Periodic Pressure Oscillations Research Articles

Patterns of direct contact condensation bubbles based on dynamic behaviors and oscillations

The occurrence of periodic pressure oscillations caused by direct contact condensation (DCC) bubbles holds great significance in engineering. To investigate and control these oscillations associated with different bubble behaviors through experimental means, a steam flow with a mass flux ranging from 0.06–0.9 kg/h is introduced into a water bath at temperatures ranging from 40–85 °C through nozzles with diameters of 3, 4, and 5 mm. In this study, apart from the internal chugging (IC) behavior, both pressure oscillation and bubble behaviors are simultaneously studied and categorized into three distinct patterns: Bubble detachment type I (BD I), bubble detachment type II (BD II), and condensation oscillation. By considering bubble dynamics and employing dimensional analysis, we establish the dimensionless criteria RenPrJa for differentiating BD I from IC, as well as the critical Reynolds number Ren for distinguishing BD I from BD II. Subsequently, a dimensionless regime diagram is constructed to predict and evaluate the bubble behavior patterns with respect to oscillation under various test conditions. This regime diagram serves as a valuable tool for predicting and assessing the bubble behavior patterns associated with pressure oscillation in different operating scenarios.

Potential infrasonic tremors in coal seam systems: Implications for the prediction of coal and gas outbursts

• Infrasonic tremors can emerge spontaneously if gas temporarily accumulates in the gas pocket. • The dynamics of gas pockets in coal seams were investigated both experimentally and theoretically. • A strong coupling relationship between infrasound and pressure oscillations was revealed. • We associated the different physical processes with different expressions of the tremor properties. One of the major goals of mining science is to develop models that connect monitoring signals with the dynamics accompanying hazards. Whether the triggering process of an outburst produces observable signatures is a fundamental question related to the potential for outburst early warning realization and probabilistic forecasting. Inspired by a recent discovery that certain volcanoes often generate infrasound waves before eruption, we hypothesize that a similar infrasound-generating mechanism occurs in select outburst-triggering processes. In the laboratory, we designed an experimental program to verify our hypothesis, which considered the presence of a gas pocket at the trigger stage. The results indicated that periodic infrasonic tremors spontaneously emerge if gas temporarily accumulates in the gas pocket before escaping from the simulated coal seam. Moreover, the tremor frequency systematically decreases with increasing pocket thickness. Therefore, we proposed a viable model to explain the origin of these periodic tremors. In particular, our model demonstrated that the pressure in the gas pocket is governed by the equation of a linear oscillator. Hence, under proper conditions, infrasonic tremors emerge due to periodic pressure oscillations in the gas pocket. More importantly, our model predicted that different outburst-triggering processes (e.g., fracture development and excavation) can leave distinct footprints in the tremor properties. If this model holds true for actual coal seams, it could be possible to predict outbursts. Our tremor model also opens new perspectives for dynamics monitoring within coal seams.

Response of Internal Flowfield to Downstream Pressure Oscillation in a Scramjet Engine

ABSTRACT The response of the internal flowfield to back pressure oscillation in a scramjet engine is studied numerically. The physical domain of interest includes both the isolator and combustor. The theoretical formulation is based on the full conservation equations in three dimensions. Turbulence closure is achieved using the SST model to improve the prediction of shear stress under adverse pressure gradient. As part of the downstream boundary condition, a high pressure is imposed to simulate the pressure condition in the situation with combustion. Periodic pressure oscillations in the frequency range of 376–1128 Hz are also imposed to mimic flow oscillations caused by combustion instability. The downstream disturbance is found to influence the supersonic core flow in the isolator through separated boundary layers. A shock train is initiated from corner separation shocks, and X-type shocks take place to adjust the pressure between the core flow and the separation bubble on the wall. When the frequency of back pressure oscillation is lower, the shock front fluctuates over a larger displacement. The shock front acceleration is correlated to the rate of change of pressure in the separation bubble on the wall, which increases with increasing back pressure frequency. However, when the oscillation increases to 1128 Hz, the rate of change of pressure in the separation bubble decreases, resulting in a more stable shock front. The shock train acts as a low-pass filter and damps flow oscillation in the isolator-combustor.

