Diffuser Wall Research Articles

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

The variable demand of the energy market requires that hydraulic turbines operate at variable conditions, which includes regimes far from the best efficiency point. The vortex rope developed at partial discharges in the conical diffuser is responsible for large pressure pulsations, runner blades breakdowns and may lead to power swing phenomena. A novel method introduced by Resiga et al. (2006, “Jet Control of the Draft Tube in Francis Turbines at Partial Discharge,” Proceedings of the 23rd IAHR Symposium on Hydraulic Machinery and Systems, Yokohama, Japan, Paper No. F192) injects an axial water jet from the runner crown downstream in the draft tube cone to mitigate the vortex rope and its consequences. A special test rig was developed at “Politehnica” University of Timisoara in order to investigate different flow control techniques. Consequently, a vortex rope similar to the one developed in a Francis turbine cone at 70% partial discharge is generated in the rig’s test section. In order to investigate the new jet control method an auxiliary hydraulic circuit was designed in order to supply the jet. The experimental investigations presented in this paper are concerned with pressure measurements at the wall of the conical diffuser. The pressure fluctuations’ Fourier spectra are analyzed in order to assess how the amplitude and dominating frequency are modified by the water injection. It is shown that the water jet injection significantly reduces both the amplitude and the frequency of pressure fluctuations, while improving the pressure recovery in the conical diffuser.

MHD Free Convection Flow in a Porous Medium with Thermal Diffusion and Ramped Wall Temperature

An exact analysis is presented to study the unsteady magnetohydrodynamic flow past an impulsively started vertical plate embedded in a porous medium in the presence of thermal diffusion and ramped wall temperature at the plate. The analytical expressions for velocity, temperature and concentration fields are obtained using the Laplace transform technique. The results for skin friction, Nusselt number and Sherwood number are also developed. The analytical results are displayed graphically and discussed.

Coherent structures and dominant frequencies in a turbulent three-dimensional diffuser

AbstractDominant frequencies and coherent structures are investigated in a turbulent, three-dimensional and separated diffuser flow at$\mathit{Re}= 10\hspace{0.167em} 000$(based on bulk velocity and inflow-duct height), where mean flow characteristics were first studied experimentally by Cherry, Elkins and Eaton (Intl J. Heat Fluid Flow, vol. 29, 2008, pp. 803–811) and later numerically by Ohlssonet al. (J. Fluid Mech., vol. 650, 2010, pp. 307–318). Coherent structures are educed by proper orthogonal decomposition (POD) of the flow, which together with time probes located in the flow domain are used to extract frequency information. The present study shows that the flow contains multiple phenomena, well separated in frequency space. Dominant large-scale frequencies in a narrow band$\mathit{St}\equiv fh/ {u}_{b} \in [0. 0092, 0. 014] $(where$h$is the inflow-duct height and${u}_{b} $is the bulk velocity), yielding time periods${T}^{\ensuremath{\ast} } = T{u}_{b} / h\in [70, 110] $, are deduced from the time signal probes in the upper separated part of the diffuser. The associated structures identified by the POD are large streaks arising from a sinusoidal oscillating motion in the diffuser. Their individual contributions to the total kinetic energy, dominated by the mean flow, are, however, small. The reason for the oscillating movement in this low-frequency range is concluded to be the confinement of the flow in this particular geometric set-up in combination with the high Reynolds number and the large separated zone on the top diffuser wall. Based on this analysis, it is shown that the bulk of the streamwise root mean square (r.m.s.) value arises due to large-scale motion, which in turn can explain the appearance of two or more peaks in the streamwise r.m.s. value. The weak secondary flow present in the inflow duct is shown to survive into the diffuser, where it experiences an imbalance with respect to the upper expanding corners, thereby giving rise to the asymmetry of the mean separated region in the diffuser.

Investigation of impact of jets with zero-net-mass flux on flow in curvilinear diffusers

The impact of the blowing of jets with a zero-net-mass-flux (JZNMF) from the wall of a plane diffuser with expansion 1.8 on a turbulent separation flow is studied using a hybrid method for solving the averaged Navier-Stokes equations and the implicit method for large-sized eddy simulation (RANS/ILES method). The total pressure losses are shown to decrease by two times with the corresponding selection of the JZNMF frequency and amplitude. In this case, the level of the total pressure fluctuations at the diffuser outlet decreases by two to three times and the level of fluctuations of the longitudinal velocity components decreases by 1.2–1.5 times. The impact of the velocity at the diffuser inlet on the JZNMF efficiency is studied. The dependences of the impact of the JZNMF frequency and amplitude on the total pressure losses and the distribution of the averaged parameters of the flow and turbulence in the outlet cross section of the diffusion are obtained.

