Frequencies Of Acoustic Oscillations Research Articles

Enhancing heat-driven acoustic characteristics of standing wave thermoacoustic engines by using corrugated stack

The present work considers the heat-driven acoustic characteristics and dynamic thermal-fluid flow fields of a standing-wave thermoacoustic engine (SWTAE) by using sinusoidally shaped corrugated stack surfaces. 3-D numerical SWTAE models are developed and validated, aiming to examine the heat-driven acoustic effects caused by different corrugation amplitudes and wave-lengths of the sinusoidal stack surface. The results demonstrate that corrugated-shaped stack channels increase the contact area of the working gas in the stack area, and the thermos-acoustic conversion is more amplified. Compared to the SWTAE with uniform-shaped stack, the corrugated-shaped stack exhibit enhanced acoustic power output while maintaining a constant acoustic oscillation frequency. With the corrugation peak and wave-length of the sinusoidal stack are 2 and 0.2 mm, respectively, the amplitude and acoustic power of the acoustic pressure oscillations increase by 10.12% and 17.31%, respectively, in comparison with that of the conventional SWTAE. However, when the corrugation peak exceeds 0.4 mm, stronger nonlinear acoustics and flow effects are observed in the stack channels, leading to a reduction in the heat-driven acoustic power output, and thermo-acoustics energy conversion efficiency. The developed model highlights the effects of the corrugated-shaped stack on enhancing acoustic power output and thermo-acoustic conversion efficiency.

The Acoustic Response of Hydrogen/Ammonia Flames

Ammonia is a carbon-free fuel which could be used in gas turbines. Ammonia’s potential as a fuel can be improved when mixed with hydrogen. However, this could cause an increased propensity to thermoacoustic oscillations in the combustor. The acoustic response of hydrogen/ammonia flames was evaluated by calculating the flame transfer functions (FTFs). A local level set approach, combined with an incompressible velocity perturbation model, was used to calculate the flame front response to different frequencies of acoustic oscillations. The unsteady heat release rate was calculated from the flame front surface area, obtained by solving the G-equation, thereby calculating the gain and phase of the FTF. The unstretched flame speed, was obtained from experimental values and two chemical kinetic models: CRECK-NH3 and GRI-Mech3.0. The models’ accuracy was assessed by comparing the modelled to experimental values from literature. CRECK-NH3 was fitter for ammonia/hydrogen modelling, as it was always within 12% of experimental values, compared to GRI-Mech3.0 which always differed by over 35%. The FTFs suggested an increase in hydrogen enrichment led to an increase in the flame acoustic response, as the gain drop off occurred at a higher frequency with higher hydrogen content. The flame acts as a low-pass filter to acoustic waves and the bandwidth of this filter (the frequency at which the gain drops off) increases with hydrogen content. This was due to higher flame speeds with higher hydrogen content. These FTFs were also compared to those of hydrogen/methane flames and the non-linear response was briefly analysed.

Оценка частот акустических колебаний в выходном диффузоре газотурбинной установки

Calculated estimation of acoustic resonances in the flow part of gas turbines in the outlet diffusers is a current problem. Its solution will improve vibration reliability of essential elements of gas turbine plant (GTP) – the rear bearing housing support and the blade apparatus. At present there are only a limited number of calculated and experimental works devoted to the study of acoustic resonance effect in the output devices of gas turbines. The objective of this paper is to estimate frequencies of acoustic oscillations in the GTP output diffuser. It allows to avoid coincidence of the indicated frequencies with the frequencies of natural oscillations of GTP structural elements. The results of the analysis of frequencies of acoustic oscillations should be confirmed in further experimental studies of the specified output device, i.e., the output diffuser. The method to solve the problem is computational. The results of numerical calculations of acoustic resonances in the GTP outlet diffuser are used for the analysis. The calculations have been performed in the Modal Acoustics ANSYS Workbench program. The model is described and the computational program to calculate the acoustic modes in the output diffuser is specified. The boundary conditions to perform the specified numerical acoustic calculations are proposed. The modes and corresponding frequencies of acoustic oscillations that are possible in the GTP outlet diffuser are considered. Justification of validity of the results of numerical calculations is their partial comparison with analytical solution for the simplest acoustic model, as well as with the preliminary result of the experimental research. The results of the acoustic calculations of the model of the GTP outlet diffuser allow to determine the modes of acoustic oscillations, and to estimate the frequency range of these modes. To substantiate the calculated estimates of frequencies of acoustic pressure pulsations, as well as to clarify the amplitudes of these pulsations, experimental aerodynamic studies of the model of the outlet diffuser with a rotating turbine stage at the inlet are necessary. Refined calculation model of acoustic resonances in the GTP outlet diffuser will contribute to the offset of frequencies of acoustic gas vibrations from the natural frequencies of vibrations of the GTP structural elements, adjacent to the diffuser. The indicated frequency offset will improve the GTP vibration reliability at the design stage.

