This paper reviews the latest findings on instability and subcritical transition to turbulence in wall-bounded flows (i.e., pipe Poiseuille flow, plane channel flow, and plane Couette flow). The main focus was on the early stage of transitional flow and the appearance of coherent structures. The scaling of threshold disturbance amplitude for the onset of natural transition was discussed. Generally, the scaling proved to be in the form of Ac = O(Reg) for Newtonian fluids where Re is the Reynolds number, g ≤ -1, and Ac is the critical perturbation amplitude. It was noted that exploration of perturbations like vortices, streaks, and traveling waves together with their amplitudes could clarify the instability and transition process. Hence, this paper focused on physical behavior and realizations of the transitional flow. Finally, a summary of consequential implications and some open issues for future works were presented and discussed.

This paper reviews the latest findings on instability and subcritical transition to turbulence in wall-bounded flows (i.e., pipe Poiseuille flow, plane channel flow, and plane Couette flow). Among the non-Newtonian fluids, viscoelastic and viscoplastic fluids were investigated. The main focus was on the early stage of transitional flow and the appearance of coherent structures. The scaling of threshold disturbance amplitude for the onset of natural transition was discussed. In addition, the transition of Newtonian fluids was compared with that of non-Newtonian fluids. Accordingly, the scaling for the transition of viscoelastic (i.e., highly elastic) fluid can be shown as Ac=O(Wig), where Wi is the Weissenberg number, g≤-1 is a scaling constant, and Ac is the critical perturbation amplitude. Moreover, the viscoelastic fluid flow at high Re numbers (i.e., Re>>1) is more stable than the Newtonian fluid flow in terms of the critical disturbance magnitude. Interestingly, the scaling for instability of viscoplastic fluid can be read as Rec=O(Bib), where Bi is the Bingham number and b≤1 is a constant. It was noted that exploration of perturbations like vortices, streaks, and traveling waves together with their amplitudes could clarify the instability and transition process. Hence, this paper focused on physical behavior and realizations of the transitional flow. Finally, a summary of consequential implications and some open issues for future works were presented and discussed.

The cold box heat exchangers are used in petrochemical and gas refinery industries. Here, an industrial complex cold box equipped with a number of plate-fins is simulated by computational fluid dynamics. The model predicts the outlet vapor fraction, pressure drop, and outlet temperature with average absolute relative deviations of 0.17%, 3.3%, and 12.89% for all streams, respectively. The influence of obstruction in streams B and C on the computational fluid dynamics results are studied. When stream B or C is blocked, the remaining open streams experience an increase in pressure drop, temperature, and vapor fraction, which negatively affects the heat exchanger's performance over a long time. Finally, the computational fluid dynamics results of the cold box are compared with those of commercial software Aspen-EDR. Even though Aspen-EDR predicts an acceptable mean temperature and vapor fraction (phase change) along channels, it calculates pressure drop incorrectly. So, Aspen-EDR, computationally efficient software, can be used for modeling of mean temperature and phase change of flow in an industrial multi-stream cold box.

Drag reduction of viscoelastic fluids under turbulent flow regime is an important phenomenon and well observed by many researchers. In this paper, turbulent flow characteristics under drag reduction condition are compared using four different low-Reynolds number k- ε models namely, Lam–Bremhorst, Launder–Sharma, Nagano–Hishida and Chien. The viscoelastic turbulence closure of Resende et al. (2011) is adopted. Time-averaged momentum and rheological constitutive equations based on finitely extensible nonlinear elastic-Peterlin (FENE-P) model are used and a polymeric contribution to the eddy viscosity equation is considered. The simulations are conducted for two sets of rheological parameters identified by Reτ0 = 395, β=0.9, L2 = 900 and Weτ0 = 25, 100 corresponding to drag reductions of 18% and 37%. Validation against DNS data is carried out for profiles of mean velocity, viscoelastic stress tensor, turbulent kinetic energy and its rate of dissipation. The differences are mainly limited to the viscous and buffer layer regions. While Nagano–Hishida and Chien models overpredict the DNS drag reduction results in both cases, the two other models underpredict them. The Lam–Bremhorst has the most accurate drag reduction results as a consequence of better capturing the near-wall effects.

