Showing papers on "Axial compressor published in 2009"

Hybrid propulsive engine including at least one independently rotatable compressor rotor

Abstract: One aspect relates to a hybrid propulsive technique, comprising providing at least some first thrust associated with a flow of a working fluid through at least a portion of an at least one axial flow jet engine. The hybrid propulsive technique includes extracting energy at least partially in the form of electrical power from the working fluid, and converting at least a portion of the electrical power to torque. The hybrid propulsive technique fiuther includes rotating an at least one substantially axial-flow independently rotatable compressor rotor at least partially responsive to the converting the at least a portion of the electrical power to torque.

Centrifugal compressor assembly and method

Abstract: A centrifugal compressor assembly for compressing refrigerant in a 250-ton capacity or larger chiller system comprising a motor, preferably a compact, high energy density motor or permanent magnet motor, for driving a shaft at a range of sustained operating speeds under the control of a variable speed drive Another embodiment of the centrifugal compressor assembly comprises a mixed flow impeller and a vaneless diffuser sized such that a final stage compressor operates with an optimal specific speed range for targeted combinations of head and capacity, while a non-final stage compressor operates above the optimum specific speed of the final stage compressor Another embodiment of the centrifugal compressor assembly comprises an integrated inlet flow conditioning assembly comprising a flow conditioning nose, a plurality of inlet guide vanes and a flow conditioning body that positions inlet guide vanes to condition flow of refrigerant into an impeller to achieve a target approximately constant angle swirl distribution with minimal guide vane turning

Characteristics and Control of the Draft-Tube Flow in Part-Load Francis Turbine

Abstract: Under part-load conditions, a Francis turbine often suffers from very severe low-frequency and large-amplitude pressure fluctuation, which is caused by the unsteady motion of vortices (known as "vortex ropes") in the draft tube This paper first reports our numerical investigation of relevant complex flow phenomena in the entire draft tube, based on the Reynolds-averaged Navier-Stokes (RANS) equations We then focus on the physical mechanisms underlying these complex and somewhat chaotic flow phenomena of the draft-tube flow under a part-load condition The flow stability and robustness are our special concern, since they determine what kind of control methodology will be effective for eliminating or alleviating those adverse phenomena Our main findings about the flow behavior in the three segments of the draft tube, ie, the cone inlet, the elbow segment, and the outlet segment with three exits, are as follows (1) In the cone segment, we reconfirmed a previous finding of our research group based on the turbine's whole-flow RANS computation that the harmful vortex rope is an inevitable consequence of the global instability of the swirling flow We further identified that this instability is caused crucially by the reversed axial flow at the inlet of the draft tube (2) In the elbow segment, we found a reversed flow continued from the inlet cone, which evolves to slow and chaotic motion There is also a fast forward stream driven by a localized favorable axial pressure gradient, which carries the whole mass flux downstream The forward stream and reversed flow coexist side-by-side in the elbow, with a complex and unstable shear layer in between (3) In the outlet segment with three exits, the forward stream always goes through a fixed exit, leaving the other two exits with a chaotic and low-speed fluid motion Based on these findings, we propose a few control principles to suppress the reversed flow and to eliminate the harmful helical vortex ropes Of the methods we tested numerically, a simple jet injection in the inlet is proven successful

Enhancing the Stability of Subsonic Compressors Using Casing Grooves

Abstract: Casing grooves are known to increase the stable operating range of axial compressors. The mechanism by which this stability enhancement occurs is poorly understood. This paper develops a better understanding of the behavior of casing grooves through analysis of new data. An experimental parametric study is used to demonstrate the effect of varying the axial location of a single casing groove on the stability and efficiency of the compressor. The effect that the groove has on rotor outflow blockage, blade loading, and the near-casing flow field is then investigated using both experimental and computational methods. It is found that the interaction of the groove with the flow field is different when the groove is positioned forward or aft relative to the blade. The interaction of the groove with the flow in the tip region in both of these positions is presented in detail. Finally, the implications of these findings for the design of casing grooves of different depths are discussed. © 2011 American Society of Mechanical Engineers.

