Secondary Drop Breakup Research Articles

Secondary breakup of drops at moderate Weber numbers: Effect of Density ratio and Reynolds number

Breakup of liquid drops occurs in several natural and industrial settings. Fully resolved volume of fluid based simulations presented in this study reveal the complete flow physics and droplet dynamics that lead to the breakup of a drop in the moderate Weber number regime. We have investigated the effects of density ratio and Reynolds number on the initial dynamics of drop deformation. A density ratio-Weber number phase plot is presented that indicates the variation in the deformation of the drop at various density ratios and Weber numbers. We show that the initial deformation and dynamics of the droplets at low density ratios is significantly different from that observed at high density ratios. We also study the temporal characteristics of the initial droplet deformation and motion.

Secondary Breakup of Drops

Secondary atomization of droplets generated from primary atomization is observed in high-speed flows. In natural scenarios, falling raindrops undergo aerodynamic breakup that modifies the resulting drop size distribution on the ground. In this article, we review some aspects of the drop breakup mechanisms, initial deformation, drag characteristics, and time scales of breakup. We also review some of the secondary atomization models, such as TAB and DDB, proposed in the literature. We discuss the role of new numerical algorithms for two-phase flow simulations in providing insights into the exact physical mechanisms involved during drop breakup.

Temporal properties of secondary drop breakup in the bag-stamen breakup regime

The situation of liquid drop bag-stamen breakup in a continuous air jet flow is investigated by a high speed camera. Test liquids include water, ethanol, and various glycerol mixtures. First, the morphology of bag-stamen breakup is observed and analyzed. The bag-stamen breakup range is found to be in good agreement with the model obtained by Rayleigh-Taylor instability. Then the disk and stamen deformation properties, the fragment average size, and size distribution of ring and stamen are researched in detail, respectively.

Breakup of Round Nonturbulent Liquid Jets in Gaseous Crossflow

An experimental investigation of the primary breakup of nonturbulent round liquid jets in gas crossflow is described. Pulsed shadowgraph and holograph observations of jet primary breakup regimes, conditions for the onset of breakup, properties of waves observed along the liquid surface, drop sue and velocity properties resulting from breakup and conditions required for the breakup of the liquid column as a whole, were obtained for air crossflows at normal temperature and pressure. When combined with the earlier studies of Mazallon et al. (1999), the test range included crossflow Weber numbers of 02000, liquidgas momentum ratios of 100-8000, liquidgas density ratios of 683-1021, and Ohnesorge numbers of 0.003-0.12. The results suggest qualitative similarities between the primary breakup of nonturbulent round liquid jets in crossflows and the secondary breakup of drops subjected to shock wave disturbances (e.g., bag, multimode and shear breakup regimes are observed in both instances) with relatively little effect of the liquidgas momentum ratio on breakup properties over the present test range. Effects of liquid viscosity were also small for present observations where Ohnesorge numbers were less than 0.4. Phenomenological analyses were successful for helping to interpret and correlate the properties of primary breakup of round liquid jets in gas crossflows that were measured during the present investigation.

Temporal properties of secondary drop breakup in the multimode breakup regime

Secondary drop breakup due to shock wave disturbances was studied for the multimode breakup regime, emphasizing the temporal evolution of breakup for shock wave disturbances. Measurements were carried out in a shock tube using pulsed shadowgraphy and holography to observe the mechanism and outcome of breakup. Test conditions involved water and ethanol drops, liquid/gas density ratios greater than 500, Ohnesorge numbers less than 0.1 and Weber numbers of 15–150. The evolution of properties in the multimode breakup regime with increasing Weber number begins at the end of the bag breakup regime with the appearance of a plume drop at the apex of the bag at a Weber number of roughly 15, continues in a bag/plume breakup regime which involves the presence of both bag-like structures and plume drops and transitions when bags are no longer present at a Weber number of roughly 40, and ends with a plume/shear breakup regime which involves development of plume-like structures that progressively evolve into a parent drop and ligament system as the shear breakup regime is approached at a Weber number of roughly 80. Measurements over the test range provide breakup times, drop deformation properties and drag coefficients before the onset of breakup, distributions of drop liquid and resulting drop sizes for various breakup structures, drop velocities after breakup, and liquid removal rates during breakup; all these properties are provided as a function of Weber number in the multimode breakup regime.