Developing proper turbulence models in order to predict flow characteristics ofviscoelastic drag-reducing fluids has become as a considerable portion of non-Newtonian flows researches. In this numerical study, three different low-Reynolds-number k-epsilon models namely, Launder-Sharma, Lam-Bremhorst and Malin are used based on the adopted viscoelastic turbulence closure of Cruz et al. (2004). Simulation results for friction factor, mean axial velocity, turbulent kinetic energy and Reynolds shear stress are obtained for three aqueous polymer solutions (0.3% carboxymethyl cellulose, 0.2% xanthan gum and 0.2% polyacrylamide) and validated against the corresponding experimental data. While a good agreement exists for the bulk parameters like drag reduction (DR) or, equivalently, the friction factor, some discrepancies are observed in other simulation results of turbulence characteristics, in particular for situations with higher DR levels. The Malin's model which accounts for the shear-thinning property of the polymeric solutions, leads to the closest results for carboxymethyl cellulose and xanthan gum cases, while for polyacrylamide with the lowest power index n, the friction factor is larger than other simulation results and an increasing difference from the experimental data is observed. (C) 2018 Elsevier B.V. All rights reserved.

This study investigated the vibrational behaviour of a rotating two-blade propeller at different rotational speeds by using self-tracking laser Doppler vibrometry. Given that a self-tracking method necessitates the accurate adjustment of test setups to reduce measurement errors, a test table with sufficient rigidity was designed and built to enable the adjustment and repair of test components. The results of the self-tracking test on the rotating propeller indicated an increase in natural frequency and a decrease in the amplitude of normalized mode shapes as rotational speed increases. To assess the test results, a numerical model created in ABAQUS was used. The model parameters were tuned in such a way that the natural frequency and associated mode shapes were in good agreement with those derived using a hammer test on a stationary propeller. The mode shapes obtained from the hammer test and the numerical (ABAQUS) modelling were compared using the modal assurance criterion. The examination indicated a strong resemblance between the hammer test results and the numerical findings. Hence, the model can be employed to determine the other mechanical properties of two-blade propellers in test scenarios.

A two-dimensional numerical analysis of combined heat transfer (convection and conduction) in a nanofluid filled square enclosure is investigated using two phase method. Main attention was paid to the effects of the Rayleigh number (104≤Ra≤107), diameter (25nm≤dp≤145nm), volume fractions (0≤φ≤5%) and type of the nanoparticle (Cu, Al2O3, and TiO2), orientation of conductive wall, thermal conductivity ratio (0.2≤Kr≤25) and segmentation of conductive obstacle on the velocity, temperature fields and heat transfer characteristic. The results of this investigation showed that by adding several conductive obstacles inside the enclosure, the heat transfer rate decreases especially at low Ra. Moreover, it is found that, orientation of conductive partition has a big impact on the heat transfer rate at high Ra. It is also observed that in general, internal heating/cooling system has a considerably higher rate of the heat transfer compared with external ones. Finally, it is found that, the effect of the thermophoresis force is more pronounced for solid particles with low thermal conductivity like Al2O3 and especially TiO2.

In the present work, the mixing process of shear thinning liquids in a six-blade Rushton turbine is studied. A finite volume based computational fluid dynamics (CFD) simulation has been carried out and the three-dimensional turbulent flow is numerically analyzed by using the Shear Stress Transport k-omega (k-omega SST) model. Shear thinning liquids were investigated and shear thinning behaviour was modelled by the Ostwald-de Waele law. The used stirred vessel has a cylindrical shape with a flat bottom and the liquid height was kept equal to the vessel diameter. Effects of the power law index and the angle of attack of the blade on power consumption have been investigated. The results show that decreasing the angle of attack from 90 degrees to 45 degrees not only results in an increase in the flow rate down to the bottom of the vessel, resulting in a better mixture qualification, but also reduces the power consumption of the stirring process. To verify the simulation, axial, radial and tangential velocity components were compared with other experimental data and satisfactory agreement was found.

