Flame Noise Research Articles

Validating Confined Flame Noise Simulation Using External Sensor.

Advancements in lean premixed combustion have increased the efficiency and reduced the amount of greenhouse gas emissions, but they have led to increased noise emissions due to higher turbulence and mixing fluctuations. This study used an external sensor (microphone) to validate the simulation of the combustion noise of a confined space. An experimental facility with a laboratory-scale furnace was used to carry out the measurement, and the simulation of the confined flame noise was conducted in OpenFOAM. The simulation utilized the Partially Stirred Reactor (PaSR) and a hybrid computational aeroacoustics (CAA) approach using the large eddy simulation (LES)/the Ffwocs Williams–Hawkings (FWH) method. Additionally, unsteady Reynolds-averaged Navier–Stokes (URANS)/the FWH method was tested for a comparison with the LES prediction. A sensor which was placed outside the enclosure for ease of access was then used to validate the results of the numerical model. The sensor data agreed with the LES/FWH results including the amplitude and frequency of the primary combustion peak and the overall sound pressure level (OASPL). This suggested that a sensor which was placed outside the enclosure could serve as a validation tool for the simulation of the confined flames despite the sound reflections from the walls.

Study on noise-vibration coupling characteristics of premixed methane-air flame propagation in a tube with an acoustic absorption material.

To study the influence of an acoustic absorbing material (AAM) on the noise and vibration of a methane–air deflagration flame in a square plexiglass tube, a high-speed video camera, pressure sensors, and a noise and vibration tester were used to test the deflagration flame propagation velocity, deflagration pressure, noise and wall vibration characteristics in the tube. The tube length is 540 mm with a cross section of 80 × 80 mm2, and its wall thickness is 12 mm. The experimental results indicate that under the conditions of 8.96% CH4 by volume and fixed repeating obstacles, the built-in AAM of polyester fiber cotton can reduce the peak velocity of the deflagration flame propagation by 11.3%. In addition, the average maximum sound pressure level of the deflagration flame noise is decreased by 17.6%, and the peak vertical vibration velocity of the tube outer wall is decreased by 85.6%. Therefore, using AAM can effectively attenuate the flame propagation and its harmful effects. For the case with an AAM, the flame propagation velocity and deflagration pressure reached the maximum values at 33 ms after ignition, and the values were 62.50 m s−1 and 27.74 kPa, respectively. Similarly, the time history curves of the noise and the tube wall vibration caused by deflagration presented certain correlations. The experimental results and analysis in this paper provide reference values for controlling the hazards of gas explosions in underground mines and other combustible gases in industrial pipelines.

Characterization of swirling CNG inverse jet flame in recessed coaxial burner

This paper is concerned with the experimental investigation of swirling inverse jet flame (IJF) characteristics such as visible flame appearance, visible flameheight, temperature distribution, post flame emissions and flame noise in a recessed coaxial burner. Furthermore, variation in lean blowout limits of swirling IJF with aerodynamic swirl number is also reported. Based on the observation of visible appearance of swirling IJF with aerodynamic swirl number, two flame regimes are identified. Besides this, semi-empirical correlations for flame height and lean blowout limits are attempted in this work which can be used for the design and development of future burner systems employing swirling inverse jet flame. Apart from this, an attempt has been made to understand the flow features and flame structure of swirling IJF through numerical simulations. The swirling inverse jet flame configuration studied in this work is capable of establishing compact blue flame with better thermal and emission characteristics and hence can be employed in thermal applications.

An Application of Minimum Statistical Noise Estimation in the High Rate of Fire Artillery Muzzle Velocity Measurement

The infrared starter is usually used to measure the artillery’s muzzle velocity, but due to the continued firelight near the muzzle produced by the continuous occurrence of the high rate of fire artillery projectile , so that the misjudgment and the Missing are easily produced by the infrared sensor . Continuous wave radar can effectively overcome the impact of the fire, but the detection accuracy of the leaving time is greatly affected echo because of the flame noise. Therefore the noise reduction of echo signal is an important step about the detection of the leaving time. For this problem, a noise reduction algorithm based on the minimum statistical noise estimation is proposed in this paper by the analysis of the echo signal. The noise of continuous muzzle flame is effectively reduced by the algorithm. It is verified correct and feasible by the processing of the measured signal. It lays the foundation for the measure of the artillery’s muzzle velocity and the engineering realization of muzzle velocity measurement.

