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The collision of two spheroidal drops in a shear flow is simulated in this paper using the finite difference/front tracking method. The influences of deformability, initial offset, and the size of drops on their collision dynamics are assessed. It is demonstrated that the non-dimensional relative trajectory of a pair of drops, Δz/R, is enhanced gradually as they approach and then, gains a maximum value and finally, reaches a new constant value after separation. An enhancement in the capillary number results in an increase in the deformation of the drops. The deformation and the time required for the collision of two drops are reduced as their initial offset is enhanced. It is revealed that as the ratio of major diameter to minor diameter of spheroidal drops is intensified, their deformation is enhanced. [ABSTRACT FROM AUTHOR]

Combustion instability caused by the amplification of sound waves is called acoustic or high-frequency instability, which can cause severe damage to the system. Adding baffles is one of the methods of passive instability control. Depending on the geometry of the chamber and the type of application, different baffles are used. In this research, the effect of the longitudinal corrugated baffle on the acoustic characteristics of the combustion chamber is investigated numerically. The quality of each baffle configuration is determined by examining their influence on the essential parameters such as natural frequency shift and damping factor. Modal and harmonic analyses for the acoustic field are conducted to investigate the effect of baffles installed in the combustion chamber. According to the obtained results, the addition of baffle shifts resonant frequencies. In other words, a combustor with baffles is more effective in controlling the instabilities than that without baffles. In addition, it increases the damping factor in the first–second circumferential (tangential) modes, making the system more stable. Also, a quantitative assessment of the acoustics by adding baffles shows that combustion chamber finds a better condition from stability point of view, and the bandwidth increase affects the combustion stability.

Based on couple stress theory, this study investigated non-Newtonian power-law nanofluid flows in converging, nontapered, and diverging arteries. In addition to excluding gravity effects artery, geometry included mild stenosis. The momentum equation is solved via the Galerkin method, and the results are compared with experimental and classical findings. Although the power-law couple stress theory's relations are first used in the analysis of non-Newtonian blood flow, the results of this theory are far more consistent with experimental results than classical results. Comparison of the results of the study of blood flow velocity profiles in a non-tapered artery without stenosis by the mentioned theory with experimental and classical theory results shows the difference in velocity at the center of the artery between the experimental results and the results of the classical theory is 36%, while this value has been reduced to 14% for the results of the couple stress theory. The variations in velocity profile with the power-law index (n=0.8 and n=0.85) and the dimensionless Darcy number (Da=10-10 and Da=10-7) in all three geometries indicated a flat velocity distribution with the increase in the power-law index while increasing the velocity profile with increased Darcy number. Mass transfer and energy equations are solved using the extended Kantorovich method. The solution convergence is evaluated, and the influence of parameters such as Prandtl number, Schmidt number, and dimensionless thermospheric and Brownian parameters on concentration and temperature profiles is obtained. [ABSTRACT FROM AUTHOR]

In this review article, the thermal performance of dual-tube heat exchangers with smooth walls is investigated in the presence of nanofluids. Important challenges in industrial and engineering processes, such as the failure of thermal devices to respond to higher capacities, conservation, saving, and optimization of energy, have been discussed in recent years. Heat exchangers are one of the types of thermal devices that are used in a wide range of engineering and industrial applications. The use of nanofluids is one of the most effective ways to enhance the thermal conductivity of heat exchangers in the industry. In this research, the types of heat exchangers are first introduced. Then, the methods of heat transfer enhancement (active, passive, and combined) are discussed. The introduction and method of preparing nanofluids are discussed, and finally, the studies on dual-tube heat exchangers in the presence of nanofluids are described. This review article examines previous studies on dual-tube heat exchangers and the use of nanofluids in them (216 references and 73 journals). The purpose of this article is to familiarize the readers with the types of heat exchangers and to understand the mechanisms of heat transfer in the context of using nanofluids in smooth dual-tube heat exchangers. It can be concluded that nanofluids are a very good substitute for other fluids because the use of nanofluids in heat exchangers leads to an improvement in their performance, a reduction in their energy consumption and costs, a decrease in their volume, a reduction in environmental effects, etc. Eventually, the challenges in the use of nanofluids in flat dual-tube heat exchangers are discussed. The most important ones include the economic costs of using nanofluids, deposition and accumulation of nanoparticles over time, stability of nanofluids, and lack of standardization among various researches and evaluations. [ABSTRACT FROM AUTHOR]

