Propagation Of Pressure Fluctuations Research Articles

Exploring the mechanism of improving the pressure fluctuation of double suction centrifugal pump by impeller stagger.

The pressure fluctuation in the volute can be effectively reduced when the impeller of the double-suction pump is staggered, but the mechanism of this reduction is still unclear. At the same time, the traditional analysis method cannot realize the visualization of pressure fluctuation. The purpose of this article is to explore the spatial distribution, propagation, and attenuation law of pressure fluctuation, and on this basis, to research the reason why staggered impeller reduce pressure fluctuation. A new method called Pulse tracking network (PTN) was used in this article. Compared with the traditional method, which only analyzes the pressure fluctuation at scattered points, this method greatly improves the spatial resolution of the pressure fluctuation. In particular, the phase analysis is a major highlight of the method. Staggered impeller significantly reduced the pressure fluctuation intensity dominated by blade passing frequency. At the same time, the propagation of the pressure fluctuation in the volute changed from radial to circumferential in the volute cross-section. Staggered impeller can effectively reduce pressure fluctuation, and the circumferential propagation caused by it is considered to be the main reason for it.

VELOCITY AND WALL PRESSURE FLUCTUATIONS OF TURBULENT BOUNDARY LAYER

The results of the study of the velocity pulsation field and the near-wall pressure of the turbulent boundary layer, which is formed on a flexible elongated cylinder and a hydraulically smooth flat plate, are presented. Peculiarities of the interaction of the velocity and pressure fields in the boundary layer are shown; sound and pseudosonic sources of hydrodynamic noise are characterized. The integral, correlation and spectral characteristics of the field of velocity pulsations and wall pressure in the turbulent boundary layer are analyzed. The relationship between these statistical characteristics is shown, as well as the mechanisms and sources that influence the behavior of these characteristics. It is noted that large-scale vortex structures and small-scale vortices are formed in the turbulent boundary layer, which form the outer and inner regions of the boundary layer. It is established that large-scale vortex structures generate pressure fluctuations, whose probability density distribution law is close to Gaussian law, and small-scale vortices that generate high-frequency or low-wave fluctuations have a non-Gaussian law of probability distribution of pulsation density. The interaction of coherent vortex structures with each other and with the streamlined surface leads to changes in the integral, correlation and spectral characteristics of the velocity and pressure fields. It is noted that with increasing the distance between the pressure fluctuation sensors, the correlation of signals decreases, and the delay time of correlated signals increases. Thus, with the increasing of the distance between the measuring points, the correlated fluctuations are those generated by the large-scale vortex structures. It is determined that the highest levels of velocity and pressure fluctuations are generated by the large-scale vortex structures that have the highest coherence. The results obtained make it possible to characterize coherent vortex structures, their features of formation, places of origin, scales, directions and velocities of transfer.

Classification of the Hydraulic Behavior Along the No-Load Curve of Francis Turbines

Abstract For hydraulic turbines, no-load (NL) is considered a homogeneous family of operating conditions, although the literature exposes a wide variety of flow structures depending on many factors. A better understanding of the flow structures developed during NL operation is necessary, since they generate pressure fluctuations in the turbine causing significant fatigue damage and reducing the life expectancy of the machines. Hydraulic turbines at model scale show that behavioral trends can be identified for NL conditions. This paper presents a classification of NL operating conditions following the swirl level at the runner outlet. The main tendencies linking the cavitation level to the runner speed and the discharge for operating points along NL curves of different turbines are also detailed. To study the NL conditions, data from 26 Francis turbines, measured between 2007 and 2020 at the laboratory of Andritz Hydro Canada Inc., are analyzed. This study demonstrates that NL operating conditions exhibit flow features very similar to those at regular operation with similar runner outlet swirl. The runner acceleration or deceleration with cavitation is related to the flow topology at the runner outlet.

