Recent Volume
Vol13 , No 1 (in press)
Heat Conduction Characteristic of Rarefied Gas in Nanochannel
Pages : 1-13
Authors : R. Rabani,  G. Heidarinejad,  J. Harting,  E. Shirani, 
Nonequilibrium molecular dynamics simulations is applied to investigate the simultaneous effect of rarefaction and wall force field on the heat conduction characteristics of nano-confined rarefied argon gas. The interactive thermal wall model is used to specify the desired temperature on the walls while the Irving–Kirkwood expression is implemented for calculating the heat flux. It is observed that as the temperature differences between the walls increases by lowering the temperature of the cold wall, the number of adsorbed gas atoms on the cold wall increases notably due to the increment in the residence time of the gas atoms. Consequently, the interfacial thermal resistance between the gas and the cold wall reduces which results in a reduction of the temperature jump. Meanwhile, the increase in the temperature of the hot wall leads to a reduction of the residence time of gas atoms in the near-wall region which decreases the number of absorbed gas atoms on the hot wall. This results in an increase in interfacial thermal resistance which leads to a higher temperature jump. It is observed that the bulk, wall force field and interface regions form approximately 10%, 45% and 45% of the total thermal resistance, respectively. Furthermore, unlike the interfacial thermal resistance, the bulk and the wall force field thermal resistance are approximately independent of the implemented temperature difference.
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Numerical Study of Particle Dispersion in the Turbulent Recirculation Zone of a Sudden Expansion Pipe using Stokes Numbers and Mean Drift Parameter
Pages : 15-23
Authors : M. J. Torres,  J. García,  Y. Doce, 
The dispersion of solid particles in zones of turbulent recirculation flow is of interest in various technological applications. Many experimental studies have been developed in order to know the contribution of Stokes numbers and mean drift parameter on the entering and dispersion of particles in the recirculation zone however to our knowledge there are not numerical studies reported about it. In this work, we made a numerical study of the incompressible turbulent flow laden with solid particles in sudden expansion pipes with different expansion ratios and different Reynolds number upstream of the pipe, using LES and Germano dynamic model with JetCode program for the continuous phase (air). The solid particles movement (different diameters were considered) was solved by using a Lagrangian tracking algorithm coupled to JetCode taking into account only drag and gravity forces supposing one way coupling. Finally, we calculated Stokes numbers based on the different fluid time scales and the mean drift parameter for all the solved cases and studied their isolated effect on the solid particle dispersion in the recirculation zones by computing the concentration by means of the particle number within the recirculation zones. Our results coincided with the experimental findings reported by others authors: the particle concentration exhibits a maximum value as the Reynolds number upstream in the pipe is decreased, the pipe expansion ratio is increased and particle size is decreased. Regarding the results obtained numerically about the solid particle dispersion within turbulent recirculation zones in terms of Stokes numbers and the mean drift parameters, coincided adequately with the experimental results.
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Investigation on Flow Characteristics and Performance Estimation of a Hybrid SVC Nozzle
Pages : 25-38
Authors : J. Shi,  Z. Wang,  Z. Li,  S. Xiao-lin, 
Higher vector efficiency of fluidic thrust vectoring (FTV) technology results in less requirement on secondary flow mass, which helps to reduce the influence of secondary flow on the performance of an aero-engine. In the paper, a new concept of FTV, named as a hybrid shock vector control (SVC) nozzle, was proposed to promote the vector efficiency of a SVC nozzle. It adopts a rotatable valve with a secondary flow injection to enhance the jet penetration, so as to improve the vector performance. The flow characteristics of a hybrid SVC nozzle were investigated numerically by solving 2D RANS equations. The influence of secondary pressure ratio (SPR) and rotatable valve angle on vector performance were conducted. Then, the coupling performance of a hybrid SVC nozzle and an aero-engine was estimated, by using the approximate model of a hybrid SVC nozzle and the performance simulation model of an aero-engine. Results show that, a desirable vector efficiency of 2.96 º/ %-ω (the vector angle achieved by using secondary flow of 1% of primary flow) of a hybrid SVC nozzle was obtained. In the coupling progress, when a secondary flow of 5.3% of primary flow was extracted from fan exit to a hybrid SVC nozzle, a vector angle of 14.1°, and a vector efficiency of 2.91º/ %-ω were achieved. Meanwhile the thrust of the aero-engine thrust decreased by 5.6% and the specific fuel consumption (SFC) increased by 0.5%.
