

مجله مكانيك سيالات كاربردي
Journal Of Applied Fluid Mechanics
ISSN 17353572
دوماهنامه داراي رتبه علمي  پژوهشي (علوم پايه) به زبان انگليسي
سال يازدهم، شماره 1، 2018




 Effect of Solvent Contribution on Thermally Developing Flow of FENEP Fluids between A. Filali , L. Khezzar , M. Alshehhi , Z. Nemouchi , Pages 19 Abstract Full Text [PDF 530KB]   Numerical computation of thermally developing laminar flow of viscoelastic FENEP fluids flowing between two stationary parallel plates is investigated using the finite element technique. The influence of the effect of the solvent contribution as well as the fluid rheology on the flow field and heat transfer enhancement is investigated for the case of imposed constant wall heat flux and neglected viscous dissipation. Numerical results for flow field are compared first against available analytical solutions with and without inclusion of the solvent contribution. The obtained results for the viscoelastic case show that increasing Weissenberg number (We) leads to an increase in Nusselt number (Nu) while high values of the extensibility parameter (L2) decrease the Nusselt number. Fully developed Nusselt number values for FENEP fluids flowing between two fixed parallel plates are obtained for several values of polymer concentration and the study confirms quantitatively that polymer concentration enhances heat transfer rates in FENEP fluids
  
 Effect of Separation Angle and Nozzle Radial Position on Mixing Time in Ladles with Two Nozzles A. S. Gó, , mez , A. N. Conejo , R. Zenit , Pages 1120 Abstract Full Text [PDF 456KB]   Chemical، thermal and mechanical homogenization of both slag and steel during the ladle furnace process depends on the design of the gas injection system in gas bottom stirred ladles. In the past، a large number of variables have been investigated، nevertheless due to the importance of the slag layer during the process، it has been incorporated in water modeling studies in more recent investigations. In large industrial size ladles is common to use two porous plugs. The configuration of the injection system with two porous plugs requires optimization of both nozzle radial position and nozzle separation angle. In this work the effect of nozzle radial position، nozzle separation angle، gas flow rate and slag thickness on mixing time has been investigated using a water model. The effect of tracer concentration on mixing time was also explored. It is shown that a separation angle of 60 degrees provides the best mixing efficiency.
  
 A Thoroughgoing Design of a Rapidcycle Microfluidic Dropletbased PCR Device to Amplify Rare DNA Strands M. Mollajan , S. Razavi Bazaz , A. Abouei Mehrizi Pages 2129 Abstract Full Text [PDF 458KB]   DNA is a molecule and assortment of fruitful information of organisms and a wide range of viruses. Polymerase chain reaction (PCR) is a process used to amplify DNA strands in order to generate millions of them and extract the applicable information. Although conventional methods for PCR are flourishing to a certain extent، they have such major drawbacks as contamination، high material consumption، and lowspeed function. By the combination of PCR devices with the microfluidic approach and integrating them with droplet generation technology، the mentioned problems can be eliminated. In this study، a novel twostep rapidcycle dropletbased PCR (dPCR) device، considering the design of microchannel and heat transfer system، has been presented. First، numerous studies have been conducted to select the proper droplet generator for the integration of the droplet generation with the PCR device. Then، with the careful attention to the requirements of a PCR device، the geometry of different zones of the PCR device has been، meticulously، designed. In the next and last step، the heat transfer system for the designed zones of the PCR device has been planned. Afterward، results are examined carefully which indicate that in a cycle of PCR، they are not any major discrepancies between the designed dPCR and the ideal one—the one that is intended to be created.
  
 Alterations of Cowl Lip for the Improvement of SupersonicIntake Performance B. John , P. Senthilkumar Pages 3141 Abstract Full Text [PDF 862KB]   This paper discusses the performance enhancement of supersonic air intake model through the implementation of blunted leading edge to the cowl lip section of the model. A supersonic air intake model with sharp cowl leading edge is initially considered to numerically investigate its performance. Mach 3، supersonic intake flow through the base model has been simulated using commercial CFD package Ansys Fluent15. Comparison of numerical predictions and experimental measurements is presented to demonstrate the correctness and accuracy of numerical frame work followed in the present study. Higher order spatial accuracy of the solver along with suitably refined mesh helped in accurate capturing of the flow field. Modification to the cowl lip is proposed as an effective method to improve the performance of the supersonic air intake. Two different blunted cowl leading edge geometries were investigated to identify the possible enhancement in performance parameters. Improvement in mass capture and combustion stability attained through the use of forward shifted blunt cowl leading edge is presented. It is also revealed through the present study that the blunt cowl leading edge can reduce the intensity of shock wave boundary layer interaction occurring at the isolator entry section. Deviation in total pressure recovery and flow distortion observed with different supersonic air intake models are also discussed with reasons for the same. This study demonstrates the scope of overall improvement in scramjet engine performance through the use of suitably positioned blunt cowl leading edge.
  
