Showing papers on "Diffuser (thermodynamics) published in 2015"

Experimental Characterization of a Pump–Turbine in Pump Mode at Hump Instability Region

Abstract: The unsteady phenomena of a low specific speed pump–turbine operating in pump mode were characterized by dynamic pressure measurements and high-speed flow visualization of injected air bubbles. Analyses were carried out on the pressure signals both in frequency and time–frequency domains and by bispectral protocol. The results obtained by high-speed camera were used to reveal the flow pattern in the diffuser and return vanes channels The unsteady structure identified in the return vane channel appeared both at full and part load condition. Furthermore, a rotating stall structure was found and characterized in the diffuser when the pump operated at part load. The characteristics of these two unsteady structures are described in the paper.

A New Optimization Method for Centrifugal Compressors Based on 1D Calculations and Analyses

Abstract: This paper presents an optimization design method for centrifugal compressors based on one-dimensional calculations and analyses. It consists of two parts: (1) centrifugal compressor geometry optimization based on one-dimensional calculations and (2) matching optimization of the vaned diffuser with an impeller based on the required throat area. A low pressure stage centrifugal compressor in a MW level gas turbine is optimized by this method. One-dimensional calculation results show that D3/D2 is too large in the original design, resulting in the low efficiency of the entire stage. Based on the one-dimensional optimization results, the geometry of the diffuser has been redesigned. The outlet diameter of the vaneless diffuser has been reduced, and the original single stage diffuser has been replaced by a tandem vaned diffuser. After optimization, the entire stage pressure ratio is increased by approximately 4%, and the efficiency is increased by approximately 2%.

Improvement in Solar Chimney Power Generation by Using a Diffuser Tower

Abstract: The solar chimney prototype, operated in Spain from 1982 to 1989, verified the concept of the solar chimney. The power generation mechanism in this system is to turn the wind turbine placed inside a high rise cylindrical hollow tower by an induced thermal updraft. As long as the thermal updraft is induced inside the tower by the solar radiation, this system can produce electricity. The disadvantage of this system is the low power generation efficiency compared to other solar energy power generation systems. To overcome this disadvantage, we improved the mechanism in order to augment the velocity of the air which flows into the wind turbine. By applying a diffuser tower instead of a cylindrical one, the efficiency of the systems power generation is increased. The mechanism that we investigated was the effect of the diffuser on the solar chimney structure. The inner diameter of the tower expands as the height increases so that the static pressure recovery effect of the diffuser causes a low static pressure region to form at the bottom of the tower. This effect induces greater airflow within the tower. The laboratory experiment, as does the computational fluid dynamics (CFD) analysis of the laboratory sized model, shows that the proposed diffuser type tower induces a velocity approximately 1.38‐1.44 times greater than the conventional cylindrical type. The wind power generation output is proportional to the cube of the incoming wind velocity into the wind turbine; therefore, approximately 2.6‐3.0 times greater power output can be expected from using the diffuser type tower. [DOI: 10.1115/1.4029377]

High performance hydraulic design techniques of mixed-flow pump impeller and diffuser

Abstract: In this paper, we describe a numerical study about the performance improvement of a mixed-flow pump by optimizing the design of the impeller and diffuser using a commercial computational fluid dynamics (CFD) code and design-of-experiments (DOE). The design variables of impeller and diffuser in the vane plane development were defined with a fixed meridional plane. The design variables were defined by the vane plane development, which indicates the blade-angle distributions and length of the impeller and diffuser. The vane plane development was controlled using the blade-angle in a fixed meridional plane. The blade shape of the impeller and diffuser were designed using a traditional method in which the inlet and exit angles are connected smoothly. First, the impeller optimum design was performed with impeller design variables. The diffuser optimum design was performed with diffuser design variables while the optimally designed impeller shape was fixed. The importance of the impeller and diffuser design variables was analyzed using 2k factorial designs, and the design optimization of the impeller and diffuser design variables was determined using the response surface method (RSM). The objective functions were defined as the total head (Ht) and the total efficiency (?t) at the design flow rate. The optimally designed model was verified using numerical analysis, and the numerical analysis results for both the optimum model and the reference model were compared to determine the reasons for the improved pump performance. A pump performance test was carried out for the optimum model, and its reliability was proved by a comparative analysis of the results of the numerical analysis and an experiment using the optimum model.

