Showing papers on "Diffuser (thermodynamics) published in 2007"

The science of making more torque from wind: Diffuser experiments and theory revisited

Abstract: History of the development of DAWT's stretches a period of more than 50 years. So far without any commercial success. In the initial years of development the conversion process was not understood very well. Experimentalists strived at maximising the pressure drop over the rotor disk, but lacked theoretical insight into optimising the performance. Increasing the diffuser area as well as the negative back pressure at the diffuser exit was found profitable in the experiments. Claims were made that performance augmentations with a factor of 4 or more were feasible, but these claims were not confirmed experimentally. With a simple momentum theory, developed along the lines of momentum theory for bare windturbines, it was shown that power augmentation is proportional to the mass flow increase generated at the nozzle of the DAWT. Such mass flow augmentation can be achieved through two basic principles: increase in the diffuser exit ratio and/or by decreasing the negative back pressure at the exit. The theory predicts an optimal pressure drop of 8/9 equal to the pressure drop for bare windturbines independent from the mass flow augmentation obtained. The maximum amount of energy that can be extracted per unit of volume with a DAWT is also the same as for a bare wind turbine. Performance predictions with this theory show good agreement with a CFD calculation. Comparison with a large amount of experimental data found in literature shows that in practice power augmentation factors above 3 have never been achieved. Referred to rotor power coefficients values of CP,rotor =2.5 might be achievable according to theory, but to the cost of fairly large diffuser area ratio's, typically values of β >4.5.

Numerical Flow Simulation in a Centrifugal Pump at Design and Off-Design Conditions

Abstract: The current investigation is aimed to simulate the complex internal flow in a centrifugal pump impeller with six twisted blades by using a three-dimensional Navier-Stokes code with a standard k-ε two-equation turbulence model. Different flow rates were specified at inlet boundary to predict the characteristics of the pump. A detailed analysis of the results at design load, Qdesign, and off-design conditions, Q = 0.43 Qdesign and Q = 1.45 Qdesign, is presented. From the numerical simulation, it shows that the impeller passage flow at design point is quite smooth and follows the curvature of the blade. However, flow separation is observed at the leading edge due to nontangential inflow condition. The flow pattern changed significantly inside the volute as well, with double vortical flow structures formed at cutwater and slowly evolved into a single vortical structure at the volute diffuser. For the pressure distribution, the pressure increases gradually along streamwise direction in the impeller passages. When the centrifugal pump is operating under off-design flow rate condition, unsteady flow developed in the impeller passage and the volute casing.

Modification and experimental research on vortex tube

Abstract: Vortex tube (VT) is a simple energy separating device which is compact and simple to produce and to operate. Although intensive research has been carried out in many countries over the years, the efficiency is still low. In order to improve the energy separate efficiency of vortex tubes, three innovative technologies were applied to vortex tubes. A new nozzle with equal gradient of Mach number and a new intake flow passage of nozzles with equal flow velocity were designed and developed to reduce the flow loss. A new kind of diffuser invented by us was installed for reducing friction loss of air flow energy at the end of the hot end tube of vortex tube, which can greatly improve the performance of vortex tube. The experiment results indicated that these modifications could remarkably improve the performance of vortex tube. The developed vortex tube was not only superior to the conventional vortex tube but also superior to that made by two companies in world under big cold gas mass flow ratio.

Modeling and scaling of aeration bubble plumes: A two-phase flow analysis

Abstract: In this paper, we focus on bubble plumes aimed at providing oxygen to, and at mixing unstratified water bodies. First, we discuss scaling relations obtained from a three-dimensional (3D) theoretical model for multi-component fluids.An expression for a length scale that represents a balance between inertia and buoyancy is derived, and it is used to scale vertical as well as horizontal (radial) distances. This length scale is then analyzed in terms of previous bubble-plume non-dimensional numbers. For the practical case in which the bubble diameter remains almost constant throughout the bubble plume, a one-dimensional theoretical model—which can be derived by integrating the 3D model in a horizontal plane—is used to show that the length scale (D) is well suited for defining zones in bubble plumes. These zones include a region near the diffuser where the size of the diffuser is important, an intermediate zone where the diffuser size and D determine the flow features, and an asymptotic region where plume vari...

