Structure and atomization properties of dense turbulent sprays

Abstract

Aspects of the structure and atomization properties of the near-injector (dense-spray) region of turbulent sprays are reviewed, considering: spray breakup regimes, dense-spray structure, and liquid breakup processes. The discussion is limited to nonevaporating round pressure-atomized sprays—a fundamental configuration that is representative of dense sprays since they normally occupy the cool portions of combusting sprays where vaporization rates are modest. Due to the complexity of spray breakup, criteria for breakup regime transitions are still not well developed. More information is particularly needed concerning effects of flow properties at the injector exit, ambient turbulence levels, and high Ohnesorge numbers where viscous effects become important. Existing measurements of dense-spray structure are limited to atomization breakup, since most practical applications involve this regime. The dense-spray region for atomization breakup consists of a liquid core (like the potential core of a single-phase jet) surrounded by a multiphase mixing layer that begins right at the injector exit. Recent measurements within the multiphase mixing layer show that the flow is surprisingly dilute and support the classical view of atomization, i.e., primary breakup at the liquid surface followed by secondary breakup of ligaments and large drops with only minor effects of collisions. In contrast, some recent computational studies suggest significant effects of collisions in dense sprays—clearly, this controversy must be resolved. Accepting the classical view, characteristic secondary-breakup and residence times are comparable in dense sprays; therefore, breakup dominates dense sprays much like drop vaporization dominates dilute sprays. Continued primary breakup along the liquid core implies significant separated-flow effects within the multiphase mixing layer, with locally-homogeneous flow only approached at large Weber numbers for sufficiently low Ohnesorge numbers. Thus, many unresolved problems of separated flow in dilute sprays are relevant to dense sprays, while breakup and modification of turbulence by dispersed phases are particularly important for dense sprays. Primary and secondary breakup have been widely studied but the dense-spray environment highlights new problems. In particular, more information is needed concerning breakup regime transitions, the temporal evolution and outcome of breakup processes, breakup mechanisms when the liquid core is turbulent, and effects of gas-phase turbulence on breakup.