The Design of Highly Loaded Axial Compressors

Abstract: Increasing compressor pressure ratios (thereby gaining a benefit in cycle efficiency), or reducing the number of stages (to reduce weight, cost, etc.), will require an increase in pressure rise per stage. One method of increasing the pressure rise per stage is by increasing the stage-loading coefficient and it is this topic which forms the focus of the present paper. In the past, a great deal of effort has been expended in trying to design highly loaded blade rows. Most of this work has focused on optimizing a particular design rather than looking at the fundamental problems associated with high loading. This paper looks at the flow physics behind the problem, makes proposals for a new design strategy and explains sources of additional loss specific to highly loaded designs. Detailed experimental measurements of three highly loaded stages (Δh0 /U2 ≈ 0.65) have been used to validate a CFD code. The calibrated CFD has then been used to show that as the stage loading is increased the flow in the stator passages breaks down first. This happens via a large corner separation which significantly impairs the stage efficiency. The stator can be relieved by increasing stage reaction, thus shifting the burden to the rotor. Fortunately, the CFD calculations show that the rotor is generally more tolerant of high loading than the stator. Thus, when stage loading is increased, it is necessary to increase the reaction to achieve the optimum efficiency. However, the design exercise using the calibrated CFD also shows that the stage efficiency is inevitably reduced as the stage loading is increased (in agreement with the experimental results). In the second part of the paper, the role profile loss plays in the reduction in efficiency at high stage loading is considered. A simple generic velocity distribution is developed from first principles to demonstrate the hitherto neglected importance of the pressure surface losses in highly loaded compressors.Copyright © 2009 by ASME

The Fouling of Axial Flow Compressors: Causes, Effects, Susceptibility, and Sensitivity

Abstract: Increased fuel costs have created a strong incentive for gas turbine operators to understand, minimize and control performance deterioration. The most prevalent deterioration problem faced by gas turbine operators is compressor fouling. Fouling causes a drop in airflow, pressure ratio and compressor efficiency, resulting in a “re-matching” of the gas turbine and compressor and a drop in power output and thermal efficiency. This paper addresses the causes and effects of fouling and provides a comprehensive treatment of the impact of salient gas turbine design parameters on the susceptibility and sensitivity to compressor fouling. Simulation analysis of ninety two (92) gas turbines of ranging from a few kW to large engines rated at greater than 300 MW has been conducted. It is hoped that this paper will provide practical information to gas turbine operators.Copyright © 2009 by ASME

Automated Multidisciplinary Optimization of a Transonic Axial Compressor

Abstract: The current paper describes DLR’s optimizer AutoOpti, the implementation of the metamodel “Kriging” as accelerating technique, and the process chain in the automated, multidisciplinary optimization of fans and compressors on basis of a recent full stage optimization of a highly loaded, transonic axial compressor Methods and strategies for an aerodynamic performance map optimization coupled with a finite element analysis on the structural side are presented The high number of 231 free design parameters, a very limited number of CFD simulations, and conflicting demands both within the aerodynamic requirements and between the disciplines are a challenging optimization task To navigate such a multi-dimensional search space, metamodels have successfully been used as accelerating technique Using four aerodynamic operating points at two rotational speeds allows adjusting a required stability margin and optimizing the working line performance under this constraint The investigated compressor concept is a highly loaded transonic stage with a single row rotor and a tandem stator, designed for a very high total pressure ratio A Introduction ompressors for aircraft engines are constantly developed towards higher aerodynamic loading to reduce the installation length, weight, and number of parts with no degradation in efficiency This leads to more complex geometries and consequently to more complex flow structures An automated optimization approach is to be preferred in order to take advantage of new design freedoms, while reducing or at least maintaining development time Automated optimization is also suggested by recent progress in simulation technologies in several fields such as steady and unsteady computational fluid dynamics (CFD), structural and thermal finite element analysis (FEM) Moreover, processors have become increasingly powerful, and parallel computing on huge clusters can be considered state of the art technology for CFD and FEM applications Thus, it has become possible to employ optimization methods in the design of various parts of heavy duty gas turbines and aircraft engines, even when calculations require large computational resources

Unsteady Simulation of an Axial Compressor Stage With Casing and Blade Passive Treatments

