Properties Of Pressure Fluctuations Research Articles

Effect of correlation time of combustion noise on early warning indicators of thermoacoustic instability.

In this paper, we analyze the effects of finite correlation time (noise color) of combustion noise on noise-induced coherence and early warning indicators (EWIs) via numerical and experimental studies. We consider the Rijke tube as a prototypical combustion system and model combustion noise as an additive Ornstein-Uhlenbeck process while varying noise intensity and correlation time. We numerically investigate corresponding effects on coherence resonance and multi-fractal properties of pressure fluctuations. Subsequently, we experimentally validate results and elucidate the influence of noise color and intensity on trends in coherence resonance and multi-fractal measures that can be expected in a practical scenario using an electroacoustic simulator. We find that the coherence factor, which quantifies the relative contribution of coherent oscillations in a noisy signal, increases as the system approaches the thermoacoustic instability-irrespective of the correlation time. It works at most levels of combustion noise (except for too low and too high noise levels). The Hurst exponent reduces as the system approaches thermoacoustic instability only when the correlation time is small. These results have implications on the prediction and monitoring of thermoacoustic instability in practical combustors.

Pressure Fluctuation near the Limiting Characteristics in a Sonic Flow around NACA0012 Airfoil

Pressure fluctuation for flow around an airfoil has been well studied for subsonic, transonic and supersonic flows. In this paper, the sonic flow case is studied using a NACA0012 airfoil. It is known that such a flow has a limiting characteristic line which is known to separate the supersonic region into an upstream zone(U-zone) and a downstream zone(D-zone) where the pressure waves propagate into different directions, thus it is interesting to investigate whether the pressure fluctuation also exhibits special behavior along the limiting characteristic line. From an analysis of the pressure fluctuation properties by detached eddy simulation and method of characteristics, it is found that the pressure fluctuation exhibits different behavior in these two zones, and displays interesting properties along the limiting characteristic line. The fluctuation pressure is the largest along the limiting characteristic line, while the correlation coefficient between two adjacent points is the smallest along the limiting characteristic line. Away from the limiting characteristic line, the fluctuation pressure decays. Moreover, there is a spatial variation of the pressure fluctuation across the boundary layer. This spatial variation is in the mid-frequency band in the U-zone, in the high-frequency band in the D-zone, and in the entire-frequency band along the limiting characteristics line.The special behavior of the pressure fluctuation along the limiting characteristic line revealed by this study enriches our knowledge about transonic flow.

Intermittent statistics of the 0-mode pressure fluctuations in the near field of Mach 0.9 circular jets at low and high Reynolds numbers

The present paper reports an investigation of the statistical properties of pressure fluctuations in the near field of subsonic compressible jets. The data-base analyzed has been obtained numerically by DNS and LES of two single-stream circular jets, having diameter-based Reynolds numbers of 3125 and 100,000 and Mach number 0.9, respectively, initially laminar and highly disturbed. Pressure fluctuations are extracted from several virtual probes positioned in the near field of the jets and covering a region from 0 to 20 diameters in the axial direction and from 0.5 to 3 diameters in the radial. An azimuthal decomposition of the pressure fluctuations is performed, and the statistical analysis is applied to the axisymmetric 0-mode component and compared to the results obtained from the full original signals. The intermittent behavior is investigated by the estimation of standard statistical indicators, such as probability distribution functions and flatness factor, as well as through conditional statistics based on the application of the wavelet transform. It is shown that downstream of the potential core, intermittency estimated through the traditional indicators is relevant even at the lowest Re for the full signals, whereas it is apparently not significant for the 0-mode component. The wavelet analysis provides an estimation of intermittency scale-by-scale and allows for the calculation of a frequency-dependent FF. This approach reveals that the 0-mode component has a relevant degree of intermittency around the frequencies associated with the Kelvin–Helmholtz instability. The statistics of the intermittent events, in terms of their temporal appearance and energy content, are shown to be weakly sensitive to the jet Reynolds number and the universal behavior can be reproduced by simple stochastic models.

