Showing papers in "Journal of Fluids Engineering-transactions of The Asme in 2012"

Computational Fluid Dynamics Analysis of a Hydrokinetic Turbine Based on Oscillating Hydrofoils

Abstract: The performance of a new concept of hydrokinetic turbine using oscillating hydrofoils to extract energy from water currents (tidal or gravitational) is investigated using URANS numerical simulations. The numerical predictions are compared with experimental data from a 2 kW prototype, composed of two rectangular oscillating hydrofoils of aspect ratio 7 in a tandem spatial configuration. 3D computational fluid dynamics (CFD) predictions are found to compare favorably with experimental data especially for the case of a single-hydrofoil turbine. The validity of approximating the actual arc-circle trajectory of each hydrofoil by an idealized vertical plunging motion is also addressed by numerical simulations. Furthermore, a sensitivity study of the turbine’s performance in relation to fluctuating operating conditions is performed by feeding the simulations with the actual time-varying experimentally recorded conditions. It is found that cycle-averaged values, as the power-extraction efficiency, are little sensitive to perturbations in the foil kinematics and upstream velocity.

Preliminary Design and Performance Estimation of Radial Inflow Turbines: An Automated Approach

Abstract: A comprehensive one-dimensional meanline design approach for radial inflow turbines is described in the present work. An original code was developed in Python that takes a novel approach to the automatic selection of feasible machines based on pre-defined performance or geometry characteristics for a given application. It comprises a brute-force search algorithm that traverses the entire search space based on key non-dimensional parameters and rotational speed. In this study, an in-depth analysis and subsequent implementation of relevant loss models as well as selection criteria for radial inflow turbines is addressed. Comparison with previously published designs, as well as other available codes, showed good agreement. Sample (real and theoretical) test cases were trialed and results showed good agreement when compared to other available codes. The presented approach was found to be valid and the model was found to be a useful tool with regards to the preliminary design and performance estimation of radial inflow turbines, enabling its integration with other thermodynamic cycle analysis and three-dimensional blade design codes.

Unsteady Pressure Analysis of a Swirling Flow With Vortex Rope and Axial Water Injection in a Discharge Cone

Abstract: The variable demand of the energy market requires that hydraulic turbines operate atvariable conditions, which includes regimes far from the best efficiency point. The vortexrope developed at partial discharges in the conical diffuser is responsible for large pres-sure pulsations, runner blades breakdowns and may lead to power swing phenomena. Anovel method introduced by Resiga et al. (2006, “Jet Control of the Draft Tube in FrancisTurbines at Partial Discharge,” Proceedings of the 23rd IAHR Symposium on HydraulicMachinery and Systems, Yokohama, Japan, Paper No. F192) injects an axial water jetfrom the runner crown downstream in the draft tube cone to mitigate the vortex rope andits consequences. A special test rig was developed at “Politehnica” University of Timi-soara in order to investigate different flow control techniques. Consequently, a vortexrope similar to the one developed in a Francis turbine cone at 70% partial discharge isgenerated in the rig’s test section. In order to investigate the new jet control method anauxiliary hydraulic circuit was designed in order to supply the jet. The experimentalinvestigations presented in this paper are concerned with pressure measurements at thewall of the conical diffuser. The pressure fluctuations’ Fourier spectra are analyzed inorder to assess how the amplitude and dominating frequency are modified by the waterinjection. It is shown that the water jet injection significantly reduces both the amplitudeand the frequency of pressure fluctuations, while improving the pressure recovery in theconical diffuser. [DOI: 10.1115/1.4007074]Keywords: decelerated swirling flow, vortex rope, water injection method, unsteadypressure, experimental investigation

