Christophe Dumouchel

Influence of the viscosity on the linear stability of an annular liquid sheet

Abstract This paper reports a study of the linear stability of an annular liquid sheet evolving in a gaseous environment at rest. In this investigation, the viscosity of the liquid is taken into account. As has been found in similar approaches developed for other liquid systems, the results reveal the presence of a dominant wave and a cut-off wave number for each set of initial conditions. A complete parametric study is conducted. This shows a very specific influence of the surface tension due to the presence of the natural curvature of the liquid system. It also shows that the influence of the liquid viscosity is stabilising, and that the importance of the action of these forces is a function of the other parameters. A non-dimensional number D is derived from this study. This number, consistent with that derived for flat liquid systems in a previous investigation, allows one to account for the influence of the liquid viscosity on the linear stability of an annular liquid sheet.

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

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.

Viscous flow in a swirl atomizer

Abstract In this paper a numerical analysis is carried out of the velocity field throughout a swirl spray nozzle and more specially at the orifice. After an examination of the parameters involved in the problem, it is found that the characteristics of the conical liquid sheet produced at the nozzles orifice are mainly functions of the geometry of the nozzle: this finding agrees well with a wide range of experimental investigations. Velocity profiles at the orifice are used to determine the spray angle through a simple relation that gives results which agree quite well with the measured angles.

Use of the Maximum Entropy Formalism to Determine Drop Size Distribution Characteristics

The work reported in this paper is an extension of a previous study on the prediction of volume-based drop size distribution from the application of the maximum entropy formalism (M.E.F.). The procedure developed in that study led to the derivation of a two-parameter distribution. The present work investigated the problem of the determination of the two parameters (q and Dq0), i.e. the determination of the information required in the procedure. It was found that the relative span factor Δv and the mean drop diameter D43 of the measured distribution constitute reliable information from which q and Dq0 can be calculated. It is shown that the distributions calculated in this way generally show a better fit than distributions obtained from different information or from an extended set of constraints. This work therefore illustrates the importance of the nature of the information to be introduced in the application of the M.E.F. Furthermore, it shows how the M.E.F. can be used to determine relevant spray drop size characteristics and the extent to which this formalism could provide relevant information to study and characterize atomization processes.

Investigation on the Drop Size Distribution of SpraysProduced by a High-Pressure Swirl Injector. Measurementsand Application of the Maximum Entropy Formalism

This paper reports an investigation on the volume-based drop size distribution of sprays produced by swirl atomizers dedicated to direct-injection spark-ignited engines. Because of the use of high injection pressures to reduce the atomization time, the spatial density of the spray is high and prevents from classical measurements of spray drop size distribution. This problem is overcome by combining an experimental approach to the application of the maximum entropy formalism (M.E.F.). Based on the determination of correction factor series to correct the measurements from multiple light scattering, the experimental procedure allows obtaining some distribution characteristic features. According to a previous study, two of these characteristics are used as information in a M.E.F. procedure to derive the spray volume-based drop size distribution. This characteristic is of paramount importance for evaluating the large drop population with accuracy. The overall procedure is presented in detail and discussed. It was applied to a series of four swirl atomizers in order to study the influence of the nozzle geometry and of the injection pressure on the injector performances. Conducted under both stationary and transient working conditions, this study allows a more precise understanding of the performances of GDI injectors as far as the spray drop size distributions are concerned.

Behavior of cylindrical liquid jets evolving in a transverse acoustic field

This paper presents a theoretical and an experimental investigation of low-velocity cylindrical liquid jets submitted to transverse planar acoustic waves. For this purpose, the behavior of a liquid jet traversing the section of a Kundt tube was examined. Experiments reported that the liquid jet could be either deviated from its trajectory or deformed as a succession of lobes oriented in space and whose length and width depend on the jet acoustic environment. Furthermore, for a sufficient acoustic velocity, the jet deformation increases in such proportion that a premature and vivid atomization mechanism disintegrates the liquid flow. Theoretical models are proposed to understand these behaviors. The first one calls out for acoustic radiation pressure to explain the jet deviation. The second one consists in a modal analysis of the vibrations of a jet when submitted to a transverse stationary acoustic field. As a first approach, a simplified two-dimensional model is proposed. This model reports that a sudden...

Development of a Three‐parameter Volume‐based Spray Drop Size Distribution through the Application of the Maximum Entropy Formalism

This work is an extension of a previous investigation on the determination of mathematical volume-spray drop size distributions by the application of the maximum entropy formalism. A two-parameter drop size distribution was derived and was found to give reasonable fits with experimental distributions obtained under different experimental conditions. However, as it is discussed, this two-parameter distribution shows critical limitations and cannot be applied in any situations of interest as far as drop size distributions in liquid sprays are concerned. To overcome this problem, a third parameter, equivalent to a drop diameter, is introduced into the procedure. This correction leads to a three-parameter drop size distribution with independent mean, width and symmetry. This function is a generalized gamma distribution and it can cover more practical situations than the previous two-parameter distribution. Furthermore, it is found that, contrary to the two-parameter distribution, the new volume-based drop size distribution shows a corresponding number-based drop size distribution with a physical behavior as the drop diameter decreases. This last result shows the importance of using three parameters to describe spray drop size distributions and that one of these parameters must represent the population of small drops.

Behaviour of free falling viscoelastic liquid jets

The authors acknowledge the financial support from the Frend National Research Agency (ANR) through the program Investissement d’Avenir (ANR-10 LABX-09-01), LABEX EMC3

Atomization and deviation of cylindrical water jets in a transverse acoustic field

This work deals with a particular breakup mode experienced by cylindrical liquid jets when submitted to an intense transverse acoustic wave. Experiments on low speed water jets (< 1 m/s) of diameters 3 mm and 6 mm show that sound waves with a frequency ranging from 500 Hz to 1800 Hz can produce bulges along the jet. When the sound level is high enough, these bulges can trigger an effective atomization mechanism where the jet flattens as a liquid sheet before disintegrating. Sound field can also induce steady deviation of the jet. Both phenomena are theoretically studied. A first model, which treats bulges as outward marks of one particular mode of vibration of the liquid column, is proposed. This model leads to a criterion for the onset of atomization that satisfactorily agrees with experimental observations of the present work. A second analysis identifies deviations as radiation pressure effects. It predicts the direction of experimental deviations with success.