One-Dimensional Analysis of Full Load Draft Tube Surge

Abstract: One-dimensional stability analysis of a hydraulic system composed of a penstock, a runner and a draft tube was carried out to determine the cause of the full load draft tube surge. It is assumed that the cavity volume at the runner exit is a function of the pressure at the vortex core evaluated from the instantaneous local pressure at the runner exit and an additional pressure decrease due to the centrifugal force on the swirling flow. It was found that the diffuser effect of the draft tube has a destabilizing effect over all flow rates while the swirl effects stabilize/destabilize the system at larger/smaller flow rates than the swirl free flow rate. Explanations of the destabilizing mechanism are given for the diffuser and swirl flow effects. The effect of finiteness of sound velocity in the penstock is also discussed

Numerical Investigation on Pressure Fluctuations for Different Configurations of Vaned Diffuser Pumps

Abstract: Numerical simulations on impeller-diffuser interactions in radial diffuser pumps are conducted to investigate the unsteady flow, and more attention is paid to pressure fluctuations on the blade and vane surfaces. Calculations are performed at different operating points, different blade number configurations, and different radial gaps between the impeller and diffuser to examine their effects on the unsteady flow. Computational results show that a jet-wake flow structure is observed at the impeller outlet. The biggest pressure fluctuation on the blade is found to occur at the impeller trailing edge, on the pressure side near the impeller trailing edge, and at the diffuser vane leading edge, independent of the flow rate, radial gap, and blade number configuration. All of the flow rate, blade number configuration, and radial gap influence significantly the pressure fluctuation and associated unsteady effects in the diffuser pumps.

Separation control using plasma actuators: application to a free turbulent jet

Abstract: This experimental work deals with active airflow control using non-thermal surface plasma actuators in the case of a rectangular cross section turbulent jet. A wide-angle diffuser composed of two adjustable hinged baseplates is linked at the jet exit. Two types of actuators are considered: the DC corona discharge and the dielectric barrier discharge (DBD). In both cases, an ionic wind with a velocity of several m s−1 is generated tangentially to the wall surface. Thus, this induced aerodynamic effect is applied in order to create the separation along the lower hinged baseplate. The effects of both actuators on the flow separation are measured by means of particle image velocimetry for velocity up to 30 m s−1. The main results show that the DBD seems more efficient than the DC corona discharge but the effect decreases with the jet velocity. However, in increasing the discharge frequency up to 1500 Hz, it is possible to separate a 30 m s−1 jet. Finally, by reducing the actuators' length in the spanwise direction, results lead to a visualization of the three-dimensional effects on the separation along the lower hinged baseplate.

Header design for flow equalization in microstructured reactors

Abstract: To enhance the uniformity of fluid flow distribution in microreactors, a header configuration consisting of a cone diffuser connected to a thick-walled screen has been proposed. The thick-walled screen consists of two sections: the upstream section constitutes a set of elongated parallel upstream channels and the downstream section constitutes a set of elongated parallel downstream channels positioned at an angle of 90° with respect to the upstream channels. In this approach the problem of flow equalization reduces to that of flow equalization in the first and second downstream channels of the thick-walled screen. In turn, this requires flow equalization in the corresponding cross sections of the upstream channels. The computational fluid dynamics analysis of the fluid flow maldistribution shows that eight parallel upstream channels with a width of 300–600 μm are required per 1 cm of length for flow equalization. The length to width ratio of these channels has to be >15. The numerical results suggest that the proposed header configuration can effectively improve the performance of the downstream microstructured devices, decreasing the ratio of the maximum flow velocity to the mean flow velocity from 2 to 1.005 for a wide range of Reynolds numbers (0.5–10). © 2006 American Institute of Chemical Engineers AIChE J, 2007

Numerical simulation of an axial blood pump.