Seeds of phase transition to thermoacoustic instability

Tackling the problem of emissions is at the forefront of scientific research today. While industrial engines designed to operate in stable regimes produce emissions, attempts to operate them at ‘greener’ conditions often fail due to a dangerous phenomenon known as thermoacoustic instability. Hazardous high amplitude periodic oscillations during thermoacoustic instability lead to the failure of these engines in power plants, aircraft, and rockets. To prevent this catastrophe in the first place, identifying the onset of thermoacoustic instability is required. However, detecting the onset is a major obstacle preventing further progress due to spatiotemporal variability in the reacting field. Here, we show how to overcome this obstacle by discovering a critical condition in certain zones of the combustor, which indicates the onset of thermoacoustic instability. In particular, we reveal the critical value of the local heat release rate that allows us to distinguish stable operating regimes from hazardous operations. We refer to these zones as seeds of the phase transition because they show the earliest manifestation of the impending instability. The increase in correlations in the heat release rate between these zones indicates the transition from a chaotic state to a periodic state. Remarkably, we found that observations at the seeds of the phase transition enable us to predict when the onset occurs, well before the emergence of dangerous large-amplitude periodic acoustic pressure oscillations. Our results contribute to the operation of combustors in more environment-friendly conditions. The presented approach is applicable to other systems exhibiting such phase transitions.

Dynamics of bubbles under stochastic pressure forcing.

Several studies have investigated the dynamics of a single spherical bubble at rest under a nonstationary pressure forcing. However, attention has almost always been focused on periodic pressure oscillations, neglecting the case of stochastic forcing. This fact is quite surprising, as random pressure fluctuations are widespread in many applications involving bubbles (e.g., hydrodynamic cavitation in turbulent flows or bubble dynamics in acoustic cavitation), and noise, in general, is known to induce a variety of counterintuitive phenomena in nonlinear dynamical systems such as bubble oscillators. To shed light on this unexplored topic, here we study bubble dynamics as described by the Keller-Miksis equation, under a pressure forcing described by a Gaussian colored noise modeled as an Ornstein-Uhlenbeck process. Results indicate that, depending on noise intensity, bubbles display two peculiar behaviors: when intensity is low, the fluctuating pressure forcing mainly excites the free oscillations of the bubble, and the bubble's radius undergoes small amplitude oscillations with a rather regular periodicity. Differently, high noise intensity induces chaotic bubble dynamics, whereby nonlinear effects are exacerbated and the bubble behaves as an amplifier of the external random forcing.

Hurst Exponents for Intra- and Intercycle Thermoacoustic Oscillations

The detrended fluctuation analysis, a technique for detecting long-range correlations in fractal time series, is used to characterize thermoacoustic oscillations of impending combustion instabilities. The slope of the linear regime of the structure function with respect to the time scale in a logarithmic plot, referred to as the Hurst exponent, takes a different value across a critical time scale corresponding to one pressure cycle. The slope for pressure oscillations over one pressure cycle, termed as the intercycle Hurst exponent, is a power-law scaling parameter for long-range oscillations; however, the slope for pressure oscillations within one pressure cycle, termed as the “intracycle Hurst exponent,” is no longer a power-law scaling parameter but resultant from the nonstationary, local average of highly nonlinear (as a function of time), seemingly sinusoidal, nearly periodic pressure oscillations. Both parametric studies and analyses of experimental data show that the intracycle Hurst exponent consistently increases with the signal-to-noise ratio and peaks at 1.6, thus having potentials for assessments of the safety margin to the onset of combustion instabilities. Given the same observation window, the measurement error (or uncertainty) of the intracycle Hurst exponent is significantly lower than that of the intercycle Hurst exponent.

Effect of ignition position and inert gas on hydrogen/air explosions

The effects of inert gas (i.e., He, Ar, and N2) and ignition position on flame dynamics in a half-open duct with an aspect-ratio of 10 are analyzed for hydrogen/air mixtures with constant laminar burning velocity SL. The results indicate that hydrodynamic and thermo-diffusive instabilities dominate flame propagations with ignition at the right-half part of the duct, while Rayleigh–Taylor instability dominates with ignition at the left-half part of the duct. The flame-sound interaction results in the periodic pressure oscillations. Due to decreased instability, the He-diluted flame exhibits a weaker sensitivity of explosion parameters to the ignition position. The maximum pressure Pmax is dominated by different mechanisms depending on the ignition position. Although constant SL is used, Pmax for the worst case with N2 dilution is two times that with He dilution, demonstrating the considerable effect of flame instabilities. Finally, a chemical kinetic calculation is performed to clarify the flame stabilities.

