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Due to current and future environmental and safety issues in space propulsion, typical propellants for upper stage or satellite rocket engines such as the toxic hydrazine are going to be replaced by green propellants like the combination of liquid oxygen and hydrogen or methane. The injection of that kind of cryogenic fluids into the vacuum atmosphere of space leads to a superheated state, which results in a sudden and eruptive atomization due to flash boiling. For a detailed experimental investigation of superheated cryogenic fluids, the new cryogenic test bench M3.3 with a temperature controlled injection system was built at DLR Lampoldshausen. After a first test campaign with high-speed shadowgraphy of flash boiling liquid nitrogen sprays, a laser-based Phase Doppler system was set-up to determine the spatial distributions of droplet velocities and diameters in highly superheated sprays. The spatial distributions revealed a core region with high mean velocities close to the injector orifice. With increasing distance from the injector orifice, the sprays develop a more and more monodisperse pattern. These distributions also showed that atomization due to flash boiling generates finer sprays with growing degrees of superheat. In certain spray regions, two droplet populations varying in their direction of motion, velocity and diameter due to possible recirculation zones were observed. The experimental data of flash boiling liquid nitrogen generated within this study provide a comprehensive data base for the validation of numerical models and further numerical investigations., Deutsche Forschungsgemeinschaft, Projekt DEAL

Collisions between a stream of drops and a continuous jet of a different liquid are experimentally investigated. In contrast to previous studies, our work focuses on the effects of liquid miscibility and wettability on the collision outcomes. Thus, miscible and immiscible liquids providing total and partial wetting are used. We show that, as long as the jet surface tension is smaller than the drop surface tension, the drops can be encapsulated by the jet, providing the so-called drops-in-jet structure. The transitions between the different regimes remain similar in nature with a capillary fragmentation responsible for the jet break-up and an inertial fragmentation causing the drops (and then possibly the jet) to break up. The dimensionless numbers proposed in the literature to model the inertial fragmentation thresholds do not bring the results obtained with different liquids at the same critical value. We explain the reason via a detailed analysis of the collisions, accounting for the drop and jet extensions and their kinetics. The drop fragmentation is found to occur during the recoil phase, leading us to propose a new dimensionless parameter that successfully reproduced all our experimental data obtained with immiscible liquids. Finally we demonstrate that the most dramatic change of the collision outcomes is produced by using drops that totally wet the jet. In this case, the encapsulation of the drops cannot be achieved, constituting a true limit to some applications based on the solidification of the drops-in-jet structure., Comment: 34 pages, 15 figures

The rapid implementation of Selective-Catalytic-Reduction (SCR) technologies into light passenger and commercial vehicles, led to the omission of fundamental research creating several reliability issues that are largely related to the research field of droplet dynamics. In this study, the splashing behaviour of an AdBlue droplet impacting onto thin urea-water solution films is experimentally investigated over a range of impact parameters. In particular, the crown-type splashing threshold, the number of fingers and the characteristics of the ejected secondary droplets are evaluated for various drop impact velocities, wall-film thicknesses and urea concentrations in the liquid film. The results show that impact parameters that are able to enhance the energy dissipation in the wall-film, e.g. film thickness and viscosity, influence negatively the intensity of splashing. On the other hand, as the droplet kinetic energy increases or the wall-film thickness decreases, more energy is available to intensify the splashing outcome, and consequently, the upward ejected secondary droplet volume. The obtained trends are correlated in simple empirical expressions providing a remarkable industrial design tool, and they are also compared to the state-of-the-art literature of single- and binary-droplet/wall-film interactions aiming to draw generalised theories and paradigms that will support the connection between SCR applications and academic research.

Mixing characteristics of supercritical injection studies were analyzed with regard to the necessity to include diffusive fluxes. Therefore, speed of sound data from mixing jets were investigated using an adiabatic mixing model and compared to an analytic solution. In this work, we show that the generalized application of the adiabatic mixing model may become inappropriate for subsonic submerged jets at high-pressure conditions. Two cases are discussed where thermal and concentration driven fluxes are seen to have significant influence. To which extent the adiabatic mixing model is valid depends on the relative importance of local diffusive fluxes, namely Fourier, Fick and Dufour diffusion. This is inter alia influenced by different time and length scales. The experimental data from a high-pressure n-hexane/nitrogen jet injection were investigated numerically. Finally, based on recent numerical findings, the plausibility of different thermodynamic mixing models for binary mixtures under high pressure conditions is analyzed.

