Recent Volume
Vol13 , No 2 (in press)
Analysis on the Performance Improvement of Reciprocating Pump with Variable Stiffness Valve using CFD
Pages : 387-400
Authors : G. Zhu,  S. M. Dong, 
As a key component of reciprocating pump, the valve has a significant influence on its performance. However, it is difficult for the existing valve to simultaneously solve the problems such as fatigue, erosion and cavitation in engineering application. In this paper, a solution to these problems of using variable stiffness spring is proposed. And three new structures of the valve are designed. Furthermore, based on Computational Fluid Dynamics (CFD) method, a three-dimensional dynamic simulation model considering fluid-structure interaction in the suction stroke of reciprocating pump is established by using dynamic grid technique and User-Defined Functions (UDF). The performance of these new valves are compared with that of conventional valve respectively. The result shows that the new valves have significant influence on the motion characteristics of the valve disc, flow field distribution and cavitation. Besides, the simulation and experimental results of the maximum lift are compared, and it is found that they are basically in agreement. The new structures provide a new research direction for improving the performance of reciprocating pump. Simultaneously, the above simulation method can also provide guidance for valve design, structural optimization and service life improvement.

Performance Comparison of Novel Single and Bi-Diaphragm PZT Based Valveless Micropumps
Pages : 401-412
Authors : H. Asadi Dereshgi,  M. Z. Yildiz,  N. Parlak, 
A commercial micropump should provide properties that justify the simple structure and miniaturization, high reliability, simple working principle, low cost and no need for complex controller. In this study, two novel piezoelectric actuated (lead zirconate titanate-PZT) valveless micropumps that can achieve high flow rates by pumping chambers and fixed reservoirs were designed and fabricated. Extensive experiments were conducted to investigate the effects of hydrodynamic and electromechanical on flow rates of the Single Diaphragm Micropump (SDM) and the Bi-diaphragm Micropump (BDM). BDM had two actuators facing to the same chamber at 180-degree phase shift. The primary features of the proposed designs were the high flow rates at low driving voltages and frequencies with the help of innovative design geometry. 3D-printing technique providing one-step fabrication for integrated micropumps with fixed reservoir was used. The micropump materials were biocompatible and can be used repeatedly to reduce costs. Mechanical parameters such as tensile test for silicon diaphragm, surface topography scanning by microscopy techniques and drop shape analysis for hydrophobic property were investigated to reveal surface wetting and flow stability. In addition, the effect of reservoir height was investigated and the calibration flow rates were measured during the inactive periods. The maximum diaphragm displacements were obtained at 45 V and 5 Hz. The maximum flow rate of SDM and BDM at 45 V and 20 Hz were 32.85 ml/min and 35.4 ml/min respectively. At all driving voltage and frequency levels, BDM had higher flow rates than of SDM.

A Comparative CFD Study on Laminar and Turbulent Flow Fields in Dual-Rushton Turbine Stirred Vessels
Pages : 413-427
Authors : L. C. Li,  N. Chen,  K. F. Xiang,  B. P. Xiang, 
A computational fluid dynamics(CFD) simulation was carried out to study on flow field characteristics in dual-Rusthton turbine stirred vessels in laminar and turbulent regimes. Model validation was conducted using experimental data in the literature. The simulation results show that flow pattern and dimensionless velocity distribution vary with Reynolds number in laminar regime, while these parameters remain almost unchanged for different Reynolds numbers in turbulent regime. For vessels with a certain geometrical configuration, flow pattern, dimensionless velocity distribution and impeller power number depend mainly on Reynolds number, and are little affected by working medium and enlargement scale. By changing impeller spacing and off-bottom clearance of lower impeller, it is obtained the parallel, merging and diverging flow patterns in turbulent regime, and the changing processes of flow patterns in laminar regime for the three configurations. Total power number has the order of parallel>diverging>merging for the three configurations at the same Reynolds number. With increasing of Reynolds number, the power number of merging configuration shows the largest drop, followed by diverging configuration, and the lowest drop for parallel configuration in laminar regime, while power number rises slightly for the three configurations in turbulent regime.

