Dottorato in ENERGIA E AMBIENTE

Titolo della tesi: Fluid flow and heat transfer through pore-structure porous media by using Lattice Boltzmann Method

Abstract The heat transfer process plays a crucial role across various industries, including electronic devices, medical equipment, vehicles, aerospace and marine applications. Enhancing the heat transfer and efficiency of cooling and heating modules in these applications, is vitally important to decrease energy consumption and consequently reduce pollution to save the environment. In addition, improving efficiency and extending the lifespan of components not only benefits the environment but also reduces costs from an economic perspective. In recent decades, fluid flow and heat transfer through the porous media has become much more important than ever. As the porous materials have high amount of contact surface per volume, play a significant role to increasing the heat transfer. In fact, enhancing the heat transfer and fluid flow performance of these materials is crucial. On the other hand, the Lattice Boltzmann Method (LBM) is a robust numerical approach for analysing fluid flow and heat transfer, particularly in complex geometries such as pore scale porous media and challenging boundary conditions. It is particularly effective for simulating porous media at both the macro and mesoscales. The study of fluid flow and heat transfer through porous media using LBM can be categorized into two approaches, Representative Elementary Volume (REV) and Pore-Structure (PS). With the (REV) approach, porous medium is viewed as a continuum medium. While the Pore-Structure simulations (PS), can provide a detailed flow field within the pores, since some obstacles can represent the solid part of the porous media and fluid is flowing through them. Although in the first approach (REV) the computing time and cost is much lower than the second one (PS), in the second approach the influence of solid part of the porous media such as, configuration, shape and size of the solid section on the fluid flow and heat transfer is more visible. Which is vitally important to enhance the performance of the cooling module systems. Due to the significance of the discussed topic, this study investigates fluid flow and heat transfer through porous media using the Lattice Boltzmann Method. The porous media in the present study includes part of a channel which is filled with some obstacles in different configurations, as a pore structure porous medium. In the present study, a wide range of relevant literature is categorized and reviewed. In addition, variety of boundary conditions is discussed and explained. Consequently, the Single Relaxation Time (SRT) Lattice Boltzmann Method (LBM), incorporating the Bhatnagar-Gross-Krook (BGK) approximation and the D2Q9 model, was employed in the present study for advection-diffusion simulations. This approach is widely recognized for its computational efficiency and suitability for such numerical simulations. In addition, the double distribution function (DDF) model used to simulate fluid flow and heat transfer. In the present work, two series of numerical simulations by the topic of fluid flow and heat transfer through pore structure porous media are conducted. The pore structure porous media in these two simulations are square obstacles in different sizes and positions. But, in the same porosity. To investigate the effect of obstacle size and position on the flow behaviour and heat transfer. The configurations and boundary conditions in these two series of simulations are different aims to investigate different conditions. In the first series of numerical simulations, the effects of the size and arrangement of cold square obstacles located within a heated wall channel are investigated for incompressible flow with a fixed porosity degree, ε=75%. The results are presented in terms of velocity and temperature fields, streamlines, tortuosity and the Nusselt number. The results show that, for the same porosity degree, changing the number of obstacles or their arrangement alters the tortuosity, which in turn modifies the velocity and temperature fields, as well as the local and average Nusselt numbers. In more detail, the results illustrate that in the same porosity degree, by changing the quantity of the obstacles or their arrangements, the value of 〖Nu〗_avg of the channel walls and tortuosity increased up to 24.7% and 9%, respectively, in comparison with the Poiseuille flow. In the second series of numerical modelling, pore structure porous media includes 4 configurations with one obstacle as well as 10 different inline and staggered configurations with six obstacles. In this study, the Fourier’s law is extended to the Lattice Boltzmann Method to implement a novel (to the best of the authors’ knowledge) node-base heat flux boundary condition. Also, at the inlet of the channel, fully developed velocity profile is applied. The consequences are presented in terms of streamlines, tortuosity, as well as the local and average Nusselt number of the obstacles walls. The results show that the LBM and the presented heat flux boundary conditions are capable to be applied in practical applications. The effects of different arrangements of obstacles are evaluated for the same porosity degree and it is highlighted that the different configurations can change the average Nusselt number (tortuosity effect). In addition, by changing the configuration from inline to a well-designed staggered configuration in porous media, the average Nusselt number of the obstacles could increase up to 73%. Also, an upward trend is observed between tortuosity and the Nusselt number. The results of both series of numerical simulations, in addition to their significant relevance to porous media, which is the focus of this study, could also be useful in various practical applications. These include heat exchanger and heating/cooling module design, as well as investigations of flow behaviour around buildings concerning their block layout. Keywords: Lattice Boltzmann method; Heat transfer; Heat flux; Constant temperature; Neumann and Dirichlet boundary conditions; Porous media; Pore-Scale; Pore-Structure; Porosity; Tortuosity