Numerical and Theoretical Investigations on Twin Standing-wave Thermoacoustic Prime Movers

In this work, we perform computational fluid dynamics investigations topredict the performance of the twin thermoacoustics engine (TAE) undervarious working gas/medium and at different charge pressures. The numerical model is validated by comparing with those of DeltaEC (Design Environment for Low-amplitude ThermoAcoustic Energy Conversion) predictions and previous experimental results. Additionally, the effects of (1) the geometry shape of the twin TAE and (2) different types of working gases on its performance are examined. The results indicate that the working gas/medium and charge pressure in the engine are critical to the twin thermoacoustic engine. Although the acoustic pressure amplitude is higher with air or nitrogen as the working gas, the acoustic oscillation frequency is higher with helium or argon and the value of the acoustic growth rate is larger, i.e., the limit cycle is observed growing faster in the twin TAE. Furthermore, the pressure-ratio is increased in the tapered resonator, because its velocity amplitude is decreased. It means the tapered resonator is the best choice for the twin TAE. The acoustic characteristics predicted are much closer to the experimental results than those of DeltaEC estimations, which provides a design tool for designing and predicting performances of twin TAEs.

Influence of acoustic oscillations on the speed of a cellular hydrogen-air flame in a horizontal channel

The results of an experimental investigation of the effect of acoustic oscillations with a frequency of 250-7000 Hz and an intensity of 105 dB on the dynamics of the acceleration of a hydrogen-air flame in an open channel are presented. Dependences of the position of the flame front and cell sizes were obtained from shadow photographs at different frequencies of acoustic oscillations. The frequency values when the average flame speed increases up to 3 times are found. Acoustic action with a frequency of 250 Hz leads to a slight decrease in the speed of the flame. Keywords: Hydrogen, duct combustion, acoustic impact, cellular flame.

Влияние акустических колебаний на скорость ячеистого водородно-воздушного пламени в горизонтальном канале

The results of an experimental study of the effect of acoustic oscillations with a frequency of 250-7000 Hz and an intensity of 105 dB on the dynamics of the acceleration of a hydrogen-air flame in an open channel are presented. Dependences of the position of the flame front and cell sizes were obtained from shadow photographs at different frequencies of acoustic oscillations. The frequency values when the average flame speed increases up to 3 times are found. Acoustic action with a frequency of 250 Hz leads to a slight decrease in the speed of the flame.

A Reduced-order Model for Lock-on via Vortex-combustion-acoustic Closed-loop Coupling in A Step Combustor

ABSTRACT The lock-on of the frequency of acoustic oscillations to the frequency of the vortex shedding in a reacting flow in a backward-facing step combustor is demonstrated using a reduced-order model. Individual models for the flow, flame and acoustic models are developed. A two-dimensional flow field with growth and shedding of vortices is modeled using a potential flow model derived via a conformal mapping of the step geometry. Combustion and acoustic models are derived using thermo-diffusive equations and the one-dimensional Galerkin method, respectively. A modulation of heat release rate fluctuations is thus enabled by vortices advecting past the flame, overcoming the time and space-localized assumptions in vortex kicked oscillator models. Comparisons with results from kicked oscillator models show a better prediction of the dominant frequencies during stable and unstable operations. A Reynolds number sweep is performed to reveal the transition from stable to unstable operation and thus the onset of instability. A shift in the dominant frequency of pressure fluctuations from the natural duct acoustic mode to the natural vortex shedding mode is observed, indicative of a lock-on. The results compare well with past experimental and computational data for similar geometry and flow conditions.