Upgrading a Shrouded Wind Turbine with a Self-Adaptive Flanged Diffuser

Abstract: In this paper, a self-adaptive flange is proposed for the wind turbine shrouded by a flanged diffuser to reduce the wind loads acting on the flanged diffuser at high wind velocities. The self-adaptive flange can maintain the advantages of the flanged diffuser at wind velocities lower than the rated velocity and reduce the wind loads acting on the diffuser and blades at higher wind velocities. Numerical analyses of fluid-structure interactions are carried out to investigate the flow field around the diffuser with a self-adaptive flange and the variation of wind load acting on the diffuser due to the reconfiguration of the self-adaptive flange at various wind velocities. Numerical results show that the wind load acting on the total flanged diffuser can be reduced by about 35% at 60 m/s due to the reconfiguration of the self-adaptive flange.

CFD investigation and PIV validation of flow field in a compact return diffuser under strong part-load conditions

Abstract: The internal flow fields in a compact return diffuser under strong part-load conditions are investigated both numerically and experimentally. For numerical simulation, three-dimensional unsteady Reynolds-Averaged Navier-Stokes equations are solved on high-quality structured grids in conjunction with the shear stress transport k-ω turbulence model by employing the computational fluid dynamics (CFD) software ANSYS-Fluent 14.5. For flow field measurements, a special test rig is designed and the two-dimensional particle image velocimetry (PIV) measurements are conducted in the diffuser midplane to capture the complex flow field and for validation of the CFD results. The analysis of the results has been focused on the flow structure in the diffuser, especially under part-load conditions. The detailed comparison between CFD and PIV results is performed. Vortical flow and recirculation flow patterns in the diffuser are captured and analyzed. Large flow separation and backflow appear under the part-load flow conditions. This paper provides a good data set for developing as well as evaluating the accuracy of various CFD models for capturing the complex flow field in a compact return diffuser used with multistage pumps.

Flow control structures for turbomachines and methods of designing the same

Abstract: Flow control devices and structures designed and configured to improve the performance of a turbomachine. Exemplary flow control devices may include various flow guiding channels, ribs, diffuser passage-width reductions, and other treatments and may be located on one or both of a shroud and hub side of a machine to redirect, guide, or otherwise influence portions of a turbomachine flow field to thereby improve the performance of the machine.

Use of multiple local recirculations to increase the efficiency in diffusers

Abstract: A flow control strategy to improve the efficiency of plane diffusers, based on the introduction of local flow recirculations along the diffuser diverging walls, is investigated. Three reference diffuser configurations, all with laminar flow and with the same area ratio ( A R = 2 ) but with different divergence half-angles ( α = 2 ∘ , α = 3 . 5 ∘ and α = 5 ∘ ), are considered. The variation of the divergence angle produces different flow patterns: in the diffuser with α = 2 ∘ the flow is attached along the diverging walls, while the diffusers with α = 3 . 5 ∘ and α = 5 ∘ are characterized by asymmetric zones of separated flow with different extents. Local recirculations are obtained by means of properly contoured cavities, whose geometries are optimized in order to maximize the pressure recovery in the diffusers. The introduction of the optimal cavities leads to an increase in pressure recovery for all the considered configurations, even when no separation is present without the introduction of the cavities. The success of the control is due both to a virtual geometry modification of the diffuser and to a reduction of the momentum losses in the small recirculation regions inside the cavities. Classical shape optimizations of the diverging walls are also carried out by using Bezier curves. If an adequate number of degrees of freedom is used in the optimization, the optimal geometry corresponds to the presence of one or more localized recirculation regions along the diffuser optimized diverging walls, i.e. to a flow configuration that is very similar to the one characterizing the diffusers with optimized cavities. The efficiency increases provided by the shape optimization are comparable to those given by the contoured cavities.