Effects of nozzle length on flame and emission behaviors of multi-fuel-jet inverse diffusion flame burner

An experimental study was performed to investigate the effects of the nozzle length on the air-pollutant-emission and noise-radiation behaviors of a burner utilizing a multi-fuel-jet inverse diffusion flame (MIDF). Comparison of the experimental results obtained from two MIDF burners, one with a long nozzle and the other with a short nozzle, operating under the same air/fuel supply conditions ( Re air and Ф) shows rather significant differences in the flame appearance, flame centerline temperature, CO/CO 2 concentrations and the noise radiation. The nozzle length influences development of the jets and hence interaction between the air/fuel jets including their mixing process. The short nozzle produces a flame with a shorter base height and a smaller potential core due to the enhanced air/fuel mixing. It also leads to faster and more complete combustion at the inner reaction cone of the flame due to the stronger and faster air/fuel mixing. The nozzle length affects the CO and CO 2 concentrations, and higher peak values are obtained with the short-nozzle flame. Flame noise of the MIDF is defined as the noise radiation at different flame heights, which is of varying strength but of the same dominant frequency in the range of 250–700 Hz. The noise radiation from the inner reaction cone of the flame is stronger than that from the lower and upper parts of the flame, and the maximum noise radiation occurs when the total amounts of air and fuel in the combustion zone are at the stoichiometric air/fuel ratio. For all the experiments conducted in the present study, the MIDF produced by the long nozzle is always noisier than its counterpart and it is due to the increase of the low-frequency noise components.

Analysis on acoustic characteristics of opposed multi-burner gasifier

Hilbert–Huang Transformation and Statistical Theory are used to analyze the impinging flame noise signals of a gasifier. The characteristic of the flame noise energy is studied by standard deviation and probability density, and frequency–time distribution is generated by the empirical mode decomposition (EMD) method and Hilbert spectrum. The results show that, in different conditions, the value of standard deviation of four burners impinging flame noise is greater than that of the two burners, but not for the probability density. The change of the standard deviation and probability density of four burners is smaller than that of two burners. It means that the flame of four burners combusts severely but stably. Under the condition of low feed and oxygen ratio, the two flame burner noise signals frequency and energy distribute mainly in 100 Hz regions; and four burners flame noise distribution is concentrated in the low-frequency bands below 45 Hz and the 45–100 Hz bands.

Combustion Noise from Non-premixed and Lean-premixed Swirl Flames

The development of modern gas turbines and jet engines is focused on the reduction of pollutant emissions and, increasingly, on the reduction of overall noise emission, including combustion noise. This requires the minimization of the noise sources, namely noise from the turbulent flow, combustion noise and noise caused by periodic instabilities and fluctuations of the ignition zone. This has to be achieved under conservation of the benefits of swirl flames, e.g. high ignition stability and broad operation ranges. This paper is focused on the description and characterisation of fluctuations of the ignition zone due to different mixing and stabilization characteristics of noise from swirl flames. Another goal was the description of flame noise caused by the formation and burning of coherent, periodic flow structures, often detected in swirl flows and flames [1, 2, 3, 4]. The physical model offers the possibility to understand the formation mechanism of the coherent flow structures and their effect on the combustion of non-premixed and premixed swirl flames.

Study of Spectral Noise Emissions from Standard Turbulent Nonpremixed Flames

The occurrence of oscillating combustion and combustion instability has led to resurgence of interest in causes, mechanisms, suppression, and control of flame noise. Nonpremixed flame noise is low frequency and difficult to control using conventional acoustic liner and so significant noise reduction needs to be achieved by altering the source. Noise generated by enclosed flames is of greater practical interest but is more complicated than the noise generated by open flames, which itself is not clearly understood. Improved understanding of the noise generated by open flames is attempted by studying spectral content of the sound for flames for which detailed velocity, scalar property, and radiation measurements are available in the literature (International Turbulent Non-Premixed Workshop flames; http://www.ca.sandia.gov/TNF/abstract.html). Comparisons were made with the spectral content of sound generated by air jets at identical Reynolds numbers. The spectral sound pressure levels (SSPL) generated by the flames were higher than those generated by corresponding air jets at all frequencies. The differences between the SSPL were the highest in the relatively low-frequency range of 400-2000 Hz. The flames emanate a constant SSPL in this frequency range for locations near the axis. The width of this range decreases at larger radial locations. The effects of flow velocity are stronger at higher frequencies. The spectra show substantial reduction in highfrequency components away from the burner exit, especially in the axial direction. These observations can be used for designing combustors with lower noise and also in the evaluation of computational aeroacoustics codes.

