Experimental Study of the Pressure Fluctuations in a Pump Turbine at Large Partial Flow Conditions

Frequent shifts of output and operating mode require a pump turbine with excellent stability. Current researches show that large partial flow conditions in pump mode experience positive-slope phenomena with a large head drop. The pressure fluctuation at the positive slope is crucial to the pump turbine unit safety. The operating instabilities at large partial flow conditions for a pump turbine are analyzed. The hydraulic performance of a model pump turbine is tested with the pressure fluctuations measured at unstable operating points near a positive slope in the performance curve. The hydraulic performance tests show that there are two separated positive-slope regions for the pump turbine, with the flow discharge for the first positive slope from 0.85 to 0.91 times that at the maximum efficiency point. The amplitudes of the pressure fluctuations at these unstable large partial flow conditions near the first positive slope are much larger than those at stable operating condtions. A dominant frequency is measured at 0.2 times the impeller rotational frequency in the flow passage near the impeller exit, which is believed to be induced by the rotating stall in the flow passage of the wicket gates. The test results also show hysteresis with pressure fluctuations when the pump turbine is operated near the first positive slope. The hysteresis creates different pressure fluctuations for those operation points even though their flow rates and heads are similar respectively. The pressure fluctuation characteristics at large partial flow conditions obtained by the present study will be helpful for the safe operation of pumped storage units.

Numerical Simulation of Pressure Fluctuations in a Large Francis Turbine Runner

The pressure fluctuation caused by unsteady flow in runner is one of the main reasons of vibration for a large Francis hydraulic turbine. It directly affects the steady operation of the hydraulic turbine unit. The existing research of the pressure fluctuation in hydraulic turbine mainly focuses on the unsteady flow in draft tube. Accurate distribution of pressure fluctuations inside a runner is not very clear. In this paper, the numerical method for predicting the pressure fluctuations in runner is investigated and the numerical simulation is performed for a large Francis hydraulic turbine. It is proved that the combination of shear-stress transport（SST） k-o） turbulence model and pressure-implicit with splitting of operators（PISO） algorithm could give more reliable prediction of pressure fluctuations in runner. The frequencies of pressure fluctuations in runner are affected by the flow in guide vane and the flow in draft tube The first dominant frequency is significantly determined by the flow in draft tube, especially at part load condition. This frequency is approximately equal to one-third of the runner rotating frequency. The evident second dominant frequency is exactly equal to the guide vane passing frequency. The peak-to-peak amplitudes of pressure fluctuations in runner at small guide vane open angle are larger than that at large open angle at the same operating head. The amplitudes at points on blade pressure surface are generally greater than that on suction surface. The research results could be used to direct the hydraulic design and operation stability improvement of a large Francis hydraulic turbine.

Time-series analysis of the characteristic pressure fluctuations in a conical fluidized bed with negative pressure

The negative pressure conical fluidized bed is widely used in the pharmaceutical industry.In this study,experiments based on the negative pressure conical fluidized bed are carried out by changing the material mass and particle size.The pressure fluctuation signals are analyzed by the time and the frequency domain methods.A method for absolutely characterizing the degree of the energy concentration at the main frequency is proposed,where the calculation is to divide the original power spectrum by the average signal power.A phenomenon where the gas velocity curve temporarily stops growing is observed when the material mass is light,and the particle size is small.The standard deviation and kurtosis both rapidly change at the minimum fluidization velocity and thus can be used to determine the flow regime,and the variation rule of the kurtosis is independent of both the material mass and particle size.In the initial fluidization stage,the dominant pressure signal comes from the material movement;with the increase in the gas velocity,the power of a 2.5 Hz signal continues to increase.A method of dividing the main frequency by the average cycle frequency can conveniently determine the fluidized state,and a novel concept called stable fluidized zone proposed in this paper can be obtained.Controlling the gas velocity within the stable fluidized zone ensures that the fluidized bed consistently remains in a stable fluidized state.