The present work summarises the main findings of a systematic study on the behaviour of liquid droplets embedded in a high pressure and temperature environment. Thanks to the availability of quantitative temperature data, it was possible for the first time to assess the conceptual soundness of different theoretical approaches. In all cases, the predictions from non-equilibrium models agree fairly well with Raman-extracted temperatures. By performing a thermodynamic analysis of interfacial transport in the framework of the Onsager theory, the following was found. Models that assume vapour-liquid equilibrium fail to provide a good estimation of the droplet temperature. This is due to the description of the interfacial fluxes that does not include the interdependency between heat and mass transfer, as prescribed by the Onsager theorem. Under non-equilibrium conditions, interfacial temperature jumps may exist without leading to the disintegration of the immiscible interface, as corroborated by several theoretical and experimental studies. The inception of thermal losses is controlled by the interfacial resistivities rather than by the Knudsen number. With decreasing droplet size and increasing ambient temperatures, the inclusion of local non-equilibrium effects in the modelling of the interfacial fluxes from the vapour side leads to higher interfacial temperature jumps.

In this study, n-hexane was compressed beyond six times its critical pressure and discharged into argon at subcritical pressure (with respect to the injectant). The injection temperature systematically varied from sub- to supercritical values to investigate near-critical disintegration phenomena of retrograde jets. Here, the ratio of the pressure in the nozzle reservoir and chamber was always above 14 leading to highly-expanded injections. We analyzed the breakup process in terms of combined shadowgraphy and light scattering measurements. Close to the critical point, different physical mechanisms have been observed. Their occurrence is predominantly determined by the expansion process resulting from the thermodynamic conditions in nozzle reservoir and chamber in combination with thermodynamic properties of the injectant. The experimental images for near- but subcritical injection temperatures imply that a fluid in liquid state expands in the nozzle and atomizes downstream of it. For sufficiently low backpressures, high liquid superheats trigger rapid vapor formation across a thin transition layer inside the nozzle that leads to a choked two-phase flow. A thermodynamic model that assumes a discontinuous phase transition layer and uses metastable fluid properties provides a physical explanation of the resulting underexpanded two-phase disintegration downstream of the nozzle exit . An increase to supercritical injection temperatures increases the compressibility of the fluid within the nozzle. An isentropic flow analysis showed that this triggers choking at thermodynamic states in the supercritical pressure regime resulting in the discharge of a sonic single-phase fluid. The expansion within the Mach barrel can lead to a supersaturated fluid state at near-critical temperature. In this case, a sharp phase transition front established approximately half a nozzle diameter downstream of the exit. We defined a dimensionless parameter to characterize the two-phase extent in underexpanded jets with near-critical phase transition based on initial injection conditions and retrograde fluid properties. We deduced the axial extent of the two-phase region from light scattering signals for a wide parameter range and demonstrate that it features a clear dependency upon the proposed parameter, which demonstrates its feasibility.

An electrohydrodynamic model for the simulation of droplet formation, detachment and motion in an electrically driven droplet generator is introduced. The numerical approach is based on the coupled solution of the multiphase flow problem with the charge continuity equation. For the latter, a modified convection-conduction model is applied, taking into account conductive, capacitive as well as convective electrical currents in the fluid. This allows for a proper description of charge relaxation phenomena in the moving fluid. In particular, the charge received by the droplet after detachment is an important parameter influencing the droplet dynamics in the test chamber. Simulation results are shown for highly conductive acetone droplets and for low conductivity pentane droplets, respectively. The operation characteristic of the droplet generator is investigated by computing droplet sizes and detachment times with respect to the applied voltage.

