Behaviour of an air-assisted jet submitted to a transverse high-frequency acoustic field

2009 ◽  
Vol 640 ◽  
pp. 305-342 ◽  
Author(s):  
F. BAILLOT ◽  
J.-B. BLAISOT ◽  
G. BOISDRON ◽  
C. DUMOUCHEL
Keyword(s):  
Heat Release ◽  
Acoustic Field ◽  
Acoustic Waves ◽  
High Frequency ◽  
Gas Flow ◽  
Liquid Core ◽  
Liquid Jet ◽  
Pressure Amplitude ◽  
Instantaneous Rate ◽  
Rate Of Heat Release

Acoustic instabilities with frequencies roughly higher than 1 kHz remain among the most harmful instabilities, able to drastically affect the operation of engines and even leading to the destruction of the combustion chamber. By coupling with resonant transverse modes of the chamber, these pressure fluctuations can lead to a large increase of heat transfer fluctuations, as soon as fluctuations are in phase. To control engine stability, the mechanisms leading to the modulation of the local instantaneous rate of heat release must be understood. The commonly developed global approaches cannot identify the dominant mechanism(s) through which the acoustic oscillation modulates the local instantaneous rate of heat release. Local approaches are being developed based on processes that could be affected by acoustic perturbations. Liquid atomization is one of these processes. In the present paper, the effect of transverse acoustic perturbations on a coaxial air-assisted jet is studied experimentally. Here, five breakup regimes have been identified according to the flow conditions, in the absence of acoustics. The liquid jet is placed either at a pressure anti-node or at a velocity anti-node of an acoustic field. Acoustic levels up to 165 dB are produced. At a pressure anti-node, breakup of the liquid jet is affected by acoustics only if it is assisted by the coaxial gas flow. Effects on the liquid core are mainly due to the unsteady modulation of the annular gas flow induced by the acoustic waves when the mean dynamic pressure of the gas flow is lower than the acoustic pressure amplitude. At a velocity anti-node, local nonlinear radiation pressure effects lead to the flattening of the jet into a liquid sheet. A new criterion, based on an acoustic radiation Bond number, is proposed to predict jet flattening. Once the sheet is formed, it is rapidly atomized by three main phenomena: intrinsic sheet instabilities, Faraday instability and membrane breakup. Globally, this process promotes atomization. The spray is also spatially organized under these conditions: large liquid clusters and droplets with a low ejection velocity can be brought back to the velocity anti-node plane, under the action of the resulting radiation force. These results suggest that in rocket engines, because of the large number of injectors, a spatial redistribution of the spray could occur and lead to inhomogeneous combustion producing high-frequency combustion instabilities.

A kinematic model of a ducted flame

1999 ◽  
Vol 394 ◽  
pp. 51-72 ◽  
Author(s):  
A. P. DOWLING
Keyword(s):  
Heat Release ◽  
Limit Cycle ◽  
Acoustic Waves ◽  
Kinematic Model ◽  
Limit Cycle Oscillation ◽  
Unsteady Combustion ◽  
Instantaneous Rate ◽  
Flame Holder ◽  
Turbulent Flame Speed ◽  
Rate Of Heat Release

A premixed ducted flame, burning in the wake of a bluff-body flame-holder, is considered. For such a flame, interaction between acoustic waves and unsteady combustion can lead to self-excited oscillations. The concept of a time-invariant turbulent flame speed is used to develop a kinematic model of the response of the flame to flow disturbances. Variations in the oncoming flow velocity at the flame-holder drive perturbations in the flame initiation surface and hence in the instantaneous rate of heat release. For linear fluctuations, the transfer function between heat release and velocity can be determined analytically from the model and is in good agreement with experiment across a wide frequency range. For nonlinear fluctuations, the model reproduces the flame surface distortions seen in schlieren films.Coupling this kinematic flame model with an analysis of the acoustic waves generated in the duct by the unsteady combustion enables the time evolution of disturbances to be calculated. Self-excited oscillations occur above a critical fuel–air ratio. The frequency and amplitude of the resulting limit cycles are in satisfactory agreement with experiment. Flow reversal is predicted to occur during part of the limit-cycle oscillation and the flame then moves upstream of the flame-holder, just as in experimental visualizations. The main nonlinearity is identified in the rate of heat release, which essentially ‘saturates’ once the amplitude of the velocity fluctuation exceeds its mean. We show that, for this type of nonlinearity, describing function analysis can be used to give a good estimate of the limit-cycle frequency and amplitude from a quasi-nonlinear theory.