Three-Dimensional Effects on an Oscillating-Foil Hydrokinetic Turbine

Abstract: Three-dimensional hydrodynamic losses are assessed in this investigation for a foil oscillating sinusoidally in a combined heave and pitch motion with large amplitudes. Simulations are performed using a unsteady Reynolds-Averaged-Navier-Stokes (URANS) solver on an oscillating foil in a power-extraction mode; thus acting as a hydrokinetic turbine at high Reynolds number. Foils of various aspect ratios (span to chord length ratio) are considered, both with and without endplates for one representative operation point. Hydrodynamic forces and extracted power are compared with results from the equivalent two-dimensional (2D) computations. It is found that the relative drop of performance (cycle-averaged power extracted) due to 3D hydrodynamic losses can be limited to 10% of the 2D prediction when endplates are used on a foil of aspect ratio greater than ten. The practical consideration of an oscillating-foil hydrokinetic turbine operating in an imperfectly-aligned upstream water flow is also addressed with simulations considering an upstream flow at a yaw angle up to 30° with respect to the foil chord line. Effects on performance are found to be proportional to the projected kinetic energy flux.

Effects of Blade Wrap Angle Influencing a Pump as Turbine

Abstract: A pump is not ideally designed to operate as a turbine. To improve the efficiency of a pump as turbine (PAT), the redesign of the PAT, according to the flow of the turbine, is required. The blade wrap angle is one of the main geometric parameters in impeller design. Therefore, an investigation into the blade wrap angle to the PAT’s influence can be useful. In order to understand blade wrap angle to the influence of the PAT, this paper numerically investigated three different specific speeds of PATs with different blade wrap angles. The validity of numerical simulation was first confirmed through a comparison between numerical and experimental results. The performance change of the PATs with the blade wrap angle was acquired. A detailed hydraulic loss distribution and a theoretical analysis were performed to investigate the reasons for performance changes caused by the blade wrap angle. The results show that there is an optimal blade wrap angle for a PAT to achieve the highest efficiency and the optimal blade wrap angle decreases with an increasing specific speed. A performance analysis shows the PAT’s flow versus pressure head (Q-H) and flow versus generated shaft power (Q-P) curves are lowered with the decrease of the blade wrap angle. The hydraulic loss distribution and theoretical analysis illustrate that it is the decrease of hydraulic loss within the impeller, together with the decrease of the theoretical head, that results in the performance decrease. The decrease of hydraulic loss within the impeller is attributed to the shortened impeller blade passage and the reduced velocity gradient within the impeller flow channel. With the decrease of the blade wrap angle, the slip factor of the PAT’s impeller is decreased; therefore, its theoretical head is also decreased.

URANS calculations for smooth circular cylinder flow in a wide range of Reynolds Numbers: solution verification and validation

Abstract: The flow around circular smooth fixed cylinder in a large range of Reynolds numbers is considered in this paper. In order to investigate this canonical case, we perform CFD calculations and apply verification & validation (V&V) procedures to draw conclusions regarding numerical error and, afterwards, assess the modeling errors and capabilities of this (U)RANS method to solve the problem. Eight Reynolds numbers between Re = 10 and Re=5×105 will be presented with, at least, four geometrically similar grids and five discretization in time for each case (when unsteady), together with strict control of iterative and round-off errors, allowing a consistent verification analysis with uncertainty estimation. Two-dimensional RANS, steady or unsteady, laminar or turbulent calculations are performed. The original 1994 k-ω SST turbulence model by Menter is used to model turbulence. The validation procedure is performed by comparing the numerical results with an extensive set of experimental results compiled from the literature.

Measurement of Effective Bulk Modulus for Hydraulic Oil at Low Pressure

Abstract: Oil pr-operties are very important input parameters for the sinaulation of hydraulic components. Precise values of effective bulk modules at low pressures are especially required to improve the simulation accuracy of the pumps suction side or of cavitation in pumps or valves. So far, theoretical equations to compute the effective bulk modulus of hydraulic oil have not been experimentally verified, and only poor measured data are available to calculate the effective bulk modulus at low pressure. Therefore in this paper, the theoretical equation was verified for effective bulk moduli based on measurements of pressure change as a function of volume change at low pressures, varying temperature, entrained air content, and type of state change. Furthermore, the comparison of effective bulk moduli calculated with three different methods (mass-change, volume-change, and sound-speed method) shows that the effective bulk naodulus can be calculated well from the measurement results of all three methods. The calculated effective bulk moduli values show little variation among the methods. Additionally, the release pressure of dissolved air in oil and the change of the polytropic gas constant depending on the speed of volume change rate were identified in this study.

