Recent Volume
Vol12 , No 4 (in press)
Study of Droplet Impact on a Wall using a Sharp Interface Method and Different Contact Line Models
Pages : 1001-1012
Authors : M. Emdadi,  P. Pournaderi, 
In this research, droplet impact on a surface is simulated by using a sharp method for interface modeling. The level-set method along with the ghost fluid method is used to model interface in a sharp fashion. Different contact line models are compared and evaluated at both low and high impact velocities. On a hydrophobic surface, dynamic models developed by Hoffman and Jiang represent a more accurate prediction of droplet behavior during the impact process than the static and molecular kinetic dynamic models, especially at rebounding stage. At lower impact velocities, the Hoffman’s model represents better predictions. However, at higher impact velocities, the Jiang’s model is somewhat more accurate. The molecular dynamic model is not appropriate for high impact velocities. On a hydrophilic surface, at low impact velocities, the Jiang’s model represents satisfactory results, whereas the static and the Hoffman’s models cannot produce accurate results, after initial stages of the impact process. At high impact velocities, the static model shows considerable deviation from the experimental results. The effect of the contact angle on the dynamic behavior of the droplet is investigated. At contact angles lower than 900, the droplet only spreads on the surface after impact. However, at contact angles higher than 900, the droplet starts to recoil after spreading. In this case, it is possible that droplet rebounds from surface after recoiling. Maximum spreading radius of the droplet decreases by an increase in contact angle. At higher contact angles, less time is needed for the droplet to rebound from surface.

Numerical Investigation of Influence of Flow Rates on Combustion Characteristics using Multiphase Flamelet Combustion
Pages : 1013-1021
Authors : N. Shreekala,  S. N. Sridhara, 
Numerical analysis plays vital role in designing gas turbine combustors for improving its performance. It requires deep understanding of turbulent and multiphase reactive flow physics inside the combustor. Hence present work focusses on numerical investigation of turbulent flow and reactive interaction between liquid spray of aviation kerosene droplets and air for the experimentally investigated combustor configuration. The present analysis results agree well with the experimental test results. Mixture fraction based Probability Density Function flamelet combustion model is used to solve transport equations by generating the flamelet library for Jet-A fuel. The analysis provides good insight into the multiphase combustion process and the flow phenomena occurring inside the combustor. Further, combustion performance characteristics at different design point operations are investigated in the present work keeping the air fuel ratio constant. Results show rise in peak velocity by 2.5 times and pressure loss factor by 6 for design point operating at higher flow rates in comparison with the baseline design operating point.

Channel Blockage and Flow Maldistribution during Unsteady Flow in a Model Microchannel Plate heat Exchanger
Pages : 1023-1035
Authors : P. Dąbrowski,  M. Klugmann,  D. Mikielewicz, 
This paper describes the problem of channel blockage as a result of flow maldistribution between the channels of a model mini channel plate heat exchanger consisting of one pass on each leg. Each leg of the heat exchanger contains 51 parallel and rectangular minichannels of four hydraulic diameters namely 461 µm, 571 µm, 750 µm and 823 µm. In addition, a more complex geometry has been investigated where for the sake of breaking the development length the inclined transverse cuts have been incorporated. The moment of liquid phase transition through the exchanger (the working medium: water) was recorded for the mass fluxes ranging from 18.67 to 277.76 kg/m2s in 51 parallel channels with the use of a fast speed camera. The Reynolds numbers Re in the individual channels were from 10.76 to 90.04. The relationship between the mass flux and the size of the minichannels in the presence of the maldistribution is discussed here. The existence of the threshold in the mass flux below which the phenomenon occurs has been shown. Two mechanisms of channel blocking have been recorded and described in detail. A miniscale variation of one of them containing the extended geometry was created as well.