Investigation of shock dynamics in an axisymmetric inlet/isolator with attached boundary layers

Abstract

Experimental study on the particle flow patterns in a cyclone dipleg with a trickle valve

An experimental apparatus including a dipleg and a trickle valve was established to simulate the operation of a suspended dipleg-trickle valve system of cyclone used in fluid catalytic cracking (FCC) unit. The flow regimes in the dipleg and the discharge modes in the trickle valve were studied by combining the observation of experimental phenomena with the analysis of transient pressure fluctuation. The results show that the flow regimes in the dipleg have two types—the dilute–dense phase coexisting falling flow and the dilute falling flow. Correspondingly, the trickle valve also has two discharge modes—the intermittent periodic dumping discharge and the continuous trickling discharge. The power spectrum density of pressure fluctuation displays that the gas–solids flow in the dipleg-trickle valve system is characterized by a low-frequency pulsation. The coherence coefficient explains the origin and propagation of pressure fluctuation in the system. Eventually, a map describing the flow regimes and discharge modes related to the operation parameters was proposed, which can provide a helpful guidance for the operation of cyclone dipleg-trickle valve system in FCC unit.

An Experimental Approach to Predict the Behaviour of EHD Contacts under Harmonic Loading

Rolling element bearings, gears and other machine components operate in the lubrication regime known as elastohydrodynamic (EHD), where lubricant film thickness is governed by the hydrodynamic action of the converging surfaces, the elastic deformation of the non-conforming surfaces and the increase of lubricant’s viscosity with pressure. Classic EHD theory indicates that under the assumption of fully flooded conditions in the contact inlet zone the central film thickness increases with the product of entrainment speed and dynamic viscosity and is very little dependent of load variations. EHD lubricating films most often work under transient conditions of load, geometry or speed. These make the behaviour of the lubricant film different from steady-state conditions. Previous studies showed that the squeeze effect generates film perturbations due to entrapment of lubricant inside the contact. This phenomenon becomes significant when both entrainment and squeeze are presented. In practical applications, this type of film perturbation will generate pressure fluctuations, which in turn may influences the fatigue life of the contact. In the present paper the results of a systematic experimental study for predicting the occurrence of film thickness perturbation phenomenon under variable load condition are shown. In the case of a mean load larger than the amplitude of the load variation, it was found that the film perturbation depends of the frequency of the load cycle, the entrainment speed of the lubricant and the overall lubricant film thickness; it also depends of the ratios between the mean load and the amplitude. Film perturbations occur in contacts for which the ratio between half the load cycle period and the average time of transit of the lubricant through the contact in the loading phase of the vibration is less than 2. In the case of mean load smaller than the amplitude, film thickness perturbation has been observed in every single test regardless of entrainment speed, frequency and lubricant viscosity.

Investigation of the correlation mechanism between cavitation rope behavior and pressure fluctuations in a hydraulic turbine

Vortex ropes occurred in the draft tube when hydraulic turbines operating at off-design conditions, which can generate pressure fluctuations. Cavitation vortex rope is one of the most harmful factors to the safety of hydroturbines which may induce further pressure fluctuations. A study of the multiphase flow in a model turbine is presented in this paper using the software ANSYS CFX. The main emphasis is spending on understanding the mechanism of cavitation evolution and underlying its correlation mechanism with flow instabilities. The results indicate that two types of pressure variations can be captured with a cavitation rope: 1) The type due to the vortex rope rotating whose frequency is 1/5–1/4 times runner rotating frequency; 2) The type due to cavitation volume surge, whose frequency is less than that of vortex rope rotating. The frequency of pressure fluctuation due to cavitation keep contanst as the cavitation number decreases, while the amplitude much increases. While the frequency and amplitude of the pressure variation caused by votex rope rotating increase a little. Based on vorticity transport equation, the vortex and cavitation have a close relation. Cavitation increases the vortex production as well as the pressure fluctuation frequency caused by the rotation of vortex rope.

The study on pressure pulsation of cooling circulating channel of magnetic drive pump

In order to study the layout and developments of pressure pulsation of the main flow passage component in magnetic drive pump, the full flow field numerical research of magnetic drive pump was done with CFX software, the pressure pulsation character of cooling circulating channel and external characteristic curve of pump were obtained. The reliability of the numerical calculation method is verified by the external characteristic test, and the pressure pulsation characteristics in the main flow components of the magnetic drive pump are studied. The results show that the main source of pressure pulsation in magnetic drive pump is the dynamic and static interference between impeller channel, pressurized water chamber and baffle tongue; the main frequency of cooling circulating channel is blade frequency and the pressure fluctuation amplitude of cooling circulation channel decreases with the increase of flow and the propagation of pressure fluctuation in cooling circulation channel weakens the intensity of pressure fluctuating.

