Local Pressure Fluctuations Research Articles

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.

Origin of pressure fluctuations in an internal-loop airlift reactor and its application in flow regime detection

The origin of pressure fluctuations in an internal-loop airlift reactor (ILALR) and its application in the flow transition detection are investigated. It is found that pressure fluctuations can be characterized as global pressure fluctuations and local pressure fluctuations by frequency domain analysis and wavelet analysis. The global pressure fluctuations generated by gas compression in the gas plenum and flow fluctuations in the gas-supply system have almost a linear attenuation in the downcomer and almost no attenuation in the riser, especially in heterogeneous flow regime. However, it is found that the pressure wave from bubble eruption at bed surface has little impact on the wall pressure fluctuations. The global pressure fluctuations may be explained by Sasic's model. The local bubble-induced pressure fluctuations generated by bubble passage, coalescence and breakage can be determined by bubble passage frequency bandwidth and lower coherence. After extracting the local bubble-induced pressure fluctuations from the origin wall pressure fluctuations, it is shown that the Hurst exponent of the local pressure fluctuations increases faster in the homogeneous flow regime than in the heterogeneous flow regime, which can be employed to indicate the flow regime transition.

CO oxidation on Pd(100) at technologically relevant pressure conditions: First-principles kinetic Monte Carlo study

The possible significance of oxide formation for the catalytic activity of transition metals in heterogeneous oxidation catalysis has evoked a lively discussion over the recent years. On the more noble transition metals (such as Pd, Pt, or Ag), the low stability of the common bulk oxides primarily suggests subnanometer thin oxide films, so-called surface oxides, as potential candidates that may be stabilized under gas phase conditions representative of technological oxidation catalysis. We address this issue for the Pd(100) model catalyst surface with first-principles kinetic Monte Carlo simulations that assess the stability of the well-characterized $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R27\ifmmode^\circ\else\textdegree\fi{}$ surface oxide during steady-state CO oxidation. Our results show that at ambient pressure conditions, the surface oxide is stabilized at the surface up to $\mathrm{C}\mathrm{O}:{\mathrm{O}}_{2}$ partial pressure ratios just around the catalytically most relevant stoichiometric feeds $({p}_{\mathrm{C}\mathrm{O}}:{p}_{{\mathrm{O}}_{2}}=2:1)$. The precise value sensitively depends on temperature, so that both local pressure and temperature fluctuations may induce a continuous formation and decomposition of oxidic phases during steady-state operation under ambient stoichiometric conditions.

A critical review of the complex pressure fluctuation phenomenon in gas–solids fluidized beds

The complex pressure fluctuation phenomenon in gas–solid fluidized beds is systematically examined in this paper based on a comprehensive review of the literature data. The local pressure fluctuations are composed of multiple sources, including local bubble induced fluctuations, global bed oscillations and propagating pressure waves originating in other locations (e.g. bed surface, distributor and windbox). The interaction and coupling among bubble motion, under-damped oscillations of fluidized particles and bed surface, propagating compressible pressure waves and flow pulsation in gas–solid fluidized beds creates the complexity of local pressure fluctuations, and is likely responsible for the formation of complex but unique flow patterns. A few attempts have been reported in the literature on examining the interaction between bed oscillations, plenum chamber air pulsation and propagating pressure waves in fluidized beds, showing some promises on predicting the local pressure fluctuations. Future work should be focused on predicting local and global pressure fluctuations and the formation of unique surface flow patterns by coupling different contributing mechanisms.

Wavelet filter analysis of local atmospheric pressure effects in the long-period tidal bands

It is well known that local atmospheric pressure variations obviously affect the observation of short-period Earth tides, such as diurnal tides, semi-diurnal tides and ter-diurnal tides, but local atmospheric pressure effects on the long-period Earth tides have not been studied in detail. This is because the local atmospheric pressure is believed not to be sufficient for an effective pressure correction in long-period tidal bands, and there are no efficient methods to investigate local atmospheric effects in these bands. The usual tidal analysis software package, such as ETERNA, Baytap-G and VAV, cannot provide detailed pressure admittances for long-period tidal bands. We propose a wavelet method to investigate local atmospheric effects on gravity variations in long-period tidal bands. This method constructs efficient orthogonal filter bank with Daubechies wavelets of high vanishing moments. The main advantage of the wavelet filter bank is that it has excellent low frequency response and efficiently suppresses instrumental drift of superconducting gravimeters (SGs) without using any mathematical model. Applying the wavelet method to the 13-year continuous gravity observations from SG T003 in Brussels, Belgium, we filtered 12 long-period tidal groups into eight narrow frequency bands. Wavelet method demonstrates that local atmospheric pressure fluctuations are highly correlated with the noise of SG measurements in the period band 4–40 days with correlation coefficients higher than 0.95 and local atmospheric pressure variations are the main error source for the determination of the tidal parameters in these bands. We show the significant improvement of long-period tidal parameters provided by wavelet method in term of precision.