Numerical and Experimental Study of the Interaction of a Spark-Generated Bubble and a Vertical Wall

Abstract: An understanding of the fundamental mechanisms involved in the interaction between bubbles and structures is of importance for many applications involving cavitation erosion. Generally, the final stage of bubble collapse is associated with the formation of a high-speed reentrant liquid jet directed toward the solid surface. Local forces associated with the collapse of such bubbles can be very high and can exert significant loads on the materials. This formation and impact of liquid jet is an area of intense research. Under some conditions, the presence of gravity and other nearby boundaries and free surfaces alters the jet direction and need to be understood, especially that in the laboratory, small scale tests in finite containers have these effects inherently present. In this work, experiments and numerical simulations of the interaction between a vertical wall and a bubble are carried out using Dynaflow’s three-dimensional code, 3DYNA FS-BEM , which models the unsteady dynamics of a liquid flow including the presence of highly nonlinear time evolving gas-liquid interfaces. The numerical predictions were validated using scaled experiments carried out using spark generated bubbles. These spark bubble tests produced high fidelity test data that properly scale the fluid dynamics as long as the geometric nondimensional parameters, gravity and time are properly scaled. The use of a high speed camera allowing framing rates as high as 50,000 frames per second to photograph the bubbles produced high quality observations of bubble dynamics including clear visualizations of the reentrant jet formation inside the bubble. Such observations were very useful in developing and validating the numerical models. The cases studied showed very good correlation between the numerical simulations and the experimental observations and allowed development of predictive rules for the re-entrant jet characteristics, including jet angle, jet speed, and various geometric characteristics of the jet.

Dependency on Runner Geometry for Reversible-Pump Turbine Characteristics in Turbine Mode of Operation

Abstract: The primary goal for this PhD project has been to investigate instability of reversiblepump turbines (RPTs) as a phenomenon and to find remedies to solve it. The instability occurs for turbines with s-shaped characteristics, unfavourable waterways and limited rotating inertia. It is only observed for certain operation points at either high speed or low load. These correspond to either high values of Ned or low values of Qed. The work done in this PhD thesis can be divided into the three following categories.Investigate and understand the behaviour of a pump turbine: A model was designed in order to investigate the pump turbine behaviour related to its characteristics. This model was manufactured and measurements were performed in the laboratory. By using throttling valves or torque as input the full s-shaped characteristics was measured. When neither of these techniques is used, the laboratory system has unstable operation points which result in hysteresis behaviour. Global behaviour of the RPT in a power plant system was investigated through analytical stability analysis and dynamic system simulations. The latter included both rigid and elastic representation of the water column.Turbine internal flow: The flow inside the runner was investigated by computer simulations (CFD). Two-dimensional analysis was used to study the inlet part of the runner. This showed that a vortex forming at the inlet is one of the causes for the unstable characteristics. Three-dimensional analyses were performed and showed multiple complex flow structures in the unstable operation range. Measurements at different pressure levels showed that the characteristics were dependent on the Reynolds number at high Ned values in turbine mode. This means that the similarity of flows is not sufficiently described by constant Qed and Ned values at this part of the characteristics.Design modifications: The root of the stability problem was considered to be the runner’s geometric design at the inlet in turbine mode. Therefore different design parameters were investigated to find relations to the characteristics. Methods used were measurements, CFD modelling and analytical models. The leading edge profile was altered on the physical model and measurements were performed in the laboratory. Results showed that the profiles have significant influence on characteristics and therewith stability at high speed operation points. Other design parameters were investigated by CFD analysis with special focus on the inlet blade angle.

