Analysis Of Pressure Fluctuations Research Articles

Pressure fluctuation analysis for identifying the CHF approaching condition during subcooled flow boiling through a mini-channel

This study presents a new method for predicting the Critical Heat Flux (CHF) approaching condition in subcooled flow boiling based on pressure fluctuation analysis. Through experimental investigations, water flow in an upward direction was conducted within a mini-channel with one side heated to analyze the sequential boiling phenomena, including ONB (Onset of Nucleate Boiling), OFI (Onset of Flow Instability), and CHF. While ONB was determined using the slope of the wall temperature–heat flux, OFI was determined by considering the pressure drop through the channel and outlet pressure fluctuation. By analyzing the inlet pressure fluctuations, a new condition called “the CHF approaching condition” was introduced, which serves as a CHF precursor. The CHF approaching condition was found to be approximately 90% of the CHF value across all experimental scenarios. The proposed method provides an effective tool for predicting and preventing CHF, thereby reducing the potential risk of damage to the heating surface.

Spatiotemporal patterns corresponding to phase synchronization and generalized synchronization states of thermoacoustic instability.

Thermoacoustic instability in turbulent combustion systems emerges from the complex interplay among the flame, flow, and acoustic subsystems. While the onset of thermoacoustic instability exhibits a global order, the characteristics of local interactions between subsystems responsible for this order are not well understood. Here, we utilize the framework of synchronization to elucidate the spatiotemporal interactions among heat release rate fluctuations in the flame, velocity fluctuations in the flow, and acoustic pressure fluctuations in a turbulent combustor, across the bluff-body stabilized flame. We examine two forms of thermoacoustic instability, characterized by phase synchronization and generalized synchronization of the acoustic pressure and global heat release rate oscillations. Despite the presence of global synchrony, we uncover a coexistence of frequency synchrony and desynchrony in the local interaction of these oscillations within the reaction field. In regions of frequency-locked oscillations, various phase-locking patterns occur, including phase synchrony and partial phase synchrony. We observe that the local formation of small pockets of phase synchrony and strong amplitude correlation between these oscillations is sufficient to trigger the state of global phase synchronization. As the global dynamics approach generalized synchronization, these local regions of synchrony expand in the reaction field. Additionally, through coupled analysis of acoustic pressure and local flow velocity fluctuations, we infer that the spatial region of flow-acoustic synchrony plays a significant role in governing thermoacoustic instabilities. Our findings imply that, in turbulent combustors, an intrinsic local balance between order, partial order, and disorder within the coupled subsystems sustains the global order during thermoacoustic instability.

Interstage transmission and differential analysis of pressure fluctuations in multistage centrifugal pump-as-turbine

Pumps as turbines (PATs) are used in petroleum and chemical industries to recover high-pressure residual energy. Multistage PATs allow for a wider energy recovery interval and wider range of applications. However, because multistage centrifugal pumps were not originally designed for turbine conditions, complex pressure fluctuations occur, impacting the stable operation and performance of multistage PAT. Pressure fluctuation is essentially a wave, and by analogy with the wave intensity definition, pressure fluctuations were quantified using the pressure wave energy flow density, and the pressure fluctuation patterns at different stages were investigated. The findings reveal significant differences in the intensity of pressure fluctuations at different locations within the multistage PAT. Specifically, the pressure fluctuation intensity is significantly higher from the second to the final stage, compared to the first stage. The difference in inlet flow conditions is the main reason for this difference in pressure fluctuations between stages. The inlet inflow from the second to the final stage is subject to rotational effects that exacerbate the difference in pressure fluctuation intensity between stages. Pressure fluctuations are found to be negatively related to the distance from the source of fluctuations and positively related to the flow state. Different flow conditions and interaction regions of the impeller affect the distribution of pressure fluctuation intensity and the distribution of pressure fluctuation energy across different frequency domains within the guide vanes. The main source of fluctuations in the shaft frequency and four times the shaft frequency is the impeller inlet interaction region, whereas the fluctuations in the blade passing frequency originate from the impeller outlet interaction region. This paper provides a reference for improving the stable operation of multistage PATs.