The relationship between ‘microscopic’ velocity field and ‘macroscopic’ outcomes of liquid droplet impact on wall-films is not yet fully understood. This article reports a preliminary experimental investigation to measure the velocity field within wall-film when a droplet impacts on it, using micro-Particle Image Velocimetry (μ-PIV). The challenges associated with measuring the velocity field within the wall-film are outlined. In this context, the limitations of the traditional μ-PIV technique are discussed, leading to the adoption of high-speed μ-PIV as the suitable technique for measuring the spatio-temporal evolution of velocity within wall-film. The salient features of the high-speed μ-PIV set-up are discussed. Further, results from preliminary experimental investigations on water droplet impacting on water wall-film at moderate impact velocities are presented. It is seen that the current high-speed μ-PIV set-up can be used to obtain reliable measurements of in-plane radial velocity, V, at ‘intermediate’ values of radial, r, and temporal, t, coordinates. Within the measurement range of the current set-up, it is observed that V scales with r and t as V ∝ r/t, which is similar to that reported in literature based on analytical considerations. The limitations of the current set-up, and the requirements for further experiments and validation are highlighted.

Drop impact onto wetted surfaces is of relevance to any spray coating application since the maximum spreading diameter and the residual film thickness of the applied liquid droplets affect the efficient distribution of the coating materials. In this paper, we propose a modification to existing models for crown propagation during single drop impact onto a wall-film based on the stagnation-point flow solution of Hiemenz. This offers two main advantages: a simple estimation of the film thickness decay rate, induced by the impulse transfer from impacting droplet to resting wall-film. Besides, the self-similarity of Hiemenz’s solution allows a straightforward estimation of the momentum losses during radial liquid spreading along the wall. The incorporation of these estimations into existing inviscid models provides an excellent agreement with experiments over the entire crown elevation phase. Additionally, the effect of fluid viscosity and initial film thickness on the momentum transfer from droplet to wall-film is highlighted.

The evaporation of liquid drops in stagnant gaseous environment is modelled, accounting for the effect of drop curvature and size at the macro- and microscopic scales. At the macro-scale level, the validity of the conjectured dependence of the local fluxes on the drop surface curvature is analysed. Analytical solutions to the gas-phase conservation equations for five drop shapes (sphere, oblate and prolate spheroids and inverse oblate and prolate spheroids), under uniform Dirichlet boundary conditions, are used to calculate the local vapour and heat fluxes. The analysis shows that in general non-dimensional fluxes do not solely depend on local curvature, but possibly the effect of the whole drop shape must be taken into account. At the micro-scale level, the equilibrium vapour pressure at a convex curved surface is higher than that at a flat surface, thus leading to a considerable enhancement of the evaporation rate for nanometre sized droplets. To model the increase in equilibrium vapour pressure, we consider the Kelvin correction. Our analysis shows that the Kelvin correction is strictly required for droplet radii below 20 A, as typically encountered for modelling the growth of critical clusters in phase transition processes initiated by homogeneous nucleation. At these conditions, it is mandatory to consider also the repartition of molecules in the different phases, in order to prevent a significant overestimation of the equilibrium vapour pressure.

This work analyses whether the inclusion of interfacial temperature jumps is necessary in the modelling of droplet evaporation at high pressure. The analysis is divided into two parts. First, we revise the major findings from theoretical models and molecular dynamics simulations on the conditions leading to the inception of interfacial jumps and the main parameters affecting them. Second, an evaporation model is considered that includes a diffuse transition layer (Knudsen layer), in the order of a few mean free paths around the droplet, where transport processes are described by kinetic molecular theory. The analysis shows that discontinuities in temperature and chemical potential across the interface are important when molecular collisions control transport processes and result in large heat and mass fluxes. This may occur not only at low pressures, but also at high pressures and temperatures for conditions sufficiently far from global thermodynamic equilibrium and/or for sufficiently small droplets. On a macroscopic scale, the resulting correction to the boundary conditions for classical diffusion-controlled models may be significant at high evaporation rates.