 Effects of Reinjection on Flow Field of Open Jet Automotive Wind Tunnel Test Section Q. Li , W. Dai , L. Zhong , Z. Yang , K. Du , Y. Xu , M. M. Rashidi , Pages 4353 Abstract Full Text [PDF 1023KB]   Effects of Reinjection on Flow Field of Open Jet Automotive Wind Tunnel Test Section The distributions of axial static pressure coefficient and flow fluctuation in the test section which affect aerodynamic measurement in an open jet wind tunnel is presented. In this paper، the flow characteristics of the open jet automotive wind tunnel with passive reinjection and active reinjection were simultaneously investigated by experimental and numerical approaches. The axial static pressure coefficient variations can be reduced by passive or active reinjection، and recycle flow returns to the test section from the loophole is the main reason. The more mass flow rate improves the effect. Meanwhile، it is found that the improvement of the axial static pressure coefficient by reinjection is always better in the condition of 0° collector angle. The turbulence intensity in the collector angle of 15° is lower than that of 0°، and the reinjection increases the turbulence intensity near the collector. The increase of the turbulence intensity by active reinjection in the collector angle of 0° is greater than the collector angle of 15° for the 3.28° diffusion angle. There are some peaks emerging at the frequencies of 40 Hz and 50 Hz، which indicates that the flow field fluctuations may have induced structural vibration. The peaks at several frequencies increase when the passive and active reinjection are conducted، and the increase of peak is correlate with the increase of the reinjection flow rate. Due to the reduction of average static pressure coefficient and increase of flow fluctuations، the application of passive and active reinjection should be considered at the same time.
  
 Hydrodynamics in TwoPhase Turbulent Boundary Layers M. Dhahri , H. Aouinet , Pages 5563 Abstract Full Text [PDF 469KB]   The corrections for log law must be taken into account the presence of bubbles in the two phase turbulent boundary layer. In the present study، a logarithmic law for the wall based on the supposition of additional turbulent viscosity associated with bubble wakes in the boundary layer was proposed for bubbly flows. An empirical constant accounting both for shear induced turbulence interaction and for nonlinearity of bubble was determined for the new wall law، this constant was deduced from experimental measurements. In the case of a turbulent boundary layer with millimetric bubbles developing on a vertical flat plate، the wall friction prediction achieved with the wall law was compared to the experiences. We obtained a good concordance between experimental and numerical result. This significant agreement for wall friction prediction was particularly important for the low void fraction when bubble induced turbulence have a considerable role.
  
 Linear and Weak Nonlinear Double Diffusive Convection in a Viscoelastic Fluid Saturated Anisotropic Porous Medium with Internal Heat Source A. Srivastava , A. K. Sing Pages 6577 Abstract Full Text [PDF 1660KB]   This paper deals linear and weak nonlinear stability analysis of doublediffusive convection in an anisotropic porous layer with internal heat source saturated by viscoelastic ﬂuid. For linear stability analysis we use normal mode technique and obtained the expression for oscillatory thermal Rayleigh number which is used to plot neutral stability curve for oscillatory case. For nonlinear analysis truncated representation of Fourier series upto two terms is used. The system of time dependent nonlinear equation is solved numerically and plot the curve for heat transfer and mass transfer with respect to time for different parameters. Effect of thermal anisotropy parameter، mechanical anisotropy parameter، relaxation parameter، retardation parameter، internal heat source parameter، solute Rayleigh number، diffusivity ratio، DarcyPrandtl number on the onset of convection، heat and mass transfers have been discussed. We also draw the stream lines، isotherms، isohalines at different times.
  
