Natural convection in vertical solar heat collector nanotextured with reduced graphene oxide and silver nanowires

In this study, a vertical shaft is coated using silver nanowires (AgNWs) together with reduced graphene oxide (rGO) via the supersonic spray-coating technique to enhance the absorption of solar radiative heat for use in solar air heaters. The rGO/AgNW-coated surface induces multiple light reflections, similar to that in a perfect black body inside a Helmholtz jar, thus enhancing the collection of solar radiation. Air fed into the vertical heated shaft ascends owing to buoyancy. The air temperature difference, ΔT, between the outlet and inlet is measured for various mass flow rates to quantify the heat transferred to the air from the solar-heated vertical shaft. The longitudinal air velocity and temperature distributions inside the shaft are numerically simulated using the fire dynamics simulator. Both two- and three-dimensional simulations are performed, and the results are compared with experimental data for various mass flow rates. The results confirm that the trends observed in both the experiments and simulations agree well for all cases. Multiple metal meshes are installed inside the vertical shaft to induce turbulence, which enhances the heat transfer intensity. This turbulence enhancement is confirmed via smoke visualization and infrared images

Theoretical, numerical, and experimental investigation of smoke dynamics in high-rise buildings

Smoke kills more people than its associated fire and thus predicting smoke spreading inside high-rise buildings is of paramount importance to structural and safety engineers. Here, the velocity, temperature, and concentration fields in large-scale turbulent smoke plumes were predicted using classical self-similar turbulent plume theory, which assumes a point fire source under open-air conditions. Turbulent fires of various heat release rates in a confined system were also simulated numerically using Fire Dynamics Simulator (FDS), which was verified against experimental data before being used to validate the analytical plume jet results. Agreements between analytical, numerical, and experimental results were accurate. This demonstrates for the first time that for realistic, wide shafts, analytical results from self-similar theory of free turbulent plumes were as accurate as the numerical simulations and appropriately described the experimental data. This allows engineers to avoid lengthy, cumbersome lengthy numerical simulations to estimate the consequences of smoke spreading in high-rise buildings using simple analytical formulae. In addition, parametric studies were conducted using plume theory for building heights up to 500 m and heat release rates up to 500 MW. Smoke velocity, temperature, and concentration fields described smoke evolution at different heights

Numerical Investigations of Smoke Dynamics in Unconfined and Confined Environments

Plume dynamics in high-rise buildings are of interest to building engineers because of the safety concerns. Herein, the temperatures, velocities, and pressures of smoke rising in buildings of various sizes as a function of the fire size were simulated using the Fire Dynamics Simulator software. The numerical results were validated against the analytical solutions for confined (building enclosure) and unconfined (open-air) systems. As the building area decreased and the fire size increased, the buoyancy-driven flow was accelerated, hence increasing the overall building temperature. In addition, the low pressure at the bottom of the building due to the buoyant smoke increased the vertical pressure gradient throughout the building. These parametric studies can be used to develop design-safety guidelines for building engineers concerned with smoke dynamics

Deflagration-to-Detonation Transition in Pipes: The Analytical Theory

Herein, we discuss the fundamental aspects of the deflagration-to-detonation transition (DDT) phenomenon in the framework of analytical theory. This semi-empirical approach facilitates prediction of the pressure rise and the shock wave speed for a given fuel type and concentration, which may be of significant interest for the design and assessment of petrochemical plants by field-safety engineers. The locally observed DDT phenomenon explored in the present experiments is also discussed, and the measured pressure rise is compared with the theoretical predictions

Deflagration-to-Detonation Transition in Pipes: The Analytical Theory

Here we discuss the fundamental aspects of the Deflagration-to-Detonation Transition (DDT) phenomenon in the framework of an analytical theory. This approach is capable to predict the pressure rise and the shock wave speed for a given fuel type and concentration, which may be of significant interest to the field-safety engineers designing and assessing petrochemical plants. A locally observed DDT phenomenon observed in the present experiments is also discussed and the measured pressure rise is compared with the theoretical predictions