Numerical Simulation of Cavity Shedding from a Three-Dimensional Twisted Hydrofoil and Induced Pressure Fluctuation by Large-Eddy Simulation

Abstract: Simulation of cavity shedding around a three-dimensional twisted hydrofoil has been conducted by large eddy simulation coupling with a mass transfer cavitation model based on the Rayleigh-Plesset equation. From comparison of the numerical results with experimental observations, e.g., cavity shedding evolution, it is validated that the unsteady cavitating flow around a twisted hydrofoil is reasonably simulated by the proposed method. Numerical results clearly reproduce the cavity shedding process, such as cavity development, breaking-off and collapsing in the downstream. Regarding vapor shedding in the cavitating flow around three-dimensional foils, it is primarily attributed to the effect of the re-entrant flow consisting of a re-entrant jet and a pair of side-entrant jets. Formation of the re-entrant jet in the rear part of an attached cavity is affected by collapse of the last shedding vapor. Numerical results also show that the cavity shedding causes the surface pressure fluctuation of the hydrofoil and the force vibration. Accompanying the cavity evolution, the wave of pressure fluctuation propagates in two directions, namely, from the leading edge of the foil to the trailing edge and from the central plane to the side of the hydrofoil along the span. It is seen that the large pressure fluctuation occurs at the central part of the hydrofoil, where the flow incidence is larger.

Numerical Investigations and Performance Experiments of a Deep-Well Centrifugal Pump With Different Diffusers

Abstract: In this paper, the design methodology of a new type of three-dimensional surface return diffuser (3DRD) is presented and described in detail. The main goal was to improve the hydrodynamic performance of the deep-well centrifugal pump (DCP). During this study, a two-stage DCP equipped with two different type diffusers was simulated employing the commercial computational fluid dynamics (CFD) software ANYSY-Fluent to solve the Navier-Stokes equations for three-dimensional steady flow. A sensitivity analysis of the numerical model was performed in order to impose appropriate parameters regarding grid elements number and turbulence model. The flow field and the static pressure distribution in the diffusers obtained by numerical simulation were analyzed, and the diffuser efficiency was defined to quantify the pressure conversion capability. The prototype experimental test results were acquired and compared with the data predicted from the numerical simulation, which showed that the performance of the pump with 3DRD is better than that of the traditional cylindrical return diffuser (CRD) under all operating conditions. The efficiency and single-stage head of the pump with 3DRD have been significantly improved compared with the standard DCP of the same class.

Analysis and Optimization of a Vaned Diffuser in a Mixed Flow Pump to Improve Hydrodynamic Performance

Abstract: Hydrodynamic analysis and an optimization of a vaned diffuser in a mixed-flow pump are performed in this work. Numerical analysis is carried out by solving three-dimensional Reynolds-averaged Navier-Stokes equations using the shear stress transport turbulence model. A validation of numerical results is conducted by comparison with experimental data for the head, power, and efficiency. An optimization process based on a radial basis neural network model is performed with four design variables that define the straight vane length ratio, the diffusion area ratio, the angle at the diffuser vane tip, and the distance ratio between the impeller blade trailing edge and the diffuser vane leading edge. Efficiency as a hydrodynamic performance parameter is selected as the objective function for optimization. The objective function is numerically assessed at design points selected by Latin hypercube sampling in the design space. The optimization yielded a maximum increase in efficiency of 9.75% at the design flow coefficient compared to a reference design. The performance curve for efficiency was also enhanced in the high flow rate region. Detailed internal flow fields between the reference and optimum designs are analyzed and discussed.

Pressure Drop for Fully Developed Turbulent Flow in Circular and Noncircular Ducts

Abstract: The objective of this paper is to furnish the engineer with a simple and convenient means of estimating frictional pressure drop in ducts and the original physical behavior can be clearly reflected. Fully developed turbulent flow frictional pressure drop in noncircular ducts is examined. Simple models are proposed to predict the frictional pressure drop in smooth and rough noncircular channels. Through the selection of a novel characteristic length scale, the square root of the cross-sectional area, the effect of duct shape has been minimized. The proposed models have an accuracy of 6% for most common duct shapes of engineering practice and can be used to predict pressure drop of fully developed turbulent flow in noncircular ducts. It is found that the hydraulic diameter is not the appropriate length scale to use in defining the Reynolds number to ensure similarity between the circular and noncircular ducts. By using the Reynolds number based on the square root of the cross-sectional area, it is demonstrated that the circular tube relations may be applied to noncircular ducts eliminating large errors in estimation of pressure drop. The square root of the cross-sectional area is an appropriate characteristic dimension applicable to most duct geometries. The dimensionless mean wall shear stress is a desirable dimensionless parameter to describe fluid flow physical behavior so that fluid flow problems can be solved in the simple and direct manner. The dimensionless mean wall shear stress is presented graphically and appears more general and reasonable to reflect the fluid flow physical behavior than the traditional Moody diagram.