Numerical Analysis of a Two-Phase Flow (Oil and Gas) in a Horizontal Separator used in Petroleum Projects
Pages : 1037-1045
Authors : S. Yayla,  K. Kamal,  S. Bayraktar, 
In the present paper, two-phase three-dimensional turbulent flow simulations are carried out by applying computational fluid dynamics (CFD) technique to the internal flow of a horizontal separator that is used in petroleum industry. Two different geometries are considered; the separator with a straight plate at the top and the separator with the straight plate on the side of the separator. Effects of the location, distance between the inlet of the separator and the diverter plate and inlet velocity on the separation efficiency are investigated by employing the standard k- ε turbulence model. For these purposes, three different distances between the straight diverter plate and the inlet to the separator (0.1 m, 0.15 m and 0.2 m) and four different velocities (0.25 m/s, 0.5 m/s, 0.75 m/s, and 1 m/s) are taken into account by means of Euler mixture model. It is revealed that the maximum separation efficiency is 99.772% when the mixture enters the separator from the top with the inlet velocity of 0.25 m/s and the plate is located 0.2 m away from the inlet section of the separator. An inverse correlation is detected between the inlet velocity and the efficiency of the separation since increasing the inlet velocity decreases the efficiency of the separation.

Numerical Simulation of Flow behind Vortex Generators
Pages : 1047-1061
Authors : N. K. Singh, 
In this study, flow behind rectangular vane type vortex generators mounted on a flat plate, is numerically simulated using the immersed boundary (IB) method. In the present work, the direct forcing IB method is employed because of its simplicity and high efficiency. Vortex generators of two different heights are numerically investigated. The height of vanes in the first case is close to the definition of submerged/low-profile vortex generators while the other case is closer to the definition of a conventional vortex generator. The resultant highly three-dimensional flow and its transition to turbulence have been studied. Counter-rotating vortices generated by these passive rectangular vortex generators are characterized. Streamwise evolution of non-dimensionalised maximum values of vorticity, vortex strength, streamwise velocity and wall-normal velocity are studied. The simulations show that the IB method in conjunction with DNS effectively simulates the time-dependent flow behind an array of passive vortex generators placed in an initially laminar boundary layer.

Experimental Investigation on Acoustic Wave Generation due to Supersonic Hot Jet Impingement on an Inclined Flat Plate
Pages : 1063-1072
Authors : I. Bahman Jahromi,  K. Ghorbanian,  M. Ebrahimi, 
In the present paper, noise generation due to supersonic hot jet impingement (M=1.4 and Tt=950 K) to an inclined flat plate is experimentally investigated. In general, four types of acoustic waves are defined for jet impingement: acoustic waves generated by the shear layer of the main jet (type-A), by the impingement region (type-B), by the shear layer of the wall/jet downstream of the impinging region (type-C), and tonal acoustic waves observed in normal impingement. An attempt is made to understand the sources of the noise of the impinging jet by comparing acoustic scalograms of the impinging and free jets at the far-field. It is determined that the type-C acoustic wave images are similar to the far-field scalogram images of the free jet at the same polar angles. Further, the type-B acoustic waves in oblique jet impingement have similar acoustic signature with tonal noise due to normal jet impingement.

Simulation of Dipole Vorticity Dynamics Colliding Viscous Boundary Layer at High Reynolds Numbers
Pages : 1073-1081
Authors : E. Ezzatneshan, 
The vorticity dynamics of a Lamb-like dipole colliding with flat boundaries are investigated for high Reynolds number flows by implementation of the lattice Boltzmann method (LBM). The standard LBM based on the single-relaxation-time collision model suffers from numerical instabilities at high Reynolds numbers. Herein, a regularized collision model is employed for the LBM to preserve the stability and accuracy of the numerical solutions at such flow conditions. The computations are performed for the normal collision of the dipole with the no-slip boundary for several Reynolds numbers in the range of . The results obtained based on the regularized lattice Boltzmann (RLB) method for the statistical flow characteristics like the vorticity field and enstrophy quantity of the dipole-wall collision problem are investigated. The present study demonstrates that the shear-layer instabilities near the wall are responsible for rolling-up of the boundary layer before it is detached from the surface for high Reynolds numbers. This detachment mechanism leads to a viscous rebound and formation of small scale vortices. The shear-layer vortices formed dramatically influence the flow evolution after the collision and result strong enhancement of the total enstrophy of the flow field. By comparing the present results with those of provided by other numerical solutions, it is also concluded that the RLB scheme implemented is robust and sufficiently accurate numerical technique in comparison with the flow solvers based on the Navier-Stokes equations for predicting the statistical features of separated fluid flows even at high Reynolds numbers.

