The paper studies the dynamics of the classical susceptible-infectious-removed (SIR) model when applied to the transmission of COVID-19 disease. The model includes the classical linear incidence rate but considers a nonlinear removal rate that depends on the hospital-bed population ratio. The model also includes the effects of media on public awareness. We prove that when the basic reproduction number is less than unity the model can exhibit a number of nonlinear phenomena including saddle-node, backward, and Hopf bifurcations. The model is fitted to COVID-19 data pertinent to Saudi Arabia. Numerical simulations are provided to supplement the theoretical analysis and delineate the effects of hospital-bed population ratio and public awareness on the control of the disease.

The use of solar energy for cooling processes is advantageous for reducing the energy consumption of conventional air-conditioning systems and protecting the environment. In the present work, a solar-powered cooling system with parabolic trough collectors (PTC) and a phase change material (PCM) tank is numerically investigated in the arid climates of Saudi Arabia. The system contains a 160-kW double-effect absorption chiller powered by solar-heated pressurized water as a heat transfer fluid (HTF) and a shell and tube PCM as a thermal battery. The novelty of this paper is to investigate the feasibility and the potential of using a PTC solar field coupled to a PCM tank for cooling purposes in arid climates. The numerical method is adopted in this work, and a dynamic model is developed based on the lumped approach; it is validated using data from the literature. The functioning of the coupled system is investigated in both sunshine hours (charging period) and off-sunshine hours (discharging period). The PTC area in this work varies from 200 m² to 260 m² and the cooling capacity of the chiller ranges from 120 kW to 200 kW. Obtained results showed that the 160-kW chiller is fully driven by the 240 m²-solar PTC during the charging period and about 23% of solar thermal energy is stored in the PCM tank. It was demonstrated that increasing the PTC area from 220 m² to 260 m² leads to a reduction in the PCM charging time by up to 45%. In addition, it was found that an increase in the cooling loads from 120 kW to 200 kW induces a decrease in the stored thermal energy in the PCM tank from 450 kWh to 45 kWh. During the discharging period, the PCM tank can continue the cooling process with a stable delivered cooling power of 160 kW and an HTF temperature between 118 °C and 150 °C. The PCM tank used in the studied absorption chiller leads to a reduction of up to 30% in cooling energy consumption during off-sunshine hours.

The processing of fine and ultrafine particles using a fluidized bed is challenging in view of their unpredictable hydrodynamic behavior due to interparticle forces. The use of assisted fluidization techniques in such cases can be effective in improving the bed hydrodynamics. This work investigates the dynamics of pulsed fluidized bed of ultrafine nanosilica subjected to square-wave flow pulsations. The pulse duration used in this study is sufficient to allow the complete collapse of the pulsed fluidized bed between two consecutive flow pulsations. The proposed pulsation strategy is carefully implemented using electronic mass flow controllers with the help of analog output signals from data acquisition system. Given that the different regions of the fluidized bed exhibit varying dynamics, which together contribute to overall bed dynamics, the bed transients in the upper, central, and lower regions of the fluidized bed are monitored using several sensitive pressure transducers located along the height of the bed. The effect of the flow pulsation on the hydrodynamics of the fluidized bed is rigorously characterized. A significant reduction in the minimum fluidization velocity was obtained and an increase in the bed homogeneity was observed due to flow pulsations. The frequency domain analysis of the signals clearly delineated the frequency of the various events occurring during the fluidization.

Decoration of 2D semiconductor structures with heterogeneous metal quantum dots has attracted considerable attention due to advanced optical, electrical, and catalytic properties that result from the large surface-to-volume ratio associated with these structures. Herein, we report on silver quantum dot decorated 2D SnO₂ nanoflakes for the photocatalytic abatement of water effluents, the synthesis of which was achieved through a straightforward and mild hydrothermal procedure. The photocatalysts were systematically investigated using UV–Vis, XRD, electron microscopy (SEM, HR-TEM), EDX, XPS and FTIR. The photocatalytic activity of the nanostructures was evaluated for the abatement of water pollutant rhodamine B (RhB), under light irradiation. The mild hydrothermal synthesis (100 °C) proved highly efficient for the production of large scale Ag quantum dot (QD)/SnO₂ nanoflakes for a novel photocatalytic application. The decoration of SnO₂ with Ag QDs significantly enhances the synergetic charge transfer, which diminishes the photo-induced electron-hole reunion. Moreover, the plasmonic effect from Ag QDs and 2D-SnO₂ structures acts as an electron tank to collect the photo-induced electrons, generating a Schottky barrier between the SnO₂ structures and quantum dots. Overall, this resulted in a facile and efficient degradation of RhB, with a rate double that of pristine SnO₂.

We have developed a new particle-mixing strategy for improving the fluidization hydrodynamics of Geldart group C powders by mixing with small proportions of group B particles. Two different group C particles with widely different physical properties, i.e., 1 μm calcium hydroxide powder and 27 μm porous activated carbon, were selected for investigation in the present work. A carefully sieved sample of inert sand was used as external group B particles for mixing. Fluidization experiments were carried out, and the quality of the fluidization was assessed using the fluidization index. For the monocomponent fluidization of fine calcium hydroxide powder, pressure drop was sometimes as much as 250% higher than the effective weight of the bed. The proposed strategy of particle mixing substantially improved its fluidization hydrodynamics. On the other hand, the development of channels and cracks during the monocomponent fluidization of the activated carbon led to gas bypassing, resulting in low pressure drop and poor contact of phases. Particle mixing was found to improve fluidization behavior, and the chi-squared test showed that the best results were obtained with 13 wt% particle mixing.