Novel flame photometric detector for gas chromatography based on counter-current gas flows

A novel analytical device has been developed for gas chromatography. It is based on optical emission from a counter-current (i.e. counter-flowing) air or oxygen flame, which burns in an opposing stream of hydrogen and column effluent. The flame is typically positioned “upside down” on the upper (air) jet, which faces the lower (hydrogen+effluent) jet. It can also be positioned on the lower jet, be connected to both jets, or be suspended in the gap between them. Excellent stability can be obtained in any of these modes. Overall, this new “counter-current flame photometric detector” (ccFPD) responds to analytes in the manner of a conventional flame photometric detector (FPD); however, it can be operated over a much wider range of gas flows. For instance, the same physical ccFPD burner easily supports stable flames of air flows between 5 and 200 ml/min and corresponding hydrogen flows between 5 and 10,000 ml/min. Visual observation of the counter-current flame, in the presence of sulfur and phosphorus as test analytes, reveals intense, steady luminescence under a wide variety of conditions. Additionally, and in contrast to the commercial FPD, flame conductivity signals can be obtained that are similar in quality to those produced by a conventional flame ionization detector (FID). Thus the ccFPD is a flexible, easily optimized photometric detector. The exceptional flow stability of the ccFPD was used to explore the earlier reported phenomenon of strong signal/noise (S/N) ratios, which had been obtained for hetero-elements of the iron group from a conventional FPD with a small, stoichiometric flame. Results using the ccFPD, which also exhibits this unusual response, indicate that these high S/N ratios are only partly due to the predictable decrease in flame noise with decreasing flame size. Contrary to expectations, the absolute analyte signal often increases as the flame size decreases to the point of extinction. The signal intensity and the magnitude of the observed changes depend to some degree on the flame composition (H 2/O 2 ratio).

Parametric studies of acoustic radiation from premixed flames

Turbulent premixed combustion processes are inherently unsteady and, thus, a source of acoustic radiation. While prior studies have extensively characterized the total sound power radiated by turbulent flames, few measurements of their spectral characteristics exist. This article presents systematic measurements of the acoustic spectra over a 100 Hz-30 kHz frequency range from a turbulent, natural gas-fueled flame. Measurements were made in an anechoic environment at several burner diameters, flow velocities, and equivalence ratios. The results show that, for a fixed burner diameter, the turbulent flame's acoustic spectrum has a nearly universal shape whose total power is a function of equivalence ratio and flow velocity. The functional dependence of the power spectra on frequency does vary with burner diameter, however, as the frequency of peak acoustic emissions generally shifts to lower frequencies with increases in burner diameter. These trends are quite similar to those exhibited by the cold flow velocity spectra, suggesting that the flame noise is "forced" by turbulent velocity fluctuations. While this observation is consistent with prior studies, the complete independence of the flame noise spectra on such characteristics as flame length or speed is puzzling, however, and demonstrates the need for further theoretical analyses of the flame noise-turbulent flow velocity transfer function.

Noise Characteristics of Turbulent Diffusion Flames with Coherent Structure

Abstract In order to elucidate the interrelationship between flame structure and noise characteristics and to obtain those properties which are easily measurable and understandable, and are strongly correlated with the acoustic pressure perturbation, three kinds of flames with different structure are first formed in a plane shear layer by varying the flow and turbulence conditions: a flame within which organized eddies are clearly observed, a flame which has a rather complicated appearance and a higher eddy-formation frequency than the first due to the increased mean convection velocity, and a flame without any organized eddy, which is stabilized under the same flow conditions as the first flame, but has increased turbulence levels. Simultaneous measurements and FFT-analyses of the fluctuations of sound pressure, ion current, temperature, and CH-emission are made on these three flames. The results are shown concerning not only clear properties of the correlations between flame structure and sound intensit...

Signal-to-Noise Considerations in Flame/Furnace Infrared Emission Spectroscopy

A theoretical model for the signal-to-noise ratio (SNR) performance of a flame/furnace infrared emission (FIRE) radiometer using a PbSe detector has been developed. The model uses readily available, published PbSe detector parameters to predict the SNR for situations involving only detector noise as well as for situations involving detector noise in the presence of residual, uncorrected flame noise remaining after background subtraction. The theoretical SNR values were found to be in good agreement (6%) with previously published, experimentally determined SNR values for Freon-113®, obtained with a dual-channel FIRE radiometer operating in the carbon mode. The model was used to show the effect of source modulation frequency on the SNR obtained with the FIRE radiometer. At room temperature under detector-noise-limited conditions, it was found that: 1/ f noise > background photon noise > generation-recombination noise » Johnson noise. The PbSe detector was found to exhibit 1/ f-noise behavior out to about 1000 Hz when operated at room temperature. The model predicts that, even for a dual-channel FIRE radiometer in the presence of residual flame background noise, some SNR improvement should be possible by increasing the chopping frequency out to around 3 kHz.