In the present study, the problem of conjugate natural and mixed convection of nanofluid in a square cavity containing several pairs of hot and cold cylinders is visualized using non-homogenous two-phase Buongiorno's model. Such configuration is considered as a model of heat exchangers in order to prevent the fluids contained in the pipelines from freezing or condensing. Water-based nanofluids with Cu, Al2O3, and TiO2 nanoparticles at different diameters (25 nm <= d(p) <= 145 nm) are chosen for investigation. The governing equations together with the specified boundary conditions are solved numerically using the finite volume method based on the SIMPLE algorithm over a wide range of Rayleigh number (10(4) <= Ra <= 10(7)), Richardson number (10(-2) <= Ri <= 10(2)) and nanoparticle volume fractions (0 <= phi <= 5%). Furthermore, the effects of three types of influential factors such as: orientation of conductive wall, thermal conductivity ratio (0.2 <= K-r <= 25) and conductive obstacles on the fluid flow and heat transfer rate are also investigated. It is found that the heat transfer rate is significantly enhanced by incrementing Rayleigh number and thermal conductivity ratio. It is also observed that at all Rayleigh numbers, the total Nusselt number rises and then reduces with increasing the nanoparticle volume fractions so that there is an optimal volume fraction of the nanoparticles where the heat transfer rate within the enclosure has a maximum value. Finally, the results reveal that by increasing the thermal conductivity of the nanoparticles and Rayleigh number, distribution of solid particles becomes uniform. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B. V. and The Society of Powder Technology Japan. All rights reserved.

In this article, a numerical study is carried out to analyze the effect of nanoparticle volume fraction over flow and thermal characteristics of laminar 2-D plane jet. Al2O3-water and TiO2-water nanofluids are considered in this investigation with lowest and highest values of particle volume concentration equals to 0 and 0.02 respectively. This paper propose four correlations for describing the relation between the solid volume fraction, ?t and ?u. The results show that the cross stream thermal diffusion depth and cross stream hydraulic diffusion depth are increased when particles volume concentration is increased and mean temperature and mean velocity decreases when the solid volume fraction is increased. The effects of nanoparticle volume fraction in velocity and temperature time histories are also studied and discussed.

In this paper the results of numerical study of the mixed convection heat transfer of Al2O3–water nanofluid in a horizontal annuli are presented. Steady, laminar flows in symmetric configurations are considered. Single-phase fluid approach is adopted for nanofluid modeling. The governing equations are discretized using the finite-volume method. A SIMPLE-like algorithm has been applied for pressure–velocity coupling on the collocated arrangement. In order to validate the code performance, the numerical results are compared with those available in the literature and good agreement is achieved. The effects of some important parameters such as nanoparticle volume fraction, aspect ratio, Grashof number, and heat flux ratio are studied and discussed in detail. In general, it is observed that the local Nusselt number increases with increase in nanoparticle concentration, Grashof number, and radius ratio. However, when increasing the nanoparticle concentration there are considerable increments in pressure drop and pumping power, which are not desirable. On the other hand, changes in the skin friction coefficient are negligible.