Abstract: This paper deals with the numerical simulation of technologies to increase the compressor performances. The objective is to extend the stable operating range of an axial compressor stage using passive control devices located in the tip region. First, the behavior of the tip leakage flow is investigated in the compressor without control. The simulation shows an increase in the interaction between the tip leakage flow and the main flow when the mass flow is reduced, a phenomenon responsible for the development of a large flow blockage region at the rotor leading edge. A separation of the rotor suction side boundary layer is also observed at near stall conditions. Then, two approaches are tested in order to control these flows in the tip region. The first one is a casing treatment with nonaxisymmetric slots. The method showed a good ability to control the tip leakage flow but failed to reduce the boundary layer separation on the suction side. However, an increase in the operability was observed but with a penalty for the efficiency. The second approach is a blade treatment that consists of a longitudinal groove built in the tip of each rotor blade. The simulation pointed out that the device is able to control partially all the critical flows with no penalty for the efficiency. Finally, some recommendations for the design of passive treatments are presented.

Numerical Investigation of Tandem Airfoils for Subsonic Axial-Flow Compressor Blades

Abstract: The tandem airfoil has potential to do more work as a compressor blade than a single airfoil without incurring significantly higher losses. Although tandem blades are sometimes employed as stators, they have not been used in any known commercial rotors. While the long-term goal for this program is development of a commercially viable tandem rotor, this paper discusses tandem airfoils in subsonic, shock-free rectilinear cascade flow. Existing literature data on tandem airfoils in rectilinear cascades have been compiled and presented in a Lieblein loss versus loading correlation. Large scatter in the data gave motivation to conduct an extensive 2D computational fluid dynamics (CFD) study evaluating the overall performance as a function of the relative positions of the forward and aft airfoils. CFD results were consistent with trends in the open literature, both of which indicate that a properly designed tandem airfoil can outperform a comparable single airfoil on and off design. The general agreement of the CFD and literature data serves as a validation for the computational approach.

Theoretical study of a novel refrigeration compressor – Part II: Performance of a rotating discharge valve in the revolving vane (RV) compressor

Abstract: A new refrigeration compressor, named ‘Revolving Vane (RV) compressor’, has been introduced in Part I of this paper series For a first time in refrigeration compressors, a rotating discharge valve is employed in the RV compressor mainly due to the rotation of the entire cylinder This paper presents a theoretical investigation on the dynamic behavior of a reed-type discharge valve undergoing rotatory motion, with the primary objective of elucidating the applicability of such valves in refrigeration compressors Under the application of the Euler–Bernoulli beam theory, a mathematical model of the rotating valve is formulated and the transient response of the valve under centrifugal loads in addition to pressure forces is analyzed Results have shown that under careful design considerations, the performance as well as the reliability of the rotating discharge valve can be enhanced as compared to a non-rotating valve that has been used in all refrigeration compressors currently

Axial and Centrifugal Compressor Mean Line Flow Analysis Method

Abstract: This paper describes a method to estimate key aerodynamic parameters of single and multistage axial and centrifugal compressors. This mean-line compressor code COMDES provides the capability of sizing single and multistage compressors quickly during the conceptual design process. Based on the compressible fluid flow equations and the Euler equation, the code can estimate rotor inlet and exit blade angles when run in the design mode. The design point rotor efficiency and stator losses are inputs to the code, and are modeled at off design. When run in the off-design analysis mode, it can be used to generate performance maps based on simple models for losses due to rotor incidence and inlet guide vane reset angle. The code can provide an improved understanding of basic aerodynamic parameters such as diffusion factor, loading levels and incidence, when matching multistage compressor blade rows at design and at part-speed operation. Rotor loading levels and relative velocity ratio are correlated to the onset of compressor surge. NASA Stage 37 and the three-stage NASA 74-A axial compressors were analyzed and the results compared to test data. The code has been used to generate the performance map for the NASA 76-B three-stage axial compressor featuring variable geometry. The compressor stages were aerodynamically matched at off-design speeds by adjusting the variable inlet guide vane and variable stator geometry angles to control the rotor diffusion factor and incidence angles.