Multidimensional stability analysis of gaseous detonations near Chapman–Jouguet conditions for small heat release

2009 ◽  
Vol 624 ◽  
pp. 125-150 ◽  
Author(s):  
PAUL CLAVIN ◽  
FORMAN A. WILLIAMS
Keyword(s):  
Heat Release ◽  
Acoustic Waves ◽  
Realistic Model ◽  
Cellular Structures ◽  
Bifurcation Parameter ◽  
Leading Order ◽  
Instability Mechanism ◽  
Gaseous Detonations ◽  
Detonation Structure ◽  
Rate Of Heat Release

Multidimensional instability of planar detonations, leading to cellular structures, is studied analytically near Chapman–Jouguet conditions, in the limit of small heat release, with small (Newtonian) differences between heat capacities, by using an expansion in a small parameter representing the ratio of the heat release to the thermal enthalpy of the fresh mixture. In this limit, the dynamics of detonations is governed by the interaction between the acoustic waves and the heat-release rate inside the inner detonation structure, the entropy–vorticity wave playing a negligible role at leading order. This situation is just opposite from that considered in our 1997 study of strongly overdriven detonations. The present analysis offers a step towards improving our understanding of the cellular structures of ordinary detonations, for which both the entropy–vorticity waves and the acoustic waves are involved in the instability mechanism. The relevant bifurcation parameter is identified, involving the degree of overdrive and the sensitivity of the rate of heat release to temperature at the Neumann state, and the onset of the instability is studied analytically for a realistic model of the inner structure of gaseous detonations.

Forced Flame Response of a Lean Premixed Multi-Nozzle Can Combustor

2011 ◽  
Author(s):  
Michael T. Szedlmayer ◽  
Bryan D. Quay ◽  
Janith Samarasinghe ◽  
Alex De Rosa ◽  
Jong Guen Lee ◽  
...  
Keyword(s):  
Heat Release ◽  
High Frequency ◽  
Transfer Functions ◽  
Low Frequency ◽  
Forced Response ◽  
Operating Conditions ◽  
Velocity Fluctuations ◽  
Lean Premixed ◽  
Flame Response ◽  
Rate Of Heat Release

An experimental investigation was conducted to determine the air-forced flame response of a five-nozzle, 250 kW, lean premixed gas turbine can combustor. Operating conditions were varied over a range of inlet temperatures, inlet velocities, and equivalence ratios, while the forcing frequency was varied from 100 to 450 Hz with constant normalized velocity fluctuations of approximately 5%. The response of the flame’s rate of heat release to inlet velocity fluctuations is expressed in terms of the phase and gain of a flame transfer function. In addition, chemiluminescence imaging is used to characterize the time-averaged and phase-averaged spatial distribution of the flame’s heat release. The resulting flame transfer functions and chemiluminescence flame images are compared to each other to determine the effects of varying the operating conditions. In addition, they are compared to data obtained from a single-nozzle combustor with the same injector. The forced response of the multi-nozzle flame demonstrates a similar pattern to those obtained in a single-nozzle combustor with the same injector. An exception occurs at high frequency where the multi-nozzle flame responds to a greater degree than the single-nozzle flame. At low frequency the multi-nozzle flame dampens the perturbations while the single-nozzle flame amplifies them. A number of minima and maxima occur at certain frequencies which correspond to the interference of two mechanisms. The frequency of these minima is nearly the same for the single- and multi-nozzle cases. When plotted with respect to Strouhal number instead of frequency there is a degree of collapse that occurs around the first observed minima.

scholarly journals Crack-Length Estimation for Structural Health Monitoring Using the High-Frequency Resonances Excited by the Energy Release during Fatigue-Crack Growth

Sensors ◽  
2021 ◽  
Vol 21 (12) ◽  
pp. 4221
Author(s):  
Roshan Joseph ◽  
Hanfei Mei ◽  
Asaad Migot ◽  
Victor Giurgiutiu
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Length ◽  
Crack Growth ◽  
Health Monitoring ◽  
Acoustic Waves ◽  
High Frequency ◽  
Fem Analysis ◽  
Signal Frequency ◽  
Propagation Pattern