Stability Limits of Reversible-Pump Turbines in Turbine Mode of Operation and Measurements of Unstable Characteristics

Abstract: The primary goal for this PhD project has been to investigate instability of reversiblepump turbines (RPTs) as a phenomenon and to find remedies to solve it. The instability occurs for turbines with s-shaped characteristics, unfavourable waterways and limited rotating inertia. It is only observed for certain operation points at either high speed or low load. These correspond to either high values of Ned or low values of Qed. The work done in this PhD thesis can be divided into the three following categories.Investigate and understand the behaviour of a pump turbine: A model was designed in order to investigate the pump turbine behaviour related to its characteristics. This model was manufactured and measurements were performed in the laboratory. By using throttling valves or torque as input the full s-shaped characteristics was measured. When neither of these techniques is used, the laboratory system has unstable operation points which result in hysteresis behaviour. Global behaviour of the RPT in a power plant system was investigated through analytical stability analysis and dynamic system simulations. The latter included both rigid and elastic representation of the water column.Turbine internal flow: The flow inside the runner was investigated by computer simulations (CFD). Two-dimensional analysis was used to study the inlet part of the runner. This showed that a vortex forming at the inlet is one of the causes for the unstable characteristics. Three-dimensional analyses were performed and showed multiple complex flow structures in the unstable operation range. Measurements at different pressure levels showed that the characteristics were dependent on the Reynolds number at high Ned values in turbine mode. This means that the similarity of flows is not sufficiently described by constant Qed and Ned values at this part of the characteristics.Design modifications: The root of the stability problem was considered to be the runner’s geometric design at the inlet in turbine mode. Therefore different design parameters were investigated to find relations to the characteristics. Methods used were measurements, CFD modelling and analytical models. The leading edge profile was altered on the physical model and measurements were performed in the laboratory. Results showed that the profiles have significant influence on characteristics and therewith stability at high speed operation points. Other design parameters were investigated by CFD analysis with special focus on the inlet blade angle.

Influence of Blade Number on the Performance and Pressure Pulsations in a Pump Used as a Turbine

Abstract: The rotor-stator interaction of a rotating impeller and a stationary volute could cause strong pressure pulsations and generate flow induced noise and vibration in a pump used as a turbine (PAT). Blade number is one of the main geometric parameters of the impeller. In this paper, a numerical investigation of the PAT’s unsteady pressure field with different blade numbers was performed. The accuracy of global performance prediction by computational fluid dynamics (CFD) was first verified through comparison between numerical and experimental results. Unsteady pressure fields of the PAT with different blade numbers were simulated, and the pulsations were extracted at various locations covering the PAT’s three main hydraulic parts. A detailed analysis of the unsteady pressure field distributions within the PAT’s control volume and comparison of unsteady pressure difference caused by the increase of blade number were performed. The transient flow results provided the unsteady pressure distribution within PAT and showed that increasing the blade number could effectively reduce the amplitude of pressure pulsations. Finally, unsteady pressure field tests were performed and some unsteady results obtained by unsteady field analysis were validated.