Experimental and numerical study on pressure oscillation in a combustor with rotary valve

A rotary valve coupled in a combustor assembly can generate periodic pressure oscillations inside the combustor and can be used to study the combustion instability in the combustion chamber of a rocket engine. This paper proposes a cold gas flow experimental system based on a rotary valve and a corresponding rotation model. A 3D numerical model is proposed to obtain the transient flow inside the rotary valve, and the dynamic mesh technique and User-Defined Functions (UDFs) are adopted to implement a swing motion instead of a rotary motion. Several cold gas flow experiments are carried out at rotating speeds of 75 225, and 375 rpm to verify the validity of the numerical model. The effects of rotating speed, stroke length ratio and radius of the RED (Rotor Exhaust Duct) on the pressure oscillations are studied using this numerical model. The results show that the maximum and peak-to-peak values of the pressure oscillations gradually decrease with increasing rotating speed. The ratio of the corresponding peak-to-peak value to the maximum pressure (pressure amplitude ratio) is reduced from 1.81 to 0.6%. The stroke length ratio affects the pressure waveform because it leads to a change in the time spent in the non-exhaust stage. When the stroke length ratio is 0, the waveform closely resembles a sine wave. With the increase in the stroke length ratio, the pressure waveform exhibits a more square or a triangular wave shape. Finally, a high-frequency and high-amplitude pressure oscillation can be obtained by appropriately increasing the radius of the RED.

Acoustic Modeling and Vibration Characteristics of Supersonic Inlet Buzz

The buzz phenomenon of a typical supersonic inlet is analyzed on the basis of numerical simulations and duct acoustic theory. Considering that the choked inlet could be treated as a duct with one end closed, a one-dimensional (1D) mathematical model based on the duct acoustic theory is proposed to describe the periodic pressure oscillation of the little buzz and the big buzz. The results of the acoustic model agree well with that of the numerical simulations and the experimental data. It could verify that the dominated oscillation patterns of the little buzz and the big buzz are closely related to the first and second resonant mode of the standing wave, respectively. The discrepancies between the numerical simulation and the ideal acoustic model might be attributed to the viscous damping in the fluid oscillation system. In order to explore the damping, a small perturbation jet is introduced to trigger the resonance of the buzz system and the nonlinear amplification effect of resonance might be helpful to estimate the damping. Through the comparison between the linear acoustic model and the nonlinear simulation, the calculated pressure oscillation damping of the little buzz and the big buzz are 0.33 and 0.16, which could be regarded as an estimation of real damping.

Nonlinear dynamics of self-organising bubble departures from twin nozzles

The nonlinear dynamics of self-organising bubble departures from twin nozzles in engine oils was analysed. Air bubbles were generated from twin brass nozzles with an inner diameter equal to 1 mm. The flow of bubbles in bubble chains was recorded using high-speed camera. The time series of air pressure oscillations and signal from laser–phototransistor identifying the presence of bubbles over the nozzles outlet were recorded simultaneously. The self-organising bubble departures were observed and their stability was analysed. It was found that self-organising bubble departures become unstable because of successive (during subsequent bubble departures) decrease of the mean air pressure in one of the nozzle air supply system. It was shown that instability of self-organising bubble departures leads to equalization of pressures in both nozzles air supply systems which causes that simultaneous bubble departures appear. In the present experiment, this process was repeated in a cyclic and chaotic way. It was shown that stable self-organising bubble departures are accompanied by periodic air pressure oscillations in one of the nozzles and chaotic air pressure oscillations in the second one.

