Local Pressure Fluctuations Research Articles

Unsteady pressure measurements on the runner of a Kaplan turbine during load acceptance and load rejection

ABSTRACTThe paper addresses unsteady pressure measurements on the blades and stationary parts of a Kaplan turbine model (Porjus U9) during load variation. The turbine was studied in various load acceptance and load rejection scenarios in off-cam mode to investigate the effect of the transients on the turbine performance. The formation and mitigation processes for the rotating vortex ropes and their effects on the forces exerted on the runner were also investigated. The results show a smooth transition during load variations between high load and the best efficiency point, at which no rotating vortex ropes form in the draft tube. However, load variation to part load resulted in a draft tube surge and the formation of a rotating vortex rope with two fluctuating components: rotating and plunging. The rotating vortex ropes began to form at the end of the draft tube cone during the closure of the guide vanes and travelled upstream with further guide vane closure. The plunging mode induced flow oscillation throughout the entire turbine conduit, whereas the rotating mode resulted in local pressure fluctuations. The rotating vortex ropes induced wide-band pressure fluctuations on the suction side of the runner close to the hub section. The formation of the rotating vortex ropes near the runner resulted in a sudden change in the pressure exerted on the suction side of the blades, whereas the rotating vortex rope mitigation process proceeded in a smooth manner.

A New High Sensitivity System to Detect Instabilities During the Extrusion of Polymer Melts

AbstractThis publication is concerned with the development of a novel high sensitivity instability detection die for a laboratory size extruder. The high sensitivity system consists of piezoelectric transducers placed along the die length to monitor local pressure fluctuations, having temporal and pressure resolution of up to Δt ≈ 10−3 s and Δp ≈ 10−5 bar, at a nominal pressure of 500 bar. The system is tested on short chain branched polyethylenes, namely on the detection and characterization of the sharkskin instability. It is shown that the determined instabilities are related to characteristic peaks in the corresponding in situ pressure Fourier‐transform spectra and that other peaks are caused by different, but defined, sources. The in situ results obtained are validated through optical image analysis of the instabilities.

Observations and measurements on unsteady cavitating flows using a simultaneous sampling approach

The main purpose of this study was to shed light on the unsteady cavitating flow and corresponding wall-pressure fluctuation characteristic. A simultaneous sampling technique is used to synchronize the observations of cavitation images and the measurements of wall-pressure signals in a convergent–divergent channel. The results show that, with the decreasing cavitation number, cavitating flows in a convergent–divergent channel display several types of cavitation behavior, such as cavitation inception, sheet cavitation, and sheet/cloud cavitation. The intensity of the pressure fluctuation increases with the decrease in cavitation number. However, with decreasing cavitation number, the dominant frequency of the unsteady pressure fluctuation decreases significantly, and for sheet/cloud cavitation, the dominant frequency of pressure fluctuation is consistent with that of global cavitation area fluctuation. A typical quasi-periodic sheet/cavitation development cycle is characterized by three stages such as: (1) the growth of attached cavity, (2) the shedding of attached cavity, and (3) the development and collapse of detached cavities. In the stage one, the magnitude of pressure fluctuations under the attached cavity is small; however, it is large in the closure region of attached cavity, especially when attached cavity obtains its maximum length. In the stage two, the attached cavity begins to shed and some small detached cavities are observed, and small local pressure fluctuations with higher frequency are detected. In the stage three, a large detached cavity is formed in the rear of attached cavity. When the detached cavity collapses rapidly in the downstream region, pressure pulses with the magnitude of the order of several atmospheres are detected. The propagation speeds of pressure pulses in different cavitation regions are found to be related with the bubble density in the flow field. It is also found that the pressure impulse in the region covered by attached cavity is much lower than that in the attached cavity closure area.

