Dongjun Ma

Atomization patterns and breakup characteristics of liquid sheets formed by two impinging jets

ligament, are studied. The circumferentially space drops were shed from the periphery of the sheet, as well as the ligaments were fragmented from the leading edge of the sheet and then broke into droplets following the Rayleigh mechanism. The periodic waves from the point of impingement were apparent on the surface of the sheet. The impact waves caused early breakdown of the sheet downstream of the impingement point, whereas waves amplied by aerodynamic stresses controlled the breakdown of the rest of the sheet and the ligaments. The impinging jets for non-Newtonian uid are also investigated, two dierent ow patterns are observed.

Energy and Mass Transfer during Binary Droplet Collision

The present study focuses on binary droplet interactions over a wide range of Weber numbers and impact parameters. The formulation is based on the complete set of conservation equations for both the liquid and surrounding gas phases. An improved volume-of-fluid (VOF) technique, augmented by an adaptive mesh refinement (AMR) algorithm, is used to track the liquid/gas interfaces. Detailed information about the droplet dynamics, including collision, deformation, coalescence, separation, and mass and energy transfer, is obtained systematically. Various underlying processes of droplet dynamics are investigated by analyzing the overall energy budget. In addition, an advanced visualization technique using the Ray-tracing methodology is implemented to gain direct insight into the detailed physics of droplet interaction. Theories are also established to predict the droplet behavior after collision. Good agreement is achieved with simulation and experimental results.

Mechanism Study of Impact Wave in Impinging Jets Atomization

The physical mechanism of impact wave and mixing process in the atomization of impinging jets are computationally investigated. Using an accurate adaptive solver for surface-tension-driven interfacial flows, the simulations are found to agree well with experiment data for both wavelength of impact wave and droplet size distribution. The formation of impact wave is studied by analyzing velocity profiles and vortices contours of liquid sheet formed by impinging jets. It shows that the impact wave is caused by the interfacial shear stress which forms the capillary waves on the two sides. The interaction of waves on the two sides forces the liquid sheet to resonate at natural frequency. The effect of impact wave to mixing process of both miscible and immiscible impinging jets is also addressed.

Phenomenology of Secondary Breakup of Newtonian Liquid Droplets

In this paper, deformation and breakup of Newtonian liquid droplets at elevated pressures have been studied. Detailed physics pertaining to four different breakup regimes, oscillatory, bag, multimode and shear breakup modes , has been investigated using an incompressible interface tracking methodology. The accuracy and efficiency of the code was enhanced by incorporating an adaptive mesh refinement (AMR) techni que. In general, the aerodynamic drag force exerted by the ambient fluid causes the droplet to deform. The deformation is resisted by viscous and surface tension forces. The breakup mechanism becomes progressively violent as the We number increases, and moves from oscillatory to shear breakup regime. Quantitatively, the droplet lifetime decreases as the inertial force is increased in comparison to the deformation resisting, surface tension force. A criterion to mark the beginning of breakup has been quantified in terms of surface and kinetic energy associated with the droplet. Critical We numbers for the three regimes have also been identified for 100 atm pressure conditions. A generalized regime diagram, for Oh < 0.1, was developed to predict the various breakup modes, taking into account the pressure effect on critical We number, using data from previous experimental investigations , and simulations conducted during the current study.

Collision Outcome and Mass Transfer of Unequal-sized Droplet Collision

The investigations of the collision outcome and dynamics of mass transfer process of unequal-sized binary droplets collision are preformed computationally and theoretically. All the possible collision outcomes, bouncing, coalescence, reflexive and stretching separation are obtained numerically. The present study focuses on water droplet interactions in atmospheric air over a wide range of Weber numbers and impact parameters with two droplet diameter ratio, 0.5 and 0.25. The numerical formulation is based on the complete set of conservation equations for both the liquid and the surrounding gas phases. An improved volume-of-fluid (VOF) technique, augmented by an adaptive mesh refinement (AMR) algorithm, is used to track the liquid/gas interfaces. A novel thickness-based refinement methodology is also developed to resolve the extremely thin gas film during early approaching of droplets. Numerous high-fidelity bouncing cases are simulated using proposed efficient methodology. The smallest cell size is up to O(10 -4 ) of the small droplet diameter with minimum grid size of 0.02 μm. In addition, an advanced visualization technique using the Ray-tracing methodology is implemented to gain direct insight into the detailed physics of droplet interaction and mass transfer process. A simple theory based on the geometric relation of the unequal-sized droplet collision system is established to predict mass transfer ratio of stretching separation outcome. Good agreement is achieved with simulation results.