Origin of Shallow Volcanic Tremor: The Dynamics of Gas Pockets Trapped Beneath Thin Permeable Media

AbstractLinking volcano‐seismic signals with subsurface processes is crucial to improve the forecasting of volcanic eruptions. One of the most enigmatic signals is shallow volcanic tremor, a highly periodic ground vibration that is typically sourced beneath the crater of active volcanoes, is long lasting (from minutes to years), appears during unrest periods, and frequently precedes eruptions. In this paper, we demonstrate that shallow tremor can be produced by periodic pressure oscillations emerging spontaneously beneath permeable media (e.g., fractured magma caps). These pressure oscillations are the result of three concurrent processes: (a) the transient porous flow of gases through the permeable cap; (b) the temporary accumulation of these gases beneath the cap, forming a gas pocket; and (c) the random supply of volatiles from deeper levels. For highly permeable media (≥10−12 m2; realistic for shallow volcanic edifices), the pressure in subsurface gas pockets is governed by the equation of a linear oscillator; hence, under proper conditions, periodic pressure oscillations emerge during the transfer of gases from the Earth's interior to the surface. Our model is consistent with geophysical data showing that the tremor source is localized at a fixed region and can explain the main characteristics of ground vibrations, including frequency gliding, variations of seismic amplitude prior to eruptions, and the different types of tremor observed. For thin permeable caps (<100 m), we find that different eruption mechanisms (e.g., magma ascent vs. cap sealing) leave distinct footprints in the tremor properties, thus opening new perspectives to forecast the type and style of impending eruptions.

Lagrangian analysis of intermittent sound sources in the flow-field of a bluff-body stabilized combustor

Experiments were performed in a partially premixed bluff-body stabilized combustor in the regimes of combustion noise, intermittency, and thermoacoustic instability. Simultaneous measurements of unsteady pressure fluctuations and flow-field using time-resolved two-component particle image velocimetry reveal dominant dynamics at 141.9 Hz which is responsible for thermoacoustic instability. In the intermittent regime that presages thermoacoustic instability, there are two distinct frequencies: a low-frequency component at 30.7 Hz dominant in the velocity spectra (hydrodynamic mode) and a higher frequency component at 176.4 Hz dominant in the pressure spectra (acoustic mode). Examining the phase relationship between the two modes in the intermittent regime using a variant of the Dynamic Mode Decomposition (DMD) confirms that the appearance of bursts of periodic pressure oscillations coincide with the time instants when the hydrodynamic and the acoustic modes are phase synchronized. To identify the flow structure dynamics observed only during sound production, we compute ridges in the fields of backward-time finite time Lyapunov exponents. The roll up of shear layers from the dump plane and the leading edge of the bluff body and subsequent impingement on combustor walls are identified as the dominant features of the flow during thermoacoustic instability as well as during the bursting stage of intermittency. We show convincingly that these identified dynamics correspond to the acoustic mode using DMD filtered flow fields comprising only of the acoustic mode.

Thermoacoustic Instabilities in an Annular Rotating Detonation Combustor Under Off-Design Condition

Experiments are performed to investigate the combustion features in a laboratory-scale hydrogen ()/air annular rotating detonation combustor under the off-design condition of low air mass flow rates. The air mass flow rate is ranged from 10 to 115 g/s, and the overall equivalence ratio is ranged from 0.6 to 1.0. The unstable longitudinal pulsed deflagration (ULPD), rotating fast deflagration (RFD), and stable longitudinal pulsed deflagration (SLPD) are found to exist in the combustor according to the analysis of pressure traces obtained at different circumferential and axial positions. The RFD is such a combustion instability coupled with the first tangential acoustic mode, but it occurs very rarely. The propagation speed of the RFD waves is approximately 500 m/s, and the pressure wave amplitudes are as low as 0.2 MPa. The SLPD appears with stronger and periodic pressure oscillations, and it is coupled with the first longitudinal acoustic mode of the combustor. The ULPD found in the present experiment can be regarded as the longitudinal mode even though the main frequency band is wider. The pressure oscillations are very irregular in the ULPD situation, but they are traveling forward and backward along the axial direction of the combustor. The variation of the air mass flow rate has more influence on the oscillation intensity of the deflagration wave than the variation of the equivalence ratio. It is also found that the pressure oscillations in the air and hydrogen plenums are forced by the unstable combustion process in the annular combustor. The unchoked condition of the injectors contributes the system coupling of the combustor and plenum.