Asymptotic Effect of Initial and Upstream Conditions on Turbulence

Abstract: More than two decades ago the first strong experimental results appeared suggesting that turbulent flows might not be asymptotically independent of their initial (or upstream) conditions (Wygnanski , 1986, “On the Large-Scale Structures in Two-Dimensional Smalldeficit, Turbulent Wakes,” J. Fluid Mech., 168 , pp. 31–71). And shortly thereafter the first theoretical explanations were offered as to why we came to believe something about turbulence that might not be true (George, 1989, “The Self-Preservation of Turbulent Flows and its Relation to Initial Conditions and Coherent Structures,” Advances in Turbulence, W. George and R. Arndt, eds., Hemisphere, New York, pp. 1–41). These were contrary to popular belief. It was recognized immediately that if turbulence was indeed asymptotically independent of its initial conditions, it meant that there could be no universal single point model for turbulence (George, 1989, “The Self-Preservation of Turbulent Flows and its Relation to Initial Conditions and Coherent Structures,” Advances in Turbulence, W. George and R. Arndt, eds., Hemisphere, New York, pp. 1–41; Taulbee, 1989, “Reynolds Stress Models Applied to Turbulent Jets,” Advances in Turbulence, W. George and R. Arndt, eds., Hemisphere, New York, pp. 29–73) certainly consistent with experience, but even so not easy to accept for the turbulence community. Even now the ideas of asymptotic independence still dominate most texts and teaching of turbulence. This paper reviews the substantial additional evidence - experimental, numerical and theoretical - for the asymptotic effect of initial and upstream conditions that has accumulated over the past 25 years. Also reviewed is evidence that the Kolmogorov theory for small scale turbulence is not as general as previously believed. Emphasis has been placed on the canonical turbulent flows (especially wakes, jets, and homogeneous decaying turbulence), which have been the traditional building blocks for our understanding. Some of the important outstanding issues are discussed; and implications for the future of turbulence modeling and research, especially LES and turbulence control, are also considered.

Single-Phase and Two-Phase Flow Through Thin and Thick Orifices in Horizontal Pipes

Abstract: Two-phase flow pressure drops through thin and thick orifices have been numerically investigated with air–water flows in horizontal pipes. Two-phase computational fluid dynamics (CFD) calculations, using the Eulerian–Eulerian model have been employed to calculate the pressure drop through orifices. The operating conditions cover the gas and liquid superficial velocity ranges Vsg1⁄4 0.3–4 m/s and Vsl1⁄4 0.6–2 m/s, respectively. The local pressure drops have been obtained by means of extrapolation from the computed upstream and downstream linearized pressure profiles to the orifice section. Simulations for the single-phase flow of water have been carried out for local liquid Reynolds number (Re based on orifice diameter) ranging from 3 10 to 2 10 to obtain the discharge coefficient and the two-phase local multiplier, which when multiplied with the pressure drop of water (for same mass flow of water and two phase mixture) will reproduce the pressure drop for two phase flow through the orifice. The effect of orifice geometry on two-phase pressure losses has been considered by selecting two pipes of 60 mm and 40 mm inner diameter and eight different orifice plates (for each pipe) with two area ratios (r1⁄4 0.73 and r1⁄4 0.54) and four different thicknesses (s/d1⁄4 0.025–0.59). The results obtained from numerical simulations are validated against experimental data from the literature and are found to be in good agreement. [DOI: 10.1115/1.4007267]

Numerical Simulation of Two-Degree-of-Freedom Vortex-Induced Vibration of a Circular Cylinder Between Two Lateral Plane Walls in Steady Currents

Abstract: Vortex-induced vibration (VIV) of a circular cylinder at a low mass ratio of 1.5 between two lateral walls is investigated numerically. The focus of the study is to examine the effects of the two lateral walls on the VIV. Numerical simulations are carried out for w/D = 4, 6, 10, and 20 with D and w being the cylinder diameter and the distance between the two walls, respectively. It is found that the effects of the two walls on the VIV are obvious as w/D ≤ 6 and negligibly small as w/D = 10. The VIV amplitudes in both x- and y-directions increase with the increasing w/D in the lock-in regime.

Computational Analysis of Marine-Propeller Performance Using Transition-Sensitive Turbulence Modeling