 Shielding Gas Coaxial Jet Pipes Numerical Study of a Vertical Laser Welding Process of AZ91 Magnesium Alloy C. Boughanmi , S. Bannour , H. Mhiri , P. Bournot Pages 7994 Abstract Full Text [PDF 850KB]   The laser welding of magnesium alloys، largely used in many fabrication applications، has gained considerable interest especially in aerospace، electronics، automotive industry etc. Unfortunately، this process is associated to an undesired phenomenon which is “oxidation”. For this reason، a good shielding system of the welding zone is of major importance. This paper presents a numerical study using computational fluid dynamics (CFD) of a laser welding process employing a moving volumetric heat source. Starting with the turbulence model validity، a parametric study of this welding process in a vertical position aiming to optimize the design of protection gas device، the gas jet inclination، the appropriate welding direction and the gas type is، then، proposed. The optimum parametric combination ensuring the largest gas coverage area is the one where the shielding gas is Argon، supplied by the coaxial nozzles at a downward inclination angle with respect to the laser beam axis، and a downward welding direction.
  
 Numerical Visualization of Plunging Water Jet using Volume of Fluid Model A. Boualouache , F. Zidouni , A. Mataoui Pages 95105 Abstract Full Text [PDF 543KB]   A plunging liquid jet is defined as a moving column of liquid passing through a gaseous headspace، air in our case، before impinging a free surface of receiving liquid pool. The mechanism of air entrainment due to plunging liquid jets is very complex and the complete mechanism of air entrainment is not fully understood so far. The present paper is an unsteady numerical simulation of air entrainment by water jet plunging، using the Volume Of Fluid (VOF) model. The piece wise linear interface construction algorithm (PLIC) for interface tracking is used، to describe the phase distributions of entirely immiscible air and liquid phases. The aim of this work is to investigate the performance and accuracy of the VOF method in predicting the initial impact between the descending jet and water free surface، air entrainment and the developing flow region under free surface. Three scale models based on geometric similarities (Froude number and dimensionless free jet length) are used for validation according to Chanson (2004) experience. The simulations show with accuracy، the air cavity formation steps، caused by the initial jet impact، its deep stretching under the pool free surface، until breakdown due to the shear created by a toroidal vortex. In terms of، air entrainment estimation، bubble dispersion and radial distribution of air volume fraction، largescale models present a good agreement with the experience. However، for the smallest scale model، the results lead to suggest that air entrainment is governed by more parameters than the geometric similarities.
  
 Studies on Droplet Size Distribution of OilinWater Emulsion in SMX Static Mixer L. Muruganandam , D. Kunal , G. O. Melwyn Pages 107114 Abstract Full Text [PDF 466KB]   Oil droplet size distribution of an emulsion produced by Sulzer Chemtech's static SMX static mixer under flow condition was experimentally studied and reported. The dispersed phase of vegetable oilinwater (O/W) emulsion produced through static mixer by varying the concentration from 1 to 4 vol % oil in water، flowrate of dispersed and continuous phase and operating time. The effect of run time on oil drop sizes is characterized using the spectra obtained from the particle size analyser. The static mixer with 9 perpendicular elements made of teflon is stacked against each other had a void fraction of 0.93. The sauter mean diameter of oil droplet decreases from 8 µm to 4 µm with an increase in Reynolds number. The emulsion droplets of mean sauter diameter in the range 4.1 µm to 4.7 µm were produced by increasing the concentration of the dispersed phase from 1:100 to 1:25، within a span value of between 30 to 240 sec، at atmospheric pressure and room temperature. Performance equation for sauter mean oil droplet diameter is developed based on the experimental data has ±0.2 rms deviation.
  
 Numerical Study on the Smoke Flow Characterization and Phenomenon of PlugHoling under Lateral Smoke Exhaust in Tunnel Fire J. Yang , X. Pan , Z. Wang , M. Hua , J. Jiang Pages 115126 Abstract Full Text [PDF 1040KB]   Although the lateral smoke extraction system had been adopted in the tunnel and subway، the research on the efficiency of the lateral smoke exhaust system is still lacking. A set of numerical simulations were conducted using Fire Dynamic Simulator (FDS) to analyze the plugholing phenomenon under lateral smoke extraction system in the tunnel fire. Taking an actual lateral smoke exhaust system in a road tunnel as the prototype، a series of fire cases were simulated، wherein the heat release rate (HRR) of the fire source and mechanical exhaust rate، in particular، varied. The smoke flow characteristic in the lateral exhaust system is analyzed and phenomenon of plugholing was observed. The temperature and smoke layer near the lateral smoke vent are analyzed، and results show that the phenomenon of plugholing will decrease the lateral smoke exhaust system performance. The exhausting efficiency would not change significantly as the exhaust rate increasing. The critical Froude number we calculated to determine the phenomenon of plugholing under lateral smoke exhaust in tunnel fire is 0.48، meanwhile، a saturated Froude number 1.95 was introduced by taking account of a better exhaust efficiency.
  