Abstract: The axial blood pump with a magnetically suspended impeller is superior to other artificial blood pumps because of its small size. In this article, the distributions of velocity, path line, pressure, and shear stress in the straightener, the rotor, and the diffuser of the axial blood pump, as well as the gap zone were obtained using the commercial software, Fluent (version 6.2). The main focus was on the flow field of the blood pump. The numerical results showed that the axial blood pump could produce 5.14 L/min of blood at 100 mm Hg through the outlet when rotating at 11 000 rpm. However, there was a leakage flow of 1.06 L/min in the gap between the rotor cylinder and the pump housing, and thus the overall flow rate the impeller could generate was 6.2 L/min. The numerical results showed that 75% of the scalar shear stresses (SSs) were less than 250 Pa, and 10% were higher than 500 Pa within the whole pump. The high SS region appeared around the blade tip where a large variation of velocity direction and magnitude was found, which might be due to the steep angle variation at the blade tip. Because the exposure time of the blood cell at the high SS region within the pump was relatively short, it might not cause serious damage to the blood cells, but the improvement of blade profile should be considered in the future design of the axial pump.

Exhaust aftertreatment system with flow distribution

Abstract: An exhaust aftertreatment system has a side inlet flow diffuser and provides even flow exhaust distribution to an aftertreatment element.

Wind Tunnel Testing of an Axisymmetric Isentropic Relaxed External Compression Inlet at Mach 1.97 Design Speed

Abstract: An alternative inlet design method employing isentropic relaxed compression has been developed by Gulfstream Aerospace Corporation for improving net propulsion system performance and reducing sonic boom strength. To substantiate results from prior analytical studies and to provide an experimental database for subsequent research, a wind tunnel test was conducted using a family of subscale axisymmetric relaxed compression inlet models designed for an incoming flow speed of Mach 1.97. All tests were performed in the 1 ft by 1 ft supersonic wind tunnel at NASA Glenn Research Center. Focusing on internal flow characteristics, a large database comprising nearly 1500 test runs was acquired, covering a range of subsonic diffuser Mach numbers, diffuser lengths, mass flow rates, and angles-of-attack. Differences in flow properties between inlet models were surprisingly modest despite significant changes in hardware geometry. Strut effects were noteworthy for all model designs. Comparisons with CFD proved the analytical methodology to be reliable for this application. The large experimental database will be used for additional analytical methodology development, design work, test planning at larger scale, and the development of non-bleed surface treatment concepts for boundary layer control.

Application of particle image velocimetry to a transonic centrifugal compressor

Abstract: As part of an ongoing research project the performance and internal flow field of a high-pressure ratio centrifugal compressor is being investigated. Based on previous, primarily, point-wise laser-optical measurements the compressor was redesigned and resulted in an improved impeller and diffuser with a single-stage pressure ratio of 6:1 at 50,000 rpm. Current research activities involve the use of particle image velocimetry (PIV) to analyze and further improve the understanding of the complex flow phenomena inside the vaned diffuser given the capability of PIV of capturing spatial structures. The study includes phase-resolved measurements of the flow inside a diffuser vane passage with respect to the impeller blade position. Both, instantaneous and phase-averaged velocity fields are presented. The flow field results obtained by PIV are to be used for future validation of the related CFD calculations, which in turn are expected to lead to further improvements in compressor performance. In addition, the potential of stereo PIV for this type of turbomachinery application could be successfully demonstrated.

Quasistatic nonlinear characteristics of double-reed instruments

Abstract: This article proposes a characterization of the double reed in quasistatic regimes. The nonlinear relation between the pressure drop, Δp, in the double reed and the volume flow crossing it, q, is measured for slow variations of these variables. The volume flow is determined from the pressure drop in a diaphragm replacing the instrument’s bore. Measurements are compared to other experimental results on reed instrument exciters and to physical models, revealing that clarinet, oboe, and bassoon quasistatic behavior relies on similar working principles. Differences in the experimental results are interpreted in terms of pressure recovery due to the conical diffuser role of the downstream part of double-reed mouthpieces (the staple).