Modified flapping jet for increased jet spreading using synthetic jets

The present paper is an experimental investigation, using a PIV system, on modified rectangular jet flow co-flowing with a pair of synthetic jets placed symmetrically with respect to the geometric centerline of the main flow. The objective was to determine the optimal forcing conditions that would result in jet spreading beyond what would be obtained in a simple flapped jet. The main jet had an exit Re h = 36,000, based on the slot height, h. The synthetic jets were operated in a periodic manner with a periodic momentum coefficient of about 3.3% and at a frequency of the main jet preferred mode. A short, wide angle diffuser of half angle of about 45° was attached to the main jet. Generally for the vectored jet, much of the flow features found here resembled those reported in the literature except that the deflection angle in this study increased with downstream distances inside the diffuser and then remained roughly unchanged thereafter. Larger jet spreading was achieved when the main jet was subjected to simultaneous actuation of the synthetic jets but the flow did not achieve the initial jet spreading that was observed in the vectored jet. Further jet spreading was achieved when the synthetic jets were alternately actuated in which each synthetic jet was actuated for a number of cycles before switching. This technique allowed the jet to flap across the flow between transverse positions larger than what would be obtained in a simple flip-flop jet. Under the present flow geometry and Reynolds number, it was found that when the ratio f s / f al , where f s is the synthetic actuation frequency and f al is the alternating frequency, was larger than 10, the mean streamwise velocity of the main jet had two peaks symmetrically placed with respect to the jet axis and the jet had the appearance of flowing into two streams each moving nearly parallel to the diffuser wall. For a value of f s / f al of about 10, the optimal value in this study, the desired flow properties were achieved in that, the mean velocity was nearly uniform with an increase in the jet width compared to the simultaneous actuations, and the jet flapping was more effective in redistributing and homogenizing the turbulent kinetic energy across the main jet.

High-speed stereoscopic PIV study of rotating instabilities in a radial vaneless diffuser

This paper presents an experimental analysis of the unsteady phenomena developing in a vaneless diffuser of a radial flow pump. Partial flow operating conditions were investigated using 2D/3C high repetition rate PIV, coupled with unsteady pressure transducers. Pressure measurements were acquired on the shroud wall of the vaneless diffuser and on the suction pipe of the pump, whereas PIV flow fields were determined on three different heights in the hub to shroud direction, inside the diffuser. The classical Fourier analysis was applied to both pressure signals to identify the spectral characteristics of the developing instabilities, and the high-order spectral analysis was exploited to investigate possible non-linear interaction mechanisms between different unsteady structures. A dedicated PIV averaging procedure was developed and applied to the PIV flow fields so as to capture and visualize the topology of the spectrally identified phenomena. The influence of these phenomena on the diffuser efficiency was also investigated.

Dead Zone Analysis for Estimating Hydraulic Efficiency in Rectangular Disinfection Chlorine Contactors

To increase disinfection potential in a chlorine contactor, it is often recommended to increase the detention time in the contactor, that is, its hydraulic efficiency. This can be accomplished by installing inner devices such as baffle walls, diffuser walls, and intrabasins. To more accurately analyze the effect of such inner devices, it is necessary to conduct transient hydrodynamic analysis, which is computationally intensive. To reduce this computational burden, this study develops a simplified approach using steady-state dead zone analysis. In addition, this study uses the simplified approach to explain, with an indicator of dead zone volume percentage, why hydraulic efficiency changes with using different designs of length/width ratio, shape factor, diffuser walls, and intrabasins. This study suggests a relationship between dead zone volume percentage and hydraulic efficiency to replace length/width ratio. This will increase the estimation accuracy of hydraulic efficiency with various designs of chlorine contactors while avoiding use of computationally intensive transient hydrodynamic analysis.