 An Innovative Roof Shape in Liquid Storage Tanks to Reduce Dynamic Sloshing Effects P. Nouraeidanesh , M. M. Kabiri , M. A. Goudarzi Pages 127136 Abstract Full Text [PDF 702KB]   In this study، a new type of tank roof form is suggested to reduce the high impact forces caused by sloshing. Using this roof allows the tank designers to consider less freeboard، which is economically valuable. For this purpose، an experimental investigation has been implemented to evaluate the efficacy of the proposed roof to distribute the contained liquid impact forces in several time stages. In these experimental measurements، a series of shaking table tests are conducted for a partially filled tank under harmonic and various earthquake excitations for both typical and proposed tank roof forms. The liquid impact forces are reasonably evaluated and compared for both types of tank roof. The efficacy of the proposed roof design is validated by experimental results and it is shown that the sloshing loads can significantly be reduced up to an average of 50% for the dimensions considered in the experiments.
  
 Control of Tip Leakage Flow in Axial Flow Compressor Cascade by Suction on the Blade Tip X. Mao , B. Liu , T. Tang , Pages 137149 Abstract Full Text [PDF 1669KB]   One of the important ways of improving axial compressor performance is to control the tip leakage flow near the endwall region. Numerical computations were conducted to investigate the impact of blade tip suction on the axial compressor cascade performance in current paper. Three suction schemes located on the blade tip with different chordwise coverage were investigated in total. The results show that the cascade overall performance can be effectively enhanced by the proper suction scheme on the blade tip and the best scheme should be arranged at slightly downstream of the onset point of the tip leakage vortex (TLV). The control effectiveness and mechanisms are different for the different suction schemes. For the suction scheme covering the starting point of TLV، the onset point of TLV is shifted downstream، while an additional induced leakage flow near the blade leading edge is generated resulting in the increase of mixing loss. It is more effective when the structure of the main TLV is destroyed and divided into different parts by applying the blade tip suction arranged slightly behind the onset point of TLV. In addition، the blade loading is redistributed near the blade tip after the blade tip suction and the total pressure loss caused by the suction slots should also be considered in the design process.
  
 Numerical Investigation of Turbulent Flow around a Recent Horizontal Axis Wind Turbine using Low and High Reynolds Models A. Bouhelal , A. Smaili , O. Guerri , C. Masson Pages 151164 Abstract Full Text [PDF 1569KB]   The effects of different Reynolds Averaged Navier Stokes (RANS) turbulence models on two nearwall approaches using high and low Reynolds models on predicting performance of horizontal axis wind turbines (HAWTs) were studied for a range of wind conditions where flow over the rotor varied from fully attached to massively separated flow. This paper's main contribution is in establishing which RANS models can produce quantitatively reliable numerical predictions of turbulent flow around wind turbine rotors. The authors used measurements done by the new MEXICO (Model rotor EXperiments In COntrolled conditions) project in the German Dutch wind tunnels (DNW) in order to validate and test CFD (Computational Fluid Dynamic) codes. Four different RANS turbulence models were considered: SpalartAllmaras; kε (RNG); kω SST; and the transition γReθ model. At low wind speeds، it was found that all four models were good predictors of aerodynamic performance، and at high wind speeds، where the swirl effect was modeled using wall function corrections in both equations، the kε model was considered to be the best model: it was the most accurate within a reasonable computational time.
  
 Detached Eddy Investigation of Conical Cavity Flow at Mach 0.9 J. Teng , J. Zhang , Z. An , Y. You Pages 165176 Abstract Full Text [PDF 699KB]   An improved delayed detached eddy simulation (IDDES) is carried out to investigate both the mean and the instantaneous flow characteristics of conical cavities fixed on a cone surface in a Mach 0.9 freestream. Two model categories، a single cavity model with different length (L) to depth (D) ratio and a multicavity model that inserted the separation disk into a single cavity are studied. Results indicate that in case of a single cavity model، the cavity L/D ratio is the key parameter that influences mean flow structure and oscillatory characteristics. The bulk reverse flow structure can be broken up into smaller structures as L/D value reduces from 4.0 to 0.51 inside the cavity. Smaller longitudinal pressure gradients on the cavity wall and the thicker downstream boundary layer are observed when larger L/D values are present. The cavities with smaller L/D values can stimulate the pressure oscillation fundamental frequency as well as the acoustic tones to increase the value. Multicavity configurations through insertion of a separation disk into the single cavity are helpful in cutting the large reverse flow structure inside a cavity into a smaller، similar، coherent structure، and to reduce the overall cone drag. The insertion of the separation disk can modulate pressure oscillation peaks and sound pressure levels to higher frequency، however، they are not able to reduce the overall sound pressure inside the cavity. Pressure oscillations within each cavity of the multicavity configuration have a weak correlation.
  