Wavelet filter analysis of local atmospheric pressure effects on gravity variations

An efficient method is proposed for the analysis of atmospheric pressure effects on gravity variations. It processes gravity and pressure signals using an orthogonal filter bank derived from high-degree Daubechies wavelets. The method introduces the atmospheric pressure admittance, which is both time- and frequency-dependent, and thus provides more information about when and how the frequency components in the pressure signal influence gravity variations. We demonstrate the efficiency of the wavelet method by applying it to observations from the Wuhan (China) superconducting gravimeter station. The analysis of gravity and pressure signals in 14 sub-bands with different bandwidths covering a frequency range from 0.176 to 720 cpd (cycles per day) reveals that local atmospheric pressure fluctuations start to induce obvious effects on gravity variations in the seismic band 0.52–1.04 mHz (periods 16 to 32 min) and highly correlate with gravity variation in the long-period seismic mode band 0.26–0.52 mHz (periods 32–64 min). The harmonics of solar-heating-induced atmospheric tides play a leading role in interfering with the variation of gravity residuals in the frequency band 0.704–11.25 cpd (periods 128 min to 1.42 day). Local atmospheric pressure effects on gravity variation are very strong in the frequency band 0.176–0.704 cpd (periods 1.42–5.69 day). Accurately filtering quarter-diurnal tides into a narrow band further demonstrates the efficiency of the wavelet method. After removing secular gravity changes and long-period atmospheric pressure variations, we show that there are obvious variations of local pressure admittance on time scales of hours to days. We also reveal seasonal variability of pressure admittances in the band 0.176–0.352 cpd (periods 2.84–5.69 day) after removing the effects of solar-heating atmospheric tides.

Structure of air–water two-phase flow in helically coiled tubes

Air–water two-phase flow in helically coiled tubes is investigated experimentally to elucidate the effects of centrifugal acceleration on the flow regime map and the spatial and the temporal flow structure distribution. Three kinds of test tubes with 20 mm inner diameters including a straight tube are used to compare the turbulent flow structure. Superficial velocities up to 6 m/s are tested so that the centrifugal Froude number covers a range from 0 to 3. The interfacial structure is photographed from two directions by a high-speed video system with synchronized measurement of local pressure fluctuations. The results reveal that the flow transition line alters due to centrifugal force acting on the liquid phase in the tube. In particular, the bubbly flow regime is narrowed significantly. The pressure fluctuation amplitude gets large relatively to the average pressure loss as void fraction increases. The frequency spectra of the pressure fluctuation have plural peaks in the case of strong curvature, implying that the periodicity of slugging two-phase flow is collapsed by an internal secondary flow activated inside the liquid phase. Moreover, under large Froude number conditions, the substantial velocity of the gas phase that biases to the inner side of the helical coil is slower than the total superficial velocity because the liquid flow is allowed to pass through the outer side and so resembles a radial stratified flow.

Bubble size estimation in slurry bubble columns from pressure fluctuations

AbstractThe average large gas bubble size in slurry bubble columns is estimated using a spectral analysis method applied to measured pressure time series. A pressure time series measured in a bubble column consists of local pressure fluctuations and global pressure fluctuations. The local pressure fluctuations arise from the liquid velocity fluctuations induced by the large gas bubbles and by the changes in gas holdup. The standard deviation of these local pressure fluctuations is a measure of the average large bubble size. The coherence between the pressure time series measured at the sparger and at any other location in the column is used to separate the local pressure fluctuations from the global pressure fluctuations. The global pressure fluctuations are measured instantaneously throughout the column and form the coherent part of the pressure time series. The local pressure fluctuations, which are absent at the sparger, form the incoherent part of the pressure time series. In a 2‐D bubble column, a clear correlation is demonstrated between the incoherent standard deviation of the pressure time series and the average large gas bubble size obtained from video imaging. A good agreement is also found between this correlation and the 3‐D model proposed by Krishna et al. for predicting the average large bubble size in 3‐D (slurry) bubble columns. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Unsteadiness of transonic convex-corner flows