Finite Element Analysis of Pulsatile Blood Flow in Elastic Artery
Pages : 1083-1091
Authors : O. ElBanhawy,  A. Guaily,  M. Tosson, 
New hybrid Eulerian/Lagrangian model is presented accounting for the two-way coupling between the pulsating blood flow and the artery deformability. The Streamline-Upwind/Petrove--Galerkin (SUPG) finite element technique is used to treat for the convective nature of the momentum equation. The deformability of the artery walls is accounted for by treating the wall as an elastic beam under transverse unsteady distributed load, namely the fluid pressure. The results of the present contribution compare well against the available published data.

Effect of Fin Parameters in Cylindrical and Divergent Duct under Natural Convection
Pages : 1093-1102
Authors : S. Benkherbache,  M. Si-Ameur, 
In this paper we propose a numerical study of the natural convective heat transfer flow in a three dimensional cylindrical and divergent annular duct. The inner cylinder subjected to a volumetric heat generation is fitted with longitudinal fins. The governing equations of mass, momentum and energy equation for both the fluid and the solid are solved by the finite volume method, using the commercially available CFD software Fluent. The effect of the inclination angle ϕ of the divergent and the fin parameters on the profiles and the contour fields of temperature and velocity as well as the average Nusselt number ratio were investigated for ϕ=0°,15°,23° and 45°and a number of fins, N=1,2,3 and 4. The Simulations were carried out in the range of Rayleigh numbers (Ra = 100 to Ra=6.3x104). The results reveal that the increasing of the inclination angle of the divergent and the number of fins enhances the heat transfer.

Detection of Cavitation through Acoustic Generation in Centrifugal Pump Impeller
Pages : 1103-1113
Authors : A. K. Jaiswal,  A. U. Rehman,  A. R. Paul,  A. Jain, 
The most common device which transport fluid in industries, agriculture as well as domestic water supply is the centrifugal pump. Based on fluid transfer conditions, several let-downs are occur in the centrifugal pump, cavitation is one among them. The flow pattern at the eye of impeller deviates from the ideal case with the occurrence of cavitation. Due to cavitation, vibration occurs on blades that generates noise in pump. In this study, the acoustics generated in centrifugal pump impeller due to cavitation is detected with the sound pressure by using 3-Dimensional, steady and unsteady state computational fluid dynamics (CFD) analysis of an industrial centrifugal pump impeller. Harmonic force analysis with blade row model helps in finding the sound pressure. The acoustics generated with unsteady-state is compared with cavitation at steady-state CFD analysis. The Reynolds averaged Navier-Stokes equations model as well as Shear Stress Transport (SST) turbulence model are used for the CFD simulation. The results show that the sound pressure calculated increases with the increase in cavitation (i.e. formation of vapour bubbles and sudden drop in head) which shows that high noises are generated by centrifugal pump impeller at lower net positive suction head (NPSH) at a particular discharge.

Numerical Investigation on the three-Dimensional Flowfield in the Single Expansion Ramp Nozzle with Passive Cavity Flow Control
Pages : 1115-1126
Authors : L. Zhou,  Z. Wang, 
Single Expansion Ramp Nozzle (SERN) with large expansion ratio is normally adopted for the hypersonic aircraft in consideration of integrated aircraft / propulsion system / nozzle design. Under low Mach number and low Nozzle Pressure Ratio (NPR, the ratio of inlet total pressure to outlet static pressure) conditions, the flow in the SERN is a state of severe over-expansion, the internal resistance increased obviously, the flow quality and performance of the SERN sharply deteriorated. How to effectively improve the SERN performance under over-expanded condition has become an important issue in the integrated design of hypersonic propulsion system. The passive cavity flow control technique was introduced on the upper expansion ramp of the SERN in this paper, the three-dimensional flowfield in the passive cavity SERN was investigated numerically, suitable NPR range for passive cavity flow control and impacts of passive cavity parameters were discussed. Results show the SERN performance is effectively improved when the passive cavity is applied from NPR of 5 to 10. Compared with the baseline SERN, 3.13 % improvement can be achieved for the thrust coefficient of the passive cavity SERN when NPR is 5. The passive cavity has the capacity of regulating the induced shock intensity or restraining flow separation, the reason for the change in its function is decided by the relative position between the induced shock and the passive cavity position on the upper expansion ramp of the SERN. As for each function of the passive cavity, an optimum position for the passive cavity structure exists on the upper expansion ramp. Among the primary geometric parameters of the passive cavity structure, percent of porosity is a crucial factor to affect the SERN performance by adjusting separation zone size and its separation starting position, and the improvement effectiveness of the axial thrust coefficient drops with the decrease of percent of porosity. The cavity depth and the aperture size are not sensitive to the performance of passive cavity SERN as compared to the effect of the percent of porosity.