Summary: Shading is one of the cases that has effectivity on the household energy consumption. Shadings can be used either in horizontal or vertical form adjacent to the building's window in order to control the sunlight. In this paper, a multi‐objective optimization is conducted to simultaneously reduce the annual electricity consumption and augment the electricity production with the application of photovoltaic shadings installed over the building window. To initiate the multi‐objective optimization, four design variables based on the positioning and geometry of photovoltaic shading are selected and test samples are simulated through EnergyPlus software. The model is three‐floor building located in Tehran, Iran. Next, the Morris sensitivity analysis (MSA) is implemented to calculate the extent to which each design variable affects the objective functions. Afterwards, the relation between the design variables and objective functions is extracted through the regression modeling using GMDH type‐ANN and the equations extracted are then utilized as inputs for genetic algorithm to conduct a Pareto‐based optimization. The results demonstrate that the maximum sunlight reception is attributed to the PV shading in southern direction of the building and its optimal tilt angle is 19.6 considering an annual period. There are slight changes to the tilt angle while seasonal period is considered for the calculations. The results also show that the use of solar shadings can reduce electricity consumption by approximately 4 to 11, 1 to 4, and 10 to 22% for an annual period, winter and summer season, respectively. At the end, the numerical data for the city of Tehran is compared to the cities of Canberra and Macapa. [ABSTRACT FROM AUTHOR]

In the present study, novel channel geometries in a wavy channel heat sink (HS) are investigated using ANSYS-FLUENT software. The Ag/water-ethylene glycol (50%) nanofluid is selected for cooling the CPU in this HS. The second-order upwind method is employed to discretize the momentum Equation and the SIMPLEC algorithm is employed for coupling velocity and pressure fields. Comparison of the two HSs with and without microtube shows that the presence of the microtube increases the uniformity of the CPU surface temperature distribution and decreases the mean surface temperature of the CPU (TCPU-Mean). However, the pumping power consumption of the system increases about 10 times. The results also demonstrate that the addition of nanoparticles results in intensification in the Performance Evaluation Criterion (PEC) of the system and up to 30%, especially at high Reynolds numbers. [ABSTRACT FROM AUTHOR]

A comprehensive study was conducted on how different nanofluids affect the heat transfer characteristics of a conical spiral duct with a square section. Metallic, non-metallic and Carbon nanotube (CNT) nanoparticles were assumed to be added to water as coolant in the spiral side of heat exchangers. The combined effects of nanoparticle dispersion and path curvature on heat transfer enhancement under two different thermal boundary conditions were investigated. The effects of the flow regime on heat transfer in such a configuration were tested. The flow and energy equations were solved numerically using available commercial software ANSYS FLUENT® Academic Research, Release 16.2. The numerical procedures were verified with available data, and correlation and maximum error were determined to be less than 13%. It was found that compared to the non-curved duct with the same length; heat transfer would increase by about 15%. The addition of metallic nanoparticles also enhanced the heat transfer by 5%. In low Reynolds numbers, crossflow affects temperature distribution and thermal characteristics but in the turbulent regime, the temperature distribution is less sensitive to generated crossflow. [ABSTRACT FROM AUTHOR]

Metastasis is the main origin of epithelial cancer-based mortality. Since circulating tumor cells (CTCs) are valid biomarkers to diagnose cancer, their isolation and analysis are crucial. Microfluidic technology has experienced remarkable potential to isolate CTCs due to its unique characteristics. The present paper analyzes the influences of Newtonian and non-Newtonian fluids on the continuous separation of CTCs using standing surface acoustic waves (SSAWs). The impacts of inlet velocity, oscillation amplitude, dynamic viscosity, CTC radius, power-law index, and sheath-to-sample flow velocity ratio on the isolation process are examined. The results demonstrate that the separation efficiency declines from 88% to 84% by augmenting the inlet velocity from 1.8 mm/s to 2 mm/s. It is found that for a certain value of CTC radius, the amount of sheath flow velocity can be changed to reach a maximum separation efficiency. Besides, the amount of separation efficiency decreases as the dynamic viscosity of the Newtonian fluid is enhanced and the power-law index of the non-Newtonian fluid is reduced. [ABSTRACT FROM AUTHOR]