Simulations of turbulent flow in a plane asymmetric diffuser

Abstract: Large-eddy simulations (LES) of a planar, asymmetric diffuser flow have been performed. The diverging angle of the inclined wall of the diffuser is chosen as 8.5°, a case for which recent experimental data are available. Reasonable agreement between the LES and the experiments is obtained. The numerical method is further validated for diffuser flow with the diffuser wall inclined at a diverging angle of 10°, which has served as a test case for a number of experimental as well as numerical studies in the literature (LES, RANS). For the present results, the subgrid-scale stresses have been closed using the dynamic Smagorinsky model. A resolution study has been performed, highlighting the disparity of the relevant temporal and spatial scales and thus the sensitivity of the simulation results to the specific numerical grids used. The effect of different Reynolds numbers of the inflowing, fully turbulent channel flow has been studied, in particular, Re b = 4,500, Re b = 9,000 and Re b = 20,000 with Re b being the Reynolds number based on the bulk velocity and channel half width. The results consistently show that by increasing the Reynolds number a clear trend towards a larger separated region is evident; at least for the studied, comparably low Reynolds-number regime. It is further shown that the small separated region occurring at the diffuser throat shows the opposite behaviour as the main separation region, i.e. the flow is separating less with higher Re b . Moreover, the influence of the Reynolds number on the internal layer occurring at the non-inclined wall described in a recent study has also been assessed. It can be concluded that this region close to the upper, straight wall, is more distinct for larger Re b . Additionally, the influence of temporal correlations arising from the commonly used periodic turbulent channel flow as inflow condition (similar to a precursor simulation) for the diffuser is assessed.

Improved High Pressure Ratio Centrifugal Compressor

Abstract: The paper describes the development and the experimental as well as theoretical investigation of a new transonic, high specific speed centrifugal compressor rotor of 6.2:1 pressure ratio. Performance measurement results, laser measurements and calculated 3D results are shown for the new rotor and are compared with the corresponding data of a same type predecessor rotor. A 2% gain in stage efficiency and a 0.2 bar increase in stage pressure ratio are found at design speed by performance measurements. With the help of optical measurements and 3D stage calculations it is shown that the flow at the exit of the new rotor is more uniform/homogeneous. The degree of uniformity increases with decreasing pressure ratio, i.e. in the compressor part load region. Deeper insight into the internal rotor and the vaned diffuser flow is obtained from the 3D stage calculations showing less flow separation in the new rotor but significant secondary flow in the small span diffuser. The investigations are indicating that a further improvement of stage performance seems to be possible by an additional optimization of the vaned diffuser.Copyright © 2007 by ASME

Numerical Simulation of Turbulent Flows in Centrifugal Compressor Stages With Different Radial Gaps

Abstract: Numerical simulation of three-dimensional flow in a one-stage centrifugal compressor with a diffuser of variable geometry has been performed using the ANSYS CFX 10 code The computations were conducted using steady and unsteady flow formulations and employing the RANS two-equation turbulence models Steady-state flow simulations in the compressor were done for two vaned diffuser geometries with different radial gaps A detailed comparison with the experimental data reported in the literature for different operating points of the “Radiver” test case compressor is presented and discussed Good agreement of the computed velocity field with the measurements data is obtained at the impeller exit Downstream of the diffuser vane, prediction quality depends on the operating point Transient simulations performed for the best operating point of the compressor did not improve considerably predictions of flow characteristics in the diffuser as compared to the steady-state approachCopyright © 2007 by ANSYS Germany GmbH and GE Global Research

Design and CFD analysis of centrifugal compressor for a microgasturbine

Abstract: Purpose – This paper aims to show how a good compressor can be designed and modeled with CFD steady models and to explain reasons for discrepancies between experiment (1D design) and 3D CFD analysis.Design/methodology/approach – A model with only one impeller channel was used to compare 1D design data, which were obtained from centrifugal compressor design code, written and developed by the authors. The often used model for CFD analysis of turbo machinery, known as “frozen‐rotor” model, only yields satisfying results for efficiency and pressure ratio, at and near the point of best efficiency. For this case, the static pressure shows a nearly uniform circumferential distribution at the inlet of the diffuser, which numerically leads to more homogeneous flow rates through the single vane channels, and thus to a more realistic time averaged flow distribution.Findings – The numerical results with respect to performance data showed quite good agreement with experimental data at and near the operating point of b...