High-Fidelity Numercial Simulations of Impinging Jet Atomization

High fidelity numerical simulations are performed to study the atomization process of impinging jets over a broad range of operating conditions. An improved volume-of-fluid (VOF) method augmented with adaptive mesh refinement (AMR) is used to simulate the formation and breakup of the liquid sheet formed by two impinging jets. In addition, a thickness-based refinement method is developed and implemented to automatically and efficiently resolve the drastic changing of requested grid resolution. The behaviors in various Reynolds and Weber number regimes are studied systemically. The predicted liquid sheet topology, atomization, and droplet size distribution agree well with experimental measurements. Several different patterns of sheet and rim configurations are obtained, including liquid chain, closed rim, fish-bone, disintegrating sheet, disintegrating rim and impact wave. The instability mechanisms of sheets and rims are studied based on the concepts of absolute and convective instabilities. New knowledge is acquired about the onset of sheet and rim instabilities. This locking-on feature of Strouhal number of impact wave is found based on the previous detailed study. Finally, schematic diagrams of all kinds of instabilities happens in impinging jet atomization are obtained. I. INTRODUCTION Collision between two cylindrical jets is one of the generic configurations for the generation of liquid sheets, the dynamics and stability of which have attracted a great deal of attention due to their relevance to the spray atomization and combustion in liquid propellant engines.[1-4] The impingement of liquid jets is a very efficient method for atomization and mixing whereby the dynamic head of the propellant is used to destabilize an opposing liquid propellant stream. This in turn results in fragmenting the liquid into ligaments and droplets.[5] The oblique collision of two cylindrical laminar jets at low jet velocities leads to the liquid flowing outward from the impingement point, producing a leaf-shaped expanding sheet which lies in a plane perpendicular to the plane containing the two liquid jets,. A rich variety of flow structures, from single oscillating jet obtained at low flow rates, to the violent disintegration of flapping sheets obtained at higher flow rates, have been observed depending on the Weber and Reynolds numbers of the jets. In the specific usage of liquid propellant rocket engine, the liquid sheet shows full-developed violent breakup with quickly growing waves. The liquid sheet destabilizes, breaks and eventually disintegrates into ligaments or droplets under the influence of surface tension, viscous, inertial, and aerodynamic forces. The impinging jets can provide rapid mixing and atomization. This unstable hydrodynamic wave is usually called impact wave[6]. Impact wave dominates the breakup and atomization process in most rocket combustors using impinging jet injectors.[7] Different from another capillary wave, impact wave shows a nonlinear behavior that cannot be described linear stability analysis.

Evolution of Richtmyer-Meshkov Instability with Single-Mode Perturbation

The Richtmyer-Meshkov instability occurs when a perturbed interface between two fluids of dierent densities is impulsively accelerated by a shock wave. A major issue with the Richtmyer-Meshkov instability is the nonlinear growth of the interpenetrating mixing layer and ensuring turbulent mixing. The present paper represents a numerical investigation on the nonlinear evolution of the Richtmyer-Meshkov instability excited by a high-amplitude single-mode perturbation. The work simulates the Mach 1.15 shock tube experiment of Jourdan and Houas [Phys. Rev. Lett. 95, 204502, 2005]. An asymptotic analysis is also performed to provide more direct insight. Four dierent cases of air/SF 6, air/CO2, air/N2, and air/He are are studied, covering a wide range of Atwood numbers of 0.77 0.6. A mixture-type interface capturing method, which can eectively avoid numerical oscillations on the interface discontinuities, is used to simulate the multi-fluid flows. The high-resolution Piecewise-Parabolic-Method (PPM) with a multi-fluid Riemann solver is employed to solve the proposed model equations. Calculated results show good agreement with experimental data and theoretical predictions.