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Simulation of Waves on Boom and Oil Plume Rising using Smoothed Particle Hydrodynamics
Pages : 39-54
Authors : M. Rostami Hosseinkhani,  P. Omidvar,  S. Allahyaribeik,  M. Torabi Azad, 
Marine oil spills can cause serious damage to the marine ecological environment. In the numerical modeling of oil plume rising and its advection, a better understanding of the oil plume transport may be effective on the sea pollution reduction and removing pollutants. In this paper, the effects of waves are investigated on the oil plume convection-diffusion pattern using smoothed particle hydrodynamics (SPH). Firstly, the rising patterns of an oil plume of different densities are simulated and the results are compared with the analytical solution. Then, the concentration distribution is shown for the oil plume rising problem. Afterwards, the suitability of the SPH method is examined by a cnoidal wave on shore effect. Finally, the plume of different conditions is located in waves and the advection of pollutant is studied with a fixed boom and different angles. It will be concluded that using a boom with a zero diversion angle would lead to minimum passing pollutant.
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Optimization of Drag Reducing Shark Inspired Blade-Shape Riblet Surfaces in External Flow
Pages : 55-65
Authors : M. Monfared,  M. A. Alidoostan, 
The main goal of this research is to study the drag reduction capabilities of blade-shape riblet surfaces in external flows. For this purpose, the ability of riblet surfaces for drag reduction of an underwater hydrodynamic model has been investigated. The surface geometry has been modified by applying shark skin inspired blade-shape riblets on the surface. These riblets have been modeled in various dimensions and applied on the exterior surface of an underwater hydrodynamic model, and their effects on the exerted drag force, at different flow velocities, have been studied numerically. For validating the numerical solution, the simulation results have been compared with the experimental data obtained by testing an underwater hydrodynamic model in a towing tank laboratory; and the validity of the numerical solution results has been confirmed. The results indicated that, riblet spacing has a significant effect on the reduction of drag force. Furthermore, by increasing the riblet spacing, the drag force is increased rather than decreased. Also, as the velocity increases, the performance of riblets in reducing the drag force is enhanced. In order to minimize the drag force applied on the underwater hydrodynamic model, by analyzing the numerical results, the most optimum riblet spacing has been obtained; at which the drag force is reduced by 7%. The achieved distance is a limit value; and at distances smaller or larger than this optimal distance, the effectiveness of the blade-shape riblet surface in reducing the drag force diminishes.
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Numerical Investigation on the Effect of Dry and Wet Compression on a Linear Low Speed Compressor Cascade
Pages : 67-77
Authors : D. Narayanan,  S. Anand,  S. Anish, 
Several techniques are implemented to reduce the temperature rise in multistage compressors, which leads to the noticeable improvement in specific power output of a gas turbine. The objective of the present investigation intends to understand the effect of incidence angles on the aerodynamic performance of the compressor cascade under wet compression. Using large eddy simulations (LES) the effects of wet compression on compressor flow separation and wake formation are investigated. Experimental investigation was performed to validate the numerical results. The study reveals notable flow modifications in the separated flow region under the influence of wet compression and the total loss coefficient reduces significantly at the downstream side of the compressor for positive incidence angles. On the other hand, for negative incidence angles the wet compression enhances the total pressure losses inside the blade passage. Also, in the present investigation, particular emphasis has been given to understand the water film formation at negative and positive incidence angles.
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Mixing Enhancement of Electroosmotic Flow in Microchannels under DC and AC Electric Field
Pages : 79-88
Authors : S. Dong,  P. F. Geng,  D. Dong,  C. X. Li, 
A novel micromixer is presented in this study and the effect of DC or AC electric field on mixing efficiency is investigated numerically. Four types of AC waveforms are considered to explore the flow characteristic and mixing efficiency. The velocity field, concentration field and the mixing efficiency are analyzed in details. The results demonstrated that a pair of vortices with opposite rotating directions is generated when DC or AC voltages is applied to the electrode plates planted within the walls. The generated vortices greatly enhance the mixing of incoming fluids with different concentrations. The mixing efficiency firstly rises with time and then reaches a relative stable periodic state under different potential waveforms. As the voltage applied on the plates increases, the mixing efficiency is improved obviously. The mixing efficiency under full-wave AC signal is the highest, and it is up to 95.44% when the applied potential is 3 V and frequency is 5 Hz.