Pressure-Driven Electro-Osmotic Flow and Mass Transport in Constricted Mixing Micro-Channels
Pages : 429-441
Authors : C. Ahamed,  A. Algahtani,  I. Anjum Badruddin,  T. M. Yunus Khan,  S. Kamangar,  M. A. H. Abdelmohimen, 
Both micro electro mechanical systems (MEMS) based and lab-on-a chip (LoC) devices demand efficient micro-scale mixing mechanisms for its effective control which necessitates the quality research towards more efficient designs. A new venture is investigated in those direction with mixing micro-channel constricted with rectangular block under pressure-driven electro-osmotic flow and is numerically simulated by a modified immersed boundary method (IBM), an alternative technique in computational fluid dynamics (CFD). The electro-osmotic flow elucidated by electrical double layer theory when simultaneously considered with pressure driven flow in micro channels can be effectively figured out by the solution of Navier-Stokes equations linked with Nernst-Planck and Poisson equations for transportation of ion and electric field respectively. In this study, the effect of varying the height of rectangular block on the flow and mixing performance are analyzed. A hybrid method, which is a combination of active and passive techniques, is introduced simultaneously in the micro-channel by the electro-osmotic effects and channel constriction. The approach is on the basis of finite volume methodology on a staggered mesh. The governing equations are solved by a time-integration technique based on a fractional step method. The velocity fields are corrected by a pseudo-pressure term to ensure the continuity in each computational time step. The extent of mixing in every cross section of the micro channel is assessed by a suitable mixing efficiency parameter. This study has shed light on the most predominant factors that influence mixing efficiency in a micro-channel, such as geometry of the block, non-dimensional numbers (Reynolds number, Re and Peclet number, Pe), zeta potential, external electric field strength and electrical double layer (EDL) thickness. The maximum efficiency in this micro mixer design is found to be 51.3% for Reynolds number of 0.05 and Peclet number of 450 with the rectangular block height of 0.75. It is clear that both electro osmotic effects and flow perturbations due to channel constriction caused a remarkable improvement in mixing efficiency. The outcomes of this investigation are widely applicable in cooling of microchips, heat sinks of MEMS based devices, drug delivery applications and Deoxyribonucleic acid (DNA) hybridization. The present IBM model is validated by experimental and numerical results from the literature.

Impact of Boundary Layer Suction on Clearance Leakage Flow in a Cantilever Stator of Transonic Compressor
Pages : 443-455
Authors : B. Zhang,  B. Liu,  P. Liu,  X. Mao, 
To control secondary flow effects and enhance the aerodynamic performance of the compressor, the flow control effects of the flow suction at the endwall with different circumferential positions and at the blade tip were numerically investigated in the cantilever stator of an axial single-stage transonic compressor. The main purpose was to gain a better understanding of the application of boundary layer suction and the associated control mechanisms in the cantilever stator. The studies show that the optimal position of the endwall suction slot should be located up the stator blade, in terms of the leakage flow structures and the blade tip unloading effect. In addition, the flow control effects of the suction at the blade tip on leakage flow upstream is better than that of the endwall flow suction with the same structure. Further, the studies of compressor aerodynamic performance curves illustrate that the efficiency and pressure ratio is increased by 0.34% and 1.09% at the peak efficiency point, and are increased by 0.39% and 0.14% at the near stall point, respectively.

Improved Delayed Detached Eddy Simulation of the Slipstream and Trackside Pressure of Trains with Different Horizontal Profiles
Pages : 457-468
Authors : M. Li,  B. Liu,  T. H. Liu,  Z. J. Guo, 
The slipstream caused by high-speed trains may harm pedestrians and workers trackside. In general, the characteristics of the slipstream are influenced mainly by the nose shape of the train. The present study explores the slipstream caused by high-speed trains with three different horizontal nose profiles based on the results of three-dimensional, improved delayed detached eddy simulation (IDDES) with an unsteady turbulence model and a set of 1/8th scaled train models. The results obtained using this numerical methodology are in good agreement with those obtained from corresponding wind tunnel tests. The trackside pressure changes around the train models are also captured and analyzed. The analysis reveals that the width of the nose can significantly influence the magnitude and arrival time of slipstream velocity and pressure peaks. The results and proposed numerical methodology can be used as guidelines for the design of high-speed train nose shapes.