In the present study, two cases of a microchannel heat sink are studied: i) with 50 wavy channels, and ii) with the addition of wavy tubes. Also, the effect of nanofluid Ag/water-ethylene glycol 50% is investigated. ANSYS Fluent software was used to solve the equations expressed in the problem geometry. To solve the momentum equation, the second-order UPWIND method is used. Also, the SIMPLEC algorithm with a staggered pressure grid is employed to couple velocity and pressure fields. The results show that the addition of a microtube significantly increases the overall thermal coefficient of the system because despite the microtube and having two different geometries in a heatsink at the same time, the heat exchange between the body and the fluid increases so that in a flow without a microtube with Reynolds number 300, the average surface temperature is 315 k, but the addition of a microtube reduces this temperature to 309 k0, which is equal to 6 degrees. Also, as the Reynolds number (Re) increases, the effect of increasing the concentration of nanoparticles enhances. The results demonstrate that the thermal entropy generation (Sh) decreases at high values of Re. In addition, the decrease in frictional entropy generation (Sf) due to the increase in nanoparticles is directly related to their concentration and independent of Re. So that the Percentage of decrease in friction entropy due to an increase in nanoparticle concentration relative to the pure fluid is equal to 1% for a concentration of 0.1% and 9% for a concentration of 1%. It is revealed that total entropy generation (Stat) and sf do not exhibit the same behavior. [ABSTRACT FROM AUTHOR]

The micro-scale equipment has many advantages, including high thermal performance, high surface-to-volume ratio in heat transfer, small size, low weight, low required fluid and high design flexibility. In this study, fluid flow inside a microchannel is modeled under the assumption of laminar, incompressible, and two-dimensional flow under symmetric boundary conditions. The slip boundary condition is applied to the walls and the flow in the channel output is assumed to be fully developed. The effect of sinusoidal wall with the domain of 0.1 on the hydrodynamic and thermal behavior of the fluid is investigated and the results are compared with the results of smooth wall. The results show that for a constant Reynolds number, the maximum velocity decreases in the microchannel center by increasing the slip coefficient. Also, the comparison between the results of the wavy-wall microchannel and the microchannel with a smooth wall indicates that the heat transfer in the smooth microchannel is less than that in wavy-wall one. Considering the boundary conditions, the thermal behavior of the fluid is approximately the same for two cases in which both walls are sinusoidal and the only upper wall is sinusoidal. [ABSTRACT FROM AUTHOR]

In this study, air-water two-phase flow patterns in a two-dimensional vertical helical channel are investigated. The simulations are performed using OpenFOAM software. The single-fluid method and Eulerian−Eulerian approach are used to solve the governing equations. The paper focuses on different flow patterns based on the change in the apparent velocity of water and air, and the influence of volume fraction of the air at the entrance of the channel. The variations of turbulent kinetic energy, volume fraction and velocity profile are presented. The range of air and water apparent velocities at the channel entrance is 0.05–10 m s−1 and 0.1–1.5 m s−1, respectively. According to the results, four flow patterns including bubbly, churn, turbulent, and annular are observed. It is demonstrated that for the bubbly regime, the values of volume fraction of phases are close to unity. When the air inlet velocity is very high, the two-phase flow forms the annular flow pattern. [ABSTRACT FROM AUTHOR]

Phase Change Materials (PCMs) have been the subject of many researches in recent years due to the storage and release of energy at low temperature ranges. PCMs store or releasing a large amount of energy at a constant temperature range leads to saving energy. In this paper, the numerical modelling of a multilayer composite wall including PCM located on the southern side of a building is carried out using an implicit method. The data correspond the fifteenth day of each month in Tehran. The governing equations are discretized by the implicit Crank Nicolson method and solved by iteration method using MATLAB software. Finally, the location and volume fraction of PCM in the wall of the building are studied to achieve maximum efficiency. The results show that the effect of latent and sensible heat results in a reduction in the input heat flux and thermal load to the building. The optimum location for the PCM layer is the middle layer of the composite wall to reduce the heat transfer rate inside the building. In addition, it is found that the PCM volume fraction in gypsum does not have a significant effect on the thermal performance of the multilayer composite wall. Hence, low volume fraction reduces the costs without affecting the thermal performance of the building. [ABSTRACT FROM AUTHOR]

In the present paper, the constructal theory is employed to determine the optimal configuration of three rows of needle-shaped fins. The heat transfer across the fins is due to laminar forced convection. Second order upwind scheme is used for discretization of the diffusion terms of governing equations. The pressure-velocity coupling is performed using the SIMPLE algorithm. The heat transfer is optimized subject to constant fin volume. The effect of Reynolds number and thermal conductivity on the optimal configuration is investigated. The results obtained from the present simulations are in good agreement with the numerical results. The results show that pin-fins flow structure leads to the best performance when the pin-fin diameters and heights are non-uniform. At Re = 100 and 200, the optimal value of H3/H1is 1.3. It is revealed that at Re = 50, the optimal value for D3/D1 is approximately 1.1. The results demonstrate that heat transfer rate is an increasing function of the Reynolds number. [ABSTRACT FROM AUTHOR]