Acoustic waves are widely used in structural health monitoring (SHM) for detecting fatigue cracking. The strain energy released when a fatigue crack advances has the effect of exciting acoustic waves, which travel through the structures and are picked up by the sensors. Piezoelectric wafer active sensors (PWAS) can effectively sense acoustic waves due to fatigue-crack growth. Conventional acoustic-wave passive SHM, which relies on counting the number of acoustic events, cannot precisely estimate the crack length. In the present research, a novel method for estimating the crack length was proposed based on the high-frequency resonances excited in the crack by the energy released when a crack advances. In this method, a PWAS sensor was used to sense the acoustic wave signal and predict the length of the crack that generated the acoustic event. First, FEM analysis was undertaken of acoustic waves generated due to a fatigue-crack growth event on an aluminum-2024 plate. The FEM analysis was used to predict the wave propagation pattern and the acoustic signal received by the PWAS mounted at a distance of 25 mm from the crack. The analysis was carried out for crack lengths of 4 and 8 mm. The presence of the crack produced scattering of the waves generated at the crack tip; this phenomenon was observable in the wave propagation pattern and in the acoustic signals recorded at the PWAS. A study of the signal frequency spectrum revealed peaks and valleys in the spectrum that changed in frequency and amplitude as the crack length was changed from 4 to 8 mm. The number of peaks and valleys was observed to increase as the crack length increased. We suggest this peak–valley pattern in the signal frequency spectrum can be used to determine the crack length from the acoustic signal alone. An experimental investigation was performed to record the acoustic signals in crack lengths of 4 and 8 mm, and the results were found to match well with the FEM predictions.

An Improved Rate of Heat Release Model for Modern High-Speed Diesel Engines

2017 ◽  
Vol 139 (9) ◽  
Author(s):  
Peter G. Dowell ◽  
Sam Akehurst ◽  
Richard D. Burke
Keyword(s):  
Heat Release ◽  
Diesel Engines ◽  
Engine Speed ◽  
Fuel Injection ◽  
Maximum Pressure ◽  
Injection Model ◽  
Wall Impingement ◽  
Small Set ◽  
Rate Of Heat Release ◽  
Engine System

To meet the increasingly stringent emissions standards, diesel engines need to include more active technologies with their associated control systems. Hardware-in-the-loop (HiL) approaches are becoming popular where the engine system is represented as a real-time capable model to allow development of the controller hardware and software without the need for the real engine system. This paper focusses on the engine model required in such approaches. A number of semi-physical, zero-dimensional combustion modeling techniques are enhanced and combined into a complete model, these include—ignition delay, premixed and diffusion combustion and wall impingement. In addition, a fuel injection model was used to provide fuel injection rate from solenoid energizing signals. The model was parameterized using a small set of experimental data from an engine dynamometer test facility and validated against a complete data set covering the full engine speed and torque range. The model was shown to characterize the rate of heat release (RoHR) well over the engine speed and load range. Critically, the wall impingement model improved R2 value for maximum RoHR from 0.89 to 0.96. This was reflected in the model's ability to match both pilot and main combustion phasing, and peak heat release rates derived from measured data. The model predicted indicated mean effective pressure and maximum pressure with R2 values of 0.99 across the engine map. The worst prediction was for the angle of maximum pressure which had an R2 of 0.74. The results demonstrate the predictive ability of the model, with only a small set of empirical data for training—this is a key advantage over conventional methods. The fuel injection model yielded good results for predicted injection quantity (R2 = 0.99) and enabled the use of the RoHR model without the need for measured rate of injection.

scholarly journals Automated Insertion of Objects Into an Acoustic Robotic Gripper

Proceedings ◽  
2020 ◽  
Vol 64 (1) ◽  
pp. 40
Author(s):  
Marc Röthlisberger ◽  
Marcel Schuck ◽  
Laurenz Kulmer ◽  
Johann W. Kolar
Keyword(s):  
Acoustic Field ◽  
Acoustic Waves ◽  
Acoustic Pressure ◽  
Pressure Field ◽  
Acoustic Power ◽  
Industrial Applications ◽  
High Density ◽  
Analytical Models ◽  
Acoustic Levitation ◽  
Mechanical Contact

Acoustic levitation forces can be used to manipulate small objects and liquid without mechanical contact or contamination. To use acoustic levitation for contactless robotic grippers, automated insertion of objects into the acoustic pressure field is necessary. This work presents analytical models based on which concepts for the controlled insertion of objects are developed. Two prototypes of acoustic grippers are implemented and used to experimentally verify the lifting of objects into the acoustic field. Using standing acoustic waves and by dynamically adjusting the acoustic power, the lifting of high-density objects (>7 g/cm3) from acoustically transparent surfaces is demonstrated. Moreover, a combination of different acoustic traps is used to lift lower-density objects from acoustically reflective surfaces. The provided results open up new possibilities for the implementation of acoustic levitation in robotic grippers, which have the potential to be used in a variety of industrial applications.

High-frequency acoustic waves are not sufficient to heat the solar chromosphere

Nature ◽  
2005 ◽  
Vol 435 (7044) ◽  
pp. 919-921 ◽  
Author(s):  
Astrid Fossum ◽  
Mats Carlsson
Keyword(s):  
Acoustic Waves ◽  
High Frequency ◽  
Solar Chromosphere
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