Advanced Nonaxisymmetric Endwall Contouring for Axial Compressors by Generating an Aerodynamic Separator—Part II: Experimental and Numerical Cascade Investigation

Abstract: Modern methods for axial compressor design are capable of shaping the blade surfaces in a three dimensional way. Linking these methods with automated optimization techniques provides a major benefit to the design process. The application of non-axisymmetric contoured endwalls is considered to be very successful in turbine rotors and vanes. Concerning axial compressors non-axisymmetric endwalls are still a field of research. This two-part paper presents the recent development of a novel endwall design. A vortex created by a nonaxisymmetric endwall groove acts as an aerodynamic separator, preventing the passage vortex from interacting with the suction side boundary layer. This major impact on the secondary flow results in a significant loss reduction by means of load redistribution, reduction of recirculation areas and suppressed corner separation. Part I of this paper deals with the endwall design and its compressor application. The resulting flow phenomena and physics are described and analysed in detail. The second paper presents the detailed experimental and numerical investigation of the developed endwall groove. The measurements carried out at the transonic cascade wind tunnel of DLR in Cologne, demonstrated a considerable influence on the cascade performance. A loss reduction and redistribution of the cascade loading were achieved at the aerodynamic design point as well as near the stall condition of the cascade. This behaviour is well predicted by the numerical simulation. The combined analysis of experimental and numerical flow patterns allows a detailed interpretation and description of the resulting flow phenomena. In this context high fidelity 3D-RANS flow simulations are required to analyse the complex blade and endwall boundary layer interaction.Copyright © 2009 by ASME

Experimental study of a small wind turbine for low-and medium-wind regimes

Abstract: The results of an experimental assessment of a small prototype battery charging wind turbine designed for low- and medium-wind regimes are presented. The turbine is based on a newly designed axial flow permanent magnet synchronous generator and a three-bladed rotor with variable twist and taper blades. Overspeed control is performed by a furling mechanism. The turbine has the unique feature of being capable of operating at either 12, 24 or 48 V system voltage, requiring no load control in any case. In the 48 V configuration, the system is capable of providing 2 kWh day−1 for an average wind speed as low as 3.5 m s−1 and an air density of 85% of the standard pressure and temperature value. The experimental assessment has been conducted under field conditions with the turbine mounted on a 20 m guy-wired tubular tower. The experimental power curves are shown to be in good agreement with a detailed aerodynamical and electromechanical model of the turbine for non-furling conditions and for wind speeds above the theoretical cut-in speed. In the case of the rapidly spinning load configurations, a finite power production at wind speeds below the theoretical cut-in speed can be observed, which can be explained in terms of inertia effects. During the measurement campaigns with high loads, we were able to observe bifurcations of the power curve, which can be explained in terms of instabilities arising in situations of transition from attached to separated flow. A full experimental Cp(λ)-curve has been constructed by operating the turbine under different load conditions and the findings are in good agreement with a variable Reynolds-number blade-element momentum model. The three proposed system configurations have been found to operate with a high aerodynamic efficiency with typical values of the power coefficient in the 0.40–0.45 range. Copyright © 2009 John Wiley & Sons, Ltd.

Absolute and convective instability of cylindrical Couette flow with axial and radial flows

Abstract: Imposing axial flow in the annulus and/or radial flow through the cylindrical walls in a Taylor–Couette system alters the stability of the flow. Theoretical methods and numerical simulations were used to determine the impact of imposed axial and radial flows, homogeneous in the axial direction, on the first transition of Taylor–Couette flow in the framework of convective and absolute instabilities. At low axial Reynolds numbers the convective instability is axisymmetric, but convective helical modes with an increasing number of helices having a helicity opposite that of the base flow dominate as the axial flow increases. The number of helices and the critical Taylor number are affected only slightly by the radial flow. The flow becomes absolutely unstable at higher Taylor numbers. Absolutely unstable axisymmetric modes occur for inward radial flows, while helical absolute instability modes having a helicity identical to that of the base flow occur at high enough axial Reynolds numbers for outward radial flow.