Observations and measurements in cloud cavitating flows

The main purpose of this study is to shed light on the cloud cavitating flow and associated characteristic of pressure fluctuation near wall. A simultaneous sampling technique is used to synchronize the observations of cavitation instantaneous behaviour and the measurements of pressure signals near wall in a convergent-divergent channel. The results show that, a typical quasi-periodical sheet/cloud cavitation can be categorized into three stages: (1) the growth of attached cavity; (2) the shedding of the attached cavity; (3) the development and collapse of the detached cavities. At the stage one, the magnitudes of pressure fluctuation under the attached cavity are limited. However, they become significant in the closure region of attached cavity, especially, when attached cavity reaches its maximum length. At the stage two, the attached cavity begins to shed small detached cavity, leading to the generation of small local pressure fluctuations with higher frequency. At the stage three, a large detached cavity is gradually formed in the rear of the channel. When it collapses rapidly in the downstream, pressure pulses with the magnitudes of the order of several atmospheres are detected. The propagation speeds of pressure pulses in different region are found to be related with the bubble density in the flow field.

Hydro-acoustic resonance behavior in presence of a precessing vortex rope: observation of a lock-in phenomenon at part load Francis turbine operation

Francis turbines operating at part load condition experience the development of a cavitating helical vortex rope in the draft tube cone at the runner outlet. The precession movement of this vortex rope induces local convective pressure fluctuations and a synchronous pressure pulsation acting as a forced excitation for the hydraulic system, propagating in the entire system. In the draft tube, synchronous pressure fluctuations with a frequency different to the precession frequency may also be observed in presence of cavitation. In the case of a matching between the precession frequency and the synchronous surge frequency, hydro-acoustic resonance occurs in the draft tube inducing high pressure fluctuations throughout the entire hydraulic system, causing torque and power pulsations. The risk of such resonances limits the possible extension of the Francis turbine operating range. A more precise knowledge of the phenomenon occurring at such resonance conditions and prediction capabilities of the induced pressure pulsations needs therefore to be developed.This paper proposes a detailed study of the occurrence of hydro-acoustic resonance for one particular part load operating point featuring a well-developed precessing vortex rope and corresponding to 64% of the BEP. It focuses particularly on the evolution of the local interaction between the pressure fluctuations at the precession frequency and the synchronous surge mode passing through the resonance condition. For this purpose, an experimental investigation is performed on a reduced scale model of a Francis turbine, including pressure fluctuation measurements in the draft tube and in the upstream piping system. Changing the pressure level in the draft tube, resonance occurrences are highlighted for different Froude numbers. The evolution of the hydro-acoustic response of the system suggests that a lock-in effect between the excitation frequency and the natural frequency may occur at low Froude number, inducing a hydro-acoustic resonance in a random range of cavitation numbers.

Challenge problem: 1. Model validation of circulating fluidized beds

The National Energy Technology Laboratory (NETL) worked with Particulate Solids Research Inc. (PSRI) to conduct the third CFD Challenge Problem in granular fluid flow to evaluate the progress and state of the art in simulating gas solids flow in a circulating fluidized bed. Both Group A and B particles were tested at several gas velocities and solids circulation rates. For both particle groups pressures and particle velocities were measured within the riser. For the Group B cases local radial solids fluxes and high speed pressure fluctuations were measured. Model predictions were compared against these experimental results and vetted in the workshop at the Circulating Fluid Bed X. The modelers were given detailed descriptions of the experimental facilities as well as physical property and small scale fluidization data on the different bed materials tested. Two general types of modeling simulations were submitted: Eulerian–Eulerian and Eulerian–Lagrangian. Both types of model had successes and failures indicating that good results are strongly influenced by resources such as available time, computational facilities, and experience level of the modeler. By comparing the predicted behavior the strengths and weaknesses associated with the different modeling approaches were identified and shortcomings could be targeted for future development and improvements.