Effect of Nozzle Pressure Ratio and Control Jets Location to Control Base Pressure in Suddenly Expanded Flows
Pages : 1127-1135
Authors : K. A. Pathan,  P. S. Dabeer,  S. A. Khan, 
In this paper, computational fluid dynamic (CFD) analysis and experiments have been carried out to study the effect of nozzle pressure ratio, i.e. the ratio of inlet pressure to atmospheric pressure, and the pitch circle diameter of the control jets to regulate the base pressure. The variables considered for the analysis as well as the experiments are the nozzle pressure ratio (NPR), the Mach number (M) and the pitch circle diameter (PCD) of the control jets. The area ratio considered for the study is kept constant at 4.84 while the length to diameter (L/D) ratio of an enlarged duct is set constant at 5. The inertia parameter considered for the study is Mach number. The Mach numbers considered for study are 1.5, 2.0, and 2.5. The nozzle pressure ratio considered for study are 2, 5 and 8. Three different pitch circle diameters of control jets considered for study are 13.1 mm, 16.2 mm and 19.3 mm. From the numerical simulations and the results of the experimental tests, it is found that the control jets are very beneficial to increase the base pressure at higher NPR when the jets issuing from the nozzles are under-expanded. The control jets were able to increase the base pressure value from 160% to 400% at nozzle pressure ratio 8. It is concluded that the parameter D3 is the most effective pitch circle diameter of the control jets to increase the base pressure.

Numerical Simulation of Flow around a High-Speed Train Subjected to Different Windbreak Walls and Yaw Angles
Pages : 1137-1149
Authors : J. Zhang,  K. He,  J. Wang,  T. Liu,  X. Liang,  G. Gao, 
The prediction of flow around a high-speed train subjected to different windbreak walls and yaw angles has been investigated using steady Shear Stress Transport (SST) k-ω turbulence model at the Reynolds number of 1.0×106 based on the height of the scaled train model. The results show that an effective windbreak wall provide a favourable shielding effect for the train behind it, and force the primary positive pressure on the windward of the train to transfer on the wall. Consequently, the airflow cannot directly act on the train body, and the train is basically in an environment with small negative pressure. The inclined slope (the earth embankment type) windbreak wall shows poor anti-wind performance that should not be used along the new high-speed railways. When designing the windbreak wall, the influences of yaw angles should be taken into account.

Effect of γ-Al2O3/Water Nanofluid on Natural Convection Heat Transfer of Corrugated ┐ Shaped Cavity: Study the Different Aspect Ratio of Grooves
Pages : 1151-1160
Authors : R. Mohebbi,  S. Haghighi Khalilabad,  Y. Ma, 
In this paper, the effect of γ-Al2O3/water nanofluid on the flow pattern and natural convection heat transfer of corrugated ⅂ shaped cavity is investigated numerically by the Lattice Boltzmann Method (LBM). The effects of nanoparticles solid volume fraction (ϕ=0-0.006), Rayleigh number (Ra=103-106) and the aspect ratio of grooves cavity (h/H=0.05-0.15) on the streamlines, isotherms and averaged Nusselt number have been examined. In addition, a comparison between γ-Al2O3/water nanofluid and MWCNT-Fe3O4/Water Hybrid Nanofluid on the average Nusselt number are studied. The results showed that the heat transfer rate of γ-Al2O3/water nanofluid is higher than water. For all Rayleigh numbers and solid volume fraction of nanoparticles, increasing the height of grooves leads to an increment in average Nusselt number. Moreover, when the Rayleigh number reaches to 106, the average Nusselt number increases, which the domain mechanism of heat transfer, in this case, becomes convection. As to be expected, the influence of MWCNT-Fe3O4/Water Hybrid Nanofluid on heat transfer rate is higher than the γ-Al2O3/water nanofluid.