Aeroengine bleed valve

Abstract: A bleed valve for a gas turbine engine, the valve comprises a diffuser having a plurality of holes through which a bleed fluid flows and into a fluid stream. The diffuser is characterized in that at least some of the holes are at least partially tangentially angled relative to the bleed fluid flow through the valve so that the bleed air forms a vortex to enhance mixing with the fluid stream.

Compressor/Diffuser/Combustor Aerodynamic Interactions in Lean Module Combustors

Abstract: The paper reports an experimental investigation into the possibility of increased interactions between combustor external aerodynamics and upstream components, e.g., prediffuser, compressor outlet guide vane (OGV), and even the compressor rotor, caused by the trend in lean module fuel injectors to larger mass flows entering the combustor cowl. To explore these component interaction effects, measurements were made on a fully annular rig comprising a single stage compressor, an advanced integrated OGV/prediffuser, followed by a dump diffuser and a generic combustor flametube with metered cowl and inner/outer annulus flows. The flow split entering the cowl was increased from 30% to 70%. The results demonstrate that, with fixed geometry, as the injector flow increases, the performance of the prediffuser and feed annuli suffer. Prediffuser losses increase and at high injector flow rates, the diffuser moves close to separation. The substantial circumferential variation in cowl flow can feed upstream and cause rotor forcing. Notable differences in performance were observed inline and between injectors at the OGV exit, suggesting that geometry changes such as an increased dump gap or nonaxisymmetric prediffuser designs may be beneficial.

급가감속 운전에 따른 듀얼 모노리스형 촉매변환기 내의 유동 균일도와 압력 강하에 관한 수치적 연구

Abstract: The conversion efficiency, durability and pressure drop of the automotive exhaust catalysts are dependent on the flow distribution within the substrate. Conventional porous medium approaches assuming monolith resistance based on the one-dimensional laminar flow for simulating the flow through the automotive exhaust catalysts over-predict the flow uniformity in the monolith. In this study, additional pressure loss is also considered by accounting for entrance effects due to the oblique flow incident on the front face of monolith as a consequence of flow separation and recirculation within the diffuser. The incorporation of an additional pressure loss improves the predictions for the maximum flow velocity within the substrate. An numerical study has also been conducted for the three-dimensional unsteady incompressible non-reacting flow inside various dual-monolith catalytic converters for the rapid acceleration/deceleration driving.

Numerical Analysis of the Vaned Diffuser of a Transonic Centrifugal Compressor

Abstract: A three-dimensional Navier-Stokes solver is used to investigate the flow field of a high pressure ratio centrifugal compressor for turbocharger applications. Such a compressor consists of a double-splitter impeller followed by a vaned diffuser. Particular attention is focused on the analysis of the vaned diffuser, designed for high subsonic inlet conditions. The diffuser is characterized by a complex three-dimensional flow field, and influenced by the unsteady interaction with the impeller. Detailed Particle Image Velocimetry (PIV) flow measurements within the diffuser are available for comparison purposes.Copyright © 2007 by ASME

Effects of tapered diffuser vane on the flow field and noise of a centrifugal compressor

Abstract: The effects of a three-dimensional tapered diffuser vane on the flow field and noise radiated from a centrifugal compressor are investigated by both CFD analyses and experiments. Tapered diffuser vanes are very useful not only for the reduction of the interaction tone noise but also for the improvement of the pressure recovery characteristics within the diffuser passage. By using tapered diffuser vanes, the interaction area between the impeller-discharge flow and diffuser vanes becomes small, and then the noise level of the discrete tone can be reduced remarkably as a result. Furthermore, by utilizing the visualization technique of vortical structures based on the CFD results, the scale of vortex shedding leaving from the leading edge of the diffuser vanes is found to be contracted and a tendency for the turbulence level to decrease is observed. This may be the cause of the attenuation of broadband noise components. The secondary flow, which is considered to be an obstruction of diffuser pressure recovery, can also be suppressed by the tapered diffuser vanes, and the pressure decrease observed in the throat part of the diffuser passage is further reducible.