 Mixed Convection Heat Transfer of Al2O3 Nanofluid on the Elliptical Shapes: Numerical Study of Irreversibility K. Khajeh , L. Jahanshaloo , S. Ebrahimi , H. Aminfar Pages 177189 Abstract Full Text [PDF 437KB]   In this study the 2D laminar and steady waterbased Al2O3 nanofluid flow over a cylinder with circular، horizontal and vertical elliptical cross section by constant surface temperature boundary condition has been studied. The main goal of this research is to investigate the effects of different natural and mixed convection heat transfer mechanisms on the convective heat transfer coefficient، and the entropy generation due to the thermal and frictional origination. Conservation equations of the mass، momentum and energy under the assumption of incompressible، Newtonian nanofluid، by using the homogeneous single phase method have been solved. The impact of considered parameters in this study (alteration in cross section، convective flow direction and volume fraction of nano particles) in enhancing the heat transfer rate is studied in association with the entropy generated value in each case. Based on the results، the vertical elliptical cross section، in comparison with others، shows the highest entropy generation value and the heat transfer coefficient in all considered mechanisms. Moreover، mixed convection heat transfer type 2، in which the force flow is perpendicular to the buoyant flow direction، has the highest entropy generation and heat transfer rate for all cross sections. In addition، in all cases in the presence of the nanoparticles، the heat transfer rate and entropy generation increases.
  
 Characterization of Vortex Development and ThermoSolutal Transfers on Confined Wall Jets Submitted to Suction or Blowing: Part 2 S. Mejri , M. A. Knani Pages 191204 Abstract Full Text [PDF 1536KB]   A computational study is conducted to explore the effect of vertical wall suction or blowing on twodimensional confined wall jet hydrodynamic characteristics. Using an implicit finite volume technique in Cartesian coordinate system، several parameters have been investigated for a wide range of Lewis numbers by fixing the Prandtl number at 7 that corresponds to water. The main purpose is to analyze the control size and location effectiveness on the flow pattern as well as heat and mass transfer rates. Detailed numerical simulations demonstrated that as the local blowing is moved downstream، discrete vortex formation begins at a critical location then shedding phenomenon occurs behind the slot at advanced positions. Since the flow dynamic structure is mainly altered، averages skin friction and thermosolutal coefficients distributions are largely influenced. Approximately for x_s≤4 (upstream of the natural vortex emission position)، Nusselt and Sherwood numbers slightly increase with the control location x_s. However، they gradually decrease as the blowing slot approaches the domain exit. Optimum values were obtained when locating the slot just downstream of the uncontrolled KelvinHelmholtz instability onset. Furthermore، computations illustrated that an appropriate suction slot length selection could be a simple and efficient tool to delay or even suppress natural structure emission and development. This choice is essentially related to the recirculation cell size.
  
 Swirl Characteristics of Vortex Valve VariableThrust Solid Rocket Motor X. G. Wei , J. Li , G. Q. He Pages 205215 Abstract   In accordance with the flow characteristics of vortex valve variablethrust solid rocket motors، a cold flow experimental system based on Particle Image Velocimetry was established. A flow velocity vector diagram of vortex chamber was generated، and the vortex structure was analyzed. The results provided an experimental foundation for numerical simulation. The flow characteristics in vortex chamber and in the throat and divergent sections of the nozzle were modeled and simulated. The flow in the vortex chamber conformed to the complex Rankine vortex، and the flow field was divided into three different zones. The vortex core was the primary influence factor for thrust modulation. The resultant velocity reached Mach number 1 before gas arrived at nozzle throat، and the axial velocity still reached Mach number 1 at nozzle throat. Hence، the axial velocity can be used to judge the occurrence of choking at the nozzle throat. The intensity of swirl flow in divergent section of the nozzle was evidently lower than that in vortex chamber and throat. As a result، a lowpressure zone emerged around the central axis، thereby causing thrust losses.
  
