Vedanth Srinivasan

Investigation of the primary breakup of round turbulent liquid jets using LES/VOF technique

*The disintegration of a round turbulent liquid jet issuing from a coaxial type atomizer into a high pressure chamber is numerically studied. The liquid-gas interface is tracked using Compressive Interface Capturing Scheme for Arbitrary Meshes (CICSAM). Large Eddy Simulation techniques are used to resolve the large scale motions in the flow while modeling the small scale statistics, required to predict the breakup mechanism. A one equation eddy viscosity model has been used to resolve the sub-grid scale stresses, which solves a transport equation for sub-grid scale kinetic energy. The advantage of using the one equation model is its inherent ability to predict backscattering, which is essential in predicting the energy exchange between the resolved and modeled scales in the currently considered multiphase regime. Numerical computations are performed using this combined LES/VOF method to investigate the primary breakup mechanism often encountered in round turbulent jets. Due to fine mesh requirements, simulations are limited to a downstream distance of 10 nozzle diameters. Our numerical study concerns the effect of relative velocity between liquid and gas phases on the turbulent jet disintegration characteristics. Injecting liquid with no co-flowing gases results in development of short wavelength perturbations on the liquid surface by the action of aerodynamic forces leading to stripping of liquid surface to form discs, ligaments and finally droplets. At the onset disc formation, the disc protrusion length to thickness (neck width near the liquid surface) ratio of the discs measured 1.9, while the ratio of disc width to jet diameter is observed to vary between 0.2-0.4. With co-flowing gas of equal velocity magnitude, suppression of liquid-gas interface instability occurs. The interface stretching and the ligament alignment configurations are modified due to decreased radial spread. Primary breakup mechanism involving disc base breakup and disc tip (ligament type) breakup have been discussed. The ligament characteristics arising from different flow configurations are clarified. I. Introduction The mechanism of primary breakup of turbulent liquid jets is of fundamental importance in various industrial processes such as spray coating, combustors, metal powder formation etc. The atomization process of liquid jets is thought to consist of two consecutive steps: primary and secondary breakup. During the primary breakup, the liquid jet exhibits large scale coherent structures that interact with the gas-phase and break into both large and small scale drops. Proceeding downstream, the drops formed due to primary breakup split further into much smaller drops. This mechanism is called secondary breakup process. The process of atomization occurs in the turbulent flow environment which results in the presence of wide range of length and time scales of motion. The foresaid variation in length and time scales differentiates the treatment of primary and secondary flow structure during atomization. The characteristics of primary breakup has significant influence on the properties of dispersed phase affecting the mixing rates with the surrounding gas, mechanisms of secondary breakup and droplet collisions, among others. Over the years, many researchers have identified that the disintegration characteristics of round turbulent liquid jets, is influenced by several parameters such as nozzle design (influence of internal turbulence, cavitation etc), injection conditions, liquid and ambient gas properties etc. The mechanism of primary breakup was earlier observed by De Juhasz et. al 1 and Spenser 2 . Further studies taken up by Grant and Middleman 3 , McCarthy and Malloy 4 identified liquid turbulence properties behind the jet instability and subsequent breakup. Later, Hoyt and Taylor 5 concluded that the drop formation due to turbulent primary breakup was associated with formation of discs and ligaments along the liquid surface and that aerodynamic effects were generally of secondary importance for turbulent primary

Numerical Simulation of Piggyback Risers Under Steady Current Loading

With elevated interest in offshore drilling, transportation of production fluid to surface facilities are under heavy scrutiny. Particularly, design of marine risers either in standalone or in a piggyback configuration continues to dominate the focus of the research community in assessing the influence of hydrodynamic drag on its operation. In this study, full three dimensional Computational Fluid Dynamics (CFD) simulations were performed to estimate the flow induced drag on piggyback risers under steady current configurations. The turbulent structures generated in the flow domain are resolved using SST k-omega turbulence modeling approach.Effect of flow velocities and circumferential position of the smaller cylinder with respect to the larger cylinder, were investigated on drag coefficient. Two different piggyback riser models, with varying cylinder diameters were considered. The predicted drag coefficients under a wide range of Reynolds number were in good agreement with the experimental data. The effect of diameter ratio of the two cylinders and the position of smaller cylinder were identified to play an important role in the generation of flow structures within the domain. Numerical simulations identify and capture key hydrodynamics interference such as drag reduction and vortex shedding patterns that are critical in the design of piggyback riser configurations. For example, the drag coefficient of the largest cylinder is lowest value when it is placed in the wake of the smaller cylinder and the drag on the small cylinder depends significantly on the diameter ratio of the cylinders and their relative position. Further, detailed discussions pertaining to vortex shedding patterns under various model configurations are elaborated. This study clearly demonstrated the applicability of numerical tools to gain insight into the hydrodynamic interaction between two cylinders placed under close proximity.Copyright

The Use of Scale Model to Study Film Flow in a Rotary Atomizer Cup

The aim of this chapter is to simulate processes of liquid droplet formation and atomization in a typical automotive paint spray system with scale modeling technique. In a spray painting process, paint is sprayed by a bell sprayer cup rotating at high speed to create fine atomized paint particles. The bell sprayers typically rotate at a speed of 30,000 rpm and have liquid (paint) flowing from the center with a flow rate of 300 cc/min. Due to the centrifugal action of the rotation, the liquid flows to the exterior of the bell, where they pass through grooves leading to the formation of ligaments and henceforth droplets due to shear forces acting against the surface tension forces. We designed different types of scale models to simulate the process.

Ultrasonically Driven Cavitating Atomizer: Prototype Fabrication and Characterization

A new device, ultrasonic cavitating atomizer (UCA), has been developed that uses ultrasonically driven cavitation to produce fine droplets. In the UCA the role of cavitation is explicitly configured to enhance the breakup of the liquid jet exiting the nozzle into fine droplets; the pressure modulation also assists the breakup process. The experimental study involves the fabrication of a prototype and the building of an experimental rig to test the prototype using water as the working fluid. The parameters tested include liquid injection pressure, horn tip frequency and liquid flow rate. The result shows improvement in the atomization of water with the application of ultrasonic cavitation.Copyright