An experimental investigation was conducted to study the transonic convex-corner flow in a turbulent boundary layer. The unsteadiness of the interaction was characterized by a local peak pressure fluctuation and the deviation of higher order moments from an effective Gaussian. The peak pressure fluctuations are correlated and compared with results from other studies.

Long-time correlations of sea-level and local atmospheric pressure fluctuations at Trieste

A random walk analysis of pressure and sea-level fluctuations from daily data taken at Trieste during the period 1939–2002 is presented. It is shown that the associated time series display long-time autocorrelations up to time scales of several thousand of days, characterized by a Hurst exponent H ≈ 0.70 for sea-level and H ≈ 0.58 for pressure. The corresponding probability distribution functions depart from the Gaussian shape, displaying asymmetric tails. The series are found to be cross-correlated with a cross-correlation coefficient of about - 0.76 .

Artificial neural network approach for flow regime classification in gas–liquid–fiber flows based on frequency domain analysis of pressure signals

The feasibility of a transportable artificial neural network (ANN)-based technique for the classification of flow regimes in three phase gas/liquid/pulp fiber systems by using pressure signals as input was examined. Experimental data obtained in a vertical, circular column 1.8 m in height and 5.08 cm in diameter, with air/water/Kraft softwood paper pulp, were used. The pulp consistency (weight percent of dry pulp in the pulp–water mixture) was varied in the 0.0–1.5% range. Local pressure fluctuations were recorded at three different stations along the column using three independent but principally similar transducers. An ANN was designed, trained and tested for the classification of the flow regimes using as input some density characteristics of the power spectrum for one of the normalized pressure signals (from Sensor 1), and was shown to predict the flow regimes with good accuracy. A voting scheme was also examined in which the three sensors fed separately trained ANNs, and a correct flow regime would require a vote from at least two of the three ANNs. This scheme improved the agreement between the model predictions and the data. The ANN trained and tested for Sensor 1 predicted the flow regimes reasonably well when applied directly to the normalized pressure power spectrum density characteristics of the other two sensors, indicating a good deal of transportability. For further improvement of transportability, an ANN-based method was also developed, whereby the power spectrum density characteristics of other sensors were adjusted before they were used as input to the ANN that was based on Sensor 1 alone. The method was shown to improve the accuracy of the flow regime predictions. This method requires in practice, that the “replacement” sensor to which regime identification is transported will be, for some while, in simultaneous use with the sensor to be replaced; then the training of the “input-adjusting” ANN would be possible in industry practice. Such a situation is realistic for sensitive processes, where redundant sensors are implemented for fault tolerance.

Flow Regime Identification in Gas/Liquid/Pulp Fiber Slurry Flows Based on Pressure Fluctuations Using Artificial Neural Networks

The objective of this work is to experimentally examine the feasibility of using artificial neural networks (ANNs) for the identification of the flow regimes in three phase gas/liquid/pulp fiber systems, using the statistical characteristics of pressure fluctuations measured by a single pressure sensor. Experiments were carried out in a transparent, vertical circular column with 1.8 m height and 5.08 cm inner diameter, using mixtures of air, water, and Kraft softwood paper pulp. The pulp consistency (weight fraction of pulp in the pulp/water mixture) was varied in the low consistency range of ∼0.0−1.5%. Flow structures and other hydrodynamic characteristics were experimentally investigated, using visual observation, γ-ray densitometry, and flash X-ray photography. Local pressure fluctuations at a station 1.2 m above the test section inlet were recorded using a high-sensitivity pressure transducer that was installed in a manner not to cause flow disturbance. Various statistical characteristics of the pressure fluctuations, as well as the characteristics of their autocorrelations, were examined as potential input parameters for ANNs trained to recognize the flow regimes solely on the basis of these statistical pressure fluctuation characteristics. Three-layer, feed-forward ANNs could learn to identify four major flow patterns (bubbly, plug, churn, and slug) well, using the standard deviation, coefficients of skewness and kurtosis, and several time shift autocorrelations of normalized pressure signals as input. The results indicate the feasibility of using ANNs for flow regime identification in three-phase gas/liquid/pulp flow systems.