 SteadyState Experiment and Simulation of Intake Ports in a FourValve Direct Injection Diesel Engine D. W. Jia , X. W. Deng , J. L. Lei Pages 217224 Abstract Full Text [PDF 636KB]   In order to analyze intake port flow characteristics of a fourvalve direct injection (DI) diesel engine، steadystate flow bench experiments and numerical simulations method were coupled to investigate the following four combined intake ports: (1) helical port (left) and tangential port (right); (2) tangential port (left) and helical port (right); (3) helical port (left) port and helical (right); and (4) tangential port (left) and tangential (right) port. Results show that the simulation of port flow coefficients matches experimental findings very well، and the port coefficients of the above four combinations do not vary much، but their swirl ratios are very different. Specifically، when the valve lift is the maximum، the swirl ratio of the combination of "helical and tangential" is the greatest among the four combinations، and the swirl ratio of "tangential and tangential" is the minimum. And the3D fluid simulation method and steadystate experiment are important means to investigate the flow characteristics of the combined intake ports.
  
 Experimental Study of Subcooled Boiling Heat Transfer of Axial and Swirling Flows inside Mini Annular Gaps S. Ahangar Zonouzi , H. Safarzadeh , H. Aminfar , M. Mohammadpourfard Pages 225232 Abstract Full Text [PDF 688KB]   An experimental study of the subcooled boiling heat transfer of axial and swirling upward flows inside vertical mini annular gaps was conducted using deionized water. The subcooled boiling heat transfer coefficients and the boiling curves of the flow inside mini annular gaps with different gap sizes have been investigated. The experimental results both for the single phase heat transfer and subcooled boiling heat transfer inside mini annular gaps showed very good agreement with correlations in the literature. The results showed that the subcooled boiling heat transfer coefficient for a given heat flux increases as the size of the annular gap is decreased. The maximum wall superheat is also influenced negligibly by mass flux. Furthermore، the effects of swirl flow by using spring insets inside the mini annuli on the single phase and subcooled boiling heat transfer have been studied. The results showed that the single phase and subcooled boiling heat transfer coefficients are increased by having swirl flow inside mini annuli using spring inserts. The obtained results also showed that the heat transfer enhancement by having swirl flow inside the annuli using spring inserts decreases as the applied heat flux is increased in the subcooled boiling heat transfer region.
  
 CFD Study on Jet Stirred Oxidation Pond of Absorption Tower in Power Plant W. Li , Y. Yao , J. Chen Pages 233240 Abstract Full Text [PDF 1005KB]   In the paper، the standard kε model and the SST kω model were employed to predict the velocity field in the jet mixing tank، and the simulation results were validated by experimental data. It showed that the standard kε model can predict the velocity field of jet mixing tank more accurately than the SST kω model. The standard kε model was applied to investigate the effects of the jet inclination angle (i.e.، 0°، 10°، 20°، 30° and 40°) and the jet velocity (i.e.، 14، 16، 18 and 20m/s) on the mixing uniformity of the jet mixing oxidation pond. Based on the evaluation criterions: unprecipitated area ratio (UPAR) and nonuniform velocity coefficient (NUVC) proposed in the paper، when the jet inclination angle and jet velocity are 10° and 18m/s، respectively، the jet mixing effect in the oxidation pond is the best. The study can be helpful for the optimization of the flow field in the jet mixing oxidation pond to improve the desulfurization efficiency.
  
 On the Instability of Two Dimensional BackwardFacing Step Flow using Energy Gradient Method H. Nowruzi , S. Salman Nourazar , H. Ghassemi Pages 241256 Abstract Full Text [PDF 665KB]   In the present paper، the energy gradient method is implemented to study the instability of 2D laminar backwardfacing step (BFS) flow under different Reynolds numbers and expansion ratios. For this purpose، six different Reynolds numbers (50 ≤ Re ≤ 1000) and two various expansion ratios of 1.9423 and 3 are considered. We compared our results of the present study with existing experimental and numerical data and good agreement is achieved. To study of fluid flow instability، we evaluated the distributions of velocity، vorticity and energy gradient function K. The results of our study show that as the expansion ratio decreases the flow becomes more stable. We also found that the origin of instability in the entire flow field is located on the separated shear layer nearby the step edge. In addition، we approved that the inflection point on the profile of velocity corresponds to the maximum of vorticity resulted to the instability.
  