Phase Content and Flow Measurement of Bubble Flow based on New Experimental Pipeline
Pages : 469-478
Authors : L. Fang,  S. Wang,  S. Li,  Y. Faraj,  J. Tian,  X. Li, 
By combining near infrared spectroscopy and differential pressure meter to measure flow rate and phase volume fraction, a new structure of the near infrared system located in the position of the Venturi tube of long throat was proposed. The flow rate and volume fraction of the bubble flow were measured in a vertical experimental line. In terms of volumetric content measurement, based on typical flow characteristics, a void age model was established and experimentally verified. In terms of flow measurement, the classical measurement model is compared, and the uniform flow model is selected as the flow measurement model and corrected. The results show that the measurement error under the bubbly flow condition of the liquid volume fraction and the gas volume fraction was within ±0.52% and ±10%, respectively. The relative error of total flow measurement was within ±1.01%.

Large Eddy Simulation of Pulsed Blowing in a Supersonic Compressor Cascade
Pages : 479-490
Authors : Q. Meng,  S. Chen,  S. Ding,  H. Liu,  S. Wang, 
Large Eddy Simulation (LES) of a two dimensional supersonic compressor cascade is performed in the current study. It is found that the Shock Wave Boundary Layer Interaction causes a large scale of total pressure losses and presents strong fluctuation features. Thus the pulsed and steady excitation jets are applied to suppress the flow separations and to reduce the total pressure losses. Several impacting parameters, such as jet axial location, jet hole width, jet angle to the local blade surface and jet mass flowrate are chosen based on the primary analysis by the calculations by the Reynolds Averaged Navier-Stokes equations. In addition, based on the results of frequency spectrum and POD analysis, the excitation jet frequency is chosen for the pulsed excitation jet scheme. It is concluded that the pulsed excitation jet scheme achieves a 9.8% reduction of total pressure loss in comparison to the steady excitation jet scheme under the same time-averaged excitation jet mass flow rate. The excitation jets affect both the flow field near the jet hole on the suction surface and the flow field on the pressure surface via the management of the reflection shock wave. In addition, the excitation frequency dominates not only the time-averaged flow field, but also the second and third modes which stand for the unsteady structures in the flow field under the POD analysis. The first mode contains most energy in the flow field and the energy percentage decreases dramatically with the increase of the mode number. In comparison to the steady excitation jet scheme, the pulsed excitation jet scheme gathers more energy to the low orders of the modes, especially the first four modes. With the mixing effect and high dissipation rate of the high-frequency signals, the high-frequency signals shrink in the wake and the flow field builds up more uniformity.

An Efficient Radial Basis Function Meshfree Local Petrov–Galerkin Method for Modeling the Unidirectional Fully Developed Fluid Flow
Pages : 491-497
Authors : I. Saeedpanah,  R. Golmohamadi Azar,  H. Sarkardeh, 
In hydrodynamics the applications range of incompressible flows is very wide. In this study, a robust, high order modeling approach is introduced, based on the MLPG meshfree method -based radial basis functions (RBF- MLPG) method, for solving the incompressible flow field. In other words a MLPG meshfree method based on an interpolation function is presented to solve the 2‐D unsteady incompressible fully developed fluid flows. This meshfree method is based on the quartic (4^th order) spline. The method is then compared against the Finite Element Method on a test case of unidirectional fully developed incompressible fluid. The performance of this weight function proved that the quartic (4th order) spline gains the highest accuracy, convergence and efficiency. Finally, it can be concluded that the presented method is formidable for simulating fluid dynamics.

Prediction of Base Pressure in a Suddenly Expanded Flow Process at Supersonic Mach Number Regimes using ANN and CFD
Pages : 499-511
Authors : J. D. Quadros,  S. A. Khan, 
Sudden expansion of flow in supersonic flow regime has gained relevance in the recent pasts for a wide run of applications. A number of kinematic as well as geometric parameters have been significantly found to impact the base pressure created within the suddenly expanded stream. The current research intends to create a predictive model for base pressure that is established in the abruptly extended stream. The artificial neural network (ANN) approach is being utilized for this purpose. The database utilized for training the network was assembled utilizing computational fluid dynamics (CFD). This was done by the design of experiments based L27 Orthogonal array. The three input parameters were Mach number (M), nozzle pressure ratio (NPR) and area ratio (AR) and base pressure was the output parameter. The CFD numerical demonstrate was approved by an experimental test rig that developed results for base pressure, and used a nozzle and sudden extended axis-symmetric duct to do so. The ANN architecture comprised of three layers with eight neurons in the hidden layer. The algorithm for optimization was Levenberg-Marquardt. The ANN was able to successfully predict the base pressure with a regression coefficient R2 of less than 0.99 and RMSE=0.0032. The importance of input parameters influencing base pressure was estimated by using the ANN weight coefficients. Mach number obtained a relative importance of 47.16% claiming to be the most dominating factor.