The development of new upper orbit thrusters using cryogenic propellants requires an improved understanding of the dynamics of oxidizer and fuel injection at near vacuum conditions before ignition. Due to the low ambient pressure, the propellants enter a superheated state and flash evaporation occurs. Flash boiling of cryogenic liquid nitrogen is studied experimentally on the newly developed test bench at DLR Lampoldshausen and numerically with a newly developed OpenFOAM© solver. Here, a one-fluid approach is selected where phase properties, such as density, enthalpy and saturation conditions are determined with the thermodynamic library CoolProp and tabulated before runtime. The phase change is modeled by the homogeneous relaxation model. For highly superheated jets the flow becomes supersonic and forms a shock after the injector outlet. The solver is validated with the aid of flashing acetone spray experiments where the shock structures are more clearly visible. The results show that the developed solver is capable to predict the all important gas dynamics by matching shock structure and spray angle to the experiment. The experiments using cryogenic liquid, however, do not reveal any shock structures but regions with low negative axial velocities can be identified on the jet centerline. A comparison with the simulations now demonstrates that shocks continue to persist but the respective shadowgraph signals may be obscured in these flows. The joint experimental and numerical study thus provides a consistent understanding of the observed flow features that govern the fluid dynamics and jet breakup of cryogenic flashing flows.

Cryogenic supercritical fluids represent an intriguing category of fluids that combine mechanical and thermophysical properties of both ultralow temperature conditions and phenomena taking place well above the critical point. Recent research has demonstrated that it is a common misconception to consider the supercritical state as one homogeneous state. Instead, these fluids consist of two to four liquid and gas like phases, each with their own unique characteristics. In our work, we investigate numerically single-specie cryogenic fluid jets—initially at subcritical temperatures—which are injected into a supercritical environment (both the pressure and temperature exceed the thermodynamic critical state). For the investigation, a new solver, namely, “CoolFoam,” has been developed, which is designed for compressible non-isothermal two-fluid simulations where diffusive transport of heat and/or mass is accounted for. Real fluid thermodynamics are modeled using a polynomial fitting approach developed in our previous work. We introduce also a new phase characterization framework based on the association of phases with specific temperature ranges (rather than using a single line like the Widom line), which allowed us to better identify the similarity effects between the various conditions. We analyze the inter-dependence of the underlying phenomena: density gradient and diffusive mass transport [molecular and thermo(Soret)-diffusion] and turbulence. We also compared supercritical N2 with subcritical liquid and gas jets to highlight potential differences with respect to how these jets behave. We find that the jet dynamics are largely dictated by the thermodynamic transition of the injected fluid and the associated variation in thermophysical properties.

The present work provides an overview on the possible phase transitions associated with supercritical fluid injection and a detailed evaluation of the mixing process between injectant fluid and quiescent ambience. The experiments cover superheated liquid disintegration, pseudo-boiling transition and single-phase jets under different nozzle pressure ratios. Pseudo-boiling effects emerge when rapid, subsequent changes in pressure/temperature interact with the non-linear behavior of thermodynamic response functions across the Widom line. The associated density fluctuations cause a significant increase in the scattering cross section and may lead to thermo-convective instabilities. Our analysis of the mixing process demonstrates the limited applicability of the adiabatic mixing model, which is often restricted to short residence times even in highly turbulent jets ( Re = O ( 10 5 ) ). Specifically, our findings show the importance of considering all coupled transport processes in the analysis of mixing problems at high pressures, in particular in presence of large mass concentration and temperature gradients.

During droplet impact on thin wall-films, a liquid lamella, the so-called ‘crown’, expands radially and vertically away from the impact point. This phenomenon is often associated with the growth of fingers at the crown rim, leading to the fascinating crown-type splashing. The present study reports an experimental investigation on droplet impact on wall-films, where the droplet and wall-film are made of different silicone oils. The focus is to characterize the crown rim kinematics in terms of relevant length, velocity, and acceleration scales during the expansion phase. The analysis showed that the crown rim displacement is a physically meaningful parameter to characterize the rim expansion. From this parameter, it is possible to clearly define the maximum expansion of the crown rim as well as the average velocity and the constant deceleration of crown rim expansion. The effects of droplet and wall-film properties (especially, their viscosities), droplet impact velocity, and wall-film thickness on these length, time, velocity, and acceleration scales associated with crown rim expansion are studied. Furthermore, the experimental data extracted from our vast database are systematically compared with the theoretical predictions from relevant models reported in literature to discuss their range of applicability.

Understanding suitable boundary conditions for vapor-liquid interfaces is important for the development of physically realistic simulations of evaporation and condensation processes. This work addresses the question whether the inclusion of interfacial temperature jumps is necessary in the modeling of droplet evaporation or condensation. The analysis is divided into two parts. First, a model for coupled heat and mass