A numerical investigation on ethylene glycol-titanium dioxide nanofluid convective flow over a stretching sheet in presence of heat generation/absorption

In this article, TiO2-ethylene glycol nanofluid flow over a porous stretching sheet in presence of non-uniform generation or absorption of heat and convective boundary condition is investigated. The concentration of solute is set by the means of an isothermal model of homogeneous-heterogeneous reactions. The governing equations were simplified to ordinary differential equations and solved using Runge-Kutta-Fehlberg shooting method of fifth order. Effects of different variables such as nanoparticle volume fraction, porosity variable, and Schmidt number were studied and the results are graphically presented. The results showed that the stretching rate ratio has inverse effect of velocities in both directions. According to plots, nanoparticle volume fraction as well as convective heat intensity has a direct relation with wall heat flux, in the contrary, heat generation has an inverse effect on it. Keywords: Ethylene glycol, Titanium dioxide, Heat generation/absorption, Nonlinear stretching shee

Multi-Objective Optimization of a Small Horizontal-Axis Wind Turbine Blade for Generating the Maximum Startup Torque at Low Wind Speeds

Generating a high startup torque is a critical factor for the application of small wind turbines in regions with low wind speed. In the present study, the blades of a small wind turbine were designed and optimized to maximize the output power and startup torque. For this purpose, the chord length and the twist angle were considered as design variables, and a multi-objective optimization study was used to assess the optimal blade geometry. The blade element momentum (BEM) technique was used to calculate the design goals and the genetic algorithm was utilized to perform the optimization. The BEM method and the optimization tools were verified with wind tunnel test results of the base turbine and Schmitz equations, respectively. The results showed that from the aerodynamic viewpoint, the blade of a small wind turbine can be divided into two sections: r/R r/R ≥ 0.52, where most of the turbine power is generated. By increasing the chord length and twist angle (especially chord length) in the r/R r/R ≥ 0.52 part, a 140% rise in the startup torque of the designed blade was observed with only a 1.5% reduction in power coefficient, compared with the base blade. Thereby, the startup wind speed was reduced from 6 m/s for the base blade to 4 m/s for the designed blade, which provides greater possibilities for the operation of this turbine in areas with lower wind speeds

Compound Heat Transfer Augmentation of a Shell-and-Coil Ice Storage Unit with Metal-Oxide Nano Additives and Connecting Plates

Due to the high enthalpy of fusion in water, ice storage systems are known as one of the best cold thermal energy storage systems. The phase change material used in these systems is water, thus it is inexpensive, accessible, and completely eco-friendly. However, despite the numerous advantages of these systems, the phase change process in them is time-consuming and this leads to difficulties in their practical application. To solve this problem, the addition of nanomaterials can be helpful. This study aims to investigate the compound heat transfer enhancement of a cylindrical-shaped unit equipped with double helically coiled coolant tubes using connecting plates and nano additives as heat transfer augmentation methods. Complex three-dimensional numerical simulations are carried out here to assess the best heat exchanger material as well as the impact of various nanoparticle types, including alumina, copper oxide, and titania, and their concentrations in the PCM side of the ice storage unit. The influence of these parameters is discussed on the charging rate and the temperature evolution factor in these systems. The results suggest that using nano additives, as well as the connecting plates, together is a promising way to enhance the solidification rate by up to 29.9%

Utilization of Carbon-Based Nanomaterials and Plate-Fin Networks in a Cold PCM Container with Application in Air Conditioning of Buildings

Cold energy storage devices are widely used for coping with the mismatch between thermal energy production and demand. These devices can store cold thermal energy and return it when required. Besides the countless advantages of these devices, their freezing rate is sluggish, therefore researchers are continuously searching for techniques to improve their operating speed. This paper tries to address this problem by simultaneously combining a network of plate fins and various types of carbon-based nanomaterials (NMs) in a series of complex computational fluid dynamics (CFD) simulations that are validated by published experimental results. Horizontal, vertical, and the combination of these two plate-fin arrangements are tested and compared to the base model. Subsequently, several carbon-based NMs, including SWCNT, MWCNT, and graphene-oxide NMs are utilized to further improve the process. The influence of these fin networks, nanoparticle types, and their volume- and mass-based concentrations within the PCM container are studied and discussed. According to the results, carbon-based NMs exhibit superior performance compared to metal-oxide NMs, so that at identical NM volume and mass fractions, MWCNT particles present a 2.77% and 17.72% faster freezing rate than the CuO particles. The combination of plate-fin network and MWCNT particles is a promising technique that can expedite the ice formation rate by up to 70.14%

Thermal management of shelter building walls by PCM macro-encapsulation in commercial hollow bricks

Maintaining building indoor temperature within comfort zone has been identified as one of the major reasons for energy consumption. The incorporation of phase change materials (PCMs) in various regions in buildings is introduced as a solution that can significantly reduce energy consumption. In the current research, a three-dimensional computational fluid dynamics approach is taken to evaluate the influence of incorporating three PCMs from the Rubitherm® RT line (RT15, RT18, and RT22) in two widespread commercial hollow brick types in Iran for keeping the indoor environment of a shelter warm in a cold climate. These enhanced bricks are compared with their solid and hollow counterparts. Furthermore, efforts were made to examine various configurations of these PCMs, with melting temperatures in obedience or contrast with the thermal stratification of the bricks. Several combinations of the PCM with air layers are studied to assess the best way to place the air and PCM layers. It was found that the RT18 PCM-filled brick can present lower heat fluxes than the hollow one for over 42 h. Also, the findings showed that RT22 PCM-filled brick can provide 66.79% lower average heat flux compared to the hollow brick for the first 24 h of the process