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Optimization of the Splitter Blade Configuration and Geometry of a Centrifugal Pump Impeller using Design of Experiment
Pages : 89-101
Authors : M. Namazizadeh,  M. Talebian Gevari,  M. Mojaddam,  M. Vajdi, 
Centrifugal pumps are among the most applicable machines in a wide variety of industrial systems for fluid pumping and transportation. Therefore their optimization has always been of great importance. Pump impellers play an important role in these machines as the energy transfer takes place in this part. In the present study, the impeller of a centrifugal pump is optimized by investigating the effect of adding splitter blades and modifying their geometry. A centrifugal pump is experimentally tested and numerically simulated and the characteristic curves are obtained. In the first stage, two different sets of splitter blades with different lengths are added to the impeller and the effect of splitter blade lengths on the results are explored. The case with the highest total head and overall efficiency is selected for the optimization process. The main blade and the splitter blade leading edge position and also the splitter blade distance between two successive blades are selected for the optimization process in the second stage. Efficiency and total head of the pump are considered as the optimization objectives. Using Design of Experiment (DoE) technique, the design space is created and response surface method is utilized to find the optimum geometry. The results show adding splitters can improve total head by about 10.6% and by modifying the geometry using DoE technique it could increase further by 4.4% with the negligible effect on the pump overall efficiency.
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Experiments and Large-Eddy Simulations of Lobed and Swirling Turbulent Thermal Jets for HVAC's Applications
Pages : 103-117
Authors : A. Bennia,  H. Fellouah,  A. Khelil,  L. Loukarfi,  H. Naji, 
The mixing improvement by passive control is of wide practical interest. The lobed diffuser, which mixes the primary and secondary streams with high efficiency, has been widely used for heat and mass transfer in the field of fluid engineering. In addition, the jets through lobed generate streamwise vortices, which mix the ambient air and the jet fluid more effectively. The main objective of the present work is to develop new air diffusers for heating, ventilation, air conditioning (HVAC) systems using different jet geometries, in order to improve the users’ thermal comfort. Three free jets of air diffusers emitted from a tubular lobed, with six and five lobes, and from a swirl nozzle have been both studied experimentally and numerically. All diffusers have the same throat diameter. It turns out that the results obtained with the LES/WALE and LES/K-ET turbulence models are respectively in good agreement with the experimental results of the lobed and swirling jets. These results indicate that the best mixture is obtained using the six-lobed nozzle with respect to the five-lobed nozzle and the swirling nozzle. In addition, the importance of the jet type on the mixing capacity is highlighted.
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Parameter and Design Analysis for Gasoline Direct Injection Injector using CFD and Design of Experiments
Pages : 119-132
Authors : B. Biçer,  A. Yurtkuran, 
Numerical modeling of internal nozzle flow can be regarded as an essential investigation in the field of gasoline direct injection system of combustion engines since it is directly connected with fuel spray atomization and subsequently efficiency of exhaust gas emission. Internal nozzle flow can be changed and formed according to several parameters such as; system pressure, chosen fuel type, the orientation of spray holes according to injector axis, conicity of spray holes and distribution of spray holes on valve-seat, etc. The changes in these parameters also affect the formation of cavitation inside of whole domain, spray angle and create wall-wetting on the spray hole surfaces. The present work investigates the parameter and design analysis in the valve-seat region of direct gasoline injection (GDI) injector using Computational Fluid Dynamics (CFD) and Design of Experiments (DOE). CFD is employed to study the behaviors of internal flow inside the valve-seat region according to several design parameters, whereas a mixed-level factorial design is used to test the significance of the effects on the response variables. In conclusion, the effects of the most significant factors on response parameters as amount of vapor formation, spray (Tau) angle, and pre-hole wall wetting are determined for further efficient design.
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Optimization Design of the Integral Inertial Particle Separator Based on Response Surface Method
Pages : 133-145
Authors : Z. Li,  Z. Wang,  J. Shi, 
Inertial Particle Separator (IPS) is widely used as an important inlet protection device for turbo-shaft aero engine to protect the core engine in seriously polluted environment. In order to improve the separation efficiency of the IPS, an investigation was conducted to study the influence of critical geometrical and aerodynamic parameters on IPS performance, and Response Surface Method(RSM)was applied to explore the interaction between different parameters and obtain the response of the IPS performance on different parameters. Results show the separation of the sand particle in the IPS is achieved by the inertial accumulation of the sand particle, the trajectories of particles with small size are dominated by flow direction while paths of particles with larger size are dominated by the individual particle inertia and bounce characteristics from the IPS walls. The separation efficiency of the IPS is not only affected by the single geometrical parameters or aerodynamic parameter, but also apparently influenced by the interaction effects between different parameters. The most conspicuous influencing factor for the IPS separation efficiency on the AC-Coarse dust is Math and the interaction effects between Ro1 and Math. IPS separation efficiency on the AC-Coarse dust is improved by 3.8% by multi-factors optimization based on RSM, and the sand particle with size larger than 8 micron can be completely separated.
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