Elastic Turbulence: An Experimental View on Inertialess Random Flow

A viscous solvent laminar flow may be strongly modified by the addition of a tiny amount of long polymer molecules, resulting in a chaotic flow called elastic turbulence (ET). ET is attributed to polymer stretching, which generates elastic stress and its back reaction on the flow. Its properties are analogous to those observed in hydrodynamic turbulence, although the formal similarity does not imply a similarity in physical mechanisms underlining these two types of random motion. Here we review the statistical and spectral properties and the spatial structure of the velocity field, the statistical and spectral properties of pressure fluctuations, and scaling of the friction factor of ET in wall-bounded and unbounded flow geometries, as observed in experiments and numerical simulations and described by theory for a wide range of control parameters and polymer concentrations.

Combustion modes and driving mechanisms of pressure fluctuation in a novel vortex-tube combustor with quasi-steady and stratified properties

The operating limit and pressure fluctuations of a localized stratified vortex-tube combustor (LSVC) are investigated experimentally by taking methane as fuel at the inlet temperature of 300 K and the combustor pressure of 1 atm over a range of equivalence ratios (0.1–1.0). The combustion modes are distinguished according to the properties of pressure fluctuations and the driving mechanisms of each combustion mode are explored in combination with numerical simulation. The results show that the LSVC can realize stable combustion with pressure fluctuation amplitudes always less than 4 kPa in a large operating range, and the flame front is always continuous. The operating flammability limit experimental range can be divided into five regimes with different combustion modes. The first and fifth combustion modes are steady combustion with pressure fluctuation amplitudes less than 1 kPa, and the second to fourth ones are quasi-steady combustion with pressure fluctuation amplitudes between 1–4 kPa. The spectrum analyses of pressure fluctuations show that there are one low-frequency peak around 300 Hz and one high-frequency peak around 1500 Hz, which are dominated by the first and third axial natural acoustic modes of resonant oscillation combustion, respectively. In the first and fifth combustion modes, the resonances are both weak due to the influences of low flow rate and laminarization respectively. In the second mode, the thermo-acoustic coupling oscillation and the resonance are excited simultaneously, yielding the highest pressure fluctuation amplitude in the entire operating range. The high-frequency resonance causes the high-frequency pressure fluctuation of the third mode. Both the unsteady heat release and flow field affect the pressure fluctuation in the fourth mode. The former produces the low-frequency fluctuation, which can resonate with the natural frequency and excite a weak thermal-acoustic coupling.

기-고 유동층에서 Gaussian 분포 입자군의 표준편차에 따른 유출 특성

기-고 유동층에서 다입자경 입자의 입자분포 변화에 따른 비산유출 특성을 연구하였다. 다입자경 입자분포는 Gaussian 분포를 사용하여 실험을 수행하였다. 최소유동화속도에 대한 유속비와 Gaussian 입자분포의 표준편차에 따른 비산유출상수를 구하였으며, 이때 조업시간에 따른 압력요동의 특성치를 구하였다. 측정된 유동층의 압력요동 특성치로부터 압력요동의 표준편차, 평균압력, Power spectrum density function, 주진동수 등을 계산하였다. 입자분포군에 따라서 유출입자의 입자분포 및 압력요동 특성치는 크게 영향을 받는 것으로 나타났으며 이러한 결과로부터 압력요동 특성치로부터 유출특성의 해석이 가능함을 알 수 있었다. The elutriation characteristics of particle size distribution were investigated in a gas-solid fluidized bed. Experiments were carried out with the mulit-sized particles of Gaussian distributions. The elutriation rate constant obtained from the experiment was correlated with the standard deviation of particle size and the dimensionless group of the velocity ratio. The standard deviation of pressure fluctuation, mean pressure, major frequency and power spectrum density function were calculated by pressure fluctuation properties. Size distribution of elutriated particles and pressure fluctuations were measured for the particle size distribution of particle system depended largrly on the size distribution. Characteristics of fluidization and elutriation were greatly influenced by the particle size distribution and these characteristics could be interpreted with pressure fluctuation properties.

