Acoustic pyrometry in flames
This paper investigates temperature measurement using acoustic pyrometry in hot zones and flames. For experiments in flameless hot zones, a Hartmann whistle is used as the acoustic source. Time of flight of sound waves in the heated path is calculated by cross-correlating the signals received by two microphones placed at pipe ends. Acoustic pyrometry is also performed for estimation of flame temperature in kerosene wick and candle flames, using a speaker as the acoustic source. Since length scales of single flames are small, leading to short times-of-flight, appropriate temperature profiling strategies for flame temperature estimation are identified. © 2012 Elsevier Ltd. All rights reserved.
Acoustic characteristics of external chamfered Hartmann whistles
Experimental investigations are carried out on Hartmann whistles to explore the effect of external chamfer at the cavity mouth. The acoustic performance depends upon the cavity length, jet-to-cavity-spacing and external chamfer angle (15, 30, and 45). The modifications in spectral and directional characteristics of external chamfered Hartmann whistles are studied in detail and are compared with a regular cavity. The frequencies are observed to attain a minimum value at a chamfer angle of 30 along with modification in the acoustic spectra. In general, it is noticed that the external chamfered whistles are directive at an emission angle of around 45 where it is around 37 for a regular whistle. Numerical simulations portray the flow/shock oscillation features in external chamfered cavities. Simulations depict intense flow diversion at the mouth of chamfered cavities and elucidate the directivity shift as well as the enhancement of acoustic power observed experimentally. Thus, it is observed that Hartmann whistles with external chamfering could radiate acoustic power up to 2.3 times that of a non-chamfered whistle. © 2013 Elsevier Ltd. All rights reserved.
Acoustic heating effects in Hartmann whistle
This paper systematically explores the heating effects in a Hartmann whistle. Effects of thermal losses on the end-wall heating of the whistle are demonstrated by conducting experiments on bare metal cavities, insulated metal cavities, and in wooden cavities. The heating studies are done on wooden cavities of three different configurations, namely, regular cylindrical, stepped and conical ones. The cavity with conical geometry is observed to provide the maximum heating effect at the end-wall. Further, the effects of various parameters such as nozzle pressure ratio, cavity length, and stand-off distance on the end-wall heating of the cavities are also studied. The relationship between acoustic resonance and heating effects are established.
Spectra and directivity of a Hartmann whistle
Experimental and theoretical studies of the spectral and directivity characteristics of a Hartmann whistle are presented for pressure ratios in the range of 4.92-6.88. Cavity resonance is observed for the range of pressure ratios studied and the sound pressure amplitudes are larger than those of free jets. Distinct spectra with high sound pressure levels are observed at the fundamental frequency as well as the higher harmonics. Based on dimensional analysis, an expression for the fundamental frequency is derived in terms of the stagnation sound speed, cavity length, and stand-off distance. The analogy between Hartmann whistle and a classical Helmholtz resonator is discussed. Numerical simulations of the flow field, carried out to supplement the experimental findings, show that the jet regurgitance is significant at smaller values of cavity-stand-off distances. The correspondence between flow pattern seen from numerical simulations and the maximum directivity observed from acoustical measurements is highlighted. The flow diversion around the cavity explains the observed shift in directivity towards higher angles for the whistle, as compared to the free jet flow. The acoustic power and efficiency are high for small values of stand-off distances and larger cavity lengths. © 2008 Elsevier Ltd. All rights reserved.
Aeroacoustic studies on chamfered resonance tubes
This paper investigates the effect of chamfer angles on the acoustic spectra and directivity of resonance tubes, kept axi-symmetrically in the flow field of a supersonic jet. Such tubes can be used for effective flow control, mixing, ignition etc. The jet impinges at the open end of the tube which is closed at the other end. The parameters being considered are the chamfer angle of the tube, nozzle pressure ratio and spacing (S) between nozzle exit and the tube inlet. The jet diameter and the tube inlet diameter are kept constant. Nozzle pressure ratio is varied from 4 to 6 in steps of 0.5. The chamfer angles considered are 15°, 30°, 45°. Acoustic pressure is measured in the far field region at emission angles varying from 37° to 135°, from the jet flow direction. The spectra clearly illustrates that the resonance tubes with chamfer has higher fundamental frequency than that of its absence. The fundamental frequency is observed to decrease with L/Dj for all chamfer angles. The frequencies obtained from experiments are compared with standard quarter wavelength theory. It is clear that the frequencies of the chamfered tubes are almost closer to the theory. At large tube lengths all the frequencies match well with theory but at small tube lengths only 30o chamfer is almost close to the theory. The fundamental frequency of 45o chamfer is found to be almost near to that of 0o chamfer. The minimum location of fundamental frequency as marked in Fig. 4 with S/Dj is found to be same for all L/Dj studied. The shadowgraph sequence (Fig. 5) shows that the low frequency components ∼2 kHz in the waterfall spectra (Fig. 6) are due to jet regurgitance. It is observed from Fig.7 that the directivity is seen to be higher for a tube with α = 30°. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.