Effect of J-Groove on the Suppression of Swirl Flow in a Conical Diffuser

The purpose of this study is to examine the validity of J-grooves in controlling and suppressing the swirl flow in a conical diffuser, for draft surge suppression in a Francis turbine, which is caused by the swirl flow from the runner outlet into the draft tube. “J-groove” composed of shallow grooves and mounted parallel to the pressure gradient on the diffuser wall is a very simple passive device to suppress several abnormal phenomena in turbomachinery. The experimental study has been performed using the conical diffuser with a divergent angle of 20 deg. The measured results of the velocity distribution in the diffuser show that a considerable reduction in the swirl intensity is attained by using J-grooves. Besides, the amplitude of pressure fluctuation caused by the rotation of the vortex core around the dead water region near the diffuser inlet is reduced by J-grooves.

Direct numerical simulation of separated flow in a three-dimensional diffuser

A direct numerical simulation (DNS) of turbulent flow in a three-dimensional diffuser at Re = 10000 (based on bulk velocity and inflow-duct height) was performed with a massively parallel high-order spectral element method running on up to 32768 processors. Accurate inflow condition is ensured through unsteady trip forcing and a long development section. Mean flow results are in good agreement with experimental data by Cherry et al. (Intl J. Heat Fluid Flow, vol. 29, 2008, pp. 803–811), in particular the separated region starting from one corner and gradually spreading to the top expanding diffuser wall. It is found that the corner vortices induced by the secondary flow in the duct persist into the diffuser, where they give rise to a dominant low-speed streak, due to a similar mechanism as the ‘lift-up effect’ in transitional shear flows, thus governing the separation behaviour. Well-resolved simulations of complex turbulent flows are thus possible even at realistic Reynolds numbers, providing accurate and detailed information about the flow physics. The available Reynolds stress budgets provide valuable references for future development of turbulence models.

Optimum Design of a Two Step Planar Diffuser: A Hybrid Approach

This study attempts to optimize a planar diffuser with two steps using a hybrid approach. It uses CFD softwares: Gambit for mesh generation, Fluent for carrying out hydrodynamic analysis, and employs a Genetic Algorithm (GA) for optimization. Here, the GA and CFD softwares combine each other seamlessly and exchange data among the commercial softwares (namely Gambit and Fluent) and optimization tool (that is, GA) in a fully automated manner without any human intervention. The hydrodynamic analysis is used to calculate pressure drop across the diffuser for a fully turbulent flow, while the GA determines its optimum design, which minimizes the same. The optimum dimensions of the diffuser wall have been determined for different values of Reynolds number and area ratios between the outlet and inlet of the diffuser. Finally, relationships of optimum geometrical dimensions with the Reynolds number and area ratio have been developed for the ready use of designers.

Experimental Study of Oblong Exhaust Ejectors With Multiring Oblong Entraining Diffusers

This paper presents experimental data for the performance of two oblong, straight, air-air ejectors with four-ring oblong entraining diffusers. The effects of inlet swirl angle, nozzle diameter, and flow temperature on the ejector pumping, backpressure, wall pressure distribution, diffuser pressure recovery, and surface film cooling effectiveness were studied. The experiments were carried out on a hot gas wind tunnel that could provide primary mass flow rates up to 2.2kg∕s at ambient temperature and 1.8kg∕s at 500°C. Velocity, pressure, and temperature were measured in the annulus upstream of the primary nozzle, on the mixing tube and diffuser walls, at the diffuser gap inlets, and at the diffuser exit. A comparison between the performance of the oblong ejector and a round ejector indicated that for a short length ejector, the oblong ejector provided better overall performance in terms of pumping and velocity and temperature distributions.

Effect of Wakes and Secondary Flow on Re-attachment of Turbine Exit Annular Diffuser Flow