Identifying the optimal operating regime of production wells based on the analysis of wellhead pressure fluctuations

The article presents the results of field experimental studies of wellhead and bottomhole pressure fluctuations at different operating modes of gas-liquid lift. As a result of the research, it was found that at the optimal operating mode of gas lift well there is minimum amplitude and maximum frequency of wellhead and bottomhole pressure fluctuations, which agrees well with the results of previously, conducted theoretical and experimental laboratory studies. It is shown that the increase in the amplitude and decrease in the frequency of wellhead and bottomhole pressure fluctuations is observed when gas injection deviates from the optimal one, both in the direction of increasing and decreasing gas injection. The possibility of assessing the optimal operating mode of gas-lift and flowing wells on the basis of analyzing the fluctuations of technological parameters during normal operation without special testing operations has been established. A methodology for assessing the efficiency of gas-lift and flowing wells operation mode based on the application of various methods of time series analysis was created. Keywords: gas lift well; gas-liquid flow; optimal rate; wellhead pressure; fluctuation.

Numerical evaluation of the bubble dynamic influence on the characteristics of multiscale cavitating flow in the bluff body wake

Unsteady cavitating flow around the bluff body widely existed in practical engineering fields. This paper aims to investigate the physics involved in the multi-scale cavitating flow in the wake of a wedge-shaped bluff body, with special emphasis on the bubble effects. Numerical simulation is conducted using the traditional Eulerian-Eulerian (E-E) and newly developed Eulerian-Lagrangian (E-L) methods in OpenFOAM. It is found that, compared with the experimental results, the E-L method can predict more accurate cavitation characteristics than the E-E method due to the contribution of discrete bubbles. Bubbles can significantly influence the behaviors of the multi-scale cavitating flow. Specially, analysis of pressure fluctuations indicates that the bubbles induce a stronger power spectral density in the middle- and high-frequency regions. With the bubble effects considered, the vortex structure is stretched and more extensive in the near wake regions. The strength of time-averaged vorticity distribution decreases in the far wake region. Further analysis of the vortex transport equation shows that bubble dynamics significantly increase the intensity of the stretching term by enhancing the spanwise velocity fluctuations. The intensity of the baroclinic torque term predicted by the E-L method is higher than that by the E-E method near the center line due to the influence of bubbles. On the other hand, the analysis of turbulent kinetic energy distribution indicates that bubbles can induce turbulence by increasing cross-stream velocity fluctuations. The increment of the shear Reynold stress, u′xu′y‾, suggests that the bubbles intensify the coupling between the streamwise and cross-stream velocity fluctuations.

Energy distribution and chaotic pressure pulsation analysis of vortex ropes in Francis-99

Francis turbines, essential for stability in diverse operating conditions and variable-speed scenarios, encounter efficiency-compromising vortex rope formations in the draft tube, leading to substantial pressure fluctuations. This research delves into the analysis of energy loss and pressure fluctuations associated with these vortex ropes. Employing the local entropy generation rate (LEGR) method and chaos theory, we scrutinize the behaviour of vortex ropes and their resultant pressure fluctuations. Notably, vortex ropes exhibit maximum LEGR near the runner cone, with secondary vortices escalating instability downstream. In the elbow section, the collision of vortex ropes with the outer elbow amplifies LEGR, primarily driven by fluctuating velocities (approximately 90%). Leveraging the GWO-VMD algorithm, non-stationary signals are decomposed, unveiling a significant 1.6 Hz vortex rope frequency under partial load (PL) conditions and isolating external noise frequencies, such as the prominent 300 Hz. Following decomposition, chaos theory tools, including phase space reconstruction and phase trajectory graphs, unveil the chaotic nature of PL conditions attributed to spiral vortex ropes, resulting in profound pressure fluctuations. This study enhances our understanding of such systems and provides methodologies for improved noise reduction and optimization of turbine performance.