Experimental characterizing combustion emissions and thermodynamic properties of a thermoacoustic swirl combustor

Many practical lean-premixed combustion systems involved in land-based power plants, gas turbines and boilers are susceptible to self-excited combustion instability, which is characterized by detrimental periodic pressure oscillations. Little attention has been paid on experimentally characterizing the chemical emissions and thermodynamic properties of a thermoacoustic swirl combustor. In this work, the effects of (1) fuel-air equivalence ratio Φ and fuel flow rate QCH4 on generating such combustion instability and its impact on chemical emissions and thermodynamic properties in a swirling combustor are experimentally studied. For this, a methane-fueled lab-scale swirl combustor is designed and tested. To monitor the thermodynamic properties of the combustor, an acoustic pressure sensor, an infrared thermal imaging camera, K-type thermal couples, and an infrared flue gas analyzer are applied. It is found that the fuel-air ratio plays an important role on generating combustion instability at different frequencies and amplitudes. This is confirmed by conducting autocorrelation and frequency spectrum analyses of the acoustic pressure time trace. Furthermore, the dominant mode swap (mode-switching) between a low frequency ω1 and a high non-harmonic one is experimentally observed. Whether the mode switching from high (harmonic or non-harmonic) to low frequency or low to high frequency is found to depend strongly on QCH4. As the equivalence ratio is changed from lean to rich, i.e. 0.8 ≤ Φ ≤ 1.2, NOx emission is increased from 1 ppm to 37 ppm. However, CO emission is decreased by 2 order of magnitudes from 1000 ppm first and then increased. The minimum CO emission is approximately 3.0 ppm. In addition, O2 concentration is decreased by more than 80% with increased Φ, depending on the methane flow rate. This means that the combustion efficiency characterized by the O2 emission is decreased dramatically from 99.5% to 68% with Φ increased from 0.6 to 1.2. The present work sheds light on the characteristics of chemical emissions and thermodynamic properties, when a thermoacoustic swirl combustor is operated with methane-air equivalence ratio Φ varied from lean to rich condition and different QCH4. It opens up a practical means to design a stably operated but low-emission thermoacoustic swirl combustor.

Operation of a liquid-fueled and valveless pulse detonation rocket engine at high frequency

To increase the operating frequency and enhance the propulsive performance of a pulse detonation rocket engine (PDRE), a valveless mode with the purge gas based on liquid fuel was developed. Liquid gasoline, oxygen-enriched air, and nitrogen were utilized as fuel, oxidizer, and purge gas respectively. Supply of these propellants was controlled by the periodic pressure oscillations in the detonation tube. The purge gas with the highest supply pressure was filled into the detonation tube first. The fuel and oxidizer were injected after the inner pressure decaying to a relatively low level. This delay and the vaporization of the liquid fuel prevented pre-ignition of the fresh detonable mixture. The PDRE can work stably from 10Hz to 160Hz in this scheme. Fully-developed detonation waves were proved to be obtained successfully at a maximum frequency of 130Hz. Both oxygen volume fraction and supply pressure of the purge gas were tested to determine the limits for stable operation. The propulsive performance of a practical PDRE at a wide operating frequency range was also tested and the maximum thrust was obtained at 140Hz. Compared to the results without purge gas, introduction of a purge gas extends the ranges of supply pressure and operating frequency.

Precursors to self-sustained oscillations in aeroacoustic systems

In this study, we attempt to provide a repertoire of measures to forewarn the onset of impending flow-induced mechanical oscillations via online health monitoring. To illustrate the principles, the flow of air through a pipe terminated by a circular orifice plate is investigated at various flow velocities using a suitably placed pressure transducer. It is observed that the regimes corresponding to the production of a tone is presaged by operating conditions that display temporarily intermittent bursts of periodic pressure oscillations that emerge from a background of lower-amplitude aperiodic fluctuations. The various model-free measures prescribed in this paper serve as efficient precursors by characterizing these intermittent states, which can potentially arise in aeroacoustic systems when the flow is highly unsteady or turbulent.