In this paper numerical results of wake and secondary flow interaction in diffuser flow fields are discussed. The wake and secondary flow are generated by a rotating wheel equipped with 30 cylindrical spokes with a diameter of 10 mm as a first approach to the turbine exit flow environment. The apex angle of the diffuser is chosen such that the flow is strongly separated according to the well-known performance charts of Sovran and Klomp (1967, “Experimentally Determined Optimum Geometries for Rectilinear Diffusers With Rectangular, Conical or Annular Cross-Section,” in Fluid Mechanics of Internal Flow, Elsevier, New York, pp. 272–319). This configuration has been tested in an experimental test rig at the Leibniz University Hannover (Sieker and Seume 2007, “Influence of Rotating Wakes on Separation in Turbine Exhaust Diffusers,” Paper No. ISAIF8-54). According to these experiments, the flow in the diffuser separates as free jet for low rotational speeds of the spoke-wheel, as expected by theory. However, if the 30 spokes of the upstream wheel rotate beyond the value of 500 rpm the measurements indicate that the flow remains attached to the outer diffuser wall. It will be shown by the present numerical analysis with the commercial solver ANSYS CFX-10.0 that only an unsteady approach using the elaborate scale adaptive simulation with the shear stress transport turbulence model is capable of predicting the stabilizing effect of the rotating wheel to the diffuser flow at larger rotational speeds. The favorable comparison with the experimental data suggests that the mixing effect of wakes and secondary flow pattern is responsible for the reattachment. As a result of our studies, it can be stated that the considerably higher numerical costs associated with unsteady calculations must be accepted in order to increase the understanding of the physical flow phenomena in turbine exit flow and its interaction with the downstream diffuser.

Effect of intracomponental aerodynamic interaction on the performance of a centrifugal compressor

This paper presents some of the aerodynamic interactions between stationary components and the rotating impeller of a centrifugal compressor. This experimental study involved three types of inlet shapes and five types of diffuser configurations fitted to the same radial impeller with stationary shroud. The volute casing and exit duct for the facility also remained unchanged. The investigation included full flow range of these systems using traverses of a three-hole probe and static pressure measurements along the diffuser walls as well as diffuser blade surfaces. The results depict that the inlet duct shape, diffuser type, and shape affect the wall static pressure rise both in vaneless and vaned configurations. The vane surface pressure is grossly affected by the diffuser shape and to some extent by the inlet duct. It is also evident from the study that aerodynamics within the impeller is affected by the geometries of the upstream and downstream components due to the intra-componental aerodynamic interaction among these with the impeller.

J0501-1-1 小弦節比翼列ディフューザにおける二次流れの挙動解析(流体機械の研究開発におけるEFD/CFD(1))

A low-solidity cascade diffuser was proposed for achieving a wide operating range as well as a high pressure ratio. However, noise increased by means of LSD due to vortex shedding from the LSD blade surface. The objectives of the present study are to show that the noise can be reduced effectively by means of a small tip-groove located at the shroud side alone without deterioration of the LSD performance, and to clarify the mechanism of high blade loading of the LSD blade with the shroud side tip-groove limited near the LSD blade leading edge. The effect of a small groove on the stagnation area, the vortex generated in the tip groove and the secondary flow behavior on the suction surface of the LSD blade and the diffuser walls as well are analyzed numerically. It is found that the formation of the recirculating secondary flow generated additionally and significantly by the shroud tip-groove is a key point of the high LSD performance.

Finite diffusion wall film evaporation model for engine simulations using continuous thermodynamics

The formation of engine pollutants directly relates to the local composition of air/fuel mixture within the combustion chamber. The evaporation of wall film, formed by spray/wall impingement, is much slower than free droplets and has strong effects on engine emission. A finite diffusion wall film evaporation model using continuous thermodynamics is developed for multi-dimensional engine modeling, accounting the multi-component nature of practical fuels, which is essentially a complicated liquid mixture. The vaporization rate is evaluated using the turbulent boundary-layer assumption and a quasi-steady approximation. Third-order polynomials are used to model the fuel composition and the temperature profiles within the liquid phase for accurate predictions of surface properties, which are important for evaluating the mass fraction, moments and heat flux. The governing equations for the film are then reduced to a set of ordinary differential equations and significantly reduce the computational cost while retaining adequate accuracy in the solution. The model is verified against high precision numerical solutions obtained with the finite difference method that solves the governing partial differential equations using a fine grid over the same computational domain. Good agreement is obtained for the evaporation of a film on a flat plate under various conditions. The boiling point of a practical fuel is found to increase steadily with evaporation and may surpass the wall temperature. The evaporation process can therefore be reverted to non-boiling evaporation. This phenomenon is unique to multi-component liquids and not observed during the evaporation process of single component liquids. The presented model, with improved computational efficiency and adequate accuracy, is applicable in multi-dimensional engine simulations. Finally, the model is used for a spray/wall impingement simulation. The results show that the multi-component nature of a practical fuel can have significant implications on the distributions of fuel concentration and composition during the spray/wall interaction.