Numerical simulation and analysis of solid-liquid two-phase three-dimensional unsteady flow in centrifugal slurry pump

Abstract: Based on RNG k-e turbulence model and sliding grid technique, solid-liquid two-phase three-dimensional (3-D) unsteady turbulence of full passage in slurry pump was simulated by means of Fluent software. The effects of unsteady flow characteristics on solid-liquid two-phase flow and pump performance were researched under design condition. The results show that clocking effect has a significant influence on the flow in pump, and the fluctuation of flow velocity and pressure is obvious, particularly near the volute tongue, at the position of small sections of volute and within diffuser. Clocking effect has a more influence on liquid-phase than on solid-phase, and the wake-jet structure of relative velocity of solid-phase is less obvious than liquid-phase near the volute tongue and the impeller passage outlet. The fluctuation of relative velocity of solid-phase flow is 7.6% smaller than liquid-phase flow at the impeller outlet on circular path. Head and radial forces of the impeller are 8.1% and 85.7% of fluctuation, respectively. The results provide a theoretical basis for further research for turbulence, improving efficient, reducing the hydraulic losses and wear. Finally, field tests were carried out to verify the operation and wear of slurry pump.

Design and experimental study of a novel three-way diffuser/nozzle elements employed in valveless piezoelectric micropumps

Abstract: A novel type of three-way diffuser/nozzle element of valveless piezoelectric micropump is presented, and an orthogonal design of the tube is done by computational fluid dynamics. Comparison of the simulation results between the traditional diffuser/nozzle element and the three-way diffuser/nozzle element shows that the latter has a better performance. The λ defined as the ratio of the total pressure loss coefficient for flow in the negative direction to that in the positive direction of the novel element is 1.2 and 16 % larger than that of the former traditional one as Δp = 10 kPa and Δp = 50 kPa, respectively. Then a three-way diffuser/nozzle element is fabricated, and the experiment is carried out. The results show that the simulation results are in good agreement with the experiment results. The maximum differences between the simulation and experiment are 6.23 % at Δp = 20 kPa in the negative direction and 3.53 % at Δp = 100 kPa in the positive direction when the pressure differences are given from 10 to 100 kPa. The micropump is fabricated, and the experiment results show that the maximum flow rate and back pressure are 0.451 ml/min and 3.11 kPa with the frequency of 225 Hz when the sinusoidal voltage is 100 VP-P.

An Experimental Study of the Flow Field Inside the Diffuser Passage of a Laboratory Centrifugal Pump

Abstract: Measurements are processed on a centrifugal pump model, which works with air and performs with the vane-island type diffuser of a real hydraulic pump, under five flow rates to investigate the internal flow characteristics and their influence on overall pump performance. The mean flow characteristics inside the diffuser are determined by using a miniature three-hole probe connected to an online data acquisition system. The flow structure at the inlet section of the diffuser is analyzed in detail, with a focus on the local pressure loss inside the vaneless gap and incidence angle distributions along the hub-to-shroud direction of the diffuser. Some existing calculations, including leakage effects, are used to evaluate the pressure recovery downstream of the impeller. Furthermore, particle image velocimetry (PIV) measurement results are obtained to help analyze the flow characteristics inside the vane-island diffuser. Each PIV measuring plane is related to one particular diffuser blade-to-blade channel and is analyzed by using the time-averaged method according to seven different relative positions of the impeller. Measurement results show that main loss is produced inside the vaneless part of the diffuser at low flow rates, which might have been caused by the strong rotor–stator interaction. When the impeller flow rate is greater than the diffuser design flow rate, a large fluctuating separated region occurs after the throat of the diffuser on the pressure side. Mean loss originates from the unsteady pressure downstream of the diffuser throat. For better characterization of the separations observed in previous experimental studies, complementary unsteady static pressure measurement campaigns have been conducted on the diffuser blade wall. The unsteadiness revealed by these measurements, as well as theirs effects on the diffuser performance, was then studied.

