Preface 1.General Considerations 2.Basic Processes in Atomization 3.Drop Size Distributions of Sprays 4.Atomizers 5.Flow in Atomizers 6.Atomizer Performance 7.External Spray Charcteristics 8.Drop Evaporation 9.Drop Sizing Methods Index

Atomization and Sprays

Simulation of liquid jet primary breakup: Dynamics of ligament and droplet formation

The production of a liquid spray can be summarized as the succession of the following three steps; the liquid flow ejection, the primary breakup mechanism and the secondary breakup mechanism. The intermediate step—the primary breakup mechanism—covers the early liquid flow deformation down to the production of the first isolated liquid fragments. This step is very important and requires to be fully understood since it constitutes the link between the flow issuing from the atomizer and the final spray. This paper reviews the experimental investigations dedicated to this early atomization step. Several situations are considered: cylindrical liquid jets, flat liquid sheets, air-assisted cylindrical liquid jets and air-assisted flat liquid sheets. Each fluid stream adopts several atomization regimes according to the operating conditions. These regimes as well as the significant parameters they depend on are listed. The main instability mechanisms, which control primary breakup processes, are rather well described. This review points out the internal geometrical nozzle characteristics and internal flow details that influence the atomization mechanisms. The contributions of these characteristics, which require further investigations to be fully identified and quantified, are believed to be the main reason of experimental discrepancies and explain a lack of universal primary breakup regime categorizations.

On the experimental investigation on primary atomization of liquid streams

Numerical simulation of primary break-up and atomization: DNS and modelling study

Transverse combustion instabilities: Acoustic, fluid mechanic, and flame processes

Up to now, measurement of drop size remains difficult in dense sprays such as those encountered in Diesel applications Commonly used diagnostics are often limited due to multi-scattering effects, high drop velocity and concentration and also nonspherical shapes The advantage of image-based techniques on the others is its ability to describe the shape of liquid particles that are not fully atomized or relaxed In the present study, a model is developed to correct the main drawbacks of imaging It permits to define criteria for the correction of the apparent size of an unfocused drop and to determine a measurement volume independent of the drop size This considerably reduces the over-estimation of large drops in the drop size distribution Drop shapes are also characterized by four morphological parameters The image-based granulometer is satisfactorily compared to a PDPA and a diffraction-based granulometer for measurements on an ultrasonic spray Then, the new granulometer is applied to a diesel spray One of the results of the analysis is that even if mean drop size distributions are stable 30 mm downstream from the nozzle outlet, the shape of the drops is still evolving towards the spherical shape The atomization process is thus not totally established at this position in opposition to what can be deduced from the drop size distribution alone

Droplet size and morphology characterization for dense sprays by image processing : application to the diesel spray

Acoustic instabilities with frequencies roughly higher than 1 kHz remain among the most harmful instabilities, able to drastically affect the operation of engines and even leading to the destruction of the combustion chamber. By coupling with resonant transverse modes of the chamber, these pressure fluctuations can lead to a large increase of heat transfer fluctuations, as soon as fluctuations are in phase. To control engine stability, the mechanisms leading to the modulation of the local instantaneous rate of heat release must be understood. The commonly developed global approaches cannot identify the dominant mechanism(s) through which the acoustic oscillation modulates the local instantaneous rate of heat release. Local approaches are being developed based on processes that could be affected by acoustic perturbations. Liquid atomization is one of these processes. In the present paper, the effect of transverse acoustic perturbations on a coaxial air-assisted jet is studied experimentally. Here, five breakup regimes have been identified according to the flow conditions, in the absence of acoustics. The liquid jet is placed either at a pressure anti-node or at a velocity anti-node of an acoustic field. Acoustic levels up to 165 dB are produced. At a pressure anti-node, breakup of the liquid jet is affected by acoustics only if it is assisted by the coaxial gas flow. Effects on the liquid core are mainly due to the unsteady modulation of the annular gas flow induced by the acoustic waves when the mean dynamic pressure of the gas flow is lower than the acoustic pressure amplitude. At a velocity anti-node, local nonlinear radiation pressure effects lead to the flattening of the jet into a liquid sheet. A new criterion, based on an acoustic radiation Bond number, is proposed to predict jet flattening. Once the sheet is formed, it is rapidly atomized by three main phenomena: intrinsic sheet instabilities, Faraday instability and membrane breakup. Globally, this process promotes atomization. The spray is also spatially organized under these conditions: large liquid clusters and droplets with a low ejection velocity can be brought back to the velocity anti-node plane, under the action of the resulting radiation force. These results suggest that in rocket engines, because of the large number of injectors, a spatial redistribution of the spray could occur and lead to inhomogeneous combustion producing high-frequency combustion instabilities.

Behaviour of an air-assisted jet submitted to a transverse high-frequency acoustic field

A method to distinguish a hidden object from a perturbing environment is to use an ultrashort femtosecond pulse of light and a time-resolved detection. To separate ballistic light containing information on a hidden object from multiscattered light coming from the surrounding environment that scrambles the signal, an optical Kerr gate can be used. It consists of a carbon disulfide (CS2) cell in which birefringence is optically induced. An imaging beam passes through the studied medium while a pump pulse is used to open the gate. The time-delayed scattered light is excluded from measurements by the gate, and the multiple-scattering scrambling effect is reduced. In previous works, the two beams had the same wavelength. We propose a new two-color experimental setup for ballistic imaging in which a second harmonic is generated and used for the image, while the fundamental is used for gate switching. This setup allows one to obtain better resolution by using a spectral filtering to eliminate noise from the pump pulse, instead of a spatial filtering. This new setup is suitable for use in ballistic imaging of dense sprays, multidiffusive, and large enough to show scattered light time delays greater than the gate duration (τ=1.3 ps).

Time gate, optical layout, and wavelength effects on ballistic imaging

A diesel jet visualisation based on an ultra-short light pulse and a time-resolved detection is proposed. An optical gate, consisting of a beta barium borate crystal, allows to separate ballistic, refractive and scattered light. An imaging pulse passes through the studied medium, while a gating pulse is used to open the gate. With this configuration, a spatial resolution of 10 μm and a temporal resolution of 270 fs are obtained. The gate duration is compatible with the typical dimensions of a liquid jet generated by a commercial automotive fuel injector. Preliminary results show that spatio-temporal diagrams of transmitted light through a spray may be acquired, showing clearly the separation between ballistic, refracted and scattered light.

Jean-Bernard Blaisot

Papers

Droplet size and morphology characterization for dense sprays by image processing : application to the diesel spray

Behaviour of an air-assisted jet submitted to a transverse high-frequency acoustic field

Time gate, optical layout, and wavelength effects on ballistic imaging

Sub-picosecond ballistic imaging of a liquid jet

Instabilities on a free falling jet under an internal flow breakup mode regime