Advanced Non-Axisymmetric Endwall Contouring for Axial Compressors by Generating an Aerodynamic Separator—Part I: Principal Cascade Design and Compressor Application

Abstract: Modern methods for axial compressor design are capable of shaping the blade surfaces in a three dimensional way. Linking these methods with automated optimization techniques provides a major benefit to the design process. The application of non-axisymmetric contoured endwalls is considered to be very successful in turbine rotors and vanes. Concerning axial compressors non-axisymmetric endwalls are still a field of research. This two-part paper presents the recent development of a novel endwall design. An aerodynamic separator, generated by a non-axisymmetric endwall groove, interacts with the passage vortex. This major impact on the secondary flow results in a significant loss reduction because of load redistribution, reduction of recirculation areas and suppressed corner separation. The first paper deals with the development of the initial endwall design using a linear compressor cascade application. A brief introduction of the design methods is provided, including the automated optimization, the 3D process chain with a focus on the endwall contouring tool. Hereafter the resulting flow phenomena and physics due to the modified endwall surface are described and analyzed in detail. Additionally, the endwall design principal is transferred to an axial compressor stage. The endwall groove is applied to the hub and casing endwalls of the stator and the initial numerical investigation is presented. For highly loaded operating points the flow behaviour at the hub region can be improved in accord with the cascade results. Obviously, the casing region is dominated by the incoming tip vortex generated by the rotor and still remains an area for further investigations concerning non-axisymmetric endwall contouring.Copyright © 2009 by ASME

Numerical Investigations of the Coupled Flow Through a Subsonic Compressor Rotor and Axial Skewed Slot

Abstract: In order to advance the understanding of the fundamental mechanisms of axial skewed slot casing treatment and their effects on the subsonic axial-flow compressor flow field, the coupled unsteady flow through a subsonic compressor rotor and the axial skewed slot was simulated with a state-of-the-art multiblock flow solver. The computational results were first compared with available measured data, that showed the numerical procedure calculates the overall effect of the axial skewed slot correctly. Then, the numerically obtained flow fields were interrogated to identify the physical mechanism responsible for improvement in stall margin of a modern subsonic axial-flow compressor rotor due to the discrete skewed slots. It was found that the axial skewed slot casing treatment can increase the stall margin of subsonic compressor by repositioning of the tip clearance flow trajectory further toward the trailing of the blade passage and retarding the movement of the incoming/tip clearance flow interface toward the rotor leading edge plane.

A gas turbine engine compressor variable stator vane arrangement

Abstract: A compressor (32) variable stator vane arrangement comprises at least one stage of variable stator vanes (42). A speed sensor (50) measures the rotational speed (N2) of the compressor rotor (36). A pressure sensor (54) measures the outlet pressure (PS26) of the compressor (32). A second pressure sensor (47), a temperature sensor (51) and a third pressure sensor (55) measure the total inlet pressure (P20), the temperature (T20) and the ambient pressure (Pamb) at the inlet of the gas turbine engine (10). A processor (58) determines a target operating line as a function of ambient pressure (Pamb) and total inlet pressure (P20). The target operating line is defined to ensure the gas turbine engine (10) operates simultaneously at both the minimum required compressor speed and the minimum required compressor outlet pressure when commanded to idle to minimise idle thrust and fuel burn. The processor (58) determines if the operating point of the compressor, defined in terms of corrected outlet pressure (PS26/DP20) and corrected rotational speed (N2RTHT20) of the compressor rotor (36) is above or below the target operating line of the compressor (32). The processor (58) adjusts the angle of the variable stator vanes (42) to make the operating point operate on the target operating line.

Shock/Boundary-Layer/Tip-Clearance Interaction in a Transonic Rotor Blade

Abstract: A newly designed rotor was modeled from the well-known radially stacked NASA rotor 37 by applying a three-dimensional shape to the original blade stacking line. A considerable curvature toward the direction of rotor rotation was given to the new blade. A three-dimensional numerical model, developed and validated using a commercial computational fluid dynamics Reynolds-averaged Navier―Stokes code, was adopted to predict the flowfield inside the new rotor. Steady-viscous-flow calculations were run at the design speed of the baseline configuration. Compared with rotor 37, the new rotor showed a higher efficiency, mainly due to a three-dimensional modification of the shock structure. At the outer span, the new rotor developed a blade-to-blade shock front located more downstream than in the baseline rotor, with a considerable impact on the flowfield near the casing. Computational fluid dynamics flow visualizations showed a less detrimental shock/boundary-layer/tip-clearance interaction at low-flow operating conditions, with a considerable reduction of the low-momentum-fluid region after the shock.

