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In the present study, entropy generation in a non-axisymmetric steady-state incompressible viscous flow over a single rotating porous disk is investigated analytically. The dimensionless form of the local and total entropy generation caused by frictional and thermal effects are derived. The effects of the injection rate and Brinkman number on the entropy generation are revealed. It is found that there exists an optimal injection rate for each Brinkman number, which minimizes the total entropy generation. [ABSTRACT FROM AUTHOR]

This numerical investigation aims to study the turbulent characteristics and thermal enhancement parameter of CuO–water nanofluids through heat exchangers enhanced with double V-cut twisted tapes. The twist ratio of the twisted tapes is 5.25, and the cut ratio (b/c) is varied from 0 (conventional twisted tape) to 1.8. The Reynolds number is in the range of 5000–15,000, and nanoparticles volume fraction is in the range of 0 < ϕ < 1.5 % . The flow is fully turbulent, and (RNG) k − ϵ turbulent model is used for the numerical analysis. The results reveal that strong turbulent kinetic energy and additional vortex flow through the cuts of the modified twisted tapes are the main reason for better fluid mixing and heat transfer enhancement. The heat transfer enhances about 14.5% for the case of ϕ = 1.5 % . Furthermore, using double V-cut twisted tapes improves the Nusselt number of the nanofluid flow inside heat exchangers about 138% compared to conventional twisted tape without cuts. The maximum value of the thermal performance η = 1.99 is achieved by using nanofluid with ϕ = 1.5 % and b/c = 1.8 at Re = 5000. [ABSTRACT FROM AUTHOR]

A large portion of world's final energy consumption is employed in thermal form. The proper techniques can be used to improve the efficiencies of thermal energy systems and decrease the energy consumption. Phase change material (PCM) storage modules can be coupled with different thermal systems to improve their thermal efficiency. This paper reviews the real potentials of coupling PCM storage modules to heat pipes and heat pumps in terms of energy saving and thermal energy management. The different systems suggested by researchers are described, and the main findings of the studies are provided. The advantages and disadvantages of these combined systems are discussed, and some suggestions for future studies are provided. The results show that heat pipes show great heat transfer ability and have good efficiency in improving heat transfer within PCMs. Moreover, heat pipes can be incorporated into PCMs to decrease the thermal resistance. The purpose of PCM coupled with heat pipe is to incorporate the large thermal conductivity of heat pipe with the large latent heat capacity of PCM. PCMs have ability to balance the unconformity between supply and demand in heat pumps and decrease the size of storage tank used in a heat pump module. [ABSTRACT FROM AUTHOR]

Entropy generation analysis for the Cu–water nanofluid flow through a heat exchanger tube equipped with perforated conical rings is numerically investigated. Frictional and thermal entropy generation rates are defined as functions of velocity and temperature gradients. Governing equations are solved by using finite volume method, and Reynolds number is in the range of 5000–15,000. The effects of geometrical and physical parameters such as Reynolds number, number of holes and nanoparticles volume fraction on the thermal and viscous entropy generation rates and Bejan number are investigated. The results indicate that the thermal irreversibility is dominant in most part of the tube. But it decreases with increasing the nanoparticle volume fraction. Frictional entropy generation reduces with increasing the number of holes from 4 to 10. This is because of stronger velocity gradient near the perforated holes. Bejan number decreases with augment of Reynolds number. [ABSTRACT FROM AUTHOR]

In this paper, a novel study on the heat transfer characteristics in a shell and tube heat exchanger with wavy cosine corrugated wall in structure of tube bundle is investigated experimentally. The optimization procedure is performed by means of response surface methodology in different wavy starting lengths (0 mm ≤ b ≤ 120 mm), hot water flow rates (7 L min−1 ≤ Qh ≤ 11 L min−1), cold water flow rates (11 L min−1 ≤ Qc ≤ 19 L min−1) and wavy wavelengths (0 mm ≤ λ ≤ 80 mm) to obtain maximum effectiveness and the overall heat transfer coefficient. An exergetic sustainability analysis has been done to show how exergy efficiency affects sustainability. The results show that the effectiveness and the overall heat transfer coefficient increase with the cold water flow rates. Additionally, enhancement of the wavy starting lengths decreases the heat exchanger effectiveness and the overall heat transfer coefficient. From sustainability point of view, the corrugated tube is more sustainable than smooth tube. [ABSTRACT FROM AUTHOR]

