Mahesh V Panchagnula

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.

This empirical study aims to characterize the dynamical behaviour of sprays using time-series analysis of the size-velocity data acquired using a phase Doppler particle analyser. The prime motivation of this analysis is to capture the spatiooral correlations using time-series modelling paradigms that provide valuable new insights into spray dynamics. As a first step, we study long-held assumptions, especially on stationarity and time unsteadiness. We show that air-blast sprays have increased drop size as well as velocity ordering near the edge of the spray. Analysis of the inter-particle time of the droplets shows non-Poisson behaviour where droplets that are closely spaced in time are also closely spaced in the size and velocity coordinates. Temporal auto-correlation and partial auto-correlation calculations reveal the presence of inherent correlated features in the spray. This correlation is stronger and short lived in an air-blast spray and weaker but more persistent in a pressure swirl spray. These correlations render the probability density function (p.d.f.) estimate obtained from standard methods inaccurate; therefore, we propose a technically correct way of estimating the p.d.f. using a suitable downsampling and averaging method. Statistical analysis of residuals (from appropriate autoregressive integrated moving average time-series models) uncovers an interesting feature of spray data pertaining to heteroskedasticity (stochastically changing variance) of the diameter series. In order to account for heteroskedasticity, appropriate generalized autoregressive conditional heteroskedasticity models are developed. Finally, we present a utilitarian view of these results as an empirically consistent boundary condition implementation tool for computational fluid dynamics (CFD).

This empirical study aims to characterize the dynamical behaviour of sprays using time-series analysis of the size-velocity data acquired using a phase Doppler particle analyser. The prime motivation of this analysis is to capture the spatio-temporal correlations using time-series modelling paradigms that provide valuable new insights into spray dynamics. As a first step, we study long-held assumptions, especially on stationarity and time unsteadiness. We show that air-blast sprays have increased drop size as well as velocity ordering near the edge of the spray. Analysis of the inter-particle time of the droplets shows non-Poisson behaviour where droplets that are closely spaced in time are also closely spaced in the size and velocity coordinates. Temporal auto-correlation and partial auto-correlation calculations reveal the presence of inherent correlated features in the spray. This correlation is stronger and short lived in an air-blast spray and weaker but more persistent in a pressure swirl spray. These correlations render the probability density function (p.d.f.) estimate obtained from standard methods inaccurate; therefore, we propose a technically correct way of estimating the p.d.f. using a suitable downsampling and averaging method. Statistical analysis of residuals (from appropriate autoregressive integrated moving average time-series models) uncovers an interesting feature of spray data pertaining to heteroskedasticity (stochastically changing variance) of the diameter series. In order to account for heteroskedasticity, appropriate generalized autoregressive conditional heteroskedasticity models are developed. Finally, we present a utilitarian view of these results as an empirically consistent boundary condition implementation tool for computational fluid dynamics (CFD).

Understanding the breakup morphology of an expelled respiratory liquid is an emerging interest in diverse fields to enhance the efficacious strategies to attenuate disease transmission. In this paper, we present the possible hydrodynamic instabilities associated with expelling the respiratory liquid by a human. For this purpose, we have performed experiments with a cylindrical soap film and air. The sequence of the chain of events was captured with high-speed imaging. We have identified three mechanisms, namely, Kelvin-Helmholtz (K-H) instability, Rayleigh-Taylor (R-T) instability, and Plateau-Rayleigh (P-R) instability, which are likely to occur in sequence. Furthermore, we discuss the multiple processes responsible for drop fragmentation. The processes such as breakup length, rupture, ligament, and drop formation are documented with a scaling factor. The breakup length scales with We-0.17, and the number of ligaments scales as Bo. In addition, the thickness of the ligaments scales as We-0.5. Here, We and Bo represent the Weber and Bond numbers, respectively. It was also demonstrated that the flapping of the liquid sheet is the result of the K-H mechanism, and the ligaments formed on the edge of the rim appear due to the R-T mechanism, and finally, the hanging drop fragmentation is the result of the P-R instability. Our study highlights that the multiple instabilities play a significant role in determining the size of the droplets while expelling a respiratory liquid. This understanding is crucial to combat disease transmission through droplets.

Intersecting sprays are important to several practical applications from rocket combustion to spray painting. The characteristics of these sprays are examined using water as the working fluid. Three types of atomizer configurations are used for this study: (i) Single spray (ii) Double spray- two pressure swirl atomizers of same type, separated by a distance in the exit orifice plane and (iii) Triple spray- three pressure swirl atomizers of same type, placed on the corners of an equilateral triangle. The Sauter mean diameter, joint velocity and diameter distributions as well as the radial and circumferential liquid distribution are experimentally measured using phase doppler interferometry (PDI) technique. From these results, it is observed that at low injection pressures the liquid sheets emerge from exit orifices interacts. As the injection pressure is increased, the breakup length tends to decrease and a regime is identified where the liquid sheets and droplet clouds interfere. In this regime, the drop clouds formed from the different sheets collide with each other resulting in further increases in velocity and drop size remain unaltered. As a result, the hollow cone nature of the overall spray is transformed into solid cone spray of almost an even mean drop size distribution downstream of the spray intersection region. The study demonstrates that the dynamics of wave propagation on the liquid sheets in intersecting sprays is markedly different from the non-intersecting spray. The superimposition of diameter and velocity distribution of two or more single spray is different from the mutual intersection of multiple sprays.

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

Miniature spray nozzles are interesting to several applications from micro-thrusters to lithographic patterning. We investigate geometric scaling effects of pressure swirl atomizers, both theoretically and experimentally. The objective of the study is to determine the range of length scales when performance - both in terms of drop size and flow rates - deviate from accepted correlations. The experimental study considers atomizers spanning an order of magnitude in dimensional scale (1x, 3x, 5x and 10x). These atomizers were all kinematically similar and are fabricated by using micro electro mechanical systems (MEMS) technology as well as rapid prototyping technology (RPT). The geometrical parameters such as ratio of exit orifice length to diameter, ratio of swirl chamber diameter to exit orifice diameter, ratio of inlet slot width to height are maintained same for all the trails. Each atomizer is operated at varying flow conditions with water as the working fluid. Spray characteristic parameters such as sheet thickness, breakup length, and spray cone angle are captured by high speed imaging. Sauter mean diameter (SMD) as well as joint drop size and velocity distributions are two of the key characteristics measured experimentally using Phase Doppler Interferometry (PDI) technique. The performance of a small scale atomizer to a large scale atomizer is compared over a wide range of operating condition of the atomizer. This study unravels the following three conclusions. (i) A length scale at which conventional correlations governing spray performance fail, is identified. (ii) An optimum range of scale factors are identified to yield good atomization, with miniaturized spray nozzles. (iii) It is observed that the air core structure of microsprays is markedly different for microsprays from that in macrosprays.