Composite control of airfoil broadband noise based on the combination of porous material and serrated trailing edges

Improving the noise reduction capability of airfoil broadband noise through serrated trailing edge design is a challenging task. To address this, we propose a novel porous-serrated trailing edge design where the gaps between the serrations are filled with porous media. Implicit large eddy simulations were conducted at Mach number Ma=0.1631 and Reynolds number Re=96 000 under a zero incidence angle. In addition to straight trailing edges and conventional serrated trailing edges, cutting-type porous-serrated (CPS) and insert-type porous-serrated (IPS) trailing edges with different porosities were designed. The flow in the porous media is described by Darcy's law, which is related to the pressure and velocity. The results indicate that the CPS trailing edges offer limited noise reduction compared to conventional serrated trailing edges, while IPS trailing edges achieve a significant noise reduction of approximately 5.21 dB. However, the drag force increases by 8.0% in the IPS case with maximum noise reduction. The composite control mainly affects flow structures near the trailing edges, especially inducing the flow penetration across the porous surface. To investigate the noise reduction mechanism, dynamic mode decomposition was conducted to show that both the CPS and IPS designs promote energy transferring significantly from the energetic mode to the modes at other frequencies, which would partly explain the difference in the noise reduction performance to some extent. Furthermore, the analysis of the wall pressure fluctuations reveals that the reduced convection velocity on the porous surface and enhanced destructive interference between the porous and the solid surfaces in IPS cases could be identified as the key factors contributing to lower noise radiation efficiency.

Pressure fluctuations in a fluidized bed of binary particles with significant differences in particle size

Pressure fluctuation signals are investigated in a binary particle fluidized bed in which two types of particles have similar physical properties but significant differences in particle size (90 μm Vs. 1800 μm). The relationship of the measured pressure drop to gas velocity exhibits a distinct ‘‘step’’ profile, indicating that the incipient fluidization of binary particle bed can be characterized by different stages, i.e. the fixed bed, the fine particle fluidization, the coarse particle partial fluidization and the binary particle complete fluidization. And then the statistical method and scale decomposition method are used to quantify pressure fluctuation amplitude and frequency in each stage. The result shows that when binary particles are completely fluidized, the bubble size and frequency in binary particle systems are larger than that in sole fine particle systems. Additionally, binary particle systems need higher gas velocity to achieve turbulent fluidization. Furthermore, the local solids holdup measured with optical fiber probes is also employed to verify the result of pressure fluctuation analysis.

Suppressing the Vortex Rope Oscillation and Pressure Fluctuations by the Air Admission in Propeller Hydro-Turbine Draft Tube

For the purpose of automatic generation control (AGC), a portion of the propeller hydro-turbine units in China is adjusted to operate within a restricted range of 75%-85% load using computer-controlled AGC strategies. In engineering applications, it has been observed that when a propeller hydro-turbine unit operates under off-design conditions, a large-scale vortex rope would occur in the draft tube, leading to significant pressure fluctuations. Injecting air into the draft tube to reduce the amplitude of pressure fluctuations is a common practice, but its effectiveness has not been proven on propeller hydro-turbine units. In this study, a CFD model of a propeller hydro-turbine was established, and 15 cases with different guide vane openings (GVO, between 31° and 45°) under unsteady conditions were calculated and studied. Two air admission measures were introduced to suppress the vortex rope oscillation in the draft tube and to mitigate pressure fluctuations. The reason for the additional energy loss due to air admission was then explained by the entropy production theory, and its value was quantified. This study points out that when injecting air, it is necessary to first consider whether the air will obstruct the flow in the draft tube. Finally, based on simulation and experimental data under various load conditions, pressure fluctuation analysis (based on fast Fourier transform, FFT) was conducted to assess the effectiveness of air admission measures. This study can provide an additional option for balancing unit efficiency and stability when scheduling units using an AGC strategy.

Cavitation cloud of waterjet under double excitation

This study experimentally explores the interplay of active and passive excitation on double-excited cavitating waterjet clouds. High-speed imaging and high-frequency pressure sensors are used to characterize the impact of piezoelectric transducers for active excitation and nozzle lip geometries for strong, moderate, and weak passive excitation conditions. The analysis of pressure fluctuations revealed that under active excitation, the waterjet exhibited forced oscillations characterized by an amplitude amplification exceeding that of single passive excitation by an order of magnitude. High-speed imaging, combined with proper orthogonal decomposition, allowed us to observe an expansion in the volume, size, and effective standoff distance of cavitation clouds upon introducing active excitation across all passive excitation scenarios. The synergy between strong passive excitation and harmonized frequency with active excitation resulted in the most robust cavitation cloud development, characterized by the highest intensity.

