Fluid flow and heat transfer in dual-scale porous media

Porous media are omnipresent in various natural and engineered systems. The study of transport phenomena in porous media has attracted the attention of researchers from a wide variety of disciplines. In many applications such as hydrogeology, petroleum engineering and thermochemistry, porous media are encountered, in which heterogeneity exists at a multitude of length-scales. In solar thermochemical reactors, a promising approach to accomplish the thermochemical cycle is to form the reactive solid into a porous structure to promote efficient solid-gas reactions through a high specific surface area, while simultaneously achieving desired transport characteristics. Recently, in light of the apparent trade-offs between rapid reaction kinetics and efficient radiation absorption, reticulated porous ceramics (RPCs) featuring dual-scale porosity have been engineered. These structures are capable of combining the desired properties, namely uniform radiative absorption and high specific surface area. Therefore, investigations are required to understand and analyse different transport phenomena in such structures. This dissertation is motivated by the need for understanding and analysing transport phenomena dual-scale porous media appear and used in many applications such as hydrogeology, petroleum engineering, chemical reactors, and in particular, energy technologies in high-temperature thermochemistry. The main objective of this thesis is to theoretically formulate and numerically demonstrate the fluid flow and heat transfer phenomena in dual-scale porous media. The theoretical and numerical results are used to propose models in forms of effective flow and heat transfer coefficients. The models are capable of estimating the fluid flow and heat transfer phenomena taking place in dual-scale porous media with appropriate fidelity and lower computational cost. The physical understanding of the models of transport phenomena in dual-scale porous structures allows us to tailor and optimise the morphology to accomplish optimal transport characteristics for the desired applications. To determine the flow coefficients, numerical simulations are performed for the fluid flow in a dual-scale porous medium. Two numerical procedures are considered. Firstly, we perform direct pore-level simulations by solving the traditional mass and momentum conservation equations for a fluid flowing through the dual-scale porous structure. Secondly, numerical simulations are performed at the Darcy level. For this purpose, the permeability and Forchheimer coefficient of the small-scale pores are numerically determined. Then, they are implemented in Darcy-level simulations in which the volume-averaged and traditional conservation equations are solved for the small- and large-scale pores, respectively. The results of the two approaches are separately used to determine and compare the permeability and Forchheimer coefficient of the dual-scale porous media. To analyse the energy transport phenomena in dual-scale porous media, a mathematical model is developed by applying volume-averaging method to the convective-conductive energy conservation equation to derive the large-scale equations with effective coefficients. The closure problems are introduced along with the closure variables to establish the closed form of the two-equation model for heat transfer of dual-scale porous media. The closure problems are numerically solved for specific cases of dual-scale porous medium consisting of packed beds of porous spherical particles. The effective coefficients appearing in the two-equation model of heat transfer in dual-scale porous media are determined using the solution of the closure problems. The velocity field in the dual-scale porous structure is calculated using the solution of the fluid flow simulations in dual-scale porous medium. Finally, "numerical experiment" is performed to qualitatively and quantitatively analyse the accuracy of the up-scaled model.The support by the Australian Research Council through Prof Wojciech Lipiński’s Future Fellowship, award no. FT14010121

A novel application for energy efficiency improvement using nanofluid in shell and tube heat exchanger equipped with helical baffles

Hydrothermal characteristics of the water–Al₂O₃ nanofluid are numerically evaluated in shell-and-tube heat exchanger equipped with helical baffles using the two-phase mixture model. Heat transfer and pressure drop increase by increasing nanoparticle concentration and baffle overlapping, and decreasing helix angle. At smaller helix angles, changing the overlapping is more effective on the convective heat transfer coefficient and the pressure drop. Neural network is used for modeling, and based on the test data, the model predicts the convective heat transfer coefficient and the pressure drop with MRE (Mean Relative Error) values of about 0.089% and 0.65%, respectively. In order to obtain conditions of effective parameters which cause maximum heat transfer along with minimum pressure drop, optimization is performed on the neural network model using both two-objective and single-objective approaches. 15 optimal states obtain from two-objective optimization. The results obtained from single-objective optimization indicate that even when a low pressure drop is significantly important for designer, nanofluids with high concentrations can be employed. Meanwhile, when both high heat transfer and low pressure drop are important, a small helix angle can be used. In addition, using large overlapping is recommended only when the heat transfer enhancement is considerably more important than the reduction of the pressure drop

Modelling of Solar Thermochemical Reaction Systems

This article reviews the progress, challenges and opportunities in numerical modelling of thermal transport, thermochemical reactions and thermomechanics in high-temperature solar thermochemical systems. Continuum-scale models are presented in mathematical detail while highlighting the literature that uses them. The discussion is enhanced by selected examples of numerical studies of solar thermochemical systems for solar fuels and commodity material production. Property predictions necessary for the modelling of solar thermochemical reaction systems are covered

Heat transfer enhancement of nanofluid flow in a tube equipped with rotating twisted tape inserts : a two-phase approach

Al2O3–water nanofluids along with stationary and rotating twisted tape inserts are used to increase the rate of heat transfer in a plain tube. The simulations are conducted through varying the design parameters including angular velocity of twisted tape, Reynolds number and nanofluid volume concentration. It is found that inserting a twisted tape inside a tube substantially increases the heat transfer coefficient and friction factor compared to the plain tube. Compared to the stationary twisted tape, the rotating twisted tape exhibits a great potential to modify the average Nusselt number by about 32.8–39.6% at Reynolds number of 250, depending on the angular velocity. This is attributed to the formation of conical tornado-shape structures in the flow pattern, causing more effective mixing in the flow. By increasing the Reynolds number, the enhancement in the average Nusselt number increases in stationary and decreases in rotating configurations compared with the plain tube. To assess the tradeoff between heat transfer enhancement and pressure loss penalty, the performance evaluation criterion (PEC) is calculated. The results suggest that the highest PEC is obtained at Reynolds number of 250, nanofluid volume concentration of 3% and the highest studied angular velocity. © 2021 Taylor & Francis Group, LLC

Topological and hydrodynamic analyses of solar thermochemical reactors for aerodynamic-aided window protection

10.1080/19942060.2022.2078883Engineering Applications of Computational Fluid Mechanics1611195-121

Progress in heat transfer research for high-temperature solar thermal applications

High-temperature solar thermal energy systems make use of concentrated solar radiation to generate electricity, produce chemical fuels, and drive energy-intensive processing of materials. Heat transfer analyses are essential for system design and optimisation. This article reviews the progress, challenges and opportunities in heat transfer research as applied to high-temperature solar thermal and thermochemical energy systems. The topics discussed include fundamentals of concentrated solar energy collection, convective heat transfer in solar receivers, application of liquid metals as heat transfer media, and heat transfer in non-reacting and reacting two-phase solid–gas systems such as particle–gas flows and gas-saturated porous structures. © 2020 Elsevier Ltd