Studies on conical and cylindrical resonators
Spectral features of conical and cylindrical Hartmann resonators are compared in this work through a systematic parametric study. Experiments have been conducted by varying the following parameters: stand-off distance, nozzle pressure ratio and cone angle. Resonance frequencies of conical cavities are found to be higher than those of cylindrical cavities of the same length. Low (∼kHz) and high frequency (∼10 kHz) modes are observed in the spectra. Low frequency modes show an oscillatory trend with stand-off distance. The high frequency tones are found to be independent of cavity geometry and cavity length, and are similar to jet impingement tones. © 2007 Elsevier Ltd. All rights reserved.
Numerical studies on erosive burning in cylindrical solid propellant grain
This paper addresses erosive burning of a cylindrical composite propellant grain. Equations governing the steady axisymmetric, chemically reacting boundary layer are solved numerically. The turbulence is described by the two equation (k-ε) model and Spalding's eddy break up model is employed for the gas phase reaction rate. The governing equations are transformed and solved in the normalized stream function coordinate system. The results indicate that the dominant reaction zone lies within 20% of the boundary layer thickness close to the wall. The sharp gradient of the temperature profile near the wall is found responsible for bringing the maximum heat release zone near the surface and hence enhancement in the burning rate. The model reproduces the experimental observation that erosive burning commences only above a threshold value of axial velocity. © 2007 Springer-Verlag.
Atomization in the acoustic field of a Hartmann whistle
The current work experimentally investigates the effect of Hartmann cavity acoustics on the atomization of droplet sprays. Initially, the experiments are conducted on a single droplet to understand its behavior in the sound field of a Hartmann whistle. The atomization studies on single droplet reveal that the existence of sound field causes the droplet to undergo large deformation and become irregular in shape. The degree of droplet deformation is quantified based on smaller circularity and larger Feret's diameter. The increase in cone angle of spray to a higher value in the presence of acoustics in comparison to its absence shows that the acoustics enhances the atomization. The stroboscopic visualization of sprays in the presence of acoustics further reveals the breakup of ligaments, large scatter as well as the formation of more number of droplets, indicating atomization enhancement.
Aeroacoustic features of cavities with chamfered outlets
This paper investigates the effect of outer chamfer on the frequency and amplitude characteristics of resonance cavities, placed axi-symmetrically in the flow field of a supersonic jet. Such cavities can be gainfully used for flow control, atomization, mixing, ignition etc. The parameters being considered are the nozzle pressure ratio, chamfer angle at the outer surface of the cavity, cavity length and stand-off distance between nozzle exit and the cavity inlet. The cavity inlet diameter and jet diameter are kept constant. The chamfer angles adopted for the present study includes 15°, 30°, 45°. The acoustic pressure is measured in the far field region at a fixed radius of around 64Dj in order to avoid flow effects. The emission angles considered for the present study varied from 37° to 135° in steps of 2°, measured from the jet flow direction. The frequencies of non-chamfered cavity and 15° chamfered cavity are almost same thereafter it follows decreasing and increasing trend with chamfer angles. It is also noticed that all the frequencies approach a minimum value at a chamfer angle of 30°. The fundamental frequencies of all outlet chamfered cavities at a nozzle pressure ratio (NPR) of 5 are observed to decrease with increase of the non-dimensional cavity length (L/Dj). The frequencies obtained from experiments are compared with those obtained using Quarter wavelength formula (QWL) for an open - closed cavity. The decreasing trend of the frequency with L/Dj is found same for both experiments and theory, but the theoretical values are slightly higher at small L/Dj's. The spectral variation of frequency components at NPR = 4 show that there is no modification of the frequency components at L/Dj = 4.28 but the increase of L/Dj to 5.71 causes the increase of broad part between two successive peaks at 15o and 45o chamfer. It is seen that the minimum location of frequency index (frequency with chamfer/frequency without chamfer) with S/Dj at NPR = 4 is same for all chamfer angles. Shadowgraph sequence clearly illustrates the flow oscillation in front of the cavity mouth for all stand-off distances normalized with jet diameter. It is observed that the overall sound pressure level attains maximum at small L/Dj almost for all chamfer angles. It is seen that for all chamfer angles the overall sound pressure level follows a decreasing trend with S/Dj. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.
Studies on tapered resonance tubes
This paper studies the effect of spacing on the acoustic spectra and directivity of a tapered resonance cavity kept axi-symmetrically in the flow field of a supersonic jet. A parametric study is conducted for various spacings and nozzle pressure ratios. Fundamental frequencies of the tapered cavities for the range of parameters considered are about 500Hz above the fundamental frequency of corresponding case of the cylindrical cavity. The preliminary results indicate that the spacing influences the spectra, and for certain spacings, the fundamental frequency (f1) of the tapered cavity is higher by around 3 times than that of the cylindrical cavity, as manifested in the spectra. The formation of intermediate frequency component between the fundamental frequency and the first harmonic and the disappearance of the intermediate frequency with a slight change of spacing is clearly depicted in the spectra. The periodic variation of fundamental frequency and its occurrence at common positions are discussed.