Starting of a scramjet air intake

The starting of a supersonic air-intake at the design Mach number depends on the internal blockage due to flame-holder struts in the combustor. In the present work, the starting of a two-dimensional supersonic air-intake for scramjet operation was verified experimentally on an isolated full-scale truncated model. The first ramp of the supersonic air-intake was truncated and simulated from downstream of the second ramp where the local Mach number was 3.7. The flight mission dictated the opening time of the cowl plate to be fixed as 200 mS. The maximum blockage of struts which allows the intake to start was determined by varying the blockage ratio in the range of 20% to 30%. The results indicate that the intake starts when the blockage ratio was 20%, but unstarts for 25% and 30%. In the unstart condition, discrete frequencies of 45 Hz and 60 Hz occurred and pressure fluctuations in the intake showed limit cycle oscillations. A detailed analysis of unsteady pressure fluctuations confirms that the buzz could be modeled as a Van Der Pol oscillator with nonlinear damping.

Analysis of Pressure Fluctuation of Pumped Storage Units

In pump turbines, the observation in the engineering field shows that pressure fluctuation has become one of the most serious hydraulic instability phenomena. The pressure pulsation will cause vibration and fatigue failure of the unit, even vibration of the plant, which greatly affects the safe operation of the power station. In this paper, the research on pressure fluctuation by domestic and foreign researchers is introduced and discussed. It includes the characteristics of pressure pulsation in various regions of pump turbines under various operating conditions, existing research methods, and the possibility of improving pressure pulsation problems with variable-speed pumped storage units, which will serve as the basis for further research and improvement of pressure pulsation.

LES analysis clarifying the response of static pressure fluctuation characteristics in isotropic/anisotropic steady turbulence field to reducing isotropic small-scale spatial resolution

This study aims to clarify the effects of an isotropic reduction in spatial resolution on the characteristics of the static pressure fluctuation in isotropic/anisotropic statistically steady state turbulence. Although previous direct numerical analyses have investigated the static pressure fluctuation, this study considers it as a quantity that could be more straightforwardly obtained from experimental measurements and numerical analyses. To address this issue, a large-eddy simulation is used in this study. The Smagorinsky model, the Vreman model and the implicit LES model are used in this study to model sub-grid scale fluctuations. The influence of isotropic spatial resolution reduction is set by changing the accuracy order for the discretisation of the viscous terms from second to eighth order, based on a three-dimensional cubic computational domain with spatially uniform computational grid width. The accuracy order for the convection terms is also set to second and fourth order in this study. When the discretisation accuracy order of the viscous terms is second order, there is a significant reduction in the accuracy of the implicit LES. The discretisation accuracy order of the viscous terms should be set to fourth order or higher, as seen in this study. In addition, unlike the turbulent kinetic energy, different discretisation accuracy orders for the viscous and convective terms also have a non-negligible effect on the static pressure fluctuation analysis.

An active mixing enhancement technique in supersonic ejectors using pulsed streamwise injections

The current study numerically investigates an active mixing enhancement technique for a supersonic ejector with a constant area mixing chamber operating under the critical regime. Streamwise control jet injections are alternatively pulsed at the top and bottom sides of the mixing chamber entrance. The induced oscillation of the primary jet enhances the bulk mixing between the primary and secondary streams. The secondary stream penetrates the primary core flow much upstream with the control strategy compared to the no-injection case, improving the onset of mixing by 65.36%. A higher spread of the primary jet along the mixing chamber height is observed with the control strategy indicating an enhanced mixing between the two streams. Dynamic mode decomposition analysis of the fluctuations of the velocity magnitude revealed that the dominant dynamic structures are determined by the pulsation frequency and a dominant flapping mode can be observed. The frequency spectrum of the primary jet oscillation revealed that the dominant frequency of oscillation is dictated by the pulsation frequency of the injection. With an increase in the control jet pulsation frequency, the amplitude of primary jet oscillation reduces near the entrance region of the mixing chamber, whereas the amplitude of oscillation far downstream reaches nearly the same value for all the cases. The power spectral analysis of the unsteady pressure fluctuations along the mixing chamber wall revealed that the wall pressure oscillations are contributed by the control jet pulsation frequency as well as the shock wave reflections produced by the supersonic jet–jet interaction within the mixing chamber.