A Detailed Study of Boost Pressure and Injection Timing on an RCCI Engine Map Fueled with Iso-Octane and N-Heptane Fuels
Pages : 1161-1175
Authors : H. R. Fajri,  A. H. Shamekhi,  S. Rezaie,  M. J. Jafari,  S. A. Jazayeri, 
By using two types of different fuels and changing the ratios of these fuels, Reactivity Controlled Compression Ignition Engine (RCCI) is able to provide a more effective control over combustion phase at different loads and speeds. In a typical RCCI engine which could be considered as a type of homogeneous charge compression ignition (HCCI) engine, a low reactive fuel is injected into the intake port and a high reactive fuel is directly injected into the combustion chamber. In this study, a multi-dimensional model coupled with chemical mechanism is developed to simulate an RCCI engine operation fueled with iso-octane as the low reactive fuel and n-heptane as the high reactive one. Initially, the engine map was derived using different quantities of total above-mentioned fuels at different ratios and then engine inappropriate operating points were detected and improved by changing intake air pressure and injection timing strategies. The improved criteria to extend engine map are ringing intensity limit, NOx formation standard and gross indicated efficiency. It was concluded that high ringing intensity and NOx formation can be reduced by increasing intake air pressure; also badfire and misfire points can get improved by retarding the injection timing.

Effect of Property Variation on the Fluid Flow and Thermal Behavior in a Vertical Channel
Pages : 1177-1188
Authors : K. Roy,  B. Das, 
Natural convection in a vertical fin array is studied numerically for Non-Boussineśq and Boussineśq fluid with the effect of property variations. Simulations are carried out for the specified range: non-dimensional fin spacing = 0.2 to 0.5, non-dimensional clearance = 0.05 to 0.4 and Grashof number = 1.86 × 105 to 8.64 × 105. Computations are executed to plot the isotherms contours across the section close to the outlet to exemplify the effect of non-Boussineśq fluid in natural convection for variable properties. Computation demonstrates a maximum of 9% higher overall Nu for fixed property Boussineśq fluid than compared to the non-Boussineśq fluid. And for the Boussineśq fluid with the thermo-physical property maintained constant, the results obtained for local Nusselt number is consistently higher than compared to the inconsistent thermo-physical property. Also temperature drop close to the tip of the fin is higher at higher Gr, indicating higher heat transfer rate. Finally, for overall Nusselt number a correlation with the governing parametyers for the present investigation is developed.

CFD Study of the Effect of Geometrical Shape of Separation Blades on the Rotor Performance of an Annular Centrifugal Extractor (ACE)
Pages : 1189-1202
Authors : H. Ghaya,  R. Guizani,  H. Mhiri,  P. Bournot, 
Annular centrifugal extractors have a great potential in the multiphase extraction of pharmaceutical, nuclear, and many other processes. Although the widespread use of this device, the design procedures are still unavailable because of the complexity of the fluid mechanics in the rotor region called the separation zone. From a structural point of view, this region has a complicated conception due to the different internals. This study presents a three-dimensional numerical simulation of the flow field inside the rotor region of an annular centrifugal extractor ACE. The industrial CFD code (Fluent) was used to model the highly swirling fluid flow in the settling zone with various geometries of separation blades (straight blades and curved blades). Numerical predictions and experimental results were compared in order to validate the proposed models. The velocity field with the k-ε model shows a good agreement with the experimental data available in the literature. The Volume of Fluid (VOF) method was employed to simulate the physics of the interface of air/water free surface. A comparison between the flow field and the performances of the ACE model design with vertical straight blades and with vertical curved blades was further investigated to study the effect of the geometric shape of the separation blades on the parameters of liquid holdup volume, the interface radius and the pressure drop. It was found that the geometry of the separating blades has a significant impact on the pressure drop, liquid hold-up volume and interface radius and general flow in the extractor-settling zone. The predicted pressure drop proved that the geometry of the ACE rotor with curved blades leads to a lower values of pressure drop.