Numerical Study of Pressure Fluctuation and Unsteady Flow in a Centrifugal Pump

A pump is one of the most important machines in the processes and flow systems. The operation of multistage centrifugal pumps could generate pressure fluctuations and instabilities that may be detrimental to the performance and integrity of the pump. In this paper, a numerical study of the influence of pressure fluctuations and unsteady flow patterns was undertaken in the pump flow channel of three configurations with different diffuser vane numbers. It was found that the amplitude of pressure fluctuation in the diffuser was increased gradually with the increase in number of diffuser vanes. The lower number of diffuser vanes was beneficial to obtain a weaker pressure fluctuation intensity. With the static pressure gradually increasing, the effects of impeller blade passing frequency attenuated gradually, and the effect of diffuser vanes was increased gradually.

Experimental investigation of the nonlinear pressure fluctuations in a residual heat removal pump

Residual Heat Removal Pumps have very strict requirements to maintain nuclear power plant safety and therefore unsteady pump operations are very critical in determining system reliability. The operational stability has however been affected by pressure pulsations that are induced by rotor-stator interactions. A pressure measurement test was undertaken to investigate the nonlinear pressure fluctuation characteristics at angular and axial positions within the annular volute wall. Diametric mode and bispectrum analysis were subsequently applied to reveal the dominant frequencies of the pressure pulsations induced at the stationary parts, and also to analyze the nonlinear frequencies in the frequency domain due to the impeller-diffuser and diffuser-volute interactions. The pressure pulsations on the volute wall were significantly affected not only by the flow rate, but also by the sensors’ position. The highest amplitude was observed at a frequency that was four-fold the blade-passing frequency at a diameter mode of 1. When the flow rate increased, the amplitude of the high harmonics of the blade-passing frequency increased, whereas the amplitudes of the blade-passing and shaft rotating frequencies decreased. In the axial direction, flow rate greatly influenced the propagation of pressure fluctuations whose dominant frequency are blade-passing frequency, vane-passing frequency and their high harmonics. The spectral domain was dominated by high harmonics of the blade-passing frequency (3St and 4St). Conversely, the circumferential pressure fluctuation was significantly affected by rotor-stator interaction and was independent of flow rate. The study provided sufficient information for further work to reduce pressure pulsations in the residual heat removal pump.

Pressure Fluctuation and Flow Characteristics in a Two-Stage Double-Suction Centrifugal Pump

Pressure fluctuation is the primary factor that affects the stability of turbomachines. The goal of the present work is to explore the propagation of pressure fluctuations in a two-stage double-suction centrifugal pump. The pressure fluctuation characteristics of each component of a two-stage double-suction centrifugal pump are simulated under four typical flow rates based on the SST k-ω turbulence model. It is shown that the pressure fluctuation frequency at blade passing frequency and its first harmonic is the same at the suction chamber, the leading edge, and the middle of the first-stage impeller, which is different from the rotor–stator interaction. Moreover, the uneven impeller inlet flow distribution will produce fluctuations with rotation frequency and its harmonics at the leading edge of the impellers in both stages. Finally, broadband frequency is found at the trailing edge of the impellers in both stages associated with the first harmonic of the rotation frequency, especially under the part load condition. The large size backflow vortex appears in the blade flow channel leading to the low-pressure zone between the impeller, the tongue, and the start of the partition. That is why the pressure drops significantly twice in one rotation period when the blades pass through the tongue and the start of the partition.

Analysis and evaluation on pressure fluctuations in air dense medium fluidized bed

Pressure fluctuations contribute to the instability of separation process in air dense medium fluidized bed, which provides a high motivation for further study of underlying mechanisms. Reasons for generation and propagation of pressure fluctuations in the air dense medium fluidized bed have been discussed. Drift rate and collision rate of particles were employed to deduce the correlation between voidage and pressure fluctuations. Simultaneously, a dynamic pressure fluctuation measuring and analysis system was established. Based on frequency domain analysis and wavelet analysis, collected signals were disassembled and analyzed. Results show gradually intensive motion of particles increases magnitudes of signal components with lower frequencies. As a result of violent particle motion, the magnitude of real pressure signal’s frequency experienced an increase as air velocity increased moderately. Wavelet analysis keeps edge features of the real signal and eliminates the noise efficaciously. The frequency of de-noised signal is closed to that of pressure signal identified in frequency domain analysis.