The permeability and tortuosity of pore-scale two and three-dimensional random porous media were calculated using the Lattice Boltzmann method (LBM). Effects of geometrical parameters of medium on permeability and tortuosity were investigated as well. Two major models of random porous media were reconstructed by computerized tomography method: Randomly distributed rectangular obstacles in a unit-cell as two-dimensional porous media, and random granular media in a cubic unit-cell as three-dimensional porous media. Results were validated using available theoretical, experimental, and numerical results from the literature. It is observed that permeability is a weak function of porosity in low porosity regions, but a strong function of porosity at high porosities. It also depends on the aspect ratio and hydraulic diameter of obstacles.Permeability results were obtained regarding to 73 random two-dimensional samples with different porosities and obstacle aspect ratios. Also 29 random sphere-packings including three different cases with three different sphere diameters were investigated as three-dimensional cases. Employing nonlinear regression based on the “least-squares” method, two permeability correlations were proposed with minimum curve-fitting errors. Besides, the effect of porosity on required time-steps to reach the converged solutions was investigated. It is concluded that an increase in the required time-steps to convergence is seen with reaching both high and low ends of porosity.© 2015 Published by Semnan University Press. All rights reserved.

In this paper, convection heat transfer of Al2O3-water nanofluid turbulent flow through internally ribbed tubes with different rib shapes (rectangular, trapezoidal and semi- circular) is numerically investigated. For each rib shape, the optimum geometric ratio and volume fraction were calculated using entropy generation minimization technique. The governing equations in steady state and axisymmetric form have been solved using Finite Volume Method (FVM) with the SIMPLE algorithm. A uniform heat flux was applied on the wall. A single-phase approach has employed to model the nanofluid. Nanoparticles size is 20 nm and nanoparticles volume fraction and Reynolds number were within the ranges of 0-5% and 10,000-35,000 respectively. Comparisons between the numerical results and experimental data show that among different turbulence models, k-e model with enhanced wall treatment gives better results. The results indicate that the heat transfer increases with nanoparticles volume fraction and Reynolds number but it is accompanied by increasing pressure drop. The simulations demonstrate that trapezoidal and semi-circular ribbed tubes have higher Nusselt number than the rectangular ribbed tubes with the same diameters. Correlations of heat transfer have obtained for different ribbed tubes. In evaluation of thermal performance and pressure drop, it is seen that the ribbed tubes with Al2O3-water nanofluid flow are thermodynamically advantageous. For each rib shape, the optimum geometric ratios are also presented.

The forced convection heat transfer of turbulent Al2O3-water nanofluid flow inside the grooved tubeswith the different aspect ratio of the rectangular grooves is numerically investigated. The governingequations have been solved using finite volume method (FVM) coupled with SIMPLE algorithm. It isassumed the heat flux is constant on the grooved walls. The Single-phase approach is applied for thecomputation of the nanofluid flow. The Nanoparticles volume fraction is in the range of 0-5% and flowReynolds number is in the range of 10,000-35,000. Comparisons between the numerical results andavailable experimental data show that among different turbulence models, k-e model with enhanced walltreatment gives the better results. The results show that the heat transfer coefficient increases withnanoparticles volume fraction and Reynolds number but it is accompanied by pressure dropaugmentation. From the results, it is concluded that the grooved tubes with Al2O3-water nanofluid floware thermodynamically advantageous. The Correlations for heat transfer coefficients have been presentedfor grooved tubes in different aspect ratios using the numerical results. The optimum geometric ratios inwhich the entropy generation is minimized are also determined.

In this study, mixed convection heat transfer and local and global entropy generation in aventilated square cavity have been investigated numerically. The natural convection effect isachieved by a constant heat flux imposed at the bottom wall and cooled by injecting a coldfollow. In order to investigate the effect of port location, four different placementconfigurations of the inlet and outlet ports are studied. In each case, external flow enters intothe cavity through an inlet port in the left side of the cavity and exits from the opposite side.The other boundaries are assumed adiabatic. The cavity is subjected to laminar flow of water.The investigation has been carried out for the Re=1000, and the Richardson number with therange of 0.0001(Global Entropy Generation), Heat Transfer Irreversibility (HTI) and Fluid FrictionIrreversibility (FFI) are calculated and compared. Then, the optimum inlet/outlet configurationhas been selected based on the minimum GEG and the maximum heat transfer.