Numerical Analysis of Performance Deterioration of a Centrifugal Pump Operating in Two-Phase Flows
Pages : 1203-1211
Authors : A. Atif,  S. Senouci, 
Pumps handling two-phase flows are essential parts of industrial process mainly in oil and gas facilities and power plants. It is known that for centrifugal pumps the presence of gas phase in liquid flow causes the performance to deteriorate. Knowledge improvement of the highly complex internal flow is the way to design more efficient and reliable pumps. The paper describes the results of studies conducted in a centrifugal pump operating in two-phase air/water mixture flows, for performance determination and flow field investigation using numerical simulations. The aim is to provide a new highlight on the performance evolution and to identify the physical mechanism responsible for the deterioration. The work is carried out at design flow rate with varying inlet gas volume fraction. The results show significant performance deterioration compared to single-phase situation. The analysis of flow fields in case of two-phase flows reveals an accumulation of the gas in the impeller passages, causing an alteration of the conventional single-phase flow structure. The effect of interaction with volute is also investigated and it is found to play a key role in changing the flow pattern inside the impeller. At the conclusion of the study, special design features are suggested as concepts for enhancing two-phase pumping behavior of centrifugal pumps.

Effect of Gap Ratio on the Wake behind Two Side-by-Side Flat Plates
Pages : 1213-1222
Authors : B. Shin,  M. Kondo, 
The flow behind two flat plates placed normal to the flow in side-by-side arrangement was experimentally investigated by varying the gap ratio G* (the ratio of gap spacing to plate width) in the range of 0.0 ≤ G* ≤ 2.5 to examine the effect of gap ratio on the wake. The flow patterns around the plates were observed using the hydrogen bubble flow visualization in the water tunnel. Velocity and pressure components were acquired by employing hot-wire anemometers and digital manometers in the wind tunnel. From the experiment, it has been found that, at gap ratios less than 1.6, the gap flow was biased either upward or downward and maintained a stable biased flow pattern. The width of the wake on the biased side was increased with the gap ratio, while that on the unbiased side was decreased. At 1.6 ≤ G* < 2.0, the switching of the biased gap flow appeared due to the flow instability by mutual interference of the vortex shedding from the plates, while at G* of 2.0 or more, the gap flow was no longer biased. The plates on the biased side showed relatively low base pressure and high velocity, and detected periodic vortex shedding, while those on the unbiased side showed the opposite phenomena. At gap ratios less than 2.0, two Strouhal numbers indicating the bi-stable situation exist, and the difference between the two Strouhal numbers decreased with the gap ratio.

Towards Methodologies for Optimal Fluid Networks Design
Pages : 1223-1229
Authors : A. Miguel, 
Tree flow networks are ubiquitous in nature and abound in engineered systems. A parent tube branching into two daughter tubes is the main building block of these networks. These branched tubes should be designed to provide easier access to flow under different size constraints. Optimal tree networks follow a homothetic scaling where the sizes of tubes have the same ratios between successive generations. In this study, different approaches aiming at optimal design of bifurcating tubes are presented and compared. The cross-sectional area of the tubes is obtained using two methods, based on Lagrange multipliers with a size constraint to respect, and including the size limitations directly into the function to optimize via chain rule. The optimal length of the tubes is obtained based both on the equipartition of forces/resistances and on the equal thermodynamic distance. These methods can be understood as a way of connecting entropy generation and the size of branching tubes. This study shows that applying the Lagrange Multiplier Method and applying the chain rule with constraint provides the same result. A similar result is obtained when the equipartition of forces/resistances and equal thermodynamic distance design methods are applied. These results are valid for different size constraints. In summary, our paper provides a comprehensive comparison of the different methods for a better choice, and is intended to provide insights into tree networks of tubes of any shape under different size constraints, for design and analysis.