Unsteady pressure fluctuations measured on a hammerhead space vehicle and comparison with Coe and Nute's 1962 data

A detailed survey of pressure fluctuations was conducted on a replica of the generic, “hammerhead” shaped space vehicle tested more than half century ago by Coe and Nute (1962). A large number of current and past space vehicles follow this general hammerhead configuration. The present test was conducted in the 11-by-11 ft Transonic Wind Tunnel of NASA Ames Research Center over the critical transonic Mach number range of 0.6 ≤ M ≤ 1.2 where typical vehicles encounter very high levels of surface pressure fluctuations during ascent through the atmosphere. Out of the many different types of instrumentations used for this test, data from the dynamic pressure sensors are presented in this paper. In addition, shadowgraph flow visualizations, and a qualitative measure of turbulent fluctuations, estimated from high-speed photography, are also presented. The data set provided a clear insight into the local high level of fluctuations from the formation of the transonic shocks on the payload fairing and also from the impingement of the separated shear layer on the second stage. Space–time correlation showed interesting physics of upstream propagation of pressure fluctuations on the payload fairing when local shock waves were present. A comparison of the magnitude of pressure fluctuations between the present test and that done earlier by Coe and Nute showed good agreement when the limited frequency bandwidth of equipment used in the latter was accounted for. Similarly, when accounting for the differences in the free-stream dynamic pressure, the spectra of surface pressure fluctuations were in reasonable agreement between the present and Coe–Nute tests. The present test provides a far more complete picture of pressure fluctuations due to the usage of a large number of pressure transducers (216 versus 30), wider frequency bandwidth (50 kHz versus 1 kHz), and unsteady pressure-sensitive paint.

Hydroacoustic noise from different geometries

Turbulent flow around bluff bodies generates pressure fluctuations which propagate as acoustic waves. Differences in the shape of a body can affect frequencies and amplitudes of the propagating pressure signals. In the present work three elementary geometries (sphere, cube and prolate spheroid), immersed in a uniform water flow, are examined in order to analyze the differences of the resulting hydroacoustic fields. The turbulent flow at ReA=4430 (based on the cross-sectional area of the bodies) is reproduced through wall-resolving Large-Eddy Simulation and the hydroacoustic far-field is analyzed by adopting the Ffowcs Williams and Hawkings analogy. The quadrupole term of the acoustic equation is first reformulated in the convective form and then solved through direct computation of the volume integrals. This procedure is found possible in hydrodynamics where the speed of sound is very large and the flow velocities are small. In spite of the fact that the frontal section of the bodies has the same area, the analysis shows that a streamlined body is able to produce a pressure signal one order of magnitude lower than that generated by a bluff geometry. The separate analysis of the loading noise and of the quadrupole one has shown that the former is larger than the latter in case of 3D-shaped bluff body (sphere and cube), whereas the opposite is true in case of a streamlined body. A preliminary analysis between the case of an elongated square cylinder and a cube, shows that the persistence of a two-dimensionally shaped wake when compared to a three-dimensional one contributes to increase the quadrupole part of the radiated noise.

Electric Arc Fluctuations in DC Plasma Spray Torch

Direct current plasma torches for plasma spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional DC plasma torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma torch domain that can generate pressure fluctuations coupled with voltage ones.

On the relation between tonal and broadband content of hull pressure spectra due to cavitating ship propellers

Cavitating propellers generate pressure fluctuations on the hull of the ship. These pressure fluctuations are usually analyzed in the frequency domain using FFTs and the spectrum is composed of tonals at multiples of the blade passage frequency and a broadband part. The two are often considered separately but a relation between the two exists which has been investigated by theoretical signal analysis. It will be shown that the broadband part is related to the variability of the signal between blade passages and a simple procedure is proposed to quantify the variability in terms of amplitude and phase angle. The procedure has been applied to a data set obtained at sea trials.

Establishment and Evaluation of a Dynamic Pressure Measuring and Analysis System for the Air Dense Medium Fluidized Bed

Over the last 30 years, great progress has been made in the theory and applications of fluidized bed. However, very few studies haves addressed the change regulation of wall pressure during the fluidization process of magnetite powder. Pressure fluctuation is a key factor that affects the fluidization characteristic and separation effect. In the proposed work, theories about generation and propagation of pressure fluctuation had been discussed for better understanding the correlation between pressure fluctuation and the extent of fluidization as well as the particle motion behavior of magnetite powder. Simultaneously, a pressure measuring and analysis system was established to measure and analyzed pressure data in air dense medium fluidized bed, including a fluidized bed with measuring ports, air supply system, pressure sensor, data acquisition block and data processing block. Pressure sensor and data acquisition block make it easy to acquire precise pressure information. Based on MATLAB software, wavelet analysis was employed for pressure signals filtering and noise removing. Through systemic experimental study, it is proved that effective analysis results have obtained.

