Gh.a Sheikhzadeh , M. Nazififard , R. Maddahian, Kh. Kazemi ,
Volume 19, Issue 1 (1-2019)
Abstract
Today, increasing the efficiency and optimization of energy systems in terms of economic and environmental conditions is of particular importance. So far, several methods have been proposed to increase the heat transfer in thermal systems, including the use of nanofluids and types of fluid flow turbulators. In this research, the application of both nanofluid and twisted tape to improve the heat transfer coefficient were numerically investigated. Different turbulence models were used to simulate fluid turbulence. The results showed that increasing the nanoparticle volume fraction, reducing the twisting ratio, and increasing the Reynolds number resulted in an increase in heat transfer. By reducing the twisting ratio from 15 to 5, the heat transfer rate increases from 8-16%. With rising Reynolds number from 10,000 to 20,000, maximum temperature differences decreases by 4.5%. Moving downstream of the flow, the difference between the maximum temperature of the sections decreases. Increasing the heat transfer and intensifying the effects of the twisted tape to downward are the reasons for this decline.
A.h. Bolandi Kashani, M.h. Nobakhti , M. Khayat ,
Volume 19, Issue 1 (1-2019)
Abstract
Shan-Chen model is the most common model for simulation of multiphase flows using lattice Boltzmann method. The entire multiphase Lattice Boltzman models are limited to regimes, where the temperature dynamics are either negligible or their effects on the flow are unimportant. The entire multiphase LBE models are limited to regimes where the temperature dynamics are either negligible or their effects on the flow are unimportant. The multiphase isothermal lattice Boltzmann equation (LBE) model and single phase thermal LBE (TLBE) model were described. In this research, by combining these two models, the thermal two-phase LBE model was proposed. The coupling of the two models is through a suitably defined body force term. Due to the external nature of this coupling, the new model will have the same stability as the isothermal two-phase model. For this purpose, the scalar thermal model was initially neutral and, then, the Shan-Chen model was expressed in homogeneous state. Also, droplet falling on a heated solid surface and positioning droplet on heated solid surface in different Rayleigh and Reynolds number and different diameter size of droplet were considered. Results show that the temperature in the multiphase flow, as a barrier, delays achieving a stable state, and the fake speed created at the interface area in the temperature field also affects.
H. Gholami , R. Kouhikamali , N. Sharifi,
Volume 19, Issue 2 (2-2019)
Abstract
In this study, using volume of fluid method in open source software OpenFOAM, the phenomenon of evaporation in the porous medium was analyzed. At the beginning of the solution, the system consists of a water phase and a porous copper environment. In the next steps of numerical simulation and as a result of partial evaporation of water, the vapor phase appears as the second fluid phase. Water and vapor are assumed to be incompressible and incompatible, and the phenomenon of evaporation occurs unevenly. The interface between phases is modeled by the VOF method and the Lee model has been used to mass transfer between two phases of water and vapor. For surface tension between phases, the continuous surface force (CSF) method was considered. The comparison of simulation results with experimental results showed that the combined solver of porous medium evaporation would well estimate the rate of evaporation at different sections of the channel. In addition, the results of the wall temperature indicate that the channel is divided into two zones of heating and evaporation. In the region of heating, the temperature increases linearly with the channel length to reach saturation temperature. After the point of saturation, the wall temperature first remains constant and eventually forms an oscillatory shape, in which locally there are temperature jumps. The evaporated flow rate also increases at high intensity first, but in the end regions of the porous channel, its growth rate is slow.
M. Karimi , R. Kouhikamali ,
Volume 19, Issue 3 (3-2019)
Abstract
In the present study, the performance of zigzag demister has been numerically investigated for the separation of dispersed liquid droplets from the gas flow. In general, liquid droplets are dispersed from the gas flow in contact with the vane demister and the formation of the liquid film. Depending on the energy of the droplet collision to the surface, it is likely to occur splash drop into smaller droplets, which will reduce the separation efficiency of the system. In this study, by focusing on the flow collision regime near the surface, it is attempted to investigate the effect of the flow parameters and vane geometry on the separation efficiency and the pressure drop of flow. The Euler-Lagrange is used to simulate the flow and particle motion path. In this research, an experimental model is designed and constructed. Numerical solver results are validated, using the experimental data. The result of this study shows that separation efficiency decreased with increasing gas flow velocity, such that by increasing the 2.5 times of gas velocity, the separation efficiency will lead to a 10% decrease. It was also found that increasing the diameter and increasing the droplet would increase the separation efficiency. On the other hand, choosing the geometry of vane has a significant effect on the amount of the pressure drop of the passing flow. In a way that, by increasing the 50% of the vane angle, the pressure drop will increase 5 times.
