Analysis of unsteady mixed convection of Cu–water nanofluid in an oscillatory, lid-driven enclosure using lattice Boltzmann method

The unsteady physics of laminar mixed convection in a lid-driven enclosure filled with Cu–water nanofluid is numerically investigated. The top wall moves with constant velocity or with a temporally sinusoidal function, while the other walls are fixed. The horizontal top and bottom walls are, respectively, held at the low and high temperatures, and the vertical walls are assumed to be adiabatic. The governing equations along with the boundary conditions are solved through D2Q9 fluid flow and D2Q5 thermal lattice Boltzmann network. The effects of Richardson number and volume fractions of nanoparticles on the fluid flow and heat transfer are investigated. For the first time in the literature, the current study considers the mechanical power required for moving the top wall of the enclosure under various conditions. This reveals that the power demand increases if the enclosure is filled with a nanofluid in comparison with that with a pure fluid. Keeping a constant heat transfer rate, the required power diminishes by implementing a temporally sinusoidal velocity on the top wall rather than a constant velocity. Reducing frequency of the wall oscillation leads to heat transfer enhancement. Similarly, dropping Richardson number and raising the volume fraction of the nanoparticles enhance the heat transfer rate. Through these analyses, the present study provides a physical insight into the less investigated problem of unsteady mixed convection in enclosures with oscillatory walls

Special topic on turbulent and multiphase flows

International audienc

Special Issue on Multiphase and Turbulent Flows in Energy Engineering Applications

International audienc

Diesel-fired boiler performance and emissions measurements using a combination of diesel and palm biodiesel

Biodiesel is widely accepted as a fuel that is similar to diesel with various advantages. Biodiesel's low-temperature flow qualities are one of its characteristics that limits its use. The goal of this study was to see how volumetric blends of palm biodiesel and diesel, and diesel as a fuel affected the performance and emissions characteristics of a diesel fired vertical coil type, water tube, and non IBR boiler. Various volumetric blends were prepared like PB25, PB50, PB75, PB100 and test in diesel fired boiler with variation in injection pressure. Performance of PB25, PB50, PB75, and PB100 fuels was observed 62.73%, 62.45%, 62.36%, and 62.32%, respectively, compare to pure diesel the value of all blends is either slightly higher or comparative. The maximum boiler efficiency with B100 fuel is 64.98%, which is lower than the pure diesel as fuel 65.30%. Because B100 has a higher kinematic viscosity, it has a larger droplet diameter which lead to poor spray formation and thus a lower boiler efficiency. At 11 bar fuel injection pressure, maximum EGT for diesel, PB25, PB50, PB75, and PB100 fuels is 300 °C, 295 °C, 308 °C, 328 °C, and 340 °C, respectively. Other blends, with the exception of B25, have higher EGT than diesel fuel. At a same fuel injection pressure of 11 bar, CO emissions from diesel, B25, B50, B75, and pure palm biodiesel fuels are 0.037%/Vol., 0.0336%/Vol., 0.0326%/Vol., 0.033%/Vol., and 0.036%/Vol., respectively. CO emissions for PB50 are the lowest of all the fuels tested, followed by B25, diesel, and B100. CO emissions from diesel, PB25, PB50, PB75, and PB100 fuels at maximum fuel pressure are 0.0605%/Vol., 0.0616%/Vol., 0.0605%/Vol., 0.060%/Vol., and 0.05%/Vol., respectively. When compared to diesel fuel, CO emissions from B100 fuel are 21% higher. The highest HC emissions are 18 ppm, 16 ppm, 14 ppm, 13 ppm, and 12 ppm for diesel, PB25, PB50, PB75, and PB100 fuel, respectively. When utilizing B100 fuel, HC emissions are reduced by around half compared to when using diesel fuel.This work was carried by the NPRP grant # NPRP11S-1221-170116 from the Qatar National Research Fund (a member of Qatar Foundation ). The statements made herein are solely the responsibility of the authors and the publication of this article was funded by the Qatar National Library.Scopu