A CFD Study of Pressure Fluctuations to Determine Fluidization Regimes in Gas–Solid Beds

Reliable computational methods can provide valuable insight into gas–solid flow processes and can be used as a design tool. Of particular interest in this study is the hydrodynamics of a binary mixture of sand–biomass in a fluidized bed. Biomass particulates vary in size, shape, and density, which inevitably alter how well the particles fluidize. Our study will use computational fluid dynamics (CFD) to interpret the hydrodynamic states of a fluidized bed by analyzing the local pressure fluctuations of beds of sand and a binary mixture of cotton stalks and sand over long time periods. Standard deviation of pressure fluctuations will be compared with experimental data to determine different fluidization regimes at inlet gas velocities ranging from two to nine times the minimum fluidization velocity. We will use Bode plots to present the pressure spectra and reveal characteristic frequencies that describe the bed hydrodynamics for different fluidization regimes. This work will present CFD as a useful tool to perform that analysis. Other important contributions include the study of pressure fluctuations of a fluidized bed in bubbling, slugging, and turbulent regimes, and the analysis of a binary mixture using CFD.

Research on Dispersed Oil Droplets Breakage and Emulsification in the Dynamic Oil and Water Hydrocyclone

Oil and water dynamic hydrocyclone is one type of facilities that separate two phases or multiple phases applied widely in the fields such as food processing, environmental protection, biological pharmacy, petroleum and chemistry. The dispersed oil droplets in the dynamic oil and water hydrocyclone were often broken into small drops by shear force, which decreased the separation efficiency of dynamic oil-water hydrocyclone greatly. To avoid the breakage of the oil droplets, the turbulence field and the velocity field of the dynamic hydrocyclone were studied by the software of Fluent to analyze the main reason that led to breakage of oil droplets. Results indicated that the deformation of oil droplets was caused by the viscous shear force; the breakage of oil droplets was caused by the Reynolds shear stress and the local pressure fluctuations. The main area that the drops were prone to breakup of the dynamic hydrocyclone is that the rotating grating nearby, the wall boundary layer of the drum and center axis of the drum. Finally, the breakage of oil droplets and emulsification of oil and water in the dynamic hydrocyclone were verified by the experiments.

Evaluation of Hydrodynamic Behavior of a Fluidized Bed Dryer by Analysis of Pressure Fluctuation

During drying, hydrodynamics of fluidized beds can vary due to moisture evaporation. It is of practical importance to quantify the fluidized bed hydrodynamics by analysis of pressure fluctuations, especially when drying cohesive materials. In this study, spent liquor from a fermentation process for the production of yeast was mixed with corn bran at different mass ratios, and used as the fluidizing material of various stickiness levels. Local bed pressure fluctuations and the dryer exhaust air temperature as well as relative humidity were monitored over the course of drying. The pressure fluctuation data were analyzed using statistical methods. The bed hydrodynamic behavior was interpreted in terms of the time-averaged bed-pressure drop and its standard deviation. The transition from channeling to uniform fluidization occurs when both the average pressure drop and the standard deviation vary little with time. Also, the average values of the bed-pressure drop within short periods in reference to the ideal bed-pressure drop (the weight of the bed per unit cross-sectional area) is considered a potentially useful criterion to evaluate fluidization quality. Parametric studies show that fluidized bed behavior during the drying process can be improved by adjusting the operating conditions.

Modeling of Wind Pressure Spectra on Spherical Domes

Characteristics of wind pressure spectra on spherical domes have been investigated based on a series of wind tunnel experiments and a synthetic spectral model is derived for load estimation of cladding. First, wind pressure fluctuations on twelve dome models with various geometric parameters (rise/span and height/span ratios) are measured in a turbulent boundary layer. A discussion is detailedly made regarding the spectral distributions and patterns under different conditions. It is found though the pressure spectra vary with different tap locations and geometric parameters, similar spectral shapes and amplitudes can be observed. Then the statistics of pressure fluctuations are carefully checked and three typical spectra have been derived to represent the evolution of spectral pattern transforming from the windward to leeward region. Finally, a model of pressure spectra is proposed and utilized for generating local pressure fluctuations on spherical domes, which can be applied for the extreme and fatigue design of the structural roofs.