A unified theory of microseisms and hum

Interacting ocean surface waves force water column pressure fluctuations with spectral peaks at the same frequencies of primary microseisms (PM), double‐frequency microseisms (DF), and seismic hum. Prior treatment of nonlinear ocean wave interactions has focused on the DF pressure fluctuations which, in the presence of opposing waves, do not decay with depth and hence are dominant in deep waters. For an arbitrary 2‐D surface wave spectrum we integrate over all pairings of wave vectors, directions, and frequencies to obtain a full‐spectrum perturbation expansion including the first‐ and second‐order pressure waves. First‐order pressure waves generate a peak at PM frequencies and second‐order pressure waves generated by obliquely interacting surface waves generate pressure fluctuations at DF and hum frequencies. These pressure fluctuations decay with depth but interact with the seabed in shallow water. As their generation does not require precise wave states, they are likely ubiquitous in shallow water.

Carbon dioxide controlled earthquake distribution pattern in the NW Bohemian swarm earthquake region, western Eger Rift, Czech Republic – gas migration in the crystalline basement

AbstractThe ascent of magmatic carbon dioxide in the western Eger (Ohře) Rift is interlinked with the fault systems of the Variscian basement. In the Cheb Basin, the minimum CO2 flux is about 160 m3 h−1, with a diminishing trend towards the north and ceasing in the main epicentral area of the Northwest Bohemian swarm earthquakes. The ascending CO2 forms Ca‐Mg‐HCO3 type waters by leaching of cations from the fault planes and creates clay minerals, such as kaolinite, as alteration products on affected fault planes. These mineral reactions result in fault weakness and in hydraulically interconnected fault network. This leads to a decrease in the friction coefficient of the Coulomb failure stress (CFS) and to fault creep as stress build‐up cannot occur in the weak segments. At the transition zone in the north of the Cheb Basin, between areas of weak, fluid conductive faults and areas of locked faults with frictional strength, fluid pressure can increase resulting in stress build‐up. This can trigger strike‐slip swarm earthquakes. Fault creep or movements in weak segments may support a stress build‐up in the transition area by transmitting fluid pressure pulses. Additionally to fluid‐driven triggering models, it is important to consider that fluids ascending along faults are CO2‐supersaturated thus intensifying the effect of fluid flow. The enforced flow of CO2‐supersaturated fluids in the transitional zone from high to low permeability segments through narrowings triggers gas exsolution and may generate pressure fluctuations. Phase separation starts according to the phase behaviour of CO2‐H2O systems in the seismically active depths of NW Bohemia and may explain the vertical distribution of the seismicity. Changes in the size of the fluid transport channels in the fault systems caused, or superimposed, by fault movements, can produce fluid pressure increases or pulses, which are the precondition for triggering fluid‐induced swarm earthquakes.

Water hammer reduces fouling during natural water ultrafiltration

Today’s ultrafiltration processes use permeate flow reversal to remove fouling deposits on the feed side of ultrafiltration membranes. We report an as effective method: the opening and rapid closing of a valve on the permeate side of an ultrafiltration module. The sudden valve closure generates pressure fluctuations due to fluid inertia and is commonly known as “water hammer”. Surface water was filtrated in hollow fiber ultrafiltration membranes with a small (5%) crossflow. Filtration experiments above sustainable flux levels (>125 l (m 2h) −1) show that a periodic closure of a valve on the permeate side improves filtration performance as a consequence of reduced fouling. It was shown that this effect depends on flux and actuation frequency of the valve. The time period that the valve was closed proved to have no effect on filtration performance. The pressure fluctuations generated by the sudden stop in fluid motion due to the valve closure are responsible for the effect of fouling reduction. High frequency recording of the dynamic pressure evolution shows water hammer related pressure fluctuations to occur in the order of 0.1 bar. The pressure fluctuations were higher at higher fluxes (higher velocities) which is in agreement with the theory. They were also more effective at higher fluxes with respect to fouling mitigation.