Impact of intraocular pressure fluctuations on progression of normal tension glaucoma

AIM:To investigate short-and long-term intraocular pressure(IOP)fluctuations and fur ther ocular and demographic parameters as predictors for normal tension glaucoma(NTG)progression.METHODS:This retrospective,longitudinal cohort study included 137 eyes of 75 patients with NTG,defined by glaucomatous optic disc or visual field defect with normal IOP(<21 mm Hg),independently from therapy regimen.IOP fluctuation,mean,and maximum were inspected with a mean follow-up of 38 mo[standard deviation(SD)18 mo].Inclusion criteria were the performance of minimum two 48-hour profiles including perimetry,Heidelberg retina tomograph(HRT)imaging,and optic disc photographs.The impact of IOP parameters,myopia,sex,cup-to-disc-ratio,and visual field results on progression of NTG were analyzed using Cox regression models.A sub-group analysis with results from optical coherence tomography(OCT)was performed.RESULTS:IOP fluctuations,average,and maximum were not risk factors for progression in NTG patients,although maximum IOP at the initial IOP profile was higher in eyes with progression than in eyes without progression(P=0.054).The 46/137(33.5%)eyes progressed over the followup period.Overall progression(at least three progression confirmations)occurred in 28/137 eyes(20.4%).Most progressions were detected by perimetry(36/46).Longterm IOP mean over all pressure profiles was 12.8 mm Hg(SD 1.3 mm Hg);IOP fluctuation was 1.4 mm Hg(SD 0.8 mm Hg).The progression-free five-year rate was 58.2%(SD 6.5%).CONCLUSION:Short-and long-term IOP fluctuations do not result in progression of NTG.As functional changes are most likely to happen,NTG should be monitored with visual field testing more often than with other devices.

Postoperative Delirium in Elderly Patients May be Associated with Perioperative Blood Pressure Fluctuations

Postoperative delirium(PD)is a common complication of surgery in elderly patients,but its pathophysiological mechanism remains unclear.In order to clarify the role of intraoperative hypotension and fluctuation of blood pressure in the development of PD,we conducted a follow-up study in elderly patients with intraoperative hypotension and fluctuation of blood pressure.A total of 237 patients underwent hip surgery between July 2018 and September 2019,and 158 patients who were eligible for inclusion were enrolled in the study.One day before the operation,the mentality of patients was evaluated by Mini-mental State Examina­tion(MMSE),and the sex,age,height,and weight of the patients were recorded.Radial artery puncture was performed in all patients before anesthesia,intraoperative SBP,MAP and DBP were recorded,and the surgical events of the patients was recorded.The markers associated with PD(TNF-α,IL-6 and S-100β)were determined before and after surgery.Perioperative delirium(PD)was assessed by the prevailing standard of assessment,Confusion of Consciousness Assessment(CAM).Cognitive assessment was evaluated using the Mini-mental State Examination(MMSE).In addition,the timing and type of delirium were recorded.There were 158 patients which were accorded with the inclusion criteria came into the study.The results of our data showed that delirium occurred in 41 patients(25.9%)during the first week after surgery.In the compar­ison between the PD group and the non-PD group,it was found that the patients with postoperative delirium were older,lower body mass index and higher MMSE score before operation.Intraoperative blood pressure is low,usually more than 30%lower than preoperative blood pressure.The levels of TNF-α,IL-6 and Smur100βwere higher after operation.The increased incidence of postoperative delirium may be related to in­traoperative hypotension and intraoperative blood pressure fluctuation.The pathophysiological mechanism may be that hypotension causes low cerebral perfusion,which in turn causes local inflammation in the brain.In addition,postoperative delirium is also more likely to occur in older patients with lower body mass index.

High-amplitude pressure fluctuations of a pump-turbine with large head variable ratio during the turbine load rejection process

Large-head variable-amplitude pump turbines(PTs) encounter serious transient hydraulic instability issues. To explore the evolution mechanisms of pressure fluctuations(PFs) and flow patterns inside large-head variable-amplitude PTs, the load rejection process(LRP) was investigated using a one-and three-dimensional coupled flow simulation approach. The temporal,spatial, and frequency characteristics of the fluctuating pressures were analyzed for four monitoring points using a combined time-frequency analysis approach. The results indicated that PFs during the LRP of large-head variable-amplitude PTs had a new fluctuation frequency component related to Dean vortices(DVs) in the volute, in addition to the common fluctuation frequency components related to rotor-stator interaction phenomena and local backflow vortices near the impeller inlet. The PF frequency component existed throughout the LRP and had a significant influence on the transient maximum pressure at the volute end. This study provides a useful theoretical guide for the design and optimization of large-head variable-amplitude PTs.