Non-Axisymmetric End Wall Profiling in Transonic Compressors—Part I: Improving the Static Pressure Recovery at Off-Design Conditions by Sequential Hub and Shroud End Wall Profiling

Abstract: Secondary flows involving cross flow and three-dimensional separation phenomena in modern axial compressors at high stage loading contribute significantly to a reduction in overall efficiency. This two-part paper presents a numerical study on the potential aerodynamic benefits of using non-axisymmetric end walls in an axial compressor, involving both the rotor and the stator row. This first paper describes the sequential profiling of stator end walls in a transonic compressor at several operating points to suppress separation. An automated multi-objective optimizer connected to a 3-D RANS flow solver was used to find the optimal end wall geometries. As a design exercise, the stator hub end wall of Configuration I of the Darmstadt Transonic Compressor was first optimized at design conditions, keeping the shroud end wall constant. This led to an increase in efficiency of 1.8% due to the suppression of the hub-corner stall. However, this was accompanied by an increased area of reverse flow at the casing, which was even more distinct at off-design conditions near stall. The numerical surge limit of the datum axisymmetric design could no longer be reached and was then determined by the new separation close to the stator casing. A subsequent optimization of the shroud end wall was carried out using the improved profiled hub as the initial design. An operating point near stall with a strongly developed separation was chosen for this purpose. The second optimization resulted in a further improvement in the characteristic speed line over the entire off-design region. Although the shroud contour was designed at off-design conditions, the optimization gained an additional 0.03% in efficiency for the design point. The lower surge limit of the datum design could also be reached again, even at higher efficiency and pressure ratios. The investigations showed that end wall profiling in high loaded compressor stators can be considered as a good supplement to 3-D blading to control separation areas and improve the entire component’s characteristics.Copyright © 2009 by ASME

Vibration of Slender Structures Subjected to Axial Flow or Axially Towed in Quiescent Fluid

Abstract: The vibrations and stability of slender structures subjected to axial flow or axially towed in quiescent fluid are discussed in this paper. A selective review of the research undertaken on it is presented. It is endeavoured to show that slender structures subjected to axial flow or axially towed in quiescent fluid are capable of displaying rich dynamical behavior. The basic dynamics of straight and curved pipes conveying fluid (with or without motion constraints), carbon nanotubes conveying fluid, tubular beams subjected to both internal and external flows in axial direction, slender structures in axial flow or axially towed in quiescent fluid, cylindrical shells conveying or immersed in axial flow, solitary plate or parallel-plate assembly in axial flow; linear, nonlinear, and chaotic dynamics; these and many more are some of the aspects of the problem considered.

Flow Field Structures of the Impeller Backside Cavity and Its Influences on the Centrifugal Compressor

Abstract: The impeller backside cavity is one of the unique features of the centrifugal compressors, it can affect the aerodynamic performances of a centrifugal compressor in many ways. This paper presents the researches on the coupled flow fields between a centrifugal compressor main flow passage and its impeller backside cavity. The flow field structures and features of the impeller backside cavity are presented for different leakage flow patterns, and its influences on the flow field details, axial thrust, shaft power, pressure ratio and efficiency of the centrifugal compressor have been studied. Some general conclusions are drawn for different centrifugal compressor operating conditions and impeller backside cavity leakage flow patterns.Copyright © 2009 by ASME

Performance Improvement of Axial Compressors and Fans with Plasma Actuation

Abstract: This paper proposes the use of plasma actuator to suppress boundary layer separation on a compressor blade suction side to increase axial compressor performance. Plasma actuators are a new type of electrical flow control device that imparts momentum to the air when submitted to a high AC voltage at high frequency. The concept presented in this paper consists in the positioning of a plasma actuator near the separation point on a compressor rotor suction side to increase flow turning. In this computational study, three parameters have been studied to evaluate the effectiveness of plasma actuator: actuator strength, position and actuation method (steady versus unsteady). Results show that plasma actuator operated in steady mode can increase the pressure ratio, efficiency, and power imparted by the rotor to the air and that the pressure ratio, efficiency and rotor power increase almost linearly with actuator strength. On the other hand, the actuator's position has limited effect on the performance increase. Finally, the results from unsteady simulations show a limited performance increase but are not fully conclusive, due possibly to the chosen pulsing frequencies of the actuator and/or to limitations of the CFD code.