Abstract: Almost all computational fluid dynamics (CFD) simulations of flow around marine propellers use turbulence models that are only well suited for fully turbulent flows, which in some cases may lead to accuracy degradation in the prediction of propeller performance characteristics. The discrepancy between computed thrust and torque and corresponding experimental data increases with increasing propeller load. This is due in part to the fact that a large laminar flow region is found to exist and turbulence transition takes place on propeller blades of model scale and/or under high-load conditions. In these cases, it may be necessary to consider boundary-layer transition to obtain accurate results from CFD simulations. The objective of this work is to perform simulations of a marine propeller using a transition-sensitive turbulence model to better resolve the propeller flow characteristics. Fully turbulent flow simulations are also performed for comparison purposes at various propeller load conditions. Computational results are analyzed and compared with water-tunnel and open-water experimental data. It is found that the applied transition-sensitive turbulence model is better able to resolve blade-surface stresses, flow separations, and tip-vortex originations, and, consequently, improve the prediction accuracy in propeller performance, especially under high-load conditions. Furthermore, solutions obtained using the transition-sensitive turbulence model show tip-vortex flows of higher strength, whereas results by the standard k-x SST turbulence model indicate excessive dissipation of the vortex core. [DOI: 10.1115/1.4005729] Three-dimensional computational fluid dynamics (CFD) simulations employing Reynolds-averaged Navier-Stokes (RANS) turbulence models have made considerable contributions to propeller theory and become useful tools for propeller design and analysis. Among many unresolved issues in RANS calculations of marinepropeller performance, turbulence transition has been recognized as a key factor directly associated with viscous effects of propeller flows, such as boundary-layer development, scale effects, and tip and hub vortices. Nevertheless, few, if any, RANS calculations in the open literature have been performed that consider laminar-toturbulent transition in marine-propeller simulations. The objective of this study is to perform marine-propeller CFD simulations using both fully turbulent and transition-sensitive eddy-viscosity turbulence models, and to compare the results between the two approaches. Results are also compared to experimental data to determine whether or not boundary-layer transition plays an important role in the calculation of performance characteristics, and

Characteristics of Mean Droplet Size Produced by Spinning Disk Atomizers

Abstract: Characteristics of mean droplet size of spray produced by spinning disk atomizers were experimentally investigated. The phase-doppler particle analyzer (PDPA) was used to measure the droplet size of water spray in the downstream distance along the spray trajectory. Effects of various operating conditions on the mean diameter had been studied. The studied variables were: the rotational speed in the range of 838 to 1677 rad/s (8,000–16,000 rpm), the liquid flow rate in the range of 0.56 to 2.8 × 10−6 m3 /s (2–10 L/h), the disk diameter in the range of 0.04 to 0.12 m, and the downstream tangential distance along the spray trajectory of up to 0. 24 m. The Sauter mean diameter (d32 ) was used to represent the mean of generated spray droplet sizes. The results indicated that the Sauter mean diameter can be correlated with dimensionless groups, such as the Reynolds number, Weber number, flow coefficient, and the ratio of downstream distance to disk diameter. Based on this correlation, it was found that the Sauter mean diameter (d32 ) increases as the downstream tangential distance, and liquid flow rate increase. Similarly, a decrease of rotational speed and disk diameter results in an increase in the Sauter mean diameter (d32 ). A comparison between the developed correlation and correlations obtained by other researchers has been presented and discussed in detail.

Design and Experimental Validation of a Ducted Counter-Rotating Axial-Flow Fans System

Abstract: An experimental study on the design of counter-rotating axial-flowfans was carried out. The fans were designed using an inversemethod. In particular, the system is designed to have a pure axialdischarge flow. The counter-rotating fans operate in a ducted-flowconfiguration and the overall performances are measured in a nor-malized test bench. The rotation rate of each fan is independentlycontrolled. The relative axial spacing between fans can vary from17% to 310%. The results show that the efficiency is stronglyincreased compared to a conventional rotor or to a rotor-statorstage. The effects of varying the rotation rates ratio on the overallperformances are studied and show that the system has a very flexi-ble use, with a large patch of high efficient operating points in theparameter space. The increase of axial spacing causes only a smalldecrease of the efficiency. [DOI: 10.1115/1.4007591]

Boundary-Layer Flow and Heat Transfer of Nanofluid Over a Vertical Plate With Convective Surface Boundary Condition

