Souvick Chatterjee

Experimental Investigation of Breakup of Annular Liquid Sheet in a Hybrid Atomizer

Disintegration of annular liquid sheets is an important field of study that plays a crucial role in engine combustion. The focus of this study is to analyze different modes of sheets experimentally in an advanced hybrid injector, which uses both pressure swirl and air momentum to impart instability into the sheet. Apart from measurement of certain macroscopic spray physical parameters like breakup length and sheet width, the images for different sheet profiles were processed to obtain fractal dimensions, temporal frequency, and different orthogonal modes using proper orthogonal decomposition. A moderate airflow is seen to create a stable liquid sheet with increased breakup length, whereas higher airflows at low liquid flows create an air-blast type of spray with disintegration very close to the nozzle. The latter shows dominance of multiple smaller temporal frequencies with relatively lower power spectral densities. A spray with high liquid flow and relatively low airflows shows a longer and wider sheet w...

Effect of a Confined Outer Air Stream on Instability of an Annular Liquid Sheet Exposed to Gas Flow

A gas turbine combustor will act in a desired way only if its components, specially the fuel injector perform satisfactorily producing fine homogeneous droplets. Stability analysis of liquid, a rich classical fluid mechanics problem, when applied to fuel injector studies can enhance our knowledge leading towards the design of an advanced efficient atomizer. In this work, we analyzed the instability of a swirling annular liquid, exposed to co-flowing inner and outer air streams, by a temporal linear stability analysis using perturbation method. This temporal analysis discusses the effect of liquid Weber number, liquid swirl strength, both inner and outer gas-to-liquid velocity ratio and outer air gas swirl strength on the growth rate of interface instability. Another interesting inclusion in this work is the effect of confinement of the outer air stream which leads to a finite thickness of the outer air stream. Our results show a higher optimum growth rate obtained at a higher axial wave number in the presence of confinement compared to that when the outer air stream extends to infinity. This leads to the formation of smaller droplets which increases the efficiency of atomization. A comparative study between different helical modes revealed that the helical modes are dominant compared to the axisymmetric mode in presence of outer air swirl, whereas reverse phenomenon occurs in its absence.Copyright

Analysis of Static and Dynamic Contact Angles of Ferrofluid Droplets for Magnetically Actuated Micropumps

Ferrofluid plug driven micro-pumps are useful for manipulating micro-volume of liquids by providing remote actuation using a localized magnetic field gradient. Inside a microchannel, the ferrofluid experiences combined actions of different relevant body forces. While the pressure, viscous and magnetic forces can be estimated using established techniques, surface tension force cannot be readily. The presence of a second fluid (the fluid being pumped) and magnetic field (the driving dipole) alters the ferrofluid-wall contact angle (CA) in both static and dynamic fashions, which has not been reported in the literature. Therefore, realistic prediction of ferrofluid-plug driven micropump requires comprehensive data on variation of CA between the ferrofluid and glass capillary wall under different kinematic conditions. Here we perform an experimental characterization of static and dynamic contact angles of oil-based ferrofluid (EFH3) droplets on glass surface immersed in pure or surfacted distilled water. The relation between CA and the contact line velocity for ferrofluids is particularly important as the observed CA values fell beyond the classical Hoffman-Tanner equation. In the presence of an external magnetic field, a sessile ferrofluid droplet is seen to acquire interesting shapes. Growth of a droplet due to pumping of fluid from below the surface leads to a pinning and de-pinning effect. Our results also show a decrease in contact angle hysteresis (CAH) at higher contact line velocities as the droplets slide down an inclined plane.

A Comprehensive Model for Estimation of Spray Characteristics

Spray characteristics play an important role in determining efficiency of gas turbine or rocket combustors. The breakup of liquid jet is a complex nonlinear process governed by the fundamentals of well-known instabilities like Rayleigh instability and Kelvin–Helmholtz instability. Current availability of powerful computing tools has made computational fluid dynamics (CFD) along with other analytical and numerical techniques a viable tool for the design of combustors which reduced the requirement of expensive experimental studies in the design process. In this chapter, we demonstrate a system which incorporates a liquid jet that breaks up in the presence of a strong swirling field and sandwiched between two swirling air flow streams. Our work includes computational fluid dynamics, analytical technique (linear stability), and statistical tool (entropy formulation) to model the spray characteristics in the form of breakup length and droplet distribution from nozzle geometry and inlet kinematic conditions. The internal hydrodynamics of the nozzle is modeled in commercial software named Ansys. The output of this simulation in the form of flow kinematics is used to estimate the growth rate of instability associated with atomization using a linear stability analysis. The breakup length which is a function of this growth rate is found to closely match with experimental values. A statistical method known as maximum entropy formulation (MEF) is further used with inputs as the breakup length and a mean diameter value (obtained from linear stability analysis) to estimate the droplet diameter distribution. Thus, a comprehensive model is described in this chapter which is a useful prediction tool for spray characteristics and hence is a significant contribution to the spray and droplet community.