Flow Control by Dielectric Barrier Discharge Actuators: Jet Mixing Enhancement

This study is focused on a new approach to control the mixing at the exhaust of a jet nozzle. Two dielectric barrier discharges are used to enhance the turbulent quantities in the jet wake. An axisymmetric air jet is equipped with a 22-degree-angle diffuser that houses two dielectric barrier discharge actuators, symmetrically placed along the lips of the diffuser. Streamwise, cross-stream planes, and turbulent spectra for free air jet velocities from 10 to 40 m/s are investigated by stereoscopic particle image velocimetry and laser Doppler velocimetry up to 8 jet diameters. The airflow is separated naturally along the diffuser wall and the results demonstrate that a full (at 10 m/s) or partial (at 20 m/s) flow reattachment can be performed, this process being associated with the shedding of coherent structures. The turbulent spectra suggest that the frequency of the vortex shedding is dictated by the electrical frequency applied to the actuators and that the flow structures can be manipulated up to 40 m/s. The time-averaged data confirm that a significant enhancement of the jet spreading, a jet core length reduction, and a turbulent kinetic energy increase are performed up to 30 m/s for a quasi-steady actuation. Unsteady actuations are also investigated and the results highlight that a Strouhal number from 0.25 to 0.32 is particularly effective to enhance the turbulent kinetic energy in the overall flowfield.

Time‐Frequency Characterization of Rotating Instabilities in a Centrifugal Pump with a Vaned Diffuser

This paper presents acoustic and flowdynamic investigations of large‐scale instabilities in a radial pump with a vaned diffuser. Pressure fluctuations were measured with transducers placed flush at the inlet duct, at the impeller discharge, and in the vane diffuser walls. Two impeller rotation speeds were analyzed in the study, at design, and at off‐design flow rates. A spectral analysis was carried out on the pressure signals in frequency and in time‐frequency domains to identified precursors, inception, and evolution of the pressure instabilities. The results highlighted the existence of a rotating pressure structure at the impeller discharge, having a fluid‐dynamical origin and propagating both in the radial direction and inside the impeller. The experimental data were then compared with the results obtained with help of ANSYS CFX computer code; focusing on the changing flow field at part load. Turbulence was reproduced by DES model.

Simulations of Turbulent Flow in a Plane Asymmetric Diffuser

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.

Heat Transfer Influenced by Turbulent Airflow Inside an Axially Rotating Diffuser

The paper presents a study of heat transfer between the turbulent airflow and the inner wall surface of an axial diffuser rotating around its longitudinal axis. Heat transfer was assessed through the measurement of a time-dependent temperature field of the diffuser inner wall surface. Measurements of the instantaneous flow velocity components were performed by a laser–Doppler anemometry system, which delivered information on mean velocity components as well as on the turbulence intensity. A significant increase of all three mean velocity components was observed near the rotating diffuser wall in comparison with a non-rotating diffuser. Temperature field measurements were carried out by means of infrared thermography. The experiment showed a significant dependence of the temperature field on the turbulent flowfield induced by diffuser rotation. A strong influence of the flow separation and reattachment on the temperature distribution was observed, while rotation was found to suppress the occurrence of flow separation from the diffuser wall. Properties of the velocity field such as turbulent kinetic energy were directly coupled with the temperature distribution in order to gain the information on how to enhance or reduce heat transfer by changing the integral parameters of the diffuser (e.g. rotation frequency or amount of flow).

Performance optimization for two-dimensional rectangular diffuser by momentum injection using computational fluid dynamics

This article explores a non-conventional method for improvement of performance of a wide-angle two-dimensional rectangular diffuser. Computational fluid dynamics investigation has been carried out to analyse the effect of injecting momentum through a moving surface to control the boundary layer separation close to the wall of a two-dimensional rectangular diffuser. A cylinder was placed at the diffuser inlet and rotated at various speeds. It is seen that injection of momentum through the moving surface delays flow separation, thereby increasing the pressure recovery coefficient. An increase in the rotational speed of the cylinder from 1500 to 5500 rad/s improves the pressure recovery coefficient marginally. Placing another cylinder at a preidentified location and rotating both cylinders at the same speed further improves the pressure recovery. Overall improvement in pressure recovery is ∼28 per cent from 0.63 to 0.81. Besides improvement of pressure recovery, the flow distribution in the core region also improves significantly.