Gh. Olyaei, A. Kebriaee ,
Volume 19, Issue 4 (4-2019)
Abstract
The present study was performed to experimentally investigate the regime of the liquid sheet breakup and the effects of dimensionless numbers on the penetration and trajectory of the liquid sheet in cross flow condition. The shadowgraphy technique was applied to study the tests. In this work, the effect of the non-dimensional numbers (momentum ratio and Weber number) were surveyed on the breakup of the liquid sheet. Also, some equations for the injection trajectory, the length, and the height of the jet were presented based on these non-dimensional numbers. The tests were done at atmospheric pressure and temperature, where the Weber number range is from 0.8 to 12.5, the variations of the momentum ratio are from 17.4 to 250, and the changes in the Reynolds number are from 2400 to 10227. Three regimes of jet breakup were observed, defined as column breakup, column-bag breakup, and bag breakup. The Weber number is the most effective parameter in the regime change of the liquid sheet breakup. The results also indicated that the increase in the momentum ratio has a great influence on the depth of penetration of the liquid sheet, but it has a very small effect on the breakup regimes.
M. Faraji Kheyrabadi, S. Kheradmand,
Volume 19, Issue 6 (6-2019)
Abstract
In the present work, an investigation and simulation of the air flow, containing solid suspended particles in the actual impactor particles under investigation are in the micron range. The results of this work can be illustrated by simulating the motion of particles in an actual impactor, investigating the effects of temperature changes on the surrounding environment, and the impedance plates on the accumulation efficiency. In the first part, by deriving the governing equations for this phenomenon and choosing the appropriate numerical method for solving these equations, the path of motion is simulated. By determining the path of the particles, it is possible to determine the number of particles deposited on collecting plate, and to the mentioned relations, the collection efficiency is obtained a laboratory experiment, which compared with laboratory values. This comparison indicates the acceptable accuracy of the chosen employed method. In the next section, by selecting particles with different densities, of the environment temperature and inlet air variations by assuming constant plate temperature, and collector plate temperature variation on the impactor efficiency have been investigated. The results show that the particle density affects the efficiency of and reduces the diameter of cut from 2.2 to 4.2 in Due to the increased viscosity of the air, the of reduces the efficiency of The results showed that temperature variation of the collection plate could also change the particle collecting efficiency.
H. Alimoradi, M. Shams,
Volume 19, Issue 7 (7-2019)
Abstract
In this research, a numerical scheme for subcooled flow boiling with water based fluid in a channel with a hot spot was developed. The effect of nanoparticles was studied in the subcooled flow boiling. Alumina nanoparticles were used for the protection of nanofluid. The properties of nanofluid are assumed to be temperature independent. The mixture of nanofluid is studied by using Eluer–Eluer approach. In addition to considering the variable properties of temperature in this study, a model for the density of the nucleation site was used, which is the surface roughness and sedimentation rate of the nanoparticles. After verifying the model, the nanofluid boiling was modeled, using 4 roughnesses of 25, 50, 75, and 100 nm. Changes of bubble dynamics parameters were investigated in different heat fluxes and roughnesses. According to the results, it was found that with increasing surface roughness, the surface temperature drop and the density of the nucleation site density increased. Also, bubble departure diameter is increased and bubble detachment frequency is decreased by increasing surface roughness. Moreover, the results shows that bubble detachment diameter is increased by increasing the heat flux and bubble detachment waiting time.
Mohamad Safavi, S. Nourazar,
Volume 19, Issue 7 (7-2019)
Abstract
The statics of droplet hanging from the parallel fibers and the dynamics of droplet impact on the parallel fibers are investigated using high-speed imaging and volume of fluid numerical simulation. Experimental results show for the parallel fibers, the maximum volume of the droplet, which is able to hang statically from the fibers is measured to vary between 1.85 to 1.9 times of the one measured for a single fiber. The dynamics of droplet impact have been studied by varying the radius of the impacting droplet, the fibers radius, and the distance between the fibers. The threshold velocity of droplets by fibers has been obtained both experimentally and numerically with the fluid volume method. The results show that by increasing the impacting droplet radius and decreasing the fibers radius, the threshold velocity of droplet capture decreases. The maximum threshold velocity of droplet capture with parallel fibers varies in the range of 1.5 to 1.8 times of the threshold velocity of capture with a single fiber. The maximum threshold capture velocity of droplets occurs where the distance between fibers is in the range of 0.35 to 0.5 times of impacting droplet diameter. The threshold capture velocity on parallel fibers is also obtained analytically, using the energy balance method. The results of the analytical solution are in a fair agreement with experimental data and numerical simulation results.