Numerical analysis of a shrouded micro-hydrokinetic turbine unit

Abstract: Computational fluid dynamics simulations were conducted for two diffuser designs that were added to a pre-existing horizontal axis hydro-kinetic turbine design. The two diffuser designs investigated in the present study had the area ratio values of 1.36 and 2.01. Each design used a short axial length to satisfy system portability constraints. The turbine-diffuser systems steady-state performance characteristics were assessed numerically. A structured, hexahedral mesh was employed to discretize the equations governing the fluid motion. Turbulent flow structures were captured through the implementation of the k-ω Shear Stress Transport (SST) model. A 39.5% and 55.8% increase in output mechanical power was observed versus the un-augmented turbine performance. As the area ratio increases from 1.36 to 2.01, the total thrust experienced by the unit nearly doubles.

Gas turbine engine component with film cooling hole feature

Abstract: A gas turbine engine component (64) includes a wall (94) that provides an exterior surface (79) and an interior flow path surface (96). A film cooling hole (92) extends through the wall (94) and is configured to fluidly connect the interior flow path surface (96) to the exterior surface (79). The film cooling hole (92) has a diffuser (100) that is arranged downstream from a metering hole (98). The diffuser (100) includes inner and outer diffuser surfaces (104,102) opposite one another and respectively arranged on sides near the interior flow path surface (96)and the exterior surface (79). A protrusion (106) is arranged in the diffuser on the outer diffuser surface (102).

Acoustophoretic device with uniform fluid flow

Abstract: An acoustophoresis device which includes a substantially vertical flow path of the fluid mixture in order to improve separation of particles / secondary fluid from a primary fluid is disclosed. The vertical flow path reduces velocity non-uniformities in the acoustic chamber resulting from gravity forces. The device includes an acoustic chamber in which multidimensional acoustic standing waves are generated. The fluid can be introduced into the acoustic chamber using a dump diffuser in which a plurality of inlets enter near the bottom of the acoustic chamber such that flow symmetry reduces both, gravity driven flow non-uniformities, and any flow interference effects between inlet mixture flow into the acoustic chamber and the continuous gravity driven particle cluster drop out.

Numerical study of centrifugal compressor stage vaneless diffusers

Abstract: The authors analyzed CFD calculations of flow in vaneless diffusers with relative width in range from 0.014 to 0.100 at inlet flow angles in range from 100 to 450 with different inlet velocity coefficients, Reynolds numbers and surface roughness. The aim is to simulate calculated performances by simple algebraic equations. The friction coefficient that represents head losses as friction losses is proposed for simulation. The friction coefficient and loss coefficient are directly connected by simple equation. The advantage is that friction coefficient changes comparatively little in range of studied parameters. Simple equations for this coefficient are proposed by the authors. The simulation accuracy is sufficient for practical calculations. To create the complete algebraic model of the vaneless diffuser the authors plan to widen this method of modeling to diffusers with different relative length and for wider range of Reynolds numbers.

Loss Mechanisms and Flow Control for Improved Efficiency of a Centrifugal Compressor at High Inlet Prewhirl

Abstract: Variable inlet prewhirl is an effective way to suppress compressor instability. Compressors usually employ a high degree of positive inlet prewhirl to shift the surge line in the performance map to a lower mass flow region. However, the efficiency of a compressor at high inlet prewhirl is far lower than that at zero or low prewhirl. This paper investigates the performances of a centrifugal compressor with different prewhirl, discusses the mechanisms thought to be responsible for the production of extra loss induced by high inlet prewhirl and develops flow control methods to improve efficiency at high inlet prewhirl. The approach combines steady three-dimensional Reynolds average Navier-Stockes (RANS) simulations with theoretical analysis and modeling. In order to make the study universal to various applications with inlet prewhirl, the inlet prewhirl was modeled by modifying the velocity condition at the inlet boundary. Simulation results show that the peak efficiency at high inlet prewhirl is reduced compared to that at zero prewhirl by over 7.6 percentage points. The extra loss is produced upstream and downstream of the impeller. Severe flow separation was found near the inlet hub which reduces efficiency by 2.3 percentage points. High inlet prewhirl works like a centrifuge gathering low-kinetic-energy fluid to hub, inducing the separation. A dimensionless parameter C is defined to measure the centrifugal component of flow. As for the extra loss produced downstream of the impeller, the flow mismatch of impeller and diffuser at high prewhirl causes a violent backflow near the diffuser vanes’ leading edges. An analytical model is built to predict diffuser choking mass flow which proves that the diffuser flow operates outside of stable conditions. Based on the two loss mechanisms, hub curve and diffuser stager angle were modified and adjusted for seeking higher efficiency at high prewhirl. The efficiency improvement of a modification of the hub is 1.1 percentage points and that of the combined optimization is 2.4 percentage points. During optimizing, constant distribution of inlet prewhirl was found to induce reverse flow at the leading edge of the blade root, which turned out being uncorrelated with blade angle. By revealing loss mechanisms and proposing flow control ideas, this paper lays a theoretical basis for overcoming the efficiency drop induced by high inlet prewhirl and for developing compressors with high inlet prewhirl.Copyright © 2015 by ASME