유동층반응기에서 촉매를 이용한 메탄 열분해

본 논문은 유동층반응기에서 메탄 열분해에 의한 수소 생산과 탄소 생성에 대한 연구를 수행하였다. 환경에 대한 영향을 최소화한 상태에서 one-step에 의한 메탄의 전환반응을 메탄 분해촉매활성에 영향을 미치는 인자에 대하여 연구하였다. 측정된 압력요동특성치의 해석을 통하여 유동층 열분해촉매의 유동화현상을 측정하였으며, 유동화특성에 따른 메탄열분해능을 측정하였다. 메탄의 분해능는 생성되는 수소의 농도로부터 측정하였다. 유동층의 특성인 층내 입자 이동성, U-Umf, 마모, 비산유출, 유동화가스의 효율밀도에 따른 분해효율에 미치는 영향을 고찰하였다. In this paper, Thermocatalytic decomposition of methane in a fluidized bed reactor (FBR) was studied. The technical approach is based on a single-step decomposition of methane over carbon catalyst in air/water vapor free environment. The factors affecting methane decompostion catalyst activity in methane decomposition reactions were examined. The fluidization phenomena in a gas-fluidized bed of catalyst was determined by the analysis of pressure fluctuation properties, and the results were confirmed with characteristics of methane decomposition. The effect of parameters on the H2 yield was examined for methane decompostion. The decompstion rate was affected by the fluidization quality such as mobility, U-Umf, carbon attrition, elutriation and effectiveness density of fluidization gas.

Hydrogen production by the thermocatalytic decomposition of methane in a fluidized bed reactor

CO2-free production of hydrogen via thermocatalytic decomposition of methane in a fluidized bed reactor (FBR) was studied. The technical approach is based on a single-step decomposition of methane over carbon catalyst in air/water vapor free environment. The factors affecting Fe catalyst (Iron powder activity in methane decomposition reactions were examined. Carbon species produced in the process were characterized by SEM methods. The fluidization quality in a gas-fluidized bed of Fe (Iron powder) and Fe/Al2O3 catalyst was determined by the analysis of pressure fluctuation properties, and the results were confirmed with characteristics of methane decomposition. The effect of parameters on the H2 yield was examined. Fibrous carbon formed over Fe catalyst surface. The hydrogen yield increased with increasing reactor temperature, and decreased with increasing superficial velocity of methane inlet stream. The conversion rate of methane is maintained by attrition of produced carbon on Fe catalyst surface in a FBR.

Evaluating effects of wind-induced pressure fluctuations on soil-atmosphere gas exchange at a landfill using stochastic modelling

The impact of wind turbulence-induced pressure fluctuations at the soil surface on landfill gas transport and emissions to the atmosphere at an old Danish landfill site was investigated using stochastic modelling combined with soil property and gas transport data measured at the site. The impacts of soil physical properties (including air permeability and volumetric water content) and wind-induced pressure fluctuation properties (amplitude and temporal correlation) on landfill gas emissions to the atmosphere were evaluated. Soil-air permeability and pressure fluctuation amplitude were found to be the most important parameters. Wind-induced gas emissions were further compared with gas emissions caused by diffusion and by long-term pressure variations (due to passing weather systems). Here diffusion and wind-induced gas transport were found to be equally important with wind-induced gas transport becoming the most important at lower soil-air contents.

Prediction of Take-Over Velocity of a Binary Density System Using Pressure Fluctuations in a Fluidized Bed

The pressure fluctuation properties and mixing degree were investigated with a binary mixture of different density particles using polymethyl methacrylate (PMMA) and sand (or natural manganese ore (NMO)) in a gas fluidized bed. This study was restricted to systems of 50% by volume jetsam, i.e. flotsam rich system. Mixing-segregation phenomena were analyzed by pressure fluctuation properties, and the results were confirmed with the mixing degree using a sieve test. It was found that take-over velocities were obtained from the value of the specific gas velocity which was observed by transition of the slope of the standard deviation of pressure fluctuations. The mixing degree was related to the variation of the bed pressure difference by measuring the height of the axial position according to gas velocity. In addition, take-over velocities could likewise be obtained from a pressure difference profile.

Pressure fluctuation properties in combustion of mixture of anthracite and bituminous coal in a fluidized bed

The combustion characteristics of a mixture of anthracite and bituminous coal were studied in a 0.155 m i.d. fluidized bed combustor (FBC). The properties of the pressure fluctuation for the bed such as the standard deviation, cross-correlation function, dominant frequency and the power spectral density function were obtained through statistical analysis. To interpret the combustion characteristics in the FBC with uniform or multi-sized particles of anthracitebituminous coal mixture, the properties of pressure fluctuation were determined as a function of the particle size distribution and anthracite mixing fraction. In the present work, it is known that the combustion region could be obtained from the analysis of pressure fluctuation properties, and the mixed-firing of anthracite-bituminous coal is related to the reaction models of both coals and particle size distribution. Moreover, the relation between coal size distribution and static mean pressure, and the ignition region could be obtained from the mean pressure profile.