Scaling Laws in Directional Spreading of Droplets on Wettability-Confined Diverging Tracks

Spontaneous pumpless transport of droplets on wettability-confined tracks is important for various applications, such as rapid transport and mixing of fluid droplets, enhanced dropwise condensation, biomedical devices, and so forth. Recent studies have shown that on an open surface, a superhydrophilic track of diverging width, laid on a superhydrophobic background, facilitates the transport of water from the narrower end to the wider end at unprecedented rates (up to 40 cm/s) without external actuation. The spreading behavior on such surfaces, however, has only been characterized for water. Keeping in mind that such designs play a key role for a diverse range of applications, such as handling organic liquids and in point-of-care devices, the importance of characterizing the spreading behavior of viscous liquids on such surfaces cannot be overemphasized. In the present work, the spreading behavior on the aforementioned wettability-patterned diverging tracks was observed for fluids of different viscosities. Two dimensionless variables were identified, and a comprehensive relationship was obtained. Three distinct temporal regimes of droplet spreading were established: I), a Washburn-type slow spreading, II) a much faster Laplace pressure-driven spreading, and III), a sluggish density-augmented Tanner-type film spreading. The results offer design guidance for tracks that can pumplessly manage fluids of various viscosities and surface tensions.

Precise Liquid Transport on and through Thin Porous Materials

Porous substrates have the ability to transport liquids not only laterally on their open surfaces but also transversally through their thickness. Directionality of the fluid transport can be achieved through spatial wettability patterning of these substrates. Different designs of wettability patterns are implemented herein to attain different schemes (modes) of three-dimensional transport in a high-density paper towel, which acts as a thin porous matrix directing the fluid. All schemes facilitate precise transport of metered liquid microvolumes (dispensed as droplets) on the surface and through the substrate. One selected mode features lateral fluid transport along the bottom surface of the substrate, with the top surface remaining dry, except at the initial droplet dispension point. This configuration is investigated in further detail, and an analytical model is developed to predict the temporal variation of the penetrating drop shape. The analysis and respective measurements agree within the experimental error limits, thus confirming the models ability to account for the main transport mechanisms.

Surface-Wettability Patterning for Distributing High-Momentum Water Jets on Porous Polymeric Substrates

Liquid jet impingement on porous materials is particularly important in many applications of heat transfer, filtration, or in incontinence products. Generally, it is desired that the liquid not penetrate the substrate at or near the point of jet impact, but rather be distributed over a wider area before reaching the back side. A facile wettability-patterning technique is presented, whereby a water jet impinging orthogonally on a wettability-patterned nonwoven substrate is distributed on the top surface and through the porous matrix, and ultimately dispensed from prespecified points underneath the sample. A systematic approach is adopted to identify the optimum design that allows for a uniform distribution of the liquid on horizontally mounted substrates of ∼50 cm2 area, with minimal or no spilling over the sample edges at jet flow rates exceeding 1 L/min. The effect of the location of jet impingement on liquid distribution is also studied, and the design is observed to perform well even under offset jet impact conditions.

Numerical Simulation to Characterize Homogeneity of Air-Fuel Mixture for Premixed Combustion in Gas Turbine Combustor

Homogeneity in mixing of air and fuel in premixed combustion for a gas turbine combustor is a critical criterion to ensure efficient combustion and less environmental hazards. The current work deals with determining this homogenous characteristic of air-fuel mixture through computational simulation to specify homogeneity for a particular premixing length and equivalence ratio required for gas turbine combustion. A 3-D geometry of combustion chamber with combustion zone of internal diameter 6 cm is constructed. A premixing tube is augmented with the combustion chamber which has one air inlet port at the bottom and 3 fuel inlet ports. Air-fuel mixture is considered to enter the combustion zone with inlet swirl. The homogeneity of the mixture is found out at the dump plane and other important planes from simulation done with ANSYS FLUENT® for the meshed geometry. The results show whether mixing of air and fuel is full or partial and the extent of partial premixing. The parameters varied in the ANSYS FLUENT®. based simulation are the premixing length i.e. port of entry of fuel, the fuel flow rate i.e. the equivalence ratio and the air flow rate.Copyright