MODELING THE EFFECTS OF GAS DENSITY ON THE DROP TRAJECTORY AND BREAKUP SIZE OF HIGH-SPEED LIQUID DROPS

An experimental and numerical study has been conducted of drop trajectory and breakup mechanisms for liquid drops injected into high velocity gas flows with various chamber gas pressures at room temperature. In the experimental study, air-assisted liquid drop atomization processes were investigated using photographic techniques under well-controlled experimental conditions. A monodisperse stream of drops from a vibrating-orifice drop generator was injected into a transverse high velocity gas stream. The back pressures and gas velocities were adjusted independently to control the drop Weber numbers. The Weber numbers used in the experiments were 72, 148, 270, 532. High-magnification photographs and conventional spray field photography revealed the microscopic breakup mechanisms and the parent drop trajectory in the high velocity flow field, respectively. Drop sizes were measured using a Phase/Doppler particle analyzer. The experimental results were used to test and assess spray models in the KIVA3 code. The breakup model considered Kelvin-Helmholtz (K-H) instability mechanisms to account for secondary drop breakup. The computations show good agreement with experimental results of parent drop trajectories and for the spatial drop size distributions which result from secondary breakup at high gas densities. At low gas densities, it is concluded that the use of K-H model to predict drop breakup may not be justified.

Temporal properties of secondary drop breakup in the bag breakup regime

The temporal properties of secondary drop breakup in the bag breakup regime were measured as a function of time for shock-wave-initiated disturbances in air at normal temperature and pressure. The test liquids included water, ethyl alcohol and various glycerol mixtures to yield liquid/gas density ratios of 633–893, Weber numbers of 13–20, Ohnesorge numbers of 0.0043–0.0427 and Reynolds numbers of 1550–2150. Single- and double-pulse shadowgraphy and holography were used to measure the structure, size and velocity of the parent drop, and the sizes and velocities of drops produced by secondary breakup. The parent drop undergoes significant deformation and lateral growth during breakup before forming a thin bag having a basal ring that is characteristic of the bag breakup regime. The basal ring contains roughly 56% of the initial drop volume (mass) and eventually yields drops having mean diameters of roughly 30% of the initial drop diameter by a Rayleigh breakup process; the size variations of drops formed from the basal ring increases with increasing Weber number due to the appearance of large ‘node’ drops that are characteristic of the onset of the multimode breakup regime. Breakup of the bag yields nearly monodisperse drops having diameters of roughly 4% of the initial drop diameter. The velocity distributions of the drops formed from breakup of the basal ring and the bag were individually independent of drop size but varied as a function of time and differed between the two groups. Many features of these phenomena were successfully correlated using phenomenological analyses. Finally, bag breakup requires considerable time (5–6 characteristic secondary drop breakup times) and extends over considerable streamwise distances (50–100 initial drop diameters) by the end of breakup, which suggests that bag breakup should be treated as a rate process, rather than by jump conditions, in some instances.

Modeling of Gas Turbine Fuel Nozzle Spray

Satisfactory performance of the gas turbine combustor relies on the careful design of various components, particularly the fuel injector. It is, therefore, essential to establish a fundamental basis for fuel injection modeling that involves various atomization processes. A two-dimensional fuel injection model has been formulated to simulate the airflow within and downstream of the atomizer and address the formation and breakup of the liquid sheet formed at the atomizer exit. The sheet breakup under the effects of airblast, fuel pressure, or the combined atomization mode of the airassist type is considered in the calculation. The model accounts for secondary breakup of drops and the stochastic Lagrangian treatment of spray. The calculation of spray evaporation addresses both droplet heat-up and steady-state mechanisms, and fuel vapor concentration is based on the partial pressure concept. An enhanced evaporation model has been developed that accounts for multicomponent, finite mass diffusivity and conductivity effects, and addresses near-critical evaporation. The presents investigation involved predictions of flow and spray characteristics of two distinctively different fuel atomizers under both nonreacting and reacting conditions. The predictions of the continuous phase velocity components and the spray mean drop sizes agree well with the detailed measurements obtained for the two atomizers, which indicates the model accounts for key aspects of atomization. The model also provides insight into ligament formation and breakup at the atomizer exit and the initial drop sizes formed in the atomizer near field region where measurements are difficult to obtain. The calculations of the reacting spray show the fuel-rich region occupied most of the spray volume with two-peak radial gas temperature profiles. The results also provided local concentrations of unburned hydrocarbon (UHC) and carbon monoxide (CO) in atomizer flowfield, information that could support the effort to reduce emission levels of gas turbine combustors.

Continuous- and dispersed-phase structure of dense nonevaporating pressure-atomized sprays

The structure and breakup properties of the dense-spray region of nonevaporating pressure-atomized sprays were studied. The multiphase mixing layer near the injector exit was emphasized, considering large-scale (9.5mm injector diameter) water jets injected vertically downward in still room air. Phase-discriminating laser velocimetry and double-pulse holography were used to measure phase velocities and drop-size properties for both nonturbulent and turbulent jet exit conditions. Present test conditions involved two types of primary breakup: 1) aerodynamic breakup for nonturbulent jets where properties could be correlated using earlier aerodynamic breakup theories; and 2) turbulent breakup for turbulent jets where drop properties could be related to liquid turbulence properties. Both mechanisms yielded Weber numbers exceeding secondary drop breakup limits near the liquid surface. Significant effects of separated flow were observed for present test conditions; however, scaling analysis suggests reduced effects of separated flow at higher injector velocities and ambient pressures—largely due to finer atomization.