M. Khayat, M. Mohebie,
Volume 19, Issue 11 (11-2019)
Abstract
This study aims to investigate the effect of nanoparticle deposition on the boiling surface in the presence of microchannel on the characteristics of boiling heat transfer. In this experimental study, the copper boiling surfaces including polished circular surface, rectangular and trapezoidal microchannels were used. The microchannels include feeding sub-channels perpendicular to the main channel, which increases the boiling surface and separates the downward cool fluid flow and upward hot bubbles. Nuclear boiling experiments on microchannel surfaces in the presence of a hybrid water-based nanofluid containing 70% titanium oxide and 30% OH-based multi-wall carbon nanotubes in volumetric concentrations of 0.1% and 0.5% have been conducted. The results of nanofluid boiling experiments on both microchannel surfaces show that with increasing concentrations, critical heat flux and heat transfer coefficient increases and the highest increase in critical heat flux and heat transfer coefficient is related to the hybrid nanofluid with 0.5 % volumetric concentration on the surface with trapezoidal microchannel and their values are 64.64% and 344.76%, respectively, compared to pure water boiling on the polished copper surface. Also, in boiling of pure water on the deposited surfaces with nanoparticles, the greatest increase in critical heat flux and heat transfer coefficient is related to the surface with trapezoidal microchannels with 0.1% volumetric concentration and 0.5% and volumetric concentration and their values are 120.16% and 149.4% respectively, compared to pure water boiling on the polished copper surface.
A. Hadidi, M.r. Ansari,
Volume 19, Issue 12 (12-2019)
Abstract
In this research, the behaviour of a single droplet of the dielectric field under the effect of the applied external uniform magnetic field has been investigated. Previously, it was thought that no force is applied to the dielectric fluids when exposed to the uniform magnetic field. A stagnant droplet in a quiescent fluid column was considered in order to determination of the net effect of the applied uniform magnetic field. Considering that the droplet behaviour has been investigated in the horizontal plane, the net effect of the gravity on the droplet and the surrounding fluid is also zero. Therefore, any change in the condition of the considered droplet will be due to the effect of the applied magnetic field. Numerical analysis has been used to perform this research. The governing equations of the problem are the continuity, momentum, level set equations for interface simulation, re-initialization and re-construction equations of the level set equations to control the mass dissipation of this method. The governing equations have been discretized and solved by developing code in the Fortran programming environment. The behaviour of the considered droplet in various regimes has been investigated under the different magnitudes of the applied magnetic field. The results of the research in various cases show that stagnant droplet deforms under the effect of the applied magnetic field and starts to vibrate which also induces the motion in the surrounding quiescent fluid.
A. Jahangiri, S. Majidi, Kh. Roshandel, D. Borzuei, S.f. Moosavian, M. Naji Ranjbar,
Volume 20, Issue 1 (1-2020)
Abstract
Investigating the history of production and dynamics of growing or collapsing bubbles under various environmental conditions plays an important role in the correct understanding of the process of boiling, evaporation, cavitation, and condensation. In this paper, the rising shape regime the air bubble injected into the water column was studied and simulated using numerical and experimental methods. For this purpose, a column filled with water was used in the laboratory as a host fluid and using the high-speed image recording method, the most important hydrodynamic properties of the bubbles, such as velocity, size, pathway, and other bubble properties were measured. Then, using the computational fluid dynamics and the volume of fluid two-phase flow model, ascent and deformation of the single-bubble injected into a stationary reservoir were investigated and compared with previous and current experimental and numerical results. The result of this validation with a good approximation was in accordance with the reference results and it proved the correctness of the solver's and its settings. Finally, the bubble shape regime was calculated by the non-dimensional numbers of Eötvös and Morton and compared with the numerical simulation and empirical test. The regime obtained from the Clift diagram is a spherical cap regime, which at the same conditions, is in accordance with the bubble shaped regime simulated by numerical and experimental methods and this confirms the validity of the numerical solution.