The Spectrum of Natural Frequencies of Acoustic Oscillations of a Spherical Quantum Dot with a Multilayer Shell

The Spectrum of Natural Frequencies of Acoustic Oscillations of a Spherical Quantum Dot with a Multilayer Shell

ЕКСПЕРИМЕНТАЛЬНІ ДОСЛІДЖЕННЯ РОБОЧИХ ПРОЦЕСІВ В ТЕРМОАКУСТИЧНОМУ ТУРБОГЕНЕРАТОРІ

Thermoacoustic thermal machines (TATM) is a relatively new and non-common type of thermal machines that differs significantly from conventional mechanical systems due to the lack of moving elements, environmental safety, and the ability to operate from external sources of energy. Thermoacoustic engines (TAE) - direct-acting thermal machines that are capable of converting thermal energy into mechanical in the form of powerful acoustic waves. The growing interest to TATM is related to the possibility of using them in energy saving systems since the introduction of energy saving technologies is an effective method for increasing the efficiency of power plants and reducing their harmful effects on the environment. The wide introduction of TATM is hampered by their low specific power, the complexity of the transformation of acoustic oscillations into mechanical work and lack of experience.The article presents the results of experimental research on the operational characteristics of the thermoacoustic turbogenerator (TATG). The TATG consists of a thermoacoustic engine, which operates in the Brighton thermodynamic cycle and an electric generator. As an actuator of an electric generator, a pulsed bi-directional turbine was used. It is capable of converting the oscillatory motion of the working medium into mechanical energy. In the course of experiments, data were obtained on external characteristics of both TAE and TATG at different load conditions. The interaction of a pulsed bi-directional turbine with a pulsating working medium leads to the appearance of complex radial currents in the resonator that affects the efficiency of the energy conversion processes.In the work, using CFD simulation, the peculiarities of formation and development of radial currents were studied. Boundary and initial conditions were imposed based on experimental data. The results of CFD experiments allowed us to study the influence of various factors on the intensity of radial currents. It is shown that radial currents exist in the form of quasistationary pulsating structures whose length depends on the amplitude of the oscillatory motion of the pulsating medium and the frequency of acoustic oscillations. The presence of these currents leads to the "desired" flow of input into the rectifying apparatus of the impulse turbine. According to the results of the research, technical solutions are proposed that prevent the formation of such currents.

The emergence of technogenic risks in the event of a heat exchange crisis in nuclear power plants

In this article, the main problems and man-caused risks are considered in the event of a heat exchange crisis in nuclear power plants (NPP). A thermophysical booth simulating the operation of a nuclear reactor was developed. The data of acoustic diagnostics of the boiling of the coolant on the basis of the work of the laboratory stand were collected. Methods for processing and analyzing data collected from acoustic sensors have been developed. The recording and processing of signals was carried out using the application package and MATLAB programming language. The article describes the application of correlation and spectral analysis for data processing and analysis. The authors also use regression analysis to find the dependence of wall temperature on the frequency of acoustic oscillations. Forecasting the values of the wall temperature from the frequency of sound vibrations makes it possible in the future to find diagnostic features and build a mathematical model for detecting boiling and heat transfer crisis in nuclear power plants.