Pressure fluctuations in a gas-solid fluidized bed at temperatures up to 1650 °C

Fluidized beds operating at ultra-high temperatures have great application potential for synthesizing essential materials and chemicals, but they are relatively unexplored. This study, for the first time, investigates fluidization characteristics at temperatures up to 1650 °C by measurement and analysis of pressure fluctuations in a laboratory fluidized bed of 30 mm diameter with corundum particles of an average size of 900 μm. Standard statistical and spectral methods are used to analyze pressure fluctuation signals and characterize fluidization behavior based on the effect of temperature on pressure fluctuation parameters, including the probability density function, standard deviation, autocorrelation, power spectra density, amplitude, dominant frequency, and average cycle frequency. The results indicate that, for the coarse corundum particles used in this study, fluidization behavior inverts at a crucial temperature of approximately 1400 °C. Below this temperature, gas–solid flows in the bed are predominantly controlled by gas bubble movements. In this temperature range, periodicity and maximum amplitude decrease and dominant frequency increases with temperature. Above 1400 °C, interparticle forces, such as van der Waals and viscous forces, are significantly strengthened due to changes in the structural and physiochemical properties of solid materials. The greatly enhanced interparticle forces at ultra-high temperatures promote the dominance of microstructured flows over the overall pressure fluctuations, making interparticle forces the controlling factor influencing gas–solid flows in fluidized beds at ultra-high temperatures. These findings provide a better understanding of ultra-high temperature fluidized beds and can promote their research and development for industrial applications.

Experimental assessments of pyrolytic and fluid-dynamic interactions between pretreated residual biomasses and fluidized beds made up of oxygen carriers for chemical looping gasification

Oxygen carriers (OCs) and wheat straw pellets (raw or pretreated by combinations of torrefaction, CaCO3-addition, water-washing) were experimentally investigated for Chemical Looping Gasification (CLG) in the Horizon2020 project CLARA (Chemical Looping gAsification foR sustAinable production of biofuels, grant agreement n.817841). Laboratory-scale experiments included: (i) pellet devolatilizations (700 °C-900 °C) in bubbling fluidized beds made of OCs or sand; (ii) study of pressure fluctuations within OC fluidized beds (700–1000 °C) containing 10 vol% of ashes from investigated pellets, to assess OC/ash interactions and fluidization quality. Those two campaigns – performed at relevant process conditions for CLG – allowed straightforwardly screening and selecting the most eligible biomasses and OCs for future CLG demonstrations in fluidized beds at higher scales. Gas yield, H2/CO molar ratio, and carbon conversion increased as the devolatilization temperature increased from 700 °C to 900°C. Untreated and pretreated wheat straw pellets showed different pyrolytic behaviors. Analyses of pressure fluctuations and characterizations of spent beds (scanning electron microscopy, particle size distribution) suggested that biomass pretreatments limit agglomeration phenomena, depending on OCs nature (e.g., mechanical strength, composition). Ilmenite was the most mechanically and chemically stable OC. Among investigated materials, pretreated wheat straw pellets and ilmenite emerged as the most promising for future CLG demonstrations.