Experimental study on one-side confined jets from a parallel-flow outlet in a push-pull ventilation system

Abstract: This study conducted a series of experiments on the airflow characteristics of one-side confined jets from a parallel-flow outlet with a two-tier perforated plate and a honeycomb in a push–pull ventilation system. Maximum velocity, maximum temperature and airflow trajectory were analysed and compared with previous studies about different outlets. The results showed that the dimensionless turbulence coefficient of the parallel-flow outlet was smaller than either the cased axial fan with open grille’s or the swirl diffuser’s turbulence coefficients from isothermal condition. Different development laws for the parallel-flow outlet discharging a hot air jet (Ar0 = 0.024) on the floor were put forward. Furthermore, it was found that the velocity decay rate of the hot air jets with a parallel-flow outlet on the floor was slower than with the circular aperture over a flat surface. When Ar0 ranged from 0.024 to 0.044, a semi-empirical equation was established to describe the trajectory of the hot jets from a para...

Multi-objective optimization for impeller shroud contour, the width of vane diffuser and the number of blades of the centrifugal compressor stage based on the CFD calculation

Abstract: The study presents the results of multi-objective optimization for a high flow centrifugal compressor stage with impeller pressure ratio Π=1.3 and conditional flow rate coefficient Φ=0.108. The compressor stage includes an impeller with 3D backswept blades, a vane diffuser and an axial inlet. The goal of optimization is to increase polytropic efficiency and polytropic head ratio of the basic design. The CFD method is used to estimate compressor efficiency at rated duty. CAE optimization is applied based on the parametric optimization of impeller shroud contour and number of blades of the impeller and diffuser versus the original design. Parameterization of geometrics is also used to the width-ratio of vane diffuser in the range of b3/D2= 0.08-0.097. The study considers 52 cases of the optimization of impeller shroud contour and the number of blades in the search for the improved design. The optimization procedure uses the automatic generated computation grid and supplementary activation of solution to each design case. The numerical calculation for each case has been performed automatically by ANSYS CFX 14.5 soft application. The optimization allowed to obtain the improved design with total polytropic efficiency increase by 1.58% for the impeller and polytropic head coefficient increase by 0.58%. The polytropic efficiency and the polytropic head coefficient are calculated on the total parameters. The performance of the basic impeller has been exhaustibly validated by test records provided JSC "REPH ZAO". The resulted error range does not exceed 5% over the performance map, except near surge point.

Near Field Flow Dynamics of Concentrate Discharges and Diffuser Design

Abstract: The major physical aspects of near field mixing of dense jets resulting from diffuser discharges of concentrate are presented. It is proposed that any environmental impacts of such discharges will be local rather than regional, so initial mixing processes are an essential component of an effective disposal scheme. Typical international environmental criteria for concentrate are summarized; these can be readily met by well-designed diffusers. The major features of dense jets are presented, beginning with the simplest case of an inclined jet into deep stationary water, followed by the effects of shallow water. We then discuss merging jets from multiport and rosette diffusers and it is shown that their dynamic interaction can be critical and lead to significant changes in flow patterns and reduction in dilution. Design criteria are suggested to avoid impaired dilution. The effects of currents on single and multiport diffusers are then discussed. It is shown that small modifications in diffuser design can lead to significant changes in flow field and dilution. Some issues and difficulties with mathematical modeling of near field flows are discussed and how entrainment models may not adequately represent critical phenomena including dynamic jet and boundary interaction, re-entrainment, density current dynamics, and turbulence collapse. Finally, some open research issues are discussed.