Experimental investigation of the thermoacoustic traveling-wave engine

Thermoacoustic engines are devices with external heat input, in which thermal energy is converted into energy of acoustic oscillations. Their main advantages include high energy conversion efficiency (up to 35%), minimal number of moving parts, high reliability, absence of contact seals and operation from a variety of heat sources (hydrocarbon fuels, thermal emissions, nuclear sources, solar radiation, etc.). The main goal of this work is to develop, manufacture and carry out the experimental investigations of the thermoacoustic traveling wave engine with a maximum heat input from the electric heating element up to 1000 W. The thermoacoustic engine includes an inertance tube, three heat exchangers, a regenerator, a thermal buffer tube and an acoustic resonator. Helium acts as the working fluid in the engine at an average pressure of 1.0 to 3.0 MPa. The frequency of acoustic oscillations on the steady-state operating mode of the engine is 96-98 Hz. The regenerator is made in the form of a package of metal screens and is the main element of the engine, in which thermoacoustic energy conversion is carried out. In experimental research the following tasks were solved: - determination of the optimal conditions for the excitation of acoustic oscillations in the internal contour of the thermoacoustic engine; - determination of the minimum onset temperature at which acoustic oscillation are excited; - determination of the dependences of the onset temperature, the frequency and the amplitude of acoustic oscillations from the input thermal power; - determination of the efficiency of the rmoacoustic energy conversion; In experimental studies, an optimal mean helium pressure is determined, which ensures the minimum heat exchanger temperature 436 К (163 C) required to start the engine. The maximum value of the acoustic power generated by the regenerator was 90 W. At the same time, the efficiency of conversion of thermal energy into acoustic energy reached 22.5% at a heat exchanger-heater temperature of 317 C. Also during the tests, it was possible to establish the dependences of the onset temperature, the frequency of acoustic oscillations and the amplitude of the acoustic pressure on the supplied thermal power. This type of engine described in this paper can be used in many practical applications.

Study on gas-droplet heat and mass transfers in oscillating flows

A numerical study on the dynamics of evaporating droplets in oscillatory flows is performed by using an Eulerian-Eulerian model including two-way couplings. The diluted liquid fuel-gas two-phase flow is assumed to be laminar. The droplet is assumed to be spherical during its lifetime and its thermal conductivity is also assumed to be infinite. Heat and mass transfer of this two-phase flow is characterized by analyzing the effects of acoustic fields on the two-phase temperatures, droplet diameter and concentration, spray evaporation rate and vaporizing species mass fraction. Results show that acoustic forcing can substantially influence the dynamics of the two-phase flow. The presence of droplet clustering as a consequence of acoustic forcing affects the two-phase flow in two aspects. The first is the oscillation of the two-phase flow parameter which roots in the periodic variation of droplet concentration. The second is the enhancement of the droplet evaporation rate at all conditions. The maximal relative increase in spray evaporation rate can be up to 72.9% in the parameter range studied. The mechanism for the enhancement of droplet evaporation rate is the optimal distribution of the heat in the liquid phase whose direct cause is droplet clustering. The enhancement of evaporation rate is highly dependent on the acoustic oscillation amplitude; however, there is no significant relationship between the acoustic oscillation frequency and the evaporation rate. Additionally, the oscillation amplitudes of the two-phase flow parameters are found to decrease with the growth of acoustic forcing frequency. It is made clear that the evaporation of droplets has a negligible effect on the occurrence of droplet clustering despite its marked influence on the two-phase flow through the reduction of the droplet relaxation timescale. Increasing the droplet initial concentration is also found to be beneficial for the enhancement of droplet evaporation rate.

Development of Methods and Algorithms for Spectral Data Analysis for Vibroacoustic Diagnostics of Diesel-Generator Sets at NPPs

In this article, the main methods and algorithms for spectral data analysis for vibroacoustic diagnostics of diesel-generator sets at nuclear power plants are considered. To collect the diagnostic data, an experimental setup was developed, thanks to which the sound signals of the diesel generator were obtained under various operating conditions. The recording and processing of signals was carried out using the application package and MATLAB programming language. The article describes the application of correlation and spectral analysis for data processing and analysis. Also, the authors apply regression analysis to find the dependence of the speed of the diesel engine on the frequency of acoustic oscillations. The prediction of the number of revolutions from the frequency of sound vibrations makes it possible in the future to build a more accurate mathematical model of engine operation, and also to find diagnostic features for detecting malfunctions and anomalies in the operation of a diesel generator.