S.a.a. Pourahmadi, Sh. Talebi,
Volume 20, Issue 1 (1-2020)
Abstract
Electrowinning is the process of copper deposing from the intracellular electrolyte solution to the cathode by creating an electric current. In the present study, the hydrodynamic simulation of the electrowinning cell of Miduk Copper Complex is studied using computational fluid dynamics. Ansys-CFX software is used for this modeling. Navier Stokes and continuity equations are considered as the two-phase fluid and gas, turbulent, incompressible and steady states and the equation for copper concentration in the electrolyte will be solved with consideration of its specific boundary condition. Turbulence will be modeled using the k-ω method. The general and local simulations have been used together due to the large variation in the properties, close to the cathode and anode, and the large volume of the cell, to create a good mesh and increase the speed and accuracy of the solution. First, in general simulation, the entire geometry of the cell is modeled by creating a suitable mesh. Then in the local simulation, only the volume between the two cathodes of the cell is considered and modeled with higher precision. Data on boundary conditions in the local simulation of interface boundary are obtained with general simulation data, which increases the accuracy of modeling. Comparison of the results of general and local simulations shows an accuracy of up to 30% in close to the electrodes. The results of this simulation are the velocity vector, the concentration of acid and copper, the turbulence intensity, pressure and the volume fraction of the oxygen phase in the whole of the electrowinning cell. Finally, the model has been validated by experiments on the real cells. The results show the high accuracy of this modeling technique with less than 2.5% deviation.
S.m. Hosseinalipoor, H. Ami Ahmadi, A. Ebadi, M. Abdollahi Gol,
Volume 20, Issue 1 (1-2020)
Abstract
Nowadays, the interaction of oil droplets with gas bubbles plays an important role in many industrial, environmental and biological processes. Therefore, in this paper, the outcome of a collision between a silicon oil droplet and an air bubble in water has studied in order to identify the effective parameters in this process. For this purpose, an especial setup was built and four series of experiments in both dynamic (in which the relative velocity of collision is equal to the bubble velocity due to the Buoyancy force) and static conditions were carried out. The results of these experiments were presented and discussed in the form of several tables and pictures. In these experiments, a high-speed camera and image processing were used to gain a better understanding about bubble-drop coalescence qualitatively, and to obtain some quantitative information such as contact time, velocity, and kinetic and interfacial energies of bubbles and drops during the impact. The results of this study show that in addition to the spreading coefficient, the kinetic energy of bubble/droplet in the collision and their contact time, are also determinative parameters in the determination of the outcome of a collision. In the dynamic and static states, the effect of kinetic energy and contact time are more effective, respectively.
M.k. Tahmasebi, R. Shamsoddini, B. Abolpour,
Volume 20, Issue 2 (1-2020)
Abstract
The motion of the liquid free surface in a container (sloshing phenomenon) inserts a momentum on the container walls. This makes a great disorder in the movement of the carrier vehicle or inserts a large force and momentum on the container walls. The reason for this phenomenon is the establishment of destructive waves and hydrodynamic forces. The side effects of this phenomenon in various industries, such as ship industries carrying liquid fuels, liquid fuel rocket industries, fuel tanks or water tanks, increase the importance of predictions of the behaviors of this phenomenon. One way of controlling is to use baffles or plates in the transverse direction of the tank. In this study, the governing equations on this phenomenon have been solved using the OpenFOAM software. This software solves partial differential equations using the finite volume method, which by default considers geometry to be three dimensional. In order to solve the two-phase flow, a modified volume of the fluid model (VOF) is applied and the moving mesh model is used for the movement of the container body. In the VOF method, the phases are expressed as a fraction of one (volume fraction). To determine this parameter, based on the continuity equation, a differential equation is regulated and solved. For the turbulent flow model, a modified k-e model is used by considering the effects of free-surface flows. Also, an experimental model of a real moving liquid container has been used for validation of the predictions of the presented simulation. The results show that the experimental and numerical results are in good accordance. In addition, the results show that using vertical baffles up to 50% can reduce the fluctuations caused by this phenomenon.
S.a. Mahdavi, A. Ranjbar, M. Farshchi,
Volume 20, Issue 7 (6-2020)
Abstract
Variable-area injectors are suitable for developing throttleable rocket engines because it is difficult to efficiently control thrust when fixed-area injectors are used. A pintle injector is a variable-area injector that can be used to control the mass flow rate of propellants. In practice, an injector plate containing several fixed-area injectors is replaced with a single pintle injector. In this research, a two-stage pintle injector is designed, manufactured, and tested for the effects of dimensionless numbers (Momentum ratio, Weber number, and discharge coefficient) on the injector’s performance, including the spray angle change, which is an important characteristic of the spray. The tests were done at ambient temperature and pressure conditions. The Weber number ranged from 19 to 1830, and the ratio of the fuel to the oxidizer momentum was varied from 0.2 to 13. Water is used instead of the oxidizer as a central propellant, and the air is used instead of the fuel as an external propellant. Shadowgraph and photography were used to measure the spray angle and study the desired parameters. Empirical relationships between functional parameters and dimensionless numbers were obtained that can be used in the design process.