Diffuser pipe with vortex generators

Abstract: A compressor diffuser for a gas turbine engine which includes at least one diffuser pipe having a tubular body with an inner surface defining an internal flow passage extending therethrough. The tubular body includes a first portion that extends in a first direction and defines a throat therein, a second portion that extends in a second direction different from the first direction, and a curved portion fluidly linking the first portion and the second portion. A plurality of vortex generators are disposed within the diffuser pipe and extend from the inner surface into the internal flow passage. The vortex generators are disposed downstream of the throat in the first portion of the diffuser pipe and upstream of the curved portion. In operation, the vortex generators engage fluid flow in the internal flow passage to generate downstream vortices.

System for suppressing acoustic noise within a gas turbine combustor

Abstract: In one aspect the present subject matter is directed to a system (100) for suppressing acoustic noise within a combustion section of a gas turbine. The system includes at least one static structure (102) disposed forward of a combustion chamber (62) defined within the combustion section. The static structure at least partially defines a diffuser cavity (84) upstream of the combustion chamber. A baffle plate (104) is coupled to the static structure (102). The baffle plate and the static structure at least partially define an air plenum (106) within the combustion section forward of the combustion chamber. The baffle plate includes an aperture (114) that provides for fluid communication between the diffuser cavity and the air plenum. The at least one static structure and the baffle plate define a Helmholtz resonator within the combustion section.

CFD Modeling of Low Pressure Steam Turbine Radial Diffuser Flow by Using a Novel Multiple Mixing Plane Based Coupling: Simulation and Validation

Abstract: The diffuser and exhaust of low pressure steam turbines shows significant impact on the overall turbine performance. The amount of recovered enthalpy leads to a considerable increase of the turbine power output, and therefore a continuous focus of turbine manufacturers is put on this component. On the one hand, the abilities to aerodynamically design such components is improved, but on the other hand a huge effort is required to properly predict the resulting performance and to enable an accurate modeling of the overall steam turbine and therewith plant heat rate. A wide range of approaches is used to compute the diffuser and exhaust flow, with a wide range of quality. Today it is well known and understood, that there is a strong interaction of rear stage and diffuser flow, and the accuracy of the overall diffuser performance prediction strongly depends on a proper coupling of both domains. The most accurate, but also most expensive method is currently seen in a full annulus and transient coupling. However, for a standard industrial application of diffuser design in a standard development schedule, such a coupling is not feasible and more simplified methods have to be developed.The paper below presents a CFD modeling of low pressure steam turbine diffusers and exhausts based on a direct coupling of the rear stage and diffuser using a novel multiple mixing plane. It is shown that the approach enables a fast diffuser design process and is still able to accurately predict the flow field and hence the exhaust performance. The method is validated against several turbine designs measured in a scaled low pressure turbine model test rig using steam. The results show a very good agreement of the presented CFD modeling against the measurements.Copyright © 2015 by ASME and Alstom Technologie AG

Experimental investigations on effects of tip clearance in mixed-flow compressor performance:

Abstract: Experimental investigations were carried out to study the effect of tip clearance (between impeller and stationary shroud) in a mixed-flow compressor stage. Two configurations, namely constant and variable clearance gaps, between impeller and stationary shroud were considered. For the purpose of the present investigations, a mixed-flow compressor stage was designed, fabricated, and experimentally evaluated. The flow investigations were carried out in a closed-circuit compressor rig. Detailed steady and unsteady flow measurements were carried out for three clearance gaps, namely 0.5 mm, 0.75 mm, and 0.9 mm. Through the experimental investigations, it was found that the constant tip-clearance configurations showed better performance in terms of pressure ratio and efficiency as compared to variable clearance configurations. For a given configuration, the pressure ratio and efficiency of the stage decrease with increase in the tip gap without indicating any optimum value. Tip-clearance flow had considerable effect on the flow through the diffuser.