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.

K. Srinivasan

Department of Mechanical Engineering

T Sundararajan

ATOMIZATION ENHANCEMENTS

CAVITY ACOUSTICS

Cone angle

DROPLET DEFORMATION

DROPLET SPRAYS

HARTMANN WHISTLE

LARGE DEFORMATIONS

SINGLE DROPLET

ACOUSTIC FIELD MEASUREMENT

Acoustic fields

Atomization

Deformation

Drop breakup

Fulltext

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IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Atomization in the Acoustic Field of a Hartmann Whistle

International Journal of Spray and Combustion Dynamics

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.

Journal of Sound and Vibration

Spectra and directivity of a Hartmann whistle

This paper presents the effect of axial acoustic fields on an air-atomized spray. In these experiments, a spray of ethanol is established in a transparent test section in which an acoustic field is set up using acoustic drivers. Visual observation shows that a high-amplitude acoustic field (160 dB) reduces the length of the spray. The spray velocity field was characterized by particle image velocimetry (PIV) measurements. The spray velocities are considerably reduced in the presence of the acoustic oscillations, indicating the presence of smaller droplets. The presence of acoustic oscillations increases the spray cone angle. The flow field in the near field of the spray was characterized. There is considerable entrainment of air into the spray in the presence of acoustic oscillations. It is seen that the acoustic velocity rather than the acoustic pressure primarily affects the spray. © Springer-Verlag 2005.

Experiments in Fluids

An experimental investigation of interaction of sprays with acoustic fields

The Hartmann acoustic generator has been used in the present experiments to atomize high-speed diesel oil, aviation kerosene and light furnace oil. The effects of upstream air pressure (196-490 kPa), nozzle diameter, and frequency of sound field (5000-22000 Hz) on the mean droplet size of the spray have been investigated. The experiments showed that the degree of fuel atomization was much more sensitive to a variation in the upstream air pressure than to a variation in the acoustic frequency. Atomization was almost independent of fuel viscosity; thus burners incorporating these acoustic generators should prove particularly suitable for the atomization and combustion of heavy fuel oils. It was dependent mainly on surface tension and fuel density. A single correlation has been obtained for all three fuels, within the range of experimental conditions investigated, but attempts to develop a dimensionless expression incorporating surface tension and density were unsuccessful, partly because of the small range of these properties examined. © 1974.

Fuel

Experimental investigation of liquid-fuel atomization using Hartmann acoustic generator

Sensors and Actuators A: Physical

High-frequency surface acoustic wave atomizer

Acoustically excited air-assisted liquid sheets

Atomization in the acoustic field of a Hartmann whistle

Journal	International Journal of Spray and Combustion Dynamics
ISSN	17568277
Open Access	Yes