Z. Dai

Liquid breakup at the surface of turbulent round liquid jets in still gases

Abstract An experimental study of liquid column breakup lengths and turbulent primary breakup properties at the surface of turbulent round liquid jets in still air at standard temperature and pressure is described. Jet exit conditions were limited to non-cavitating water and ethanol flows, long length/diameter ratio (greater than 40:1) constant-diameter round injector passages, jet exit Reynolds numbers of 5000–200,000, jet exit Weber numbers of 235–270,000 and liquid/gas density ratios of 690 and 860, at conditions where direct effects of viscosity were small (e.g., liquid jet Ohnesorge numbers were smaller than 0.0053). Three liquid column breakup modes were observed, as follows: a weakly turbulent Rayleigh-like breakup mode observed at small jet exit Weber and Reynolds numbers, a turbulent breakup mode observed at moderate jet exit Weber numbers, and an aerodynamic bag/shear breakup mode observed at large jet Weber numbers. The turbulent primary liquid column breakup mode was associated with conditions where drop diameters resulting from turbulent primary breakup along the liquid surface were comparable to the diameter of the liquid column itself. The bag/shear liquid column breakup mode was observed when liquid turbulence caused large deformations of the liquid column so that portions of it were in a gaseous cross flow; this resulted in bag or shear liquid column breakup, very similar to the breakup of non-turbulent liquid jets in gaseous cross flow. Mean streamwise drop velocities after breakup were comparable to mean streamwise velocities within the jet whereas mean cross stream drop velocities after breakup were comparable to cross stream velocity fluctuations within the liquid. Rates of primary breakup at the liquid surface are reported as surface efficiency factors (the fraction of the maximum possible cross stream drop liquid flux at the surface based on the mean relative cross stream velocity of the drops at the surface and the liquid density). The resulting surface efficiency factors varied from small values near the onset of liquid surface breakup to values of the order of unity as conditions near breakup of the liquid column as a whole were approached.

Temporal properties of secondary drop breakup in the multimode breakup regime

Abstract 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.

Structure of Round, Fully Developed, Buoyant Turbulent Plumes

An experimental study of the structure of round buoyant turbulent plumes was carried out, emphasizing conditions in the fully developed (self-preserving) portion of the flow. Plume conditions were simulated using dense gas sources (carbon) dioxide and sulfur hexafluoride) in a still air environment. Mean and fluctuating mixture fraction properties were measured using single-and two-point laser-induced iodine fluorescence. The present measurements extended farther from the source (up to 151 source diameters) than most earlier measurements (up to 62 source diameters) and indicated that self-preserving turbulent plumes are narrower, with larger mean and fluctuating mixture fractions (when appropriately scaled) near the axis, than previously thought. Other mixture fraction measurements reported include probability density functions, temporal power spectra, radial spatial correlations and temporal and spatial integral scales.

Velocity Statistics of Round, Fully Developed, Buoyant Turbulent Plumes

An experimental study of the structure of round buoyant turbulent plumes was carried out, limited to conditions within the fully developed (self-preserving) portion of the flow. Plume conditions were simulated using dense gas sources (carbon dioxide and sulfur hexafluoride) in a still air environment. Velocity statistics were measured using laser velocimetry in order to supplement earlier measurements of mixture fraction statistics using laser-induced iodine fluorescence. Similar to the earlier observations of mixture fraction statistics, self-preserving behavior was observed for velocity statistics over the present test range, (87-151 source diameters and 12-43 Morton length scales from the source), which was farther from the source than most earlier measurements. Additionnally, the new measurements indicated that self-preserving plumes are narrower, with larger mean streamwise velocities near the axis (when appropriately scaled) and with smaller entrainment rates, than previously thought. Velocity statistics reported include mean and fluctuating velocities, temporal power spectra, temporal and spatial integral scales, and Reynolds stresses

Velocity/mixture fraction statistics of round, self-preserving, buoyant turbulent plumes

An experimental study of the structure of round buoyant turbulent plumes was carried OUL limited to conditions m the self-preserving portion of the flow. Plume conditions were simulated using dense gas sources (carbon dioxide and sulfur hexafluoride) in a still and unstratifkd air environment. Velocity/mixturefraction statistics, and other higher-order turbulence quantities, were measured using laser velocimetry and laser-induced fluorescence. Similar to earlier observations of these plumes, self-preserving behavior of all properties was observed for the present test range, which involved streamwise distances of 87-151 source diameters and 12-43 Morton length scales from the source. Streamwise turbulent fluxes of mass and momentum exhibited countergradient diffusion near the edge of the flow, although the much more significant radial fluxes of these properties satisfied gradient diffusion in the normal manner. The turbulent Prandtl/Schmidt number, the ratio of time scales characterizing velocity and mixture function fluctuations and the coefficient of the radial gradient diffusion approximation for Reynolds stress, all exhibited significant variations across the flow rather than remaining constant as prescribed by simple turbulence models. Fourth moments of velocity and velocity/mixture fraction fluctuations generally satisfied the quasi-Gaussian approximation. Consideration of budgets of turbulence quantities provided information about kinetic energy and scalar variance dissipation rates, and also indicated that the source of large mixture fraction fluctuations near the axis of

Hydrodynamic suppression of soot formation in laminar coflowing jet diffusion flames

