside recovery boilers. In this thesis, the effects of various design choices on these processes are systematically and quantitatively investigated for the first time. The first part focuses on the combustion performance in the furnace. A surrogate-based optimization method and CFD modeling are combined to understand and quantify the connection between the furnace dimensions and combustion processes. As a result of the optimization, a set of Pareto-optimal geometries is obtained and the reasons for the improved combustion performance are investigated. Thereafter, a highly systematic CFD study is performed to understand the effects of typical design choices regarding the secondary air system on the mixing, penetration, and uniformity of the vertical velocity field. Several implications for the optimal design of the air system are formulated. The second part focuses on the flow field and heat transfer in the superheater region. A surrogate-based optimization method and CFD modeling are integrated to investigate the effects of the superheater region geometry on the flow field and heat transfer. The numerical results are analyzed to explain the physical mechanisms for the performance improvements and the linkage between the geometry, flow field, and heat transfer. After this, a new fully three-dimensional CFD model is developed to simulate the complex three-dimensional flow and heat transfer phenomena in the superheater region. Two sets of industrial full-scale measurements are reported and utilized to validate the simulation results. The added-value and new implications of the three-dimensional results are demonstrated. The main added-value of this thesis is considered to consist of the following factors. Primarily, it is one of the first extensive numerical studies into the internal processes of recovery boilers. Significant new knowledge is obtained in the following areas: 1) geometrical design of the furnace, 2) geometrical design of the superheater region, and 3) design of the combustion air system. In addition, an exceptional validation study is done between the numerical simulations and experiments in the superheater region. Finally, substantial new knowledge is obtained concerning the utilization of optimization methods in the context of recovery boilers and similar very large-scale applications.
Integrated computational fluid dynamics and 1D process modelling for superheater region in recovery boiler
(2019-06-17) Kumar, Kunal
Insinööritieteiden korkeakoulu | Master's thesis
Superheaters are the last heat exchangers on the steam side in recovery boilers. Their performance is accountable for proficient recovery boiler operation. The objective of this work is to obtain thorough knowledge about the superheating process and material temperature distribution for superheater platens. The study includes the effects of 3D flue gas flow field in superheater region and generated steam properties in steam cycle. The detailed analysis for flue gas side and steam side is important for improving recovery boilers' energy efficiency, cost efficiency, safety and contribution for carbon neutral energy production. In this work, for the first time, a comprehensive 1D-process model (1D-PM) for superheated steam cycle is developed and linked with a full-scale 3D-CFD model of the superheater region flue gas flow. The developed 1D-PM is validated using reference data including mass and energy balance calculations, and measurements. The results reveal that first; the geometrical structures of headers, connecting pipes and superheater platens affect platen-wise steam distribution. Second, the integrated solution of the 3D flue gas flow field and platen heat flux distribution with the 1D-PM substantially affect both generated superheated steam properties and material temperature distribution. It is also found that the commonly used uniform heat flux distribution approach for superheating process is not accurate because it does not consider the effect of flue gas flow field in superheater region. This novel integration modelling approach is advantageous for trouble shooting, optimizing the performance of superheaters in recovery boiler and selecting their design margins for the future. It could also be applied for other large scale energy production units including industrial biomass fired boilers.
Integrated study of flue gas flow and superheating process in a recovery boiler using computational fluid dynamics and 1D-process modeling
(2020-06) Kumar, Kunal; Maakala, Viljami; Vuorinen, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Superheaters are the last heat exchangers on the steam side in recovery boilers. They are typically made of expensive materials due to the high steam temperature and risks associated with ash-induced corrosion. Therefore, detailed knowledge about the steam properties and material temperature distribution is essential for improving the energy efficiency, cost efficiency, and safety of recovery boilers. In this work, for the first time, a comprehensive one-dimensional (1D) process model (1D-PM) for a superheated steam cycle is developed and linked with a full-scale three-dimensional (3D) computational fluid dynamics (CFD) model of the superheater region flue gas flow. The results indicate that: (1) the geometries of headers and superheater platens affect platen-wise steam mass flow rate distribution (3%–7%); and (2) the CFD solution of the 3D flue gas flow field and platen heat flux distribution coupled with the 1D-PM affect the platen-wise steam superheating temperature (45%–122%) and material temperature distribution (1%–6%). Moreover, it is also found that the commonly-used uniform heat flux distribution approach for the superheating process is not accurate, as it does not consider the effect of flue gas flow field in the superheater region. These new observations clearly demonstrate the value of the present integrated CFD/1D-PM modeling approach. Application: The present integrated modeling approach is advantageous for troubleshooting, optimizing the performance of superheaters, and selecting their design margins for the future. It could also be relevant for other large-scale energy production units, such as biomass-fired boilers.
Multi-objective optimization of recovery boiler dimensions using computational fluid dynamics
(2013) Maakala, Viljami
Insinööritieteiden korkeakoulu | Master's thesis
The purpose of this work was to develop a multi-objective optimization program based on computational fluid dynamics (CFD). The program combines a CFD model with a genetic algorithm optimizer and a radial basis function network learner. The tool was applied to optimizing a furnace geometry of a recovery boiler using two approaches: an uncoupled method and a coupled method. Before solving the optimization task, a study was done on the CFD model errors and the CFD-optimization program was verified. Error analyses revealed that the simulations have substantial iteration errors. Discretization errors were studied using grid convergence index values, but iteration errors made their interpretation difficult. The program was verified in test problems and the proposed methodology was concluded to work as intended. Both optimization approaches found several geometries that deliver better performance than the original boiler design. The coupled method is more reliable, because it performs numerous CFD evaluations near the final solutions. Future research should be done to reduce iteration errors when modeling recovery boilers. It would also be useful to develop the CFD-optimization methodology further and to use it in different applications. Areas for development include improved integration of preferences into the optimization and usage of hybrid optimization methods.
Säilymismuotoiset menetelmät virtauslaskennassa
(2011) Maakala, Viljami
Insinööritieteiden korkeakoulu | Bachelor's thesis