H. Ami Ahmadi, A. Ebadi, S.m. Hosseinalipour ,
Volume 20, Issue 8 (8-2020)
Abstract
Nowadays, the interaction between gas bubbles and oil droplets plays an important role in the efficiency of many industrial processes. Therefore, it is of great importance to study the influencing factors on these processes. So, in the present paper, the effect of droplet and bubble size on the drainage time of the trapped intervening film between droplet and bubble was investigated. Six series of experiments were conducted for various sizes and three characteristic time scales including drainage time, coverage time, and rupture time were measured. Each of these experiments was repeated at least five times. The results showed that the drainage time changed independently of the droplet/bubble size. Moreover, it was observed that due to the nature of the phenomenon, the measured drainage times in each equivalent size are notably scattered, which means that the microscopic interactions in the water film and between bubble-droplet interfaces have significant impacts on the drainage time. Also, in the current experiment, it was found that the volume of the intervening film between droplet and bubble has no vital role in the drainage time of the mediate water film.
Mohamadhossein Ramezani, Mohammad Mehdi Noroozi, Reza Madahian, Mohammad Reza Ansari,
Volume 21, Issue 11 (9-2021)
Abstract
The purpose of this study is to generate two-phase flow patterns and to obtain a flow pattern map for two phases as water and air in a vertical pipe which is made of transparent Plexiglas. The pipe specification is 50 mm diameter and 390 cm length. In this attempt the average velocity of the Taylor bubble will be calculate. In order to facilitate this research work, a two phase flow was designed, built and adjusted at Tarbiat Modares University Two-phase flow laboratory. Three flow patterns as bubbly, slug and churn flow are generated and examined for 320 runs of different superficial velocities of air and water. A seven-layer distributor with the ability to change the number of bubbles produced is used to create a bubbly flow pattern at the air inlet. The effect of the superficial velocities of each phase on the flow pattern was evaluated and a flow pattern map was presented for 320 different data. By processing the images obtained from the high-speed camera, the average Taylor bubble velocity was calculated for different flow conditions with uncertainty in calculating the velocity. Also, for 5 different velocities of the liquid phase, a diagram of the average velocity of Taylor velocity with increasing gas velocity was drawn and compared with the Nicklin correlation which can be found in the literature
Sina Nikbakht, Mohammad Mahdi Heyhat,
Volume 24, Issue 12 (11-2024)
Abstract
Nowadays, fresh water scarcity is one of the major concerns of the global community. To tackle the freshwater scarcity situation, several solutions have been suggested, including the extraction of water from fog-laden flow. Fog harvesting is known as a sustainable and effective approach to supplying freshwater. Various types of fog collecting elements (FCEs) have been implemented in studies to collect water from fog-laden flow. Woven meshes with square-shaped holes are among the most frequently employed FCEs in studies. One of the major drawbacks of these types of FCEs is their low water collection efficiency, particularly at low wind velocities. In this study, two alternative mesh hole geometries, triangular and hexagonal, were proposed to enhance the collection efficiency and compared with an equivalent square mesh in terms of the shading coefficient (SC). The evaluations were conducted experimentally using an experimental setup capable of mimicking atmospheric fog-laden flow at two different air velocities. The results indicate that the water harvesting rate is highly affected by mesh hole geometry. Using triangular and hexagonal meshes, compared to square mesh, can improve the water collection rate by up to 12.6% and 29%, respectively
Maziar Shafaee Roshani, Abbas Ouni,
Volume 25, Issue 1 (12-2024)
Abstract
cost-effectiveness, and reliability. Despite numerous experimental studies on pressure-swirl atomizer spray, a comprehensive mathematical model for predicting spray characteristics has not yet been presented. Additionally, there is no consensus on the distribution function accurately describing droplet size dispersion. In the present study, the main characteristics of pressure-swirl atomizer spray, including discharge coefficient, spray cone angle and droplet size distribution, were experimentally investigated using the shadowgraph technique. The study spanned a wide range of Reynolds numbers, from 1250 to 8500, encompassing laminar, transition, and atomization regimes. The findings showed that the discharge coefficient initially declined during the transition stage, followed by a gradual increase up to the atomization regime. In the atomization regime, the discharge coefficient remained almost constant. A similar trend was observed for the spray cone angle throughout the transient and atomization phases. The gamma distribution function provided a favourable fit with the experimental drop size distribution in the near-ligament location, where primary breakup mechanisms dominate. The log-normal distribution function showed superior fitting with the experimental droplet size distribution for regions distant from the liquid sheet disintegration point, where secondary breakup mechanisms exert a more pronounced influence on droplet dispersion. Overall, these findings provide valuable insights into spray characteristics and associated uncertainties.