Prediction of sound absorption by a circular orifice termination in a turbulent pipe flow using the Lattice-Boltzmann method

The Lattice Boltzmann method was used to perform numerical simulations of the sound and turbulent flow inside a standing wave tube terminated by a circular orifice in presence of a forced mean flow. The computational domain comprised a standard virtual impedance tube apparatus in which sound waves were produced by periodic pressure oscillations imposed at one end. An orifice plate was located between the driver and the tube termination. All waves transmitted through the orifice were effectively dissipated by a passively non-reflecting (i.e. anechoic) boundary at the tube termination. A turbulent jet was formed at the discharge of the orifice by the forced mean flow inside the tube. The acoustic impedance and sound absorption coefficient of the orifice plate were calculated from a wave decomposition of the sound field upstream of the orifice. Simulations were carried out for different excitation frequencies, and orifice Mach numbers. Results and trends were in good quantitative agreement with available analytical solutions and experimental data. The Lattice Boltzmann method was found to be an efficient numerical scheme for prediction of sound absorption by realistic three dimensional orifice configurations.

Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process

In traditional PDRE (pulse detonation rocket engines), mechanical valves are used for periodic supply control and a purge process is widely used to form a buffer zone to prevent fresh fuel-oxidizer mixture from pre-ignition. However, both of them increase hardware complexity and limit increase of operating frequency. To eliminate mechanical valves and the purge process, a valveless mode without the purge process has been proposed. Multi-cycle detonations are able to create periodic pressure oscillations inside a detonation tube, which are capable to interrupt fuel and oxidizer supply. In the present study, liquid gasoline was used because vaporization of the liquid fuel would cool hot combustion products, which acted as a buffer zone. Therefore, a liquid-fueled and valveless PDRE without the purge process became possible. When oxygen-enriched air with 25% ∼ 45% oxygen by volume was employed, a maximum operating frequency of 110 Hz was achieved. It was observed that supply pressures of fuel and oxidizer were of great importance for such an operating mode. Exhaust plumes at different operating frequencies were also investigated. The results indicated that it was feasible for the valveless PDRE to run steadily without the purge process when liquid gasoline was utilized.

Numerical study of three-dimensional detonation structure transformations in a narrow square tube: from rectangular and diagonal modes into spinning modes

Three-dimensional (3-D) detonation structure transformations from rectangular and diagonal modes into spinning modes in a narrow square tube are investigated by high-resolution simulation. Numerical simulations are performed with a Riemann solver of the HLLC-type, new cell-based structured adaptive mesh refinement data structure, high-order, parallel adaptive mesh refinement reactive flow code. A simplified one-step kinetic reaction model is used to reveal the 3-D detonation structure. The four different types of initial disturbances applied in the ZND profiles lead to the structures of rectangular in phase, rectangular out of phase, rectangular partial out of phase and diagonal, respectively, during the initial stages of detonation propagation. Eventually, all these detonation structures evolve into the self-sustained spinning detonations. The asymmetric disturbance leads to a stable spinning detonation much faster than the rest. The important features in the formation of spinning detonation are revealed using a 3-D visualization, and a remarkable qualitative agreement with experimental and numerical results is obtained with respect to the transverse wave dynamics and detonation front structures. The transverse wave collisions produce the unburnt gas pockets and the energy to sustain the detonation front propagation and distortion. The periodic pressure oscillation of front plays a complex role as it shifts the reaction zone structure with an accompanying change in the driving energy of transition and the detonation parameters which result in the more distorted front and the unstable detonation. Eventually, the unstable distorted detonation evolves into a spinning detonation.

Numerical Research on The Nozzle Damping Effect by A Wave Attenuation Method

Abstract Nozzle damping is one of the most important factors in the suppression of combustion instability in solid rocket motors. For an engineering solid rocket motor that experiences combustion instability at the end of burning, a wave attenuation method is proposed to assess the nozzle damping characteristics numerically. In this method, a periodic pressure oscillation signal which frequency equals to the first acoustic mode is superimposed on a steady flow at the head end of the chamber. When the pressure oscillation is turned off, the decay rate of the pressure can be used to determine the nozzle attenuation constant. The damping characteristics of three other nozzle geometries are numerically studied with this method under the same operating condition. The results show that the convex nozzle provides more damping than the conical nozzle which in turn provides more damping than the concave nozzle. All the three nozzles have better damping effect than that of basic nozzle geometry. At last, the phase difference in the chamber is analyzed, and the numerical pressure distribution satisfies well with theoretical distribution.