P.-K. Wu

Structure and breakup properties of sprays

Abstract Multiphase flow phenomena relevant to spray combustion are reviewed, emphasizing the structure of the near-injector dense-spray region and the properties of secondary and primary breakup. Existing measurements of dense-spray structure are limited to round pressure-atomized sprays in still gases and show that the dispersed flow region is surprisingly dilute, that separated flow effects are significant because the flow is dilute and developing, and that atomization involves primary breakup at the liquid surface followed by secondary breakup, while effects of collisions are small. Available information about secondary breakup emphasizes breakup due to shock wave disturbances at large liquid/gas density ratios and shows that secondary breakup is a dominant feature of dense sprays that must be resolved as a function of time so that secondary breakup can be properly treated as a rate process. Finally, available information about primary breakup has been dominated by effects of disturbances in the injector passage; therefore, while some understanding of turbulent primary breakup has been achieved, more information about aerodynamic primary breakup is needed to address practical spray combustion processes.

Onset and end of drop formation along the surface of turbulent liquid jets in still gases

The onset and end of drop formation along the surface of turbulent liquid jets in still gases was studied for liquid/gas density ratios greater than 500 where aerodynamic effects are small. Measurements were correlated using a phenomenological turbulent breakup theory based on the interactions between the turbulence energy spectrum and the energy requirements for drop formation. The onset and end of drop formation along the surface generally was associated with turbulent eddies in the internal and large‐eddy subranges of the turbulence spectrum, respectively, although breakup of the entire liquid column ends breakup along the surface in some instances, as well.

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.

Dispersed-phase structure of pressure-atomized sprays at various gas densities

The dispersed-phase structure of the dense-spray region of pressure-atom ized sprays was studied for atomization breakup conditions, considering large-scale (9.5 mm initial diameter) water jets in still-air at ambient pressures of 1, 2, and 4 atm, with both fully developed turbulent pipe flow and nonturbulent slug flow at the jet exit. Drop sizes and velocities and liquid-volume fractions and fluxes were measured using holography. Measurements were compared with predictions based on the locally homogeneous flow (LHF) approximation as well as recent correlations of drop sizes after primary breakup of turbulent and nonturbulent liquids. The dispersed-flow region beyond the liquid surface was relatively dilute (liquid-volume fractions less than 0.1%), with significant separated-flow effects throughout, and evidence of near-limit secondary breakup and drop deformation near the liquid surface. Turbulent primary breakup predictions were satisfactory at atmospheric pressure, where the correlation was developed, but failed to predict observed trends of decreasing drop sizes with increasing gas density due to aerodynamic effects; in contrast, the laminar primary breakup predictions successfully treated the relatively small effects of gas density for this breakup mechanism. Effects of liquid turbulence at the jet exit were qualitatively similar to single-phase flows, yielding faster mixing rates with increased turbulence levels even though drop sizes tended to increase as well. LHF predictions within the dispersed-flow region were only qualitatively correct due to significant separated-flow effects, but tended to improve as the ambient pressure and the distance from the jet exit increased.