Investigation on Transonic Convex-Corner Flows

An experimental investigation was conducted to study the transonic flow past convex corners. The mean surface pressure indicated strong inviscid-viscous interactions, and the interaction region increased with the freestream Mach number and the convex-corner angle. Unsteadiness of the interaction was characterized by an intermittent region and a local peak pressure fluctuation. The peak pressure fluctuations and the tentative boundary between the attached and separated flows could be scaled with the convex-corner angle and a second-order freestream Mach number (M 2 ∞η)

Characterization of electrostatic charges in freely bubbling fluidized beds with dielectric particles

Experiments were conducted in an 89 mm inner diameter Plexiglas fluidized bed with polyethylene resin particles to study the local instantaneous electrostatic charges using a miniature collision ball probe installed inside the column. Local differential pressure fluctuations were also measured to monitor the local fluidization quality. The power spectrum and probability density distributions from both the differential pressure fluctuations and the local instantaneous electric voltage signals were similar, confirming that the amplitude of voltage signals from a ball probe is mainly induced by passing bubbles. The standard deviation of voltages from the ball probe increased with increasing superficial gas velocity, probably due to an increase in the charge density on particles surrounding the rising bubble and increasing bubble rise velocity. The net charge transfer rate, however, showed no clear change with increasing superficial gas velocity because of the balance between the increase in particle velocity and the decrease in dense phase fraction. As the relative humidity increased at a given gas velocity, the standard deviation of the voltage signals and the differential pressure fluctuations both decreased, indicating that the charge buildup in the vicinity of the bubble decreased as the bubble size decreased. The net charge transfer rate, however, increased with increasing relative humidity at RH>50%, likely due to an increase in the charge dissipation rate due to increased electrical conductivity of the fluidizing media. At relative humidities below 50%, the net charge transfer rate was insensitive to the relative humidity because the increase in conductivity was offset by the decrease in charge buildup on particle surfaces. The addition of antistatic powders was effective in reducing the electrostatic charge buildup in the polyethylene resin bed. One hour after the addition of 1 wt% Larostat powders, both the standard deviation of voltage signals and the net charge transfer rate were reduced to insignificant levels.

천이 경계층 유동의 벽면 변동 압력에 관한 실험적 연구

It has been long suspected that the transition region may give rise to local pressure fluctuations and radiated sound that are different from those created by the fully-developed turbulent boundary layer at equivalent Reynolds number. Experimental investigation described in this paper concerns the characteristics of pressure fluctuations at the transition. Flush-mounted microphones and hot wires are used to measure the pressure fluctuations and local flow velocities within the boundary layer in the low noise wind tunnel. From this experiment we could observe the spatial and temporal development process of T-S wave using Wigner-Ville method and find the relations between the characteristic frequency of T-S wave and free stream velocity and the boundary layer thickness based on nondimensional pressure spectra scaled on outer variables.

Wind Loads on Low Building Roofs: A Stochastic Perspective

A comprehensive experimental study of local wind pressure fluctuations on low building roofs has been carried out, by testing a variety of building models exposed to open and suburban boundary-layer flows, for various wind directions, to determine the overall stochastic characteristics of wind loads on roofs. The analysis of pressure data indicates that the pressure fluctuations are sensitive to tap location, roof geometry, and wind direction. Both Gaussian and non-Gaussian fluctuations have been found in all roof types; however, non-Gaussian fluctuations are generally found in flow separation zones. Zones of Gaussian and non-Gaussian pressure fluctuations are presented. Relevant stochastic characteristics of wind pressure fluctuations in time, frequency, and amplitude domains are discussed.