Comparative evaluation of the dinitrogen oxide-hydrogen flame for the analysis of clinical samples

An evaluation of a nitrogen-separated dinitrogen oxide-hydrogen (N2O–H2) flame for clinical analysis by both atomic fluorescence and emission spectrometry is described. When supported on a capillary burner this flame exhibited a large background emission in the spectral range 180–600 nm. Although flame noise was higher for this flame compared with an air-acetylene flame, the detection limits were adequate for the determination of major elements of clinical importance. Notably, the N2O–H2 flame gives improved sensitivities for calcium and magnesium, with a marked reduction in flame scatter effects. It also has potential for analytical atomic fluorescence but is not appropriate for routine spectrometric analysis.

An implicit least-square method for the inverse problem of acoustic radiation

A simple and natural method, called the implicit least-square method, is developed to solve the inverse problem, especially for first-kind Fredholm integral equations of acoustic radiation. Unlike most previous methods, there are no artificial constraints or parameters to be specified. This method is proven to be equivalent to the pseudoinverse method and is tested in three examples: (i) inversion of Poisson’s formula of harmonic continuation, (ii) inversion of the free-space turbulent flame noise radiation for the heat release rate source, and (iii) inversion of the ducted combustion system noise radiation for the heat release rate source. Comparisons performed using the results of the present prediction, the exact solution, and the experimental data in these examples are satisfactory.

Diagnostic possibilities on the basis of premixed flame noise levels

On the basis of an assumed turbulent premixed gaseous flame model a useful formula is derived for the flame noise intensity, which is considered as an effect of combustion intensity fluctuations and flame zone turbulence. The observed similarity between the change in the flame noise spectrum amplitude and the energy release rate suggests the possibility of a diagnostic interpretation of the noise in terms of combustion intensity. This information becomes less clear, however, when the flame turbulence is strongly developed. The mutual relation between combustion intensity, flame zone turbulence, and amplitude of noise spectrum components suggests an analogy with the basic diagnostic formula.

The effect of turbulence on premixed flame noise

The effects of turbulence intensity and integral length scale on combustion noise have been investigated in cylindrical burners with fully developed pipe flow and in a nozzle burner with grid induced turbulence. Mean velocity and turbulence intensity measurements in flames over a wide range of Reynolds numbers have been made by laser anemometry and the results for equivalent air jets have been verified by hot-wire anemometry. Semi-empirical relationships for combustion noise have been tested over a wide range of flow conditions and equivalence ratios and it has been confirmed that these expressions give accurate predictions for fuel lean and stoichiometric flames on cylindrical burners. Grids producing higher turbulence levels in flames give rise to higher values of turbulent burning velocity and combustion noise, indicating that the upstream turbulence is also responsible for the increase in amplitude of combustion noise. Integral length scale measurements by auto-correlation indicate that the magnitude of the macro-scale is independent of the grid gemetry. Assuming that the integral scale in equivalent air jets is identical to that in flames, it appears to have no effect on combustion noise.

Effect of Acoustic Fields on Flames

Abstract Experiments performed to investigate the effect of ultrasonic fields on gas diffusion flames show that they can either increase or decrease the amount of flame noise generated. The studies described show that a reduction in flame noise can only be obtained with a single frequency field. The presence of a field composed of more than one frequency results in an increase in flame noise at frequencies related to the beat frequency of the applied signals. Schlieren photographs of the flame indicate a relationship between the position of its transition to turbulence and the character of the applied field.

Combustion Noise from Stationary Laminar Diffusion Flames on Single Streams of Monosized Kerosene Droplets

FLAME noise has been studied experimentally using both premixed1,2 and turbulent diffusion3–5 flames and by exploding premixed gaseous mixtures contained in soap bubbles.

Determination of Phosphorus and Sulfur in Fuel Rich Air–Hydrogen Flames

Excitation conditions for the green HPO bands, the uv PO bands, and the violet S(2) bands were explored. A cylindrical glass shield placed around the tip of a Beckman (sprayer) burner provided a simple arrangement to isolate a fuel rich environment of a reversed air-hydrogen flame. Samples were converted to aerosol in a separate nebulization chamber. The HPO band at 5262 A gave a detection limit of 6 microg/ml; the S(2) band, 5 microg/ml. Owing to low flame noise, both detection limits could be improved with interference filters in place of the Beckman small quartz monochromator. The PO bands are emitted best from fuel rich oxygen-acetylene flames with the normal Beckman sprayer burner. The significant aspect of this study lies in the introduction of liquid samples and circumvention of lengthy, and often uncertain, decomposition steps when handling many types of organic and biochemical compounds containing phosphorus and sulfur.

REDUCTION OF FLAME NOISE BY APPLIED ACOUSTICS

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