Characteristics of self-excited shock wave oscillations in a supersonic isolator with exit duct configurations

The structure and dynamics of multiple shock waves inside a constant area isolator of a model scramjet engine are studied numerically using a finite volume solver. The flow is accelerated through a nozzle with an exit Mach number of 1.75 and continues in the constant area isolator where multiple shock waves are formed. To analyze the effect of duct acoustics on the dynamics of shock train oscillations, three ducts with a duct length of 0D, 50D, and 150D are attached to the exit of the isolator, where D is the isolator exit height. The wall static pressure variation along the bottom wall and four specific locations are monitored. The wall shear stress variation along the wall provides the location of separation and reattachment points in the flow field. The statistical analysis provided more insights into the oscillation mechanisms. The lower frequency components from the duct resonance are enhanced as the duct length is increased and it altered the dominant frequency of shock oscillation. The dependence of shock train length on the exit duct confirmation is analyzed by the pressure ratio method. The different frequency components in the flow field are analyzed using the power spectral density of wall static pressure fluctuations and are distinguished based on the frequency range. The cross-spectral analysis of the wall pressure fluctuations provided the correlation between pressure signals and their narrowband time delay. Thus, the origin and directions of the perturbations are obtained and discussed. The increase in exit duct length decreases the downstream disturbance velocity, indicating the duct resonance effect on shock train oscillations. • Simulation of the supersonic isolator flow field to understand the duct resonance effect on self-excited oscillations of shock train. • The duct resonance is varied by attaching ducts of different lengths to the exit of the isolator. • As the duct length is increased, the amplitude of shock oscillation is increased and the dominant frequency of shock oscillation is decreased. • The length of the shock train is increased with an increase in duct length. • The cross-spectral analysis of the wall pressure fluctuations provided the correlation between pressure signals and their narrowband time delay.

Wavenumber spectrum determination for aeroacoustic applications using FISTA

This conference paper deals with computational methods to determine the wavenumber spectrum of acoustic data measured by a phased microphone array. Such problems occur e.g. within the analysis of pressure fluctuations due to a turbulent boundary layer on a surface such as a wind-tunnel wall or the skin of an aircraft. The problem is closely related to the deconvolution of dirty beamforming maps in wavenumber domain, which seeks to determine the wavenumber spectrum by removing the influence of the shift-invariant point spread function from the beamforming result. Firstly, we recall how this task can be formulated as a minimization problem and then discuss a specific solver for this problem, provided by the framework of the generalized FISTA algorithm. The resulting method takes advantage of the convolutional structure of the gradient of the data misfit functional and allows further for a flexible regularization with L1 and L2 penalties as well as a nonnegativity constraint. Finally, the presented algorithmic framework is demonstrated with numerical examples.

Effects of Aeroelastic Walls on the Aeroacoustics in Transonic Cavity Flow

The effects of elastic cavity walls on noise generation at transonic speed are investigated for the generic M219 cavity. The flow is simulated with the Spalart–Allmaras (SA) improved delayed detached-eddy simulation (IDDES) turbulence model in combination with a wall function. The structural analysis software uses a modal formulation. The first 50 structural normal mode shapes are included in the simulation, spanning frequencies of 468–2280 Hz. Results are compared with those from a reference simulation with rigid cavity walls. A spectral analysis of pressure fluctuations from a microphone array above the cavity evinces a distinct tone at 816 Hz, which is absent in the reference simulation. Furthermore, the power of the 4th Rossiter mode at 852 Hz is depleted, implying a significant energy transfer from the fluid to the structure. Spectral proper orthogonal decomposition (SPOD) is employed for analyses of cavity wall pressure fluctuations and wall displacements. The SPOD mode energy spectra show results consistent with the spectra of the microphone array with respect to the tone at 816 Hz and the depletion of the energy at the 4th Rossiter mode. Furthermore, the SPOD mode energy spectra show energy spikes at additional frequencies, which coincide with structural eigenfrequencies.

Bubble Behaviors in a Dense Gas‐Solids Fluidized Bed: Analysis of Pressure Fluctuation

AbstractThe pressure signals were analyzed by time‐domain and frequency‐domain methods, including standard deviation, autocorrelation function, and power spectral density function, to investigate the bubble behavior and determine the regime transition. Results indicated that the autocorrelation function of pressure signals derived from bubbles was periodic fluctuation. The periodic nature of the autocorrelation function disappeared and the dominant frequency increased when the transition from bubbling to the turbulent regime occurred. The decrease of static bed height facilitated the explosion of bubbles, which accelerated the transition from the bubbling to the turbulent regime. The fluidization index was used as an indicator for the bubble state in the dense fluidized bed.