Effects of flow (hydrodynamic) properties on limiting conditions for soot-free laminar non-premixed hydrocarbon/air flames (called laminar soot-point conditions) were studied, emphasizing non-buoyantlaminar coflowing jet diffusion flames. Effects of air/fuel-stream velocity ratios were of particular interest: therefore, the experiments were carried out at reduced pressures to minimize effects of flow acceleration due to the intrusion of buoyancy. Test conditions included reactant temperatures of 300 K; ambient pressures of 3.7–49.8 kPa: methane-, acetylene-, ethylene-, propane-, and methane-fueled flames burning in coflowing air with fuel-port diameters of 1.7, 3.2, and 6.4 mm; fuel jet Reynolds numbers of 18–121; air coflow velocities of 0–6 m/s: and air/fuel-stream velocity ratios of 0.003–70. Measurements included laminar soot-point flame lengths, laminar soot-point fuel flow rates, and laminar liftoff conditions. The measurements show that laminar soot-point flame lengths and fuel flow rates can be increased, broadening the range of fuel flow rates where the flames remain soot free, by increasing air/fuel-stream velocity ratios. The mechanism of this effect involves the magnitude and direction of flow velocities relative to the flame sheet where increased air/fuel-stream velocity ratios cause progressive reduction of flame residence times in the fuel-rich soot-formation region. The range of soot-free conditions is limited by both liftoff, particularly at low pressures, and the intrusion of effects of buoyancy on effective air/fuel-stream velocity ratios, particularly at high pressures. Effective correlations of laminar soot- and smoke-point flame lengths were also found in terms of a corrected fuel flow rate parameter, based on simplified analysis of laminar jet diffusion flame structure. The results show that laminar smoke-point flame lengths in coflowing air environments are roughly twice as long as soot-free (blue) flames under comparable conditions due to the presence of luminous soot particles under fuel-lean conditions when smoke-point conditions are approached. This is very similar to earlier findings concerning differences between laminar smoke- and soot-point flame lengths in still environments.

Mixing Structure of Plane Self-Preserving Buoyant Turbulent Plumes

Measurements of the structure of plane buoyant turbulent plumes are described, emphasizing conditions in the fully developed (self-preserving) portion of the flow. Plumes were simulated using helium/air sources in a still and unstratified air environment. Mean and fluctuating mixture fractions were measured using laser-induced iodine fluorescence. Present measurements extended farther from the source (up to 155 source widths) and had more accurate specifications of plume buoyancy fluxes than past measurements and yielded narrower plume widths and different scaled mean and fluctuating mixture fractions near the plane of symmetry than previously thought. Measurements of probability density functions, temporal power spectra, and temporal integral scales of mixture fraction fluctuations are also reported

Mixture Fraction Statistics of Plane Self-Preserving Buoyant Turbulent Adiabatic Wall Plumes

Measurements of the mixture fraction properties of plane buoyant turbulent adiabatic wall plumes (adiabatic wall plumes) are described, emphasizing conditions far from the source where self-preserving behavior is approximated. The experiments involved helium/air mixtures rising along a smooth, plane and vertical wall. Mean and fluctuating mixture fractions were measured using laser-induced iodine fluorescence. Self-preserving behavior was observed 92-155 source widths above the source, yielding smaller normalized plume widths and near-wall mean mixture fractions than earlier measurements. Selfpreserving adiabatic wall plumes mix slower than comparable free line plumes (which have 58 percent larger normalized widths) because the wall prevents mixing on one side and inhibits large-scale turbulent motion. Measurements of probability density functions, temporal power spectra, and temporal integral scales of mixture fraction fluctuations are also reported

AERODYNAMIC PRIMARY BREAKUP AT THE SURFACE OF NONTURBULENT ROUND LIQUID JETS IN CROSSFLOW

An experimental investigation of nonturbulent round liquid jets in air crossflows at normal temperature and pressure was carried out, using pulsed shadowgraphs to observe jet deformation and breakup. Test liquids included water, ethyl alcohol and glycerol mixtures; liquid jet diameters were in the range 0.8-13 mm; liquid velocities were in the range 0-50 m/s; and air velocities were in the range 0-24 m/s. The corresponding test range involved Weber numbers of 2-200, Ohnesorge numbers of 0.00006-0.3, liquid/gas momentum ratios of 100-8000 and liquid/gas density ratios of 580-1020. The observations suggest qualitative similarities between the surface breakup of nonturbulent round liquid jets in crossflow and the secondary breakup of drops; for example, for Ohnesorge numbers less than 0.1, the onset of breakup occurs as bag breakup beginning at a Weber number of 5, there is a second transition to a bag/shear breakup regime at a Weber number of 60 and a third transition to a shear breakup regime at a Weber number of 110. A long-ligament shear breakup regime appears at Ohnesorge numbers greater than 0.1. At the onset of breakup, the deformation of the liquid column yields a frontal diameter roughly twice the initial jet diameter, relatively independent of the breakup regime. The characteristics of waves associated with the breakup process were also studied, finding that bag and bag/shear breakup involved both liquid column and surface waves while shear breakup involved only surface waves. The results also showed that breakup characteristics were mainly influenced by the Weber number while effects of the liquid/gas momentum ratio are small. Nomenclature

Velocity Statistics of Plane Self-Preserving Buoyant Turbulent Adiabatic Wall Plumes

Measurements of the velocity properties of plane buoyant turbulent adiabatic wall plumes (adiabatic wall plumes) are described, emphasizing conditions far from the source where self-preserving behavior is approximated. The experiments involved helium/air mixtures rising along a smooth, plane, and vertical wall. Mean and fluctuating streamwise and cross-stream velocities were measured using laser velocimetry. Self-preserving behavior was observed 92-156 source widths from the source, yielding smaller normalized plume widths and larger near-wall mean velocities than observations within the flow development region nearer to the source. Unlike earlier observations of concentration fluctuation intensities, which are unusually large due to effects of streamwise buoyant instabilities, velocity fluctuation intensities were comparable to values observed in nonbuoyant turbulent wall jets. The entrainment properties of the present flows approximated self-preserving behavior in spite of continued development of the wall boundary layer. Measurements of temporal power spectra and temporal and spatial integral scales of velocity fluctuations are also reported