0639 自由せん断乱流中の局所圧力変動の計測(OS6-8 噴流,後流およびはく離流れ現象の探求と先端的応用,OS6 噴流,後流およびはく離流れ現象の探求と先端的応用)

0639 自由せん断乱流中の局所圧力変動の計測(OS6-8 噴流,後流およびはく離流れ現象の探求と先端的応用,OS6 噴流,後流およびはく離流れ現象の探求と先端的応用)

Frequency-based characterization of liquid–solid fluidized bed hydrodynamics using the analysis of vibration signature and pressure fluctuations

This work is mainly focused on the frequency-based analyses of non-invasive vibration and pressure fluctuations signals to characterize main hydrodynamic events inside a liquid–solid fluidized bed. The power spectra of bed shell vibration and local pressure fluctuations in a frequency domain were calculated over a wide range of liquid velocities to cover the pathway of bed behavior in between three main hydrodynamic events, namely: minimum fluidization, cluster-circulating-to-individual particle motion (termed as the solid regime transition), and solid entrainment points. First rise in the mean power spectral density function (PSDF m ) of vibration fluctuations predicted the minimum fluidization accurately while a sharp level off in pressure signals showed the minimum fluidization velocity. Global maxima in the PSDF m of vibrations as well as second minima in the PSDF m of pressure fluctuations corresponded to the solid regime transition points. Finally, a plateau in the PSDF m of vibration signatures indicated a complete solid entrainment. Moreover, energy-based wavelet transform coupled with Hurst exponent analysis provided a detailed pathway of bed structure evolution, which led to the successful and accurate predictions of minimum fluidization as well as the solid regime transition inside the bed. The frequency-based analysis of non-invasive vibration and pressure fluctuations signals allows the main hydrodynamic parameters inside a liquid–solid fluidized bed (i.e., minimum U mf , solid regime transition velocity U c , and solid entrainment velocity U t ) to be determined. ► Hydrodynamic parameters of a liquid-solid two-phase fluidized bed are determined using non-intrusive vibration and pressure fluctuations. ► Wavelet transform furnishes minimum fluidization and regime transition points. ► The results can be used in the operation of multiphase reactors at severe conditions.

Study of pressure fluctuations in an internal loop airlift bioreactor

AbstractThe origin and coupling of pressure fluctuations in an internal loop airlift bioreactor are investigated. The pressure fluctuations in the reactor are divided into two categories: global pressure fluctuations and local pressure fluctuations. It is found that the coupling between global pressure fluctuations and local pressure fluctuations mainly focuses in the frequency region between 10 and 30 Hz. Local pressure fluctuations in the reactor are strongly affected by pressure waves originating from the air‐supply system, while pressure fluctuations caused by the bubble eruption at the liquid surface have less influence on local pressure fluctuations. Based on the coherence analysis, the pressure signal at a certain position in the reactor is decomposed into three different parts: coherent part, joint incoherent part and exclusive incoherent part. The energy ratios of these different parts are helpful to study the interaction among pressure fluctuations from different sources. Three flow regimes were identified from the evolution of the energy ratio of the joint incoherent part. © 2011 Canadian Society for Chemical Engineering

Measurement and analysis of turbulent liquid metal flow in a high-power spallation neutron source—EURISOL