Rotor-Stator Interaction Induced Pressure Fluctuations: CFD and Hydroacoustic Simulations in the Stationary Components of a Multistage Centrifugal Pump

Abstract: In a centrifugal pump the interaction between the rotating impeller pressure field and the stationary diffuser pressure field yields pressure fluctuations as the result of a modulation process. These fluctuations may induce hydroacoustic pressure fluctuations in the exit chamber of the pump and could cause %under resonance conditions% unacceptable vibrations. This paper presents a methodology for the prediction of hydroacoustic pressure fluctuations resulting from rotor-stator interaction in a multistage centrifugal pump. The method consists in the one-way coupling of incompressible CFD and hydroacoustic simulations. In a first step the rotor-stator pressure fluctuations are calculated using a commercial 3D-RANS CFD-code (CFX 10) for different flow rates. The acoustic simulations are performed in two consecutive steps. Initially a free oscillation analysis using white noise pressure fluctuations is performed, which provides hydroacoustic eigen frequencies and mode shapes of the outlet casing. In a second step the spatially distributed pressure fluctuations from the CFD simulation are used to perform a forced oscillation analysis. This approach allows the prediction of possible standing waves in the hydraulic collection elements in the last stage of multistage pumps.

Vehicle air-conditioner

Abstract: PROBLEM TO BE SOLVED: To provide a vehicle air-conditioner that reduces noise in a specific frequency region caused by self-excited oscillation of a separation vortex occurring in a diffuser part of a unit case. SOLUTION: The vehicle air-conditioner 1 includes a blower unit 2 and an air-conditioning unit 3 that blows out air, blown from the blower unit 2, into a vehicle interior while temperature-conditioning the air. The air-conditioning unit 3 has a unit case 11 that guides air, blown from the blower unit 2, to an evaporator inlet passage 21 through a diffuser part 22 so as to circulate the air from the evaporator inlet passage 21 to an evaporator 13. A face 23, facing an air inflow direction, of the unit case 11 formed with the evaporator inlet passage 21 is provided with a reflection-direction change part 24 that changes a reflection direction of sound waves caused by self-excited oscillation of a separation vortex occurring in the diffuser part 22. COPYRIGHT: (C)2009,JPO&INPIT

Servo-controlled variable geometry ejector pump

Abstract: A variable geometry ejector pump is configured to receive pressurized fluid from one or more pressurized fluid sources and to control fluid pressure and temperature down stream of the pump to a variety of pressure and temperature values. The variable geometry ejector pump includes a primary inlet, a secondary inlet, a variable geometry ejector, an ejector valve, an actuator, a mixing section, a diffuser section, and an actuator control mechanism. The actuator control mechanism is adapted to receive one or more control signals, and is operable, in response to the control signals, to control the actuator, to thereby control fluid pressure and temperature down stream of the pump.

On the Use of the Squire-Long Equation to Estimate Radial Velocities in Swirling Flows

Abstract: A method to estimate the radial velocity in swirling flows from experimental values of the axial and tangential velocities is presented. The study is motivated by the experimental difficulties to o ...

Dynamic Analysis of Wind Tunnel Data from an Isentropic Relaxed Compression Inlet

Abstract: A family of five axisymmetric external compression inlets was tested at Mach 1.97 in the 1-ft ×1-ft supersonic wind tunnel at the NASA Glenn Research Center. All five inlets shared the same external compression surface; they differed only in subsonic diffuser geometry. The external compression surface used an isentropic relaxed compression design developed by Gulfstream Aerospace Corporation. Steady-state pressure data were recorded on the inlet centerbody, on the cowl, and at two pitot pressure rakes located at the aerodynamic interface plane (AIP). High frequency dynamic pressure data were recorded at one static pressure tap on the inlet centerbody and at a pitot probe on the AIP. Schlieren images were also recorded. This paper analyzes the dynamic pressure data from three of the inlet models.