Three-step approach for prediction of limit cycle pressure oscillations in combustion chambers of gas turbines

Currently, gas turbine manufacturers frequently face the problem of strong acoustic combustion driven oscillations inside combustion chambers. These combustion instabilities can cause extensive wear and sometimes even catastrophic damages to combustion hardware. This requires prevention of combustion instabilities, which, in turn, requires reliable and fast predictive tools. This work presents a three-step method to find stability margins within which gas turbines can be operated without going into self-excited pressure oscillations. As a first step, a set of unsteady Reynolds-averaged Navier–Stokes simulations with the Flame Speed Closure (FSC) model implemented in the OpenFOAM® environment are performed to obtain the flame describing function of the combustor set-up. The standard FSC model is extended in this work to take into account the combined effect of strain and heat losses on the flame. As a second step, a linear three-time-lag-distributed model for a perfectly premixed swirl-stabilized flame is extended to the nonlinear regime. The factors causing changes in the model parameters when applying high-amplitude velocity perturbations are analysed. As a third step, time-domain simulations employing a low-order network model implemented in Simulink® are performed. In this work, the proposed method is applied to a laboratory test rig. The proposed method permits not only the unsteady frequencies of acoustic oscillations to be computed, but the amplitudes of such oscillations as well. Knowing the amplitudes of unstable pressure oscillations, it is possible to determine how these oscillations are harmful to the combustor equipment. The proposed method has a low cost because it does not require any license for computational fluid dynamics software.

Frequencies of gas oscillations in a pipe with a concentrated heat source

It is known that the location of the heat source significantly affects the frequency of acoustic oscillations in the channels. The case of a step change in the sound speed is investigated. In this article, linear distribution of sound speed in hot gas is considered. The well-known equations are used to calculate frequencies of the gas oscillations. The analysis shows that the movement of the flame from the down up in an open tube causes a nonmonotonic change in the resonant frequency. The calculation results are in good agreement with the experimental data.

Experimental Study Of The Development Of Hydrodynamic Instability In Round Microjet Under The Influence Of External Acoustic Field With And Without Burning

Experimental study of subsonic round micro jets in the diffusion combustion of propane and without combustion at low Reynolds numbers was performed. Development of the jet was as under natural conditions without the acoustic excitation and as acoustic excitation of high intensity (90– 100 dB), frequency of acoustic oscillations was from a few Hz to 6 kHz. The jet flow was realized through the nozzle of round cross-section with a bevel of 45° and a diameter d = 0,45 mm. Blown trough the nozzle mixture of propane-butane previously passed through the evaporator, then through flowmeter (two-channel PR4000B) gas consumption. The studies were performed using Schlieren method based on the IAB-451. The pictures of the shadow visualizations was received, the presence of a developing hydrodynamic instability was found, including the presence of convective forces (by burning). The pictures of diffusion combustion of round propane-butane micro-jet at various modes of combustion – lifted and attached flame were received. It was found that the flame in the diffusion combustion of round microjet at an external acoustic field, exposed to flattening and bifurcation as the air free round microjet, while the source of acoustic oscillations is oriented perpendicular to the jet. The pictures of the diffusion combustion for splitting impact jet by acoustic field were obtained. It was found that in this case on the limits plate two temperature maximum were realized.

Measurements of acoustic particle velocity in a coaxial duct and its application to a traveling-wave thermoacoustic heat engine.

We present theoretical solutions, based on linear acoustic theory, for axial acoustic particle velocity in an annular region of a coaxial duct. The solutions are expressed in terms of two non-dimensional parameters h/δ(ν) and R; h and δ(ν), respectively, represent the half of the spacing between two concentric ducts and the characteristic length given by kinematic viscosity of the gas and angular frequency of acoustic oscillations, and R is the radius ratio of the ducts. The validity of the solutions was verified by direct measurements using a laser Doppler velocimeter. The present results are applied to measurements of the acoustic power distribution in a traveling wave thermoacoustic engine with a coaxial duct, which provides experimental evidence for acoustic power feedback in the coaxial duct.

Acoustic instability in a two-phase mixture with temperature relaxation of particles

Theoretical calculation of acoustic instability in a two-phase mixture with temperature relaxation of particles is performed. Particles are assumed to be spherical and monodisperse, while their mass fraction does not exceed 50%. The formula for calculation of the acoustic oscillation frequency and increment, as well as the approximate formula for oscillation-amplitude calculation, are derived in this work. The heat exchange between the particles and the gas mixture is taken into consideration explicitly in these formulas.