The European Isotope Separation On- Line (EURISOL) design study completed in 2009 examined means of producing exotic nuclei for fundamental research. One of the critical components identified in the study was a high-power neutron spallation source in which a target material is impacted by a proton beam producing neutrons by a process known as spallation. Due to the high heat power deposition, liquid metal, in this case mercury, is the only viable choice as target material. Complex issues arise from the use of liquid metal. It is characterised by an unusually low Prandtl number and a higher thermal expansivity than conventional fluids. The turbulence structure in LM is thereby affected and still an object of intense research, hampered in part by measurement difficulties. The use of Computational Fluid Dynamics (CFD) allowed a satisfactory design for the neutron source to be found rapidly with little iteration. However it was feared that the development of the boundary layer and associated turbulence would not be correctly represented by the CFD since the codes were developed for standard fluids. Therefore, an experiment and associated measurements were carried out to assess the reliability of the computations and to validate the design of the target. System level measurements such as cavitation noise, target structural vibrations and cover gas pressure were recorded as local pressure fluctuations in the fluid. In this manner, both the macroscopic effect of the turbulence on the target structure, and the liquid metal turbulence itself could be measured and serve as a benchmark for assessing the reliability of the computations. The current work shows how the Shear Stress Transfer (SST) and Large Eddy Simulation (LES) turbulence models were used to simulate the liquid metal flow. LES was proved more accurate than SST in predicting large and small turbulent structures in the liquid. The prediction of the turbulence intensity generated by large eddy structures in the fluid proved to be in agreement with experimental data at frequencies lower than 40 Hz and at speeds up to 6 m/s in the liquid metal.

Numerical Investigation of Nucleating-Agent-Enhanced Heterogeneous Nucleation

Nucleating agents have long been employed in polymeric foaming processes to promote cell nucleation, increase cell density, and improve cell uniformity. This improvement in foam morphology is usually considered to result from the enhanced heterogeneous nucleation caused by the lower free energy barrier for cell nucleation. However, less is known about the underlying mechanisms of nucleating-agent-enhanced nucleation. In the polymer foaming process, pressure is a critical parameter that affects the degree of supersaturation of gas within a polymer−gas solution. In most previous theoretical studies on cell nucleation, a uniform pressure was assumed throughout the solution. Although this assumption may be acceptable when no particles have been added, its validity is questionable when nucleating agents are present. It has been speculated that growing cells that have already been nucleated generate local flow fields that induce tensile stresses around nearby particles, resulting in local pressure fluctuations....

Numerical study of turbulent trailing-edge flows with base cavity effects using URANS

Turbulent flows over lifting surfaces exhibiting trailing-edge vortex shedding often cause adverse and complex phenomena, such as self-induced vibration and noise. In this paper, a numerical study on flow past a blunt-edged two-dimensional NACA 0015 section and the same section with various base cavity shapes and sizes at high Reynolds numbers has been performed using the unsteady Reynolds-averaged Navier–Stokes (URANS) approach with the realisable κ– ε turbulence model. The equations are solved using the control volume method of second-order accuracy in both spatial and time domains. The assessment of the application of URANS for periodic trailing-edge flow has shown that reasonable agreement is achieved for both the time-averaged and fluctuating parameters of interest, although some differences exist in the prediction of the near-wake streamwise velocity fluctuation magnitudes. The predicted Strouhal numbers of flows past the squared-off blunt configuration with varying degrees of bluntness agree well with published experimental measurements. It is found that the intensity of the vortex strengths at the trailing-edge is amplified when the degree of bluntness is increased, leading to an increase in the mean square pressure fluctuations. The numerical prediction shows that the presence of the base cavity at the trailing-edge does not change the inherent Strouhal number of the 2D section examined. However, it does have an apparent effect on the wake structure, local pressure fluctuations and the lift force fluctuations. It is observed that the size of the cavity has more influence on the periodic trailing-edge flow than its shape does.