Two-phase liquid flow in micro-/mini-channels has several thermal advantages in comparison with single-phase liquid flow, such as heat transfer rate, thermosiphon effect and isothermal heat sinks. Multiphase flows in microfluidic devices are usually used as an effective way to improve the cooling process, since phase change process can provide high heat transfer coefficient in comparison with single-phase flows. On the other hand, high surface-to-volume ratio of microheat exchangers can intensify the overall heat rejection from the system. In the current research, three models, namely (a) the formation of interior and exterior phase, (b) hydrodynamics of droplets during condensation process in a diffusive oblique fin (mini-channel) with hydrophilic walls and (c) liquid evacuation from the channel, are investigated by means of a novel perspective of pseudo-potential lattice Boltzmann model. The effects of channel slope angle on velocity profiles and outlet mass flow rate of the condensed liquid are analyzed. The results show that the slope angle plays an important role at the rate of droplet nucleation and on evacuation time. It is observed that the variation of liquid profile at the channel outlet can be neglected at angles more than 45°. Finally, it is revealed that the required time for droplets coalescence and discharging liquid from the mini-channel can be controlled by optimizing the channel angle. [ABSTRACT FROM AUTHOR]

In this paper, the magnetic nanofluids and magnetic field are used to provide a coating around the wall of a channel. The magnetic field is induced by the direct current wire. Iron oxide is used as magnetic nanoparticles. A finite volume method is used to solve the Navier–Stokes equations, and the Eulerian–Lagrangian approach is employed to track the magnetic nanoparticles. The effects of magnetic strength, the position of current wire, and the diameter of magnetic nanoparticles on the trajectory of magnetic nanoparticles and coating efficiency are investigated by providing contours and diagrams. The results show that the length of coating decreases by about 55% with the increase in the particle diameter in the range of 500 nm to 1 μm. Further, the coating efficiency, defined as the ratio of the number of trapped particles on the wall to the number of injected particles at the inlet of the channel, improves by increasing the magnetic strength and decreasing the vertical position of the current wire. [ABSTRACT FROM AUTHOR]

In this study, a fluid-structure interaction (FSI) simulation of the blood flow in the femoral artery with a small occlusion is presented. For a more accurate simulation of the real conditions, computerized tomography (CT) scan was used to obtain a 3-D model of leg blood vessels, while the vessel was modeled as an isotropic elastic wall. By assuming a heartbeat period of 0.5 s, the inlet condition was considered as a time-dependent pulse using a non-Newtonian flow model. Blood flow was assumed nonlinear and incompressible, and Carreau model was used for blood rheological model. By considering unstable blood flow at the inlet, the involved hemodynamic parameters are velocity profile, vortices shapes, pressure drop, and streamlines. Furthermore, to determine the relationship between flow geometry and the vascular wall, wall shear stress (WSS) was calculated. By taking the real geometry of the vessel and fluidity of blood into account, comparison of computational results indicated a significant difference in velocity distribution and shear stress depending on whether the fluid-structure interaction is considered Newtonian or non-Newtonian. The results showed that employing Newtonian models for the blood flow does not lead to promising results at occluded areas and beyond them. [ABSTRACT FROM AUTHOR]

The present work investigates the natural convection of nanofluids in Rayleigh-Benard problem. New models, particularly some enhancement factors, are presented in this research to calculate the heat transfer characteristics of nanofluids around critical Rayleigh numbers, including the criteria in which the creation of Rayleigh-Benard and transition to turbulent flow occurs. The present models are compared with available models in the literature to reveal discrepancies of theoretical models in prediction of nanofluids heat transfer characteristics. This research is conducted for SiO2, Cu,CuO and Al2O3 as nanoparticle and water as the base fluid and discussed the SiO2-water nanofluid in more detail. Results indicate that the Nusselt number and convective heat transfer ratio are reduced for SiO2-water nanofluid for almost all volume fractions and as a result using SiO2 as nanoparticle into water (the base fluid) is not desired for a natural convection in Rayleigh-Benard problem. Moreover, the onset of natural convection is delayed due to the reduction in enhancement factor. [ABSTRACT FROM AUTHOR]