 Optimization and Biodiesel Production from Prosopis Julifera Oil with High Free Fatty Acids M. Rajeshwaran , P. Ganeshan , K. Raja Pages 257270 Abstract   Prosopis julifera is a nonedible feedstock found in the arid and semiarid regions was used for the production of biodiesel. Solvent extraction technique was used for oil extraction from Prosopis julifera .The present work mainly concentrates on the three step process of biodiesel production from Prosopis julifera oil .The acid value of Prosopis julifera oil was reduced below 1% using acid catalyst 1% v/v H2SO4 followed by esterification process using alkaline catalyst (KOH).Transesterification reaction is found to be affected by the reaction variables namely methanol to oil molar ratio، amount of catalyst used، reaction time and reaction temperature. Gas chromatography was used to analyse the Fatty acid methyl esters. The methyl ester obtained from the previous step was refined to produce biodiesel. The fuel properties of Prosopis julifera methyl ester (PJME) such as viscosity، cetane number، flash point، acid value، etc were determined and compared according to the ASTM standards. The optimum reaction conditions of Methanol/oil molar ratio of 9:1v/v، reaction temperature of 550C، reaction time of 2 hrs and 0.75% w/v of KOH usage were determined. Response surface Methodology (RSM) technique was used to optimize the maximum yield of Prosopis julifera methyl ester.
  
 Natural Convection in an Enclosure with a Discretely Heated Sidewall: Heatlines and Flow Visualization M. Saglam , B. Sarper , O. Aydin Pages 271284 Abstract   Natural convection inside a rectangular enclosure is investigated experimentally and numerically. One of the sidewalls is heated discretely by two flushmounted heat sources. The other sidewall is kept at a constant temperature، while the horizontal walls are unheated. Heat dissipation rates of the heat sources are equal to each other. The aspect ratio of the enclosure (AR) is 2 and the working fluid is air (Pr=0.71). The study is focused on the validation of the two and three dimensional computations under real test conditions against experiments for various modified Rayleigh number values. Experimental study is performed for various modified Rayleigh numbers in the range of 7.7x105 and 3.1x106 while numerical part covers the values between 104 and 5x106. Temperature measurements and flow visualization studies are performed in the experimental work، and streamlines، isotherms and heatlines are presented in the numerical part of the study. From the experimental and numerical studies، it is shown that two dimensional computations reflects the general characteristics of the problem، conduction and radiation heat transfer are not negligible، surface temperatures increase with the modified Rayleigh number and heatline approach is an important tool to analyze convective heat transfer.
  
 Experimental and Computational Analysis on Aerodynamic Behavior of a Car Model with Vortex Generators at Different Yaw Angles G. Shankar , G. Devaradjane Pages 285295 Abstract   Investigating the effects of aerodynamic characteristics in the automotive segment has been one of the thrust areas of research in the recent years. Extensive research had been carried out earlier in minimizing the aerodynamic drag of the car body and to study the effects using passive air flow deflectors. Little work has been conducted on semiactive or actively controlled air flow modification techniques. In order to contribute new knowledge in the chosen area and to draw the attention of current researchers the present work focuses on the study of aerodynamic characteristics of a typical sedan car model equipped with three numbers of delta shaped vortex generators (VGs) as an aerodynamic addon device to delay the early flow separation of air from the vehicle body. The yaw angles of the VGs are semi actively controlled using mini stepper motors. The middle VG is kept stationary، whereas the other two VGs orientation has been modified and the results have been studied. The aerodynamic property of a car model mounted with four distinct yaw angle configurations obtained by means of mini stepper motor has been quantitatively evaluated by subsonic wind tunnel tests and computational analysis. From the experiments the peak drag and lift coefficient reduction rates of 4.53% and 2.55% respectively have been observed in the case of car model with vortex generators having leading edges facing the rear end and the mid plane of the car respectively when compared with the car model without vortex generators. Numerical simulation using realizable (k ε) model predicted the drag and lift coefficient reduction rates closer to the experimental values and also it predicted the existence of magnitude of turbulent kinetic energy variation in the roof portion of the car model with four dissimilar configurations of vortex generators relative to the case of car model without vortex generators.
  
تاريخ انتشار: 18/10/96 تلفن: 33915271 (031)
تاريخ درج در سايت: 18/10/96
شمار بازديدکنندگان اين شماره: 147