Aerodynamic Performance of Blade Tip End-Plates Designed for Low-Noise Operation in Axial Flow Fans

Abstract: This study assesses the effectiveness of modified blade-tip configurations in achieving passive noise control in industrial fans. The concepts developed here, which are based on the addition of end-plates at the fan-blade tip, are shown to have a beneficial effect on the fan aeroacoustic signature as a result of the changes they induce in tip-leakage-flow behavior. The aerodynamic merits of the proposed blade-tip concepts are investigated by experimental and computational studies in a fully ducted configuration. The flow mechanisms in the blade-tip region are correlated with the specific end-plate design features, and their role in the creation of overall acoustic emissions is clarified. The tip-leakage flows of the fans are analyzed in terms of vortex structure, chordwise leakage flow, and loading distribution. Rotor losses are also investigated. The modifications to blade-tip geometry are found to have marked effects on the multiple vortex behaviors of leakage flow as a result of changes in the near-wall fluid flow paths on both blade surfaces. The improvements in rotor efficiency are assessed and correlated with the control of tip-leakage flows produced by the modified tip end-plates.

Refrigeration apparatus and method for controlling the same

Abstract: A refrigeration apparatus includes an outdoor unit having a first compressor and a second compressor connected in parallel with each other, a first oil separator attached to the first compressor and connected to a suction pipe of the second compressor via a first oil return piping including a pressure reducing device, and a second oil separator attached to the second compressor and connected to a suction pipe of the first compressor via a second oil return piping including a pressure reducing device. The suction pipes of the first and second compressors are connected together. The first compressor is a variable-speed compressor, and the second compressor is a constant-speed compressor. A solenoid valve selectively forming a bypass circuit with respect to the pressure reducing device, is connected in parallel with the pressure reducing device of the first oil return piping. The solenoid valve is opened when the first compressor is restarted.

Unsteady and Three-Dimensional Flow Phenomena in a Transonic Centrifugal Compressor Impeller at Rotating Stall

Abstract: This paper presents a combined experimental and numerical analysis of rotating stall in a transonic centrifugal compressor impeller for automotive turbochargers. Stall characteristics of the compressor were examined by two high-response pressure transducers mounted on the casing wall near the impeller inlet. The pressure traces were analyzed by wavelet transforms to estimate the disturbance waves quantitatively. Three-dimensional unsteady internal flow fields were simulated numerically by Detached Eddy Simulation (DES) coupled LES-RANS approach. The analysis results show good agreements on both compressor performance characteristics and the unsteady flow features at the rotating stall. At stall inception, spiral-type breakdown of the full-blade tip leakage vortex was found out at some passages and the brokendown regions propagated against the impeller rotation. This phenomenon changed with throttling, and tornado-type separation vortex caused by the full-blade leading edge separation dominated the flow field at developed stall condition. It is similar to the flow model of short-length scale rotating stall established in an axial compressor rotor.Copyright © 2009 by ASME

Coupled flows and oscillations in asymmetric rotating plasmas

Abstract: Nonlinear coupling among the radial, axial, and azimuthal flows in an asymmetric cold rotating plasma is considered nonperturbatively. Exact solutions describing an expanding or contracting plasma with oscillations are then obtained. It is shown that despite the flow asymmetry the energy in the radial and axial flow components can be transferred to the azimuthal component but not the vice versa, and that flow oscillations need not be accompanied by density oscillations.

A Multiple Inlet Swirler for Gas Turbine Combustors

Abstract: The central recirculation zone (CRZ) in a swirl stabilized gas turbine combustor has a dominant effect on the fuel air mixing process and flame stability. Most of state of the art swirlers share one disadvantage; the fixed swirl number for the same swirler configuration. Thus, in a mathematical sense, Reynolds number becomes the sole parameter for controlling the flow characteristics inside the combustor. As a result, at low load operation, the generated swirl is more likely to become feeble affecting the flame stabilization and mixing process. This paper introduces a new swirler concept which overcomes the mentioned weakness of the modern configurations. The new swirler introduces air tangentially and axially to the combustor through tangential vanes and an axial vanes respectively. Therefore, it provides different swirl numbers for the same configuration by regulating the ratio between the axial and tangential flow momenta. The swirler aerodynamic performance was investigated using four CFD simulations in order to demonstrate the impact of tangential to axial flow rate ratio on the CRZ. It was found that the length of the CRZ is directly proportional to the tangential to axial air flow rate ratio.