This paper investigates condensation process and liquid reaction on a moving square shape micro-object by lattice Boltzmann method. Micro-object with a horizontal velocity toward the left side of the domain is considered. Both condensation and evaporation processes are included in the simulations for three cases. For first case, it is assumed that there is no liquid on micro-object walls at initial time and appearance of liquid is only due to the condensation. For second, and third cases, a drop is considered initially on upper, and back walls of the micro-object, respectively. The effects of initial position of drop and velocity of micro-object on liquid reaction and condensation process are investigated. It is found that for all values of micro-object velocity, the difference between three cases for liquid reaction parameter will be negligible over time. Moreover, the liquid reaction grows faster for high values of micro-object velocity. [ABSTRACT FROM AUTHOR]

This paper is devoted to study a two-dimensional airfoil oscillating in pitch and plunge degrees of freedom. A nonlinear analysis is performed to investigate the effects of a discontinuous freeplay nonlinearity in pitch on the response of the airfoil system. In fact we show that in the presence of freeplay, the air velocity has a direct effect on the pitch vibrations of the airfoil system. Namely, it can generate the flutter leading to the limit cycle oscillation for the airfoil. With the aid of a fixed point of the Poincar $$\acute{\text {e}}$$ map of the system and numerical findings, we determine the flutter and the limit cycle oscillation of that. The frequency, period of the limit cycle oscillation of pitch motion and the flutter speed are calculated. Tangent points are also computed, and it is shown that these points cannot be two-fold singularities for the system. Furthermore, by using the theoretical techniques of discontinuous systems, we will obtain parametric regions for the existence of grazing bifurcation (global bifurcation). The existence of grazing bifurcation helps us to display that for some values of the air velocity, different transitions or sudden jumps can occur in the system's response. Numerical results demonstrate that these transitions are accompanied by the appearance and disappearance of a tangential contact between the trajectory and the switching boundaries. Also they can cause a change in the response of the pitch motion from simply periodic to double periodic (periodic-2). Moreover, stability regions for the airfoil system with freeplay will be found. The property of these stability regions is that inside them there exist no flutter and limit cycle oscillation. Some numerical examples are given which are in good agreement with our theoretical results. [ABSTRACT FROM AUTHOR]

In this paper, the condensation process, dynamic behaviour of fluid and phase-change inside a minichannel in the presence of square blocks are investigated numerically. The blocks are placed inside the minichannel with a tandem arrangement. The analysis is performed by using the lattice Boltzmann method based on the pseudo potential model. Fluid-fluid and fluid-surface interactions are fully considered in this model. Beside this, image processing is used to perform the validation and grid independent study. The effects of the distance between the blocks (D), the number of block (N) and the vertical blockage ratio (Z) on the condensation and nucleation processes are explored in details. The simulations are performed for different values of vertical blockage ratio ( $$0.05 \le Z \le 0.8$$ ), number of block ( $$0 \le N \le 4$$ ), and distance between the blocks (10Lu < D < 20Lu). It was found that the maximum value of the required time to transfer all the liquid phase to the bottom wall of the channel is occurred for the case of blocked channel. Moreover, the blocked channel phenomenon may be controlled with increase in N (number of block). Graphical Abstract: [ABSTRACT FROM AUTHOR]

The present study investigated fluid flow and natural convection heat transfer in an enclosure embedded with isothermal cylinder. The purpose was to simulate the three-dimensional natural convection by thermal lattice Boltzmann method based on the D3Q19 model. The effects of suspended nanoparticles on the fluid flow and heat transfer analysis have been investigated for different parameters such as particle volume fraction, particle diameters, and geometry aspect ratio. It is seen that flow behaviors and the average rate of heat transfer in terms of the Nusselt number (Nu) are effectively changed with different controlling parameters such as particle volume fraction (5 % ≤ φ ≤ 10 %), particle diameter ( d = 10 nm to 30 nm) and aspect ratio (0.5 ≤ AR ≤ 2) with fixed Rayleigh number, Ra = 10. The present results give a good approximation for choosing an effective parameter to design a thermal system. [ABSTRACT FROM AUTHOR]

Control of wake structure may lead to a reduction in the unsteady forces and vibrations in the marine structures. In this paper, the finite-volume method (FVM) is used to simulate the flow around two-dimensional obstacles with different cross-sections. Maxwell equations are used to provide the coupling between the flow field and the magnetic field. The range of Stuart ( Su and Reynolds ( Re numbers are 0-10 and 1-200, respectively. The effects of the magnetic field on the control of the wake structure and vortex shedding behind the obstacles are investigated in details. Moreover, an analogy has been performed between different configurations. Finally, several empirical equations for steadiness and disappearance Stuart numbers are presented for each obstacle. It was found that the diamond and circular configurations have the largest and smallest effects on steadiness and disappearance Stuart numbers among all the configurations. Additionally, it was observed that the drag coefficient slowly reduces by exerting the magnetic field at low Stuart numbers. [ABSTRACT FROM AUTHOR]