Cross-Spectral Characteristics of Wall Pressure Fluctuations in Flows over a Backward-Facing Step

Laboratory measurements were made of wall pressure fluctuations in a separated and reattaching flow over a backward-facing step. An array of 32 microphones along the streamwise direction was utilized. Various statistical properties of pressure fluctuations were scrutinized. The main emphasis was placed on the flow inhomogeneity along the streamwise direction. One point statistics such as the streamwise distribution of rms pressure and autospectra were shown to be generally consistent with other studies. The coherences and wavenumber spectra in the streamwise directions were indicative of the presence of dual modes in pressure; one is the large-scale vortical structure in low frequency and the other is the boundary-layer-like decaying mode in high frequency.

Chiba port tower: Full-scale measurement of wind actions Part 2. Basic properties of fluctuating wind pressures

This paper contains the study concerning the basic properties of pressure fluctuations which has been investigated on Chiba Port Tower starting about one year ago for the purpose of making clear the characteristics of wind pressure on claddings of building. The record of strong wind which we have selected as our observation object this time is taken only at the time of Typhoon whose wind speed at the top of the building is 20 m/s and over. Resultantly, the value of r.m.s. pressure coefficient depends largely on turbulence intensity of wind speed and the r.m.s. pressure coefficient is almost twice of turbulence intensity of wind speed on windward face, and about twice or three times on side face. According to power spectrum analysis, Kárman vortex is recognized to occur periodically from this building, and Strouhal number is about 0.08∼0.11 based on the one side length of the building. Also the peak factor of pressure fluctuations on side and leeward faces, compared with the values on windward face, both mode and dispersion are large. It is cleared that the peak factor of pressure fluctuations on side and leeward faces inclines to be considerably larger than the peak factor of wind speed when the averaged period of peak value is under 0.5 second.

Pressure Fluctuations on the Surface of a Hemisphere Immersed in a Thick Turbulent Boundary Layer

Statistical properties of pressure fluctuations on the surface of a hemisphere immersed in a thick turbulent boundary layer are described. The height of the hemisphere tested was 0.275 thicknesses of the boundary layer. Reynolds number based on the model diameter D and the time-mean approaching flow velocity at the level of the top Ur was 3.0 × 105. Time-mean and root-mean-square (rms) values, probability density and power spectra of the pressure fluctuations are presented and discussed. The pressure fluctuations are related to the fluctuating approaching-flow velocity in terms of the pressure-velocity admittance and the cross correlation. Main results are that the time-mean and rms pressures attained a primary maximum at the front stagnation point; that the pressure-velocity admittance near the front stagnation point was approximately unity at frequencies less than about 0.4 Ur/D; that the pressure fluctuation in front of the hemisphere is positively correlated with that in the rear side and negatively correlated with that in the middle.

Properties of pressure fluctuations in a gas—solids fluidized bed under a free bubbling condition

A dynamic model, which takes into account the internal noise source, has been proposed to explain some properties of pressure fluctuations in a gas—solids fluidized bed. The present model groups internal noises responsible for pressure fluctuations under a free bubbling condition into two categories, namely, fluctuations of the overall mean bubble residence time and fluctuations of the local mean bubble residence time. It has been concluded that pressure fluctuations at the lower part of the bed are mainly affected by fluctuations of the overall mean bubble residence time, whereas pressure fluctuations at the upper part of the bed are mainly caused by fluctuations of the local mean bubble residence time, induced by the bubble coalescence.

Properties of pressure fluctuations in an atmospheric boundary layer

From experimental data it is shown that there is a relationship between measured wind speed and associated micropressure levels. It is also shown that turbulent eddies associated with the wind vanish within distances of five to six times their size, and that these eddies exhibit normal dispersion. These results confirm, at the atmospheric scales, results previously observed only in model experiments.