Mathematical simulation of acoustic and gas-dynamical processes in the channel of an acoustoconvective dryer

This paper presents and analyzes the data of numerical simulation of nonstationary fl ow in the channel of an acoustoconvective dryer in which the acoustic fi eld is produced by a Hartmann generator. The gas-dynamical fl ow fi eld and frequency characteristics of the problem are calculated in axisymmetric and three-dimensional formulations. The jet outfl owing from the generator nozzle with change in the resonator length and in the jet pressure ratio was investigated parametrically. The results of numerical calculations are compared with experimental data. Introduction. This present paper is a sequel to earlier publications that describe investigations carried out at the S. A. Khristianovich Institute of Theoretical and Applied Mechanics, Siberian Branch of the Russian Academy of Sciences, and is devoted to the study of the mechanisms underlying the acoustoconvective drying of capillary-porous materials. The indicated method of drying offers a number of advantages over the traditional convective technique since it requires less energy for drying due to the fact that the process proceeds almost at room temperature and less time is needed for drying the material. The review of the works carried out at the Institute and devoted to jet fl ows in an acoustoconvective dryer and to the verifi cation of the results of two-dimensional calculations based on the experimental data obtained at the Institute are given in (1, 2). The results of mathematical simulation of the characteristic frequencies of gas fl ow in the considered facility obtained in plane statement (2) agree satisfactorily with the characteristic frequencies of the process obtained experimentally, which allows one to use the mathematical algorithms proposed in this work for further investigation of the process of drying materials in the indicated facility. Statement of the Problem. We consider outfl ow of a nonstationary sonic air jet into the space limited by a resonator (Fig. 1). The gas is supplied to the channel of a drying chamber through a sonic nozzle in which acoustic oscillations are produced by the resonator that consists of a hollow cylinder, closed at one end, whose axis coincides with the nozzle (jet) axis. The convergent nozzle has the shape of a cone, and the channel of the drying chamber has a square section. Thus, the total computational domain is not symmetrical. The amplitudes and frequencies of acoustic oscillations vary with the expense of resonator depth variation. The mathematical model used in numerical calculations includes complete Navier-Stokes equations that ignore the fl ow turbulence. For numerical realization of the corresponding initial boundary-value problem for the equations of the model, we used the ANSYS Fluent package in versions 13.0 and 14.0. Roe's scheme with the MUSCL discretization of the second order of accuracy was used for approximating the convective terms of these equations. The implicit scheme of calculation was used to integrate the equations in time. Two variants of calculation of the problem considered were performed: in axisymmetric and complete three- dimensional statements. The amplitude and frequency of acoustic oscillations in the jet were calculated for different lengths of the resonator. Below we consider several variants of calculations in various simplifi ed formulations. In the two-dimensional statement the problem was solved without account for the presence of the drying chamber in the facility investigated. Calculation for two regions (1 and 2) depicted in Fig. 2 was performed in the axisymmetric approximation under the following boundary conditions: in the inlet section of the nozzle the total and static pressures and the temperature in the receiver were specifi ed; the no-slip conditions for velocity and the adiabatic condition for temperature

Probing tiny convective cores with the acoustic modes of lowest degree

Solar-like oscillations are expected to be excited in stars of up to about 1.6 solar masses. Most of these stars will have convective cores during their main-sequence evolution. At the edges of these convective cores, there is a rapid variation in the sound speed, which influences the frequencies of acoustic oscillations. In this paper we build on earlier work to investigate the impact that these rapid structural variations have on different p-mode frequency combinations, involving modes of low degree. In particular, we adopt a different expression to describe the sound speed variation at the edge of the core, which we show more closely reproduces the profiles derived from the equilibrium models. We analyse the impact of this change on the frequency perturbation derived for radial modes. Moreover, we consider three different small frequency separations involving modes of degree l = 0,1,2,3, l = 0,1, and l = 0,2, and show that they are all significantly affected by the sharp sound speed variation at the edge of the core. In particular, we confirm that the frequency derivative of the diagnostic tool that combines modes of degree up to 3 can potentially be used to infer the amplitude of the relative sound speed variation directly at the edge of the core. We show that at high frequencies the other two diagnostic tools can be up to a few μHz smaller than what would be expected in the absence of the rapid structural variation at the edge of the core. Also, we show that the absolute values of their frequency derivatives are significantly increased, in a manner that strongly depends on stellar age.