Abstract: The problem of boundary layer flow and heat transfer induced due to nanofluid over a vertical plate is investigated. The transport equations employed in the analysis include the effect of Brownian motion and thermophoresis. We used a convective heating boundary condition instead of a widely employed thermal conduction of constant temperature or constant heat flux. The solution for the temperature and nanoparticle concentration depends on six parameters, viz., convective heating parameter A, Prandtl number Pr, Lewis number Le, Brownian motion Nb, buoyancy ratio parameter Nr, and the thermophoresis parameter Nt. Similarity transformation is used to convert the governing nonlinear boundary-layer equations into coupled higher order ordinary differential equations. These equations were solved numerically using Runge-Kutta fourth order method with shooting technique. The effects of the governing parameters on flow field and heat transfer characteristics were obtained and discussed. Numerical results are obtained for velocity, temperature, and concentration distribution as well as the local Nusselt number and Sherwood number. It is found that the local Nusselt number and Sherwood number increase with an increase in convective parameter A and Lewis number Le. Likewise, the local Sherwood number increases with an increase in both A and Le. A comparison with the previous study available in literature has been done and we found an excellent agreement with them.

Study of Near-Stall Flow Behavior in a Modern Transonic Fan With Compound Sweep

Abstract: Detailed flow behavior in a modern transonic fan with a composite sweep is investigated in this paper. Both unsteady Reynolds-averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES) methods are applied to investigate the flow field over a wide operating range. The calculated flow fields are compared with the data from an array of high-frequency response pressure transducers embedded in the fan casing. The current study shows that a relativel y fine computational grid is required to resolve the flow field adequately and to calculate the pressure rise across the fan correctly. The calculated flow field shows detailed flow structure near the fan rotor tip region. Due to the introduction of composite sweep toward the rotor tip, the flow structure at the rotor tip is much more stable com pared to that of the conventional blade design. The passage sho ck stays very close to the leading edge at the rotor tip even at the throttle limit. On the other hand, the passage shock becomes stronger and detaches earlier from the blade passage at the radius where the blade sweep is in the opposite direction. The interaction between the tip cleara nce vortex and the passage shock becomes intense as the fan operates toward the stall limit, and tip clearance vor tex breakdown occurs at near-stall operation. URANS calculat es the time-averaged flow field fairly well. Details of me asured RMS static pressure are not calculated with sufficient accuracy with URANS. On the other hand, LES calculates details of the measured unsteady flow features in the cu rr nt transonic fan with composite sweep fairly well and reve als the flow mechanism behind the measured unsteady flow field. INTRODUCTION Transonic fans with various blade sweeps have been developed as crucial components of modern ultra-high bypass engine concepts. The development aims to achieve higher thrust and higher aerodynamic efficiency with the potential for reducing noise and emissions. The transonic fa in this study with a composite blade sweep has been developed by the General Electric Company and tested at the NASA Glenn Research Center. The 0.559 m (22 inch) diameter model was supported and driven by the Universal Propulsion Simulator (UPS), which was designed for evaluating configurations of high bypass ratio ducted fan engines. The averaged tip clearance is 0.5% of the blade height and the tested fan model has 20 blades. The aerodynamic test was conducted at the 9x15-Foot Low Speed Wind Tunnel, which is located at the NASA Glenn Research Center in Cleveland, Ohio. The cross section of the tested fan is shown in Fig. 1. The flow field near the fan casing is very complex. Dominant features of the compressor endwall flow incl ude the tip clearance flow; interactions among the tip clearance flow, the passage shock, and the endwall boundary layers; and accumulation of low momentum fluid due to radial migration. Tip clearance flow in fans and compressors has been widely studied (for example Hah [1986], Copenhaver et al. [1996], Storor and Cumpsty [1991], Suder and Celestina [1994], Van Zante et al. [2000]). Tip clearance flow arises from the pressure difference between the pressure and t he suction side in the tip gap area. Flow through the tip gap interacts with the incoming passage flow near the suctio n side of the blade as it leaves the blade tip section, f orming STUDY OF NEAR-STALL FLOW BEHAVIOR IN A MODERN TRANSONIC FAN WITH COMPOSITE SWEEP Chunill HAH, Hyoun-Woo Shin NASA Glenn Research Center, MS 5-11, Cleveland, Ohio 44135 Aero Technology Lab. GE Aviation, Cincinnati, Ohio, 45215 ISABE-2011-1220 https://ntrs.nasa.gov/search.jsp?R=20120000842 2019-10-26T15:09:28+00:00Z