Numerical analyses of pressure fluctuations induced by interblade vortices in a model Francis turbine

Interblade vortices can greatly influence the stable operations of Francis turbines. As visible interblade vortices are essentially cavitating flows, i.e., the ones to cause interblade vortex cavitations, an unsteady simulation with a method using the RNG k- ？ turbulence model and the Zwart-Gerber-Belamri（ZGB） cavitation model is carried out to predict the pressure fluctuations induced. Modifications of the turbulence viscosity are made to improve the resolutions. The interblade vortices of two different appearances are observed from the numerical results, namely, the columnar and streamwise vortices, as is consistent with the experimental results. The pressure fluctuations of different frequencies are found to be induced by the interblade vortices on incipient and developed interblade vortex lines, respectively, on the Hill diagram of the model runner＇s parameters. From the centrifugal Rayleigh instability criterion, it follows that the columnar interblade vortices are stable and the streamwise interblade vortices are unstable in the model Francis turbine.

EXPERIMENTAL STUDY OF EFFECTS OF AIR CONTENT ON CAVITATION AND PRESSURE FLUCTUATIONS

This article studies the effects of air content on propeller cavitation and pressure fluctuations. The cavitation is observed while the pressure fluctuations on the hull are measured. When adjusting the air content, the sheet cavitation range does not change distinctly, but the pressure fluctuations see obvious differences. The amplitudes of the pressure fluctuations increase with the decrease of the air content. The results indicate that the air content has little effect on the sheet cavitation range but has an important effect on the bubble cavitation and the tip vortex cavitation. When the air content decreases, the water tensile force increases, which results in the instability of the bubble cavitation and the tip vortex cavitation and the increase of the pressure fluctuations. To minimize the scale effects, the experiments should be run at a high Reynolds number with a high nuclei content. The high Reynolds number is often realized by increasing the flow velocity and the propeller rotation speed, and the high nuclei content is often made by increasing the dissolved air content.

Experimental investigation of vortex generator influences on propeller cavitation and hull pressure fluctuations

In this paper,the transient turbulent cavitating flow around a marine propeller behind a ship was investigated experimentally with emphasis on how vortex generator(VG)influences propeller cavitation and hull pressure fluctuations.The experiments were carried out in China Large Cavitation Channel with the closed test section covering 10.5 m in length and cross-section of 2.2 m^2×2.0m^2.The experiment recorded instantaneous cavitation photos of the propeller and pressure fluctuations on the hull surface.The results demonstrate that without vortex generator,the sheet cavitation inception begins at-50°(310°),and then from-50°(310°)to 30°,the extent and area of sheet cavitation increase.When vortex generator installed on the hull,the sheet cavitation inception occurs in advance at-60°(300°),and the sheet cavitation expands to larger rotation range due to the vortex generator.It is shown that the vortex generator with proper geometry and installed location may contribute the highly nonuniform wake to be more uniform,and can make the change of propeller cavitation to be milder,which decrease the pressure fluctuations in this study.Further analysis indicates that the vortex generator may promote the energy distribution to be more uniform in physical and spectral space in some degree.

PRESSURE FLUCTUATIONS IN GAS-SOLIDS FLUIDIZED BEDS

Pressure fluctuation data measured in a series of fluidized beds with diameters of 0.05, 0.1, 0.29, 0.60 and 1.56 m showed that the maximum amplitude or standard deviation increased with increasing the superficial gas velocity and static bed height for relatively shallow beds and became insensitive to the increase in static bed height in relatively deep beds. The amplitude appeared to be less dependent on the measurement location in the dense bed. Predictions based on bubble passage, bubble eruption at the upper bed surface and bed oscillation all failed to explain all observed trends and underestimated the amplitude of pressure fluctuations, suggesting that the global pressure fluctuations in gas-solids bubbling fluidized beds are the superposition of local pressure variations, bed oscillations and pressure waves generated from the bubble formation in the distributor region, bubble coalescence during their rise and bubble eruption at the upper bed surface.