Unsteady Effects of Upstream Nozzle Wakes on Hub-Endwall Flow and Heat Transfer of an HPT Rotor
Pages : 513-526
Authors : L. Chen,  Z. F. Lu,  R. Dai, 
Nozzle wakes have significant effects on the heat transfer on the rotor blade and endwall surfaces. Numerical studies have been carried out in a subsonic high pressure turbine stage to investigate the rotor’s secondary flow field and endwall heat transfer. Both steady and unsteady RANS analyses were accomplished for the multiple blade rows using mixing-plane and domain-scaling techniques respectively. Special attention was focused on the particular nozzle wake structure of secondary passage vortex near the hub endwall and its effects on the endwall heat transfer characteristics. Unsteady solution indicates that the passage vortex near the rotor hub is transported toward the midspan due to the blade interaction and rotation effects. In the front passage region, the time-averaged result yields higher heat transfer up to 20% than a steady one, and the transient fluctuation amplitude reaches 40% of mean values along the passage vortex moving path. In the rear passage region, the difference between steady and unsteady solutions is negligible. Current study reveals that the major difference of wake effects between an actual turbine and a linear cascade with moving bars comes from the movement of the vortical endwall passage vortex in the incoming flow.

Some Insights into Drop Impacts on a Hydrophobic Surface
Pages : 527-536
Authors : A. Karn,  R. De,  A. Kumar, 
The current study reports the phenomenon of drop impacts on a hydrophobic surface in the substrate deposition regime (non-splashing), focusing on the characterization of each stage upon impact and different non-dimensional parameters involved such as spreading factor, recoil height and the durations of several phases. The results indicate that the drop dynamics is determined by an interplay of drop inertia, viscosity and surface tension. Apart from Reynolds number (Re) and Weber number (We) which are conventionally used to characterize drop impacts, a new non-dimensional impact parameter, ξ (= 〖We〗^(1/4) 〖Re〗^(1/5)) is introduced, and it is found out that the spreading factor and the different non-dimensional phase durations involved in the drop impact dynamics on a hydrophobic surface, scale fairly well with this newly defined impact parameter. Further, systematic studies into the non-dimensional durations of each phase upon impact, spreading factor and recoil factor (i.e. non-dimensional recoil height) with respect to different non-dimensional parameters are reported.

Experimental and Numerical Investigation of Air Flow Motion in Cylinder of Heavy Duty Diesel Engines
Pages : 537-547
Authors : C. Demirkesen,  U. Colak,  I. H. Savci,  H. B. Zeren, 
In recent years, numerical simulations have become key tool for diesel engine combustion system development due to the requirement of the shorter development duration for the improved performance and better emission levels. In this study, an approach, which integrates numerical and experimental methods in order to characterize the flow field in diesel engine cylinder, is presented. The steady-flow port bench testing, PIV (Particle Image Velocimetry) measurements and numerical simulation methods are used to determine the flow behavior inside the cylinder. Numerical simulation method is validated by using experimental results in terms of mass flow rate and swirl ratio in cylinder. Mass flow rate values predicted within 5 percent error and swirl ratio values predicted within 10 percent error. This proves the viability of numerical method as an important alternative to port bench measurements. In addition to that, cylinder-to-cylinder variation and effects of surface roughness are investigated by swirl ratio measurements and optical diagnostic. Results showed that surface quality and manufacturing problems have significant effects on the swirl ratio in cylinder.

Time Domain Simulation of Ship Motion in Irregular Oblique Waves
Pages : 549-559
Authors : S. M. Mousavi,  A. R. Khoogar,  H. Ghassemi, 
Ship harmonic motion is an important and practical characteristic in ship design and performance evaluation. The development and optimization of hull-form, ship’s dynamic effects, seakeeping performance, and motion control, all require the motion data that includes wave exciting forces as well as dynamic response of the ship. This paper presents a new approach for time-domain simulation of full-scale ship model with four degrees of freedom based on computational fluid dynamics using unsteady RANS method. The key objective of this paper was the full-scale simulation of ship motion in oblique waves and assessment of time domain forces, moments, and other motion parameters. The David Taylor Model Basin (DTMB) 5415 full-scale model has been used for the numerical studies in this paper. The obtained computational results showed good conformation to the results obtained using the strip theory. It is intended to extend this research to subscale test experiments for more definite validation of the results.