Atomizers find applications in diverse fields such as agriculture, pharmaceutics and combustion. Among the most commonly found atomizer classes of designs are pressure swirl, airblast and ultrasonic atomizers. However, it has thus far not been possible to identify the class of an atomizer from spray characteristics. We perform multifractal detrended fluctuation analysis on the droplet inter-arrival times, diameters and axial velocities of pressure swirl, airblast and ultrasonic nebulizer sprays to quantify the differences in complexity in the respective signals. We show that the width of the multifractal spectrum of the signals of droplet diameters and the inter-arrival times, measured at the edge of the spray are robust atomizer identifiers. Further, we show the presence of correlations among the droplet diameters which are otherwise considered as random or derived from a log-normal distribution. This study can be further generalized to classify fluid mechanical systems or biological sprays using an appropriately chosen single point measurement in the flow field. © 2019, The Author(s).

R. I. Sujith

Department of Aerospace Engineering

Mahesh V. Panchagnula

Department of Applied Mechanics

Vedasri Godavarthi

K. Dhivyaraja

Fulltext

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Scientific Reports

Analysis and classification of droplet characteristics from atomizers using multifractal analysis

Sprays are a class of multiphase flows which exhibit a wide range of drop size and velocity scales spanning several orders of magnitude. The objective of the current work is to experimentally investigate the prospect of dynamical similarity in these flows. We are also motivated to identify a choice of length and time scales which could lead towards a universal description of the drop size and velocity spectra. Towards this end, we have fabricated a cohort of geometrically similar pressure swirl atomizers using micro-electromechanical systems (MEMS) as well as additive manufacturing technology. We have characterized the dynamical characteristics of the sprays as well as the drop size and velocity spectra (in terms of probability density functions, p.d.f.s) over a wide range of Reynolds and Weber numbers using high-speed imaging and phase Doppler interferometry, respectively. We show that the dimensionless Sauter mean diameter scaled to the boundary layer thickness in the liquid sheet at the nozzle exit exhibits self-similarity in the core region of the spray, but not in the outer zone. In addition, we show that global drop size spectra in the sprays show two distinct characteristics. The spectra from varying and collapse onto a universal p.d.f. for drops of size where 1$]]>. For <![CDATA[$x/\unicode[STIX]{x1D6FF}-{o}, a residual effect of and persists in the size spectra. We explain this characteristic by the fact that the physical mechanisms that cause large drops is different from that which is responsible for the small drops. Similarly, with the liquid sheet velocity at the nozzle exit as the choice of velocity scale, we show that drops moving with a velocity such that exhibit a residual effect of collapse onto a universal p.d.f., while drops with 1$]] and . From these observations, we suggest that physically accurate models for drop size and velocity spectra should rely on piecewise descriptions of the p.d.f. rather than invoking a single mathematical form for the entire distribution. Finally, we show from a dynamical modal analysis that the conical liquid sheet flapping characteristics exhibit a sharp transition in Strouhal number at a critical . © 2018 Cambridge University Press.

Journal of Fluid Mechanics

Dynamical similarity and universality of drop size and velocity spectra in sprays

In classical literature, blowout is described as loss of static stability of the combustion system whereas thermoacoustic instability is seen as loss of dynamic stability of the system. At blowout, the system transitions from a stable reacting state to a non-reacting state, indicating loss of static stability of the reaction. However, this simple description of stability margin is inadequate since recent studies have shown that combustors exhibit complex nonlinear behaviour prior to blowout. Recently, it was shown that combustion noise that characterizes the regime of stable operation is itself dynamically complex and exhibits multifractal characteristics. Researchers have already described the transition from combustion noise to combustion instability as a loss of multifractality. In this work, we provide a multifractal description for lean blowout in combustors with turbulent flow and thus introduce a unified framework within which both thermoacoustic instability and blowout can be described. Further, we introduce a method for predicting blowout based on the multifractal description of blowout. © © 2015 Cambridge University PressA.

Multifractal characteristics of combustor dynamics close to lean blowout

The transition in dynamics from low-amplitude, aperiodic, combustion noise to high-amplitude, periodic, combustion instability in confined, combustion environments was studied experimentally in a laboratory-scale combustor with two different flameholding devices in a turbulent flow field. We show that the low-amplitude, irregular pressure fluctuations acquired during stable regimes, termed 'combustion noise', display scale invariance and have a multifractal signature that disappears at the onset of combustion instability. Traditional analysis often treats combustion noise and combustion instability as acoustic problems wherein the irregular fluctuations observed in experiments are often considered as a stochastic background to the dynamics. We demonstrate that the irregular fluctuations contain useful information of prognostic value by defining representative measures such as Hurst exponents that can act as early warning signals to impending instability in fielded combustors. © 2014 Cambridge University Press.

Multifractality in combustion noise: Predicting an impending combustion instability

Combustors with fuel-spray atomizers are particularly susceptible to pressure or velocity oscillations resulting from the occurrence of thermoacoustic oscillations. Experimental investigations of distilled water spray - swirl - acoustic interactions are presented. The presence of a swirl flow field alters the behavior of water spray drastically. Investigations showed complex behavior of water sprays in an acoustic cycle. In the presence of acoustic oscillations the phase averaged droplet diameter variation with in an acoustic cycle is more at the axial locations compared with off axis locations. The phase averaged axial velocity variation of the droplets is high compared to that of phase averaged radial and tangential velocities with in an acoustic cycle. Periodic clustering of droplets in the spray field is observed in the presence of acoustic field. A maximum particle count of 14 times the minimum count in a phase angle bin is observed within an acoustic cycle for the acoustic pressure amplitude of 3000 Pa. The droplet diameter distribution in the spray field is affected in the presence of acoustic oscillations.

Journal	Scientific Reports
Publisher	Nature Publishing Group
ISSN	20452322
Open Access	No