Laser-pulse interferometry applied to high-pressure fluid flow in micro channels

The miniaturization of hydraulic systems together with ever increasing static and dynamic fluid pressure as is happening in fuel injection systems leads to complex flow effects with very high local and temporal pressure gradients. System optimization for hydraulic efficiency, components durability or spray formation quality needs the understanding of relevant flow properties. Fluid flow simulation models support such understanding, but with the complex nature of flow conditions, they are in need for precise and comprehensive verification and validation data. This work reports on measurement methods and analysis results for local fluid density and pressure measurements under overall stationary, highly turbulent and cavitating flow conditions in planar, optically accessed, model flow experiments. Laser-pulsed interferometry is applied for the measurement of fluid density fields under high spatial (∼3 μm) and temporal (∼5 ns) resolution. Interferometric imaging and image evaluation techniques provide ensemble mean pressure field data, local pressure fluctuation and differential pressure data. This yields information about local flow features such as flow vortex generation frequency, spatial size and shape of vortices and local pressure distribution inside of vortex structures. Features of bubble collapse process and corresponding pressure shock waves have been observed. The analysis method is applied to a forward-facing step and a target flow geometry. Experimental method, evaluation procedures and results are presented in this paper.

Steady and time-periodic blowing/suction perturbations in a turbulent channel flow

Direct Numerical Simulations (DNS) of an incompressible turbulent channel flow with given local perturbations at the walls are performed. Steady and time-periodic blowing/suction are applied by means of narrow spanwise slots located at the lower and upper walls. Our previous investigations on time-periodic blowing/suction (Araya et al. (2008) [13,14] and Araya (2008) [15]) have revealed the existence of a characteristic forcing frequency of f ¯ = 0.64 or f + = 0.044 . At this frequency, the largest local increases of the skin friction in the region downstream from the slots, followed by the highest augmentation of the Stanton number, were achieved. In this study, we seek to compare the different effects of constant blowing/suction with time-periodic blowing/suction on the flow and thermal fields. For the cyclical perturbation, the normal disturbing velocity is varied sinusoidally in time at the characteristic frequency of f ¯ = 0.64 . In both cases, the forcing amplitude, A o , is fixed to 0.025. The Reynolds number of the unperturbed case is Re τ = 394 and the molecular Prandtl number is Pr = 0.71 . Isothermal conditions are assumed for the lower wall (hot) and the upper wall (cold), respectively. For the forcing amplitude considered, steady disturbances induce higher modifications on the mean time quantities than time-periodic blowing/suction does. In addition, the effects of steady perturbations were observed to last longer downstream. However, time-periodic blowing/suction disturbances at the characteristic forcing frequency of f + = 0.044 have been demonstrated to be more effective in promoting local pressure fluctuations despite its short-distance influence. Furthermore, the energy spectra analysis of fluctuations indicates that time-periodic perturbations at this forcing frequency would be more efficient to excite the very small scales in a turbulent channel flow than by steady disturbances at the same forcing amplitude.

Insights in hydrodynamics of bubbling fluidized beds at elevated pressure by DEM–CFD approach

A numerical simulation was conducted to study the effect of pressure on bubble dynamics in a gas–solid fluidized bed. The gas flow was modeled using the continuum theory and the solid phase, by the discrete element method (DEM). To validate the simulation results, calculated local pressure fluctuations were compared with corresponding experimental data of 1-mm polyethylene particles. It was shown that the model successfully predicts the hydrodynamic features of the fluidized bed as observed in the experiments. Influence of pressure on bubble rise characteristics such as bubble rise path, bubble stability, average bubbles diameter and bubble velocity through the bed was investigated. The simulation results are in conformity with current hydrodynamic theories and concepts for fluidized beds at high pressures. The results show further that elevated pressure reduces bubble growth, velocity and stability and enhances bubble gyration through the bed, leading to change in bed flow structure.

Identification of regime transitions in an inner-loop airlift reactor using local bubble-induced pressure fluctuation signals

The local bubble-induced pressure fluctuation signals, which were extracted by Zhang's method [15], were used to identify the flow regime in an inner-loop airlift reactor. For this purpose, local energy ratio and chaotic analysis were employed to extract useful information in detecting the flow regime transition. It was found that local energy ratio was able to indicate the two transition points in the downcomer and two transition points in the riser. Correlation dimension and Kolmogorov entropy can also detect two transition points in the downcomer, but only one transition points in the riser. Since the local energy ratio was easy to calculate, it had a great potential for detecting flow regime transition in an internal-loop airlift reactor or other multiphase systems.