Assessing the wall effects of packed concentric cylinders and angular walls on granular bed porosity

The effect of added wall support on granular bed porosity is systematically studied to elucidate performance enhancements in filtration processes achieved by using inserts, as demonstrated experimentally (Bandelt Riess et al. in Chem Eng Technol 2018, 2021). Packed beds of spheres are simulated through discrete element method in cylinders with different internal wall configurations. Three containing systems are generated: concentric cylinders, angular walls, and a combination of both. Variations of particle size and wall friction and thickness are also considered, and the resulting granular bed porosities are analyzed. The porosity increase is proportional to the incorporated wall support; the combination of cylindrical and angular inserts displays the greatest effect (up to 26% increase). The sinusoidal porosity values near the walls are exhibited to clarify the effects. The presented method can change and evaluate granular bed porosity increments, which could lead to filtration process improvements, and the obtained behaviors and profiles can be used to explore additional effects and further systems. Graphical abstract

Drying of Sisal Fiber: A Numerical Analysis by Finite-Volumes

Vegetable fibers have inspired studies in academia and industry, because of their good characteristics appropriated for many technological applications. Sisal fibers (Agave sisalana variety), when extracted from the leaf, are wet and must be dried to reduce moisture content, minimizing deterioration and degradation for long time. The control of the drying process plays an important role to guarantee maximum quality of the fibers related to mechanical strength and color. In this sense, this research aims to evaluate the drying of sisal fibers in an oven with mechanical air circulation. For this purpose, a transient and 3D mathematical model has been developed to predict moisture removal and heating of a fiber porous bed, and drying experiments were carried out at different drying conditions. The advanced model considers bed porosity, fiber and bed moisture, simultaneous heat and mass transfer, and heat transport due to conduction, convection and evaporation. Simulated drying and heating curves and the hygroscopic equilibrium moisture content of the sisal fibers are presented and compared with the experimental data, and good concordance was obtained. Results of moisture content and temperature distribution within the fiber porous bed are presented and discussed in details. It was observed that the moisture removal and temperature kinetics of the sisal fibers were affected by the temperature and relative humidity of the drying air, being more accentuated at higher temperature and lower relative humidity, and the drying process occurred in a falling rate period.

A Critical Review on Thermo-Hydraulic Performance of Wire Screen Matrix Packed Solar Air Heaters

Solar air heaters (SAHs) have an important role in applications such as space heating and industrial drying worldwide. The packing of SAH bed not only increases the heat transfer area but also increases the pumping power losses thereby limiting the thermo-hydraulic performance. In the present study, efforts have been made for a critical assessment of the literature dealing with the impact of collector bed and operating parameters over thermal and thermo-hydraulic performance for different configurations of wire screen matrix packed SAH. The porosity of bed and mass flow rate of the air have a major influence on the thermo-hydraulic performance of wire screen matrix packed SAH. It is found that the enhancement in the volumetric heat transfer coefficient due to a decrease in bed porosity is obtained at the expense of increase in pumping power which ultimately affects the thermo-hydraulic performance of wire screen matrix packed SAH. In general it is observed that porosity is an important parameter that affects the thermo-hydraulic performance. It is seen that matrix having porosity 0.937 yields thermo-hydraulic performance of 68% at mass flow rate 0.023 kg/s where as for the same mass flow rate porosity of 0.887 results thermo-hydraulic performance of only 42%.

Producing Steam by Spraying Water on a Heated Bed of Steel Spheres

Steam generators used in industrial baking ovens operate by pouring or spraying water on a preheated thermal mass. This paper presents a methodology to quantify the amount of steam generated from a thermal mass along with experiments to determine the effect of particle size and porosity on steam generation. Three sizes of steel spheres, 0.6 mm, 8 mm, and 16 mm in diameter, were used to construct porous media beds that were preheated in an oven after which water was sprayed onto them from a full-cone nozzle for a fixed duration. The weight of the heated bed and the impinging water were recorded during spraying. The difference in weight change when spraying on heated and unheated beds gave the rate of evaporation. Thermocouples were used to record the internal temperature of the bed. Steam generation rate increased with particle size while bed porosity had only a minor influence. The counter-current flow of steam within the media bed disrupts the downward flow of water enough to leave pockets of hot material, reducing steam production. To maximize steam generation the media size, material, and spray time should be matched to ensure the surfaces of particles remain above the boiling point of water during spraying.

The Effects of Periodicity Assumptions in Porous Media Modelling

The effects of periodicity assumptions on the macroscopic properties of packed porous beds are evaluated using a cascaded Lattice-Boltzmann method model. The porous bed is modelled as cubic and staggered packings of mono-radii circular obstructions where the bed porosity is varied by altering the circle radii. The results for the macroscopic properties are validated using previously published results. For unsteady flows, it is found that one unit cell is not enough to represent all structures of the fluid flow which substantially impacts the permeability and dispersive properties of the porous bed. In the steady region, a single unit cell is shown to accurately represent the fluid flow across all cases studied

Topology optimization of a packed bed microreactor involving pressure driven non-Newtonian fluids

Multivariable optimization is an important task for a microreactor to operate with better control and efficacy. The bed porosity in a packed bed microreactor is one of the key parameters...

Computational Study of Heat Transfer Behavior in Fluid-Solid Fluidized Beds.

Heat transfer in fluid-solid fluidized beds is investigated using a combined of computational fluid dynamics (CFD) and discrete element method (DEM) approach, incorporated with a thermal model. The approach has taken into account almost all the mechanisms in heat transfer in fluidized beds. A comparison and validation of hydrodynamic and thermal data of fluidized bed obtained using CFD-DEM thermal approach with experimental and numerical results data in the literature is carried out. The simulations results reveal a good thermal steady state during the simulation time for calculating the thermal behaviors of fluidized beds like; the mean particle temperature, bed porosity, heat transfer coefficient and mean particle Reynolds number. The simulations results are showed a good agreement and consistency with the experimental and numerical data in the literatures. Thus, the integration of combined CFD-DEM with the thermal model is a step toward for the prediction, development the heat transfer efficiency in fluid-solid system, and the decrease of energy consumption of the industrial applications.