Active removal of wind noise from outdoor microphones using local velocity measurements

Wind noise in outdoor microphone measurements can significantly degrade acoustic data. An outdoor active windscreen based upon four local anemometer velocity measurements applied to an unscreened microphone is demonstrated. A simple relationship, developed from the Bernoulli equation, is used to transform hot-film anemometer wind velocity measurements into microphone wind pressure estimates. Measurements performed outdoors on a horizontally positioned microphone demonstrate that a major part of unscreened microphone wind noise is flow noise caused by interaction between the microphone and adjacent air flow. These measurements also indicate that the dominant source of wind noise in a passively screened microphone are local pressure fluctuations that are not correlated with local velocity fluctuations. This important observation indicates that the active windscreen is unlikely to significantly quiet a microphone that has conventional passive screening and that the active windscreen applied to an unscreened microphone is unlikely to surpass the performance of a passive windscreen. Wind energy reduction performance of the active windscreen is shown to be approximately 12 dB for frequencies below 200 Hz and decreases smoothly to approximately 5 dB at 500 Hz. In addition, the active system transfer functions are observed to possess significant stability of over periods of hours. a)Currently at Texas Instruments. b)Currently at California Maritime Academy.

Monitoring the quality of fluidization using the short‐term predictability of pressure fluctuations

AbstractIn various industrial applications of bubbling fluidized beds, defluidizing parts of the bed or even of the complete bed can occur as a result of agglomeration and sintering of particles due to unintentional maloperation of the bed, changes in operating conditions, or variations in gas or solids feed. Defluidization may be prevented by increasing the gas velocity or changing the solids feed, if the change in the “quality” of the fluidized state of particles is detected early enough. An analysis method is proposed that uses the short‐term predictability of time series of (local) pressure fluctuations in the fluidized bed to detect changes early in the hydrodynamic state of the bed. It is based on the comparison of an original time series of pressure fluctuations with successive time series measured during operation of a fluidized bed. The comparison is based on a discriminating statistic computed for the original time series as well as for each successive series. The null hypothesis that the original and successive time series are similar is rejected if the discriminating statistics of both time series significantly differ. Experimental application of the method is illustrated for fluidization at elevated temperatures (ca. 120°C) of agglomerating plastic particles in a 5‐cm‐ID laboratory fluidized bed. The method recognizes the change in the hydrodynamics due to the incipient agglomeration of the particles. In this particular case the time period between the moment of detection of a significant change in the hydrodynamics and the end of the experiment when the bed becomes defluidized seems sufficiently large to take preventive measures. The average bed‐pressure drop is not a sensitive quantity to detect changes early in the fluidization behavior.

Effect of Modification to Tongue and Impeller Geometry on Unsteady Flow, Pressure Fluctuations, and Noise in a Centrifugal Pump

Particle Image Velocimetry (PIV), pressure, and noise measurements are used to study the effect of modifications to tongue and impeller geometries on the flow structure and resulting noise in a centrifugal pump. It is demonstrated that the primary sources of noise are associated with interactions of the nonuniform outflux from the impeller (jet/wake phenomenon) with the tongue. Consequently, significant reduction of noise is achieved by increasing the gap between the tongue and the impeller up to about 20 percent of the impeller radius. Further increase in the gap affects the performance adversely with minimal impact on the noise level. When the gap is narrow, the primary sources of noise are impingement of the wake on the tip of the tongue, and tongue oscillations when the pressure difference across it is high. At about 20 percent gap, the entire wake and its associated vorticity trains miss the tongue, and the only (quite weak) effect of nonuniform outflux is the impingement of the jet on the tongue. An attempt is also made to reduce the nonuniformity in outflux from the impeller by inserting short vanes between the blades. They cause reduction in the size of the original wakes, but generate an additional jet/wake phenomenon of their own. Both wakes are weak to a level that their impacts on local pressure fluctuations and noise are insignificant. The only remaining major contributor to noise is tongue oscillations. This effect is shown to be dependent on the stiffness of the tongue.

Effect of Void Fraction on Vibrational Behavior of Tubes in Tube Bundle Under Two-Phase Cross Flow

The effects of local two-phase flow parameters on the vibrational behavior of tubes have been studied in an in-line 5 × 20 tube bundle subjected to air-water cross-flow. One of the tubes was flexibly mounted and instrumented for vibration measurement and the others were rigid. Parameters obtained include local void fraction fluctuations, RMS amplitude of void fraction fluctuations, void fraction distributions across the tube bundle, flow regimes based on probability density function of void fraction signals, damping ratio, and tube vibration response as a function of mass flux, void fraction and dynamic pressure. Damping and tube vibration amplitude in two-phase flow have been found to be closely related to the RMS amplitudes of the local void fraction fluctuations and dynamic pressure fluctuations, respectively.