The present work investigates the entropy generation and enhancement of heat transfer in natural convection flow and heat transfer using Copper (Cu)–water nanofluid in the presence of a constant magnetic field. The analysis uses a two dimensional trapezoidal enclosure with the left vertical wall and inclined walls kept in a low constant temperature and a heat source with constant heat flux placed on the bottom wall of the enclosure. The governing equations were discretized by the control volume method and solved numerically by SIMPLE algorithm. The computations were carried out for a wide range of the Rayleigh number (104⩽Ra⩽107), Hartman number (0⩽Ha⩽100) and solid volume fraction (0⩽ϕ⩽0.05). The results show that at Ra = 104 and 105 the enhancement of the Nusselt number due to presence of nanoparticles increases with the Hartman number, but at higher Rayleigh number, a reduction has been observed. In addition it was observed that the entropy generation is decreased when the nanoparticles are present, while the magnetic field generally increases the magnitude of the entropy generation.

In this study, natural convection inside a square cavity filled with nanofluids with several pairs of heaters and coolers (HACs) inside is investigated numerically in the range of Rayleigh numbers between 104 and 107. Walls of the cavity are insulated and heaters and coolers walls are isothermal with temperatures of Th and Tc (Th>Tc). Two-dimensional Navier–Stokes and energy equations are solved using finite volume discretization method. Effects of various design parameters on the heat transfer rate are investigated. Design parameters considered in this study are: position, surface area, shape and orientation of HACs, volume fraction and types of nanoparticles. The results show that the highest and the lowest impacts of design parameters, on the enhancement of heat transfer rate are caused by changing the HAC position and types of nanoparticles, respectively. Moreover, it is found that for a constant surface area of the HAC at the entire range of Rayleigh number, rate of the heat transfer increases with changing orientation of the HAC from horizontal to vertical. Our simulations indicate that the heat transfer rate at all Rayleigh numbers can be enhanced more efficiently by increasing number of HACs than increasing the HAC size. The optimum value of volume fraction of nanoparticles which result in the highest rate of heat transfer in most cases found to be equal to 1%, and beyond that the heat transfer rate decreases.

The objective of this paper is to investigate the natural convection cooling of a heat source mounted inside a square cavity with special attention being paid to entropy generation. The cavity is filled with copper-water nanofluid; the right vertical wall is kept at a constant temperature, while other walls are adiabatic ones. The numerical scheme is based on the finite volume method with the SIMPLE algorithm for pressure velocity coupling.In this study, the influence of some effective parameters such as the Rayleigh number, location of the heat source, and solid concentration are studied; then, entropy generation due to the heat transfer irreversibility and fluid friction irreversibility as a function of Ra and solid concentration and heat source location is studied. The result shows that location of the heat source is an important parameter affecting the flow pattern and temperature field and variation of the entropy generation. Consequently the optimum case is selected since the thermal system could have the least entropy generation and the best heat transfer rate.

This article presents a numerical study of natural convection cooling of a heat source mounted inside the cavity, with special attention being paid to entropy generation. The right vertical wall is partially open and is subjected to copper-water nanofluid at a constant low temperature and pressure, while the other boundaries are assumed to be adiabatic. The governing equations have been solved using the finite volume approach, using SIMPLE algorithm on the collocated arrangement. The study has been carried out for a Rayleigh number in the range 10(3) < Ra < 10(6), and for solid volume fraction 0 < phi < 0.05. In order to investigate the effect of the heat source and open boundary location, six different configurations are considered. The effects of Rayleigh numbers, heat source and open boundary locations on the streamlines, isotherms, local entropy generation, Nusselt number, and total entropy generation are investigated. The results indicate that when open boundary is located up, the fluid flow augments and hence the heat transfer and Nusselt number increase and total entropy generation decreases.