This paper investigates the combustion phenomenon in catalytic micro-combustor by using a two-dimensional model. In solution of energy and mass equations, the temperature and mass dependant reaction rates are considered with an iterative procedure where the series matching conditions are exerted between neighbor zones. The reaction zone thickness is considered as a variable parameter and predicted by the solution results of the present study. The study of main parameters effects such as normalized wall temperature, Peclet number and equivalence ratio is also attained. The limitations of low and high Peclet number are determined where at high Peclet numbers the maximum temperature is decreased and the trend is inversed at low Peclet numbers. In order to validate this model the results are compared with published experimental data, showing an acceptable agreement and confirming the accuracy of the predicted data. [ABSTRACT FROM AUTHOR]

In the present work the effect of catalytic surface reaction on non-reactive bulk flow in micro-channel is investigated. The reaction surface is analyzed with assumption of constant wall temperature. To solve this problem, the energy, mole fraction and catalytic reaction equations are solved by trial and error process. The analysis show that most sensitive parameter is fuel conversion and the sensitivity of hydraulic diameter is more than longitude coordinates. [ABSTRACT FROM AUTHOR]

In the present work, conjugate heat transfer in a rectangular cavity with a heated moving lid is investigated using the lattice Boltzmann method (LBM). The simulations are performed for incompressible flow, with Reynolds numbers ranging from 100 to 500, thermal diffusivity ratios ranging from 1 to 100, and Prandtl numbers ranging from 0.7 to 7. A uniform heat flux through the top of the lid is assumed. Results show that LBM is suitable for the study of heat transfer in conjugate problems. Effects of the Reynolds number, the Prandtl number and the thermal diffusivity ratio on hydrodynamic and thermal characteristics are investigated and discussed. The streamlines and temperature distribution in flow field, dimensionless temperature and Nusselt number along the hot wall are illustrated. The results indicate that increase of thermal diffusivity yields the removal of a higher quantity of energy from lid and its temperature decreases when increasing the Reynolds and the Prandtl numbers. [ABSTRACT FROM AUTHOR]

The objective of this work is to estimate the accuracy of a predicted velocity profile which can be gained from experimental results, in comparison with the exact ones by the methodology of entropy generation. The analysis is concerned with the entropy generation rate in hydrodynamic, steady, laminar, and incompressible flow for Newtonian fluids in the insulated channels of arbitrary cross section. The entropy generation can be calculated from two local and overall techniques. Adaptation of the results of these techniques depends on the used velocity profile. Results express that in experimental works, whatever the values of local and overall entropy generation rates are close to each other, the results are more accuracy. In order to extent the subject, different geometries have been investigated. Also, the influence of geometry on the entropy generation rate is studied, and the distribution of volumetric local entropy generation rate for the selected geometries is drawn. [ABSTRACT FROM AUTHOR]

In this article we intend to find the optimal shape of a nozzle respecting to some given target flow fields including viscosity effect. Via an approach based on measure theory which is not an iterative method and need not to any initial guess, each shape optimization problems are solved and consequently each geometry of the nozzle corresponding to prescribed flow fields is determined. Analyzing several case studies make us to confident on the use of the presented approach, because the obtained results give entirely the same as what we expect physically. [ABSTRACT FROM AUTHOR]

This paper is an attempt to simulate the mathematical modelling on drop deformation in a saturated medium due to its movement and collision onto the liquid film. The reactions between fluid-fluid interaction and wall are completely simulated. Computations are performed for a two-dimensional domain under the influence of a saturation situation (that is saturation vapor and liquid co-exist) by the lattice Boltzmann method. The effects of different parameters, such as Weber number, collision angle on the drop deformation and its collision onto the liquid film, radius and initial position of the droplet are investigated. A stronger wave forms in the liquid film after collision of the drop for the case of inclined collision in comparison with the normal one. Additionally, a 5.5% increment in the maximum height of the created wave for R dp =22 Lu and normal collision when the initial position of the drop moves from H = 0.5 L to H = 0.25 L is found. [ABSTRACT FROM AUTHOR]

A correction to the article "Analytical two-dimensional modeling of hydrogen-air mixture in catalytic micro-combustor" that was published online on October 20, 2015 is presented.