Numerical study on charging process inside a grid-structure thermal storage

Heat transfer enhancement in different engineering systems is a major challenge nowadays. Phase change ma-terials can be used for this purpose due to moderate phase change temperature, great thermal capacity, and great latent melting energy. The melting process of phase change materials that can be used in the management and reserving solar energy has been studied numerically in various grided solar storages. The novelty of this research is a numerical analysis of the phase change process in a grid structure with different small cells having imper-meable heat-conducting walls. The characteristic equations including continuity equation, motion, and energy equations were solved by the finite element method as the melting process was developing. The influence of grid sizes and thermal conductivity of the solid material of thermal storage partitions on the fluid flow and heat transfer performance was studied. The heat transfer rate in a thermal storage system filled with the phase change material depends on the grid sizes. Moreover, a growth of the abovementioned small cells filled with phase change material reflects a diminution of the convective flow rate and as a result, the charging time increases. At the same time, a reduction of the inner solid walls heat conductivity characterizes a significant raise of the system charging time. As a result, it is possible to find optimal structure of such a storage system with effective energy transport parameters

Numerical simulations of ultra-low-Re flow around two tandem airfoils in ground effect: isothermal and heated conditions

The advent of pico-aerial vehicles (PAVs) for thermal surveillance has necessitated a better understanding of the flow field around airfoils at ultra-low Reynolds numbers (102 to 103). Previous studies have shown that two airfoils arranged in a tandem configuration can exhibit better aerodynamic performance than two identical airfoils in isolation, but this improvement has only been confirmed at relatively high Reynolds numbers (105 and above). In this parametric study, we numerically simulate the two-dimensional flow field around two tandem NACA 0012 airfoils in ground effect, at a Reynolds number low enough to be relevant to PAVs (Re = 500). With the angle of attack fixed at α = 5° on both airfoils, we investigate the effects of three control parameters, namely the stagger distance, the gap height and the ground clearance, for both isothermal airfoils and fore-heated airfoils. Results show that consistent with previous studies at higher Re, two tandem airfoils are more aerodynamically efficient than two identical airfoils in isolation, especially when the gap height is positive, i.e., when the fore airfoil is higher than the aft airfoil. The aerodynamics of the tandem-airfoil system are strongly influenced by the airfoil-to-airfoil interference arising from the downwash generated by the fore airfoil. The presence of a laminar separation bubble on the suction surface of both airfoils is found to alter the lift and drag coefficients as well as the overall lift-to-drag ratio. The wake of the fore airfoil is often seen impinging on the aft airfoil, which is a key mechanism by which the lift and drag forces are altered. The gains in aerodynamic efficiency achieved by the tandem airfoils become smaller as the stagger distance increases owing to weakened airfoil-to-airfoil interference. The effect of ground clearance on the tandem airfoils is found to be similar to that on two isolated airfoils, with both the lift and drag coefficients increasing with decreasing ground clearance. Heating the fore airfoil of a tandem-airfoil system in ground effect is found to decrease the lift coefficient without much affecting the drag coefficient, resulting in a drop in the lift-to-drag ratio. Overall these results lend new insight into the ultra-low-Re aerodynamics of tandem airfoils under both isothermal and heated conditions, advancing the development of the next generation of PAVs for thermal surveillance and other assorted applications

Predicting the effects of environmental parameters on the spatio-temporal distribution of the droplets carrying coronavirus in public transport – a machine learning approach

Human-generated droplets constitute the main route for the transmission of coronavirus. However, the details of such transmission in enclosed environments are yet to be understood. This is because geometrical and environmental parameters can immensely complicate the problem and turn the conventional analyses inefficient. As a remedy, this work develops a predictive tool based on computational fluid dynamics and machine learning to examine the distribution of sneezing droplets in realistic configurations. The time-dependent effects of environmental parameters, including temperature, humidity and ventilation rate, upon the droplets with diameters between 1 and 250μm are investigated inside a bus. It is shown that humidity can profoundly affect the droplets distribution, such that 10% increase in relative humidity results in 30% increase in the droplets density at the farthest point from a sneezing passenger. Further, ventilation process is found to feature dual effects on the droplets distribution. Simple increases in the ventilation rate may accelerate the droplets transmission. However, carefully tailored injection of fresh air enhances deposition of droplets on the surfaces and thus reduces their concentration in the bus. Finally, the analysis identifies an optimal range of temperature, humidity and ventilation rate to maintain human comfort while minimising the transmission of droplets