Dynamic characteristics of bubbling fluidization through recurrence rate analysis of pressure fluctuations

Pressure fluctuations signals of a lab-scale fiuidized bed （15 cm inner diameter and 2 m height） at different superficial gas velocities were measured. Recurrence plot （RP） and recurrence rate （RR）, and the simplest variable of recurrence quantification analysis （RQA） were used to analyze the pressure signals. Different patterns observed in RP reflect different dynamic behavior of the system under study. It was also found that the variance of RR （a2R） Could reveal the peak dominant frequencies （PDF） of different dynamic systems： completely periodic, completely stochastic, Lorenz system, and fluidized bed. The results were compared with power spectral density. Additionally, the diagram of σ^2RR provides a new technique for prediction of transition velocity from bubbling to turbulent fluidization regime.

WALL PRESSURE FLUCTUATIONS OF TURBULENT FLOW OVER BACKWARD-FACING STEP WITH AND WITHOUT ENTRAINMENT: MICROPHONE ARRAY MEASUREMENT

Wall pressure fluctuations in turbulent boundary layer flow over backward-facing step with and without entrainment were investigated. Digital array pressure sensors and multi-arrayed microphones were employed to acquire the time-averaged static pressure and fluctuating pressure, respectively. The differences of two flows were scrutinized in terms of static pressure characteristics, pressure fluctuations, cross-correlation and coherence of wall pressure. Introduction of the entrainment increased scale of large-scale vortical structure and reduced its convection velocity. However, shedding frequency of large-scale vortical structures was found to be the same for both flows.

MECHANISM OF WALL PRESSURE FLUCTUATIONS BENEATH THE OPEN CHANNEL FLOW

Based on the measured results that wall pressure fluctuations are mainly de- cided by coherent structures of turbulence, the relationship between root-mean- square wall pressure and wall shear stress in turbulent shear flow and that between the intensities of pressure and fluctuating velocity in homogeneous and isotropic turbulence are established in this paper. These relationships are consistent with former works, and have good agreement with experimental data. The paper also dis- cusses the concept of 'apparent pressure' on the wall in mean flow.

ITTC COMPARATIVE TESTS ON PRESSURE FLUCTUATIONS

Pressure fluctuation induced by unsteady propeller cavitation is one of the most predominant vibration sources. Cavitation Committee of International Towing Tank Conference (ITTC) has been continuously reviewing the theoretical and experimental prediction methods. In this paper, the accuracy of the existing prediction methods and the parameters which affect the model test results are discussed, especial]y with referring to the results of the comparative tests on the 'Sydney Express' propeller, organized by Cavitation Committee.

Experimental Investigation of the Rail Pressure Fluctuations Correlated with Fuel Properties and Injection Settings

Injection-induced rail pressure fluctuations are proven to cause nonuniform spray development.These fluctuations are also responsible for generating lower injection pressures,to the detriment of jet penetration length and break-up timing.Despite the vast literature dealing with such issues,several aspects of rail pressure fluctuations remain unclear.Additionally,the need for compliance with the emission legislation has shed light on the potential of alternative fuels,which represent a pathway for sustainable mobility.This scenario has motivated the present study dealing with the assessment of the time history of rail pressure correlated with fuel properties.Tests have been performed using a last-generation common rail injection equipment under various injection settings,employing diesel and 2-methylfuran-diesel blend.This paper describes the research activity and aims to provide new insights into the correlation of rail pressure fluctuations with fuel properties.

Interior noise field of a viscoelastic cylindrical shell excited by TBL pressure fluctuations:Ⅰ.Production mechanisms of the noise

Applying the wavenumber frequency transfer function to describe the whole system including tht elastic cylindrical shell and the fluid loading, a general expression of the cross spectrum of the interior noise induced by the TBL (turbulent boundary layer) pressure fiuctuations is derived. There are two production mechanisms of the noise: one is direct transfer of the convective ridge of the pressure fluctuations through the shell, the other is the reradiation of resonance modes excited by the pressure fluctuations. At low frequencies the noise produced by the latter mechanism is dominant. Solving the frequency equation of the cylindrical shell with liquid loading, the two Stoneley-type poles in the complex K plane are presented. They are the major sources of the reradiation of shell at low frequencies. Finally, effects of the shell radius, shell thickness, absorption of material and the flow speeds on the noise reduction are computed by numerical iniegration.

Interior noise field of a viscoelastic cylindrical shell excited by TBL pressure fluctuations:Ⅱ. finite hydrophone and array

Finite hydrophone and hydrophone array are the wave vector filter and can re-duce the flow noise. In this paper the responses of the cylindrical area hydrophone and two-circular area hydrophone within viscoelastic cylindrical shell to the TBL (turbulent boundary layer) pressure fluctuations are investigated. Applying the method based on the wavenumber frequency spectrum analysis, the expressions of 1) the noise power spectrum of a single hy-drophone; 2) the space correlation of two hydrophones; 3) the noise power spectrum of array are derived. The dependencies of the noise reduction on hydrophone shape, dimension, element amount and separation of hydrophones of array are calculated by numerical integration. The wide-band and narrow-band correlation for two hydrophones is also calculated. The numerical results show that hydrophone array can effectively reduce the interior noise.

Noise radiation of an elastic plate excited by the turbulent boundary layer pressure fluctuations

A unified theory for calculating the noise radiation of an infinite elastic plate excited by the turbulent boundary layer pressure fluctuations is presented . Using the wave number frequency transfer function to desribe the whole system , consisting of the plate and the liquid loading, a general expression of the cross spectrum was derived. It is an integral in the complex wave number plane and leads to a sum of the residues at the poles of two types. One pole introduced by the convective ridge of the pressure fluctuations yields a direct transfer component , which is an evanescent wave in liquid because the pole lies in the high wave number region . The other poles introduced by the transfer function of structure produce the radiation field components with the resonance modes of a liquid loaded plate. The pole positions and their residues can be computed approximately by use of the Resonance Scattering Theory . For the case of hydrodynamic noise, where the range of the frequency-thickness product of interest is relatively low , the symmetric zero-order mode dominates the noise radiation.

Characteristics of pressure fluctuations and fine coal preparation in gas-vibro fluidized bed

To improve the separation efficiency of air dense medium fluidized beds tor dry coal preparation, a gasvibro fluidized bed has been proposed in which magnetic powder is used as the heavy medium. Pressure fluctuations in the gas-vibro fluidized bed were investigated using time- and frequency-domain analysis methods. The relationship between pressure fluctuations, bubble behavior, and separation efficiency was established. The low amplitude of the standard deviation, the power spectral density （PSD）, the incoherent-output PSD, and the high amplitude of the coherent-output PSD, which corresponds to the bubble behavior in the bed, were improved for coal preparation. The coal ash content was reduced from 42.5570 to 16.54% by using the gas-vibro fluidized bed.

Influence of the Axial Position of the Guide Vane on the Fluctuations of Pressure in a Nuclear Pump

The influence of the axial mount position of the guide vane on the pressure fluctuation in a nuclear pump(AP1000)is investigated.The characteristics of the three-dimensional flow inside the nuclear pump are analyzed by means of numerical simulation.Results indicate that when the axial relative distance between the guide vane and the pumping chamber is reduced,in conditions of“small flow,”the efficiency of the pump increases,the pressure inside the pumping chamber decreases,while the losses related to the guide vane grow.Under large flow conditions,as the efficiency of the pump decreases,the losses for the guide vane and the pumping chamber increase.The pressure fluctuation in the annular pumping chamber is basically determined by the rotation frequency and the blade passing frequency.The magnitude of these fluctuations is affected by the guide vane axial position.In particular,the smallest possible amplitude is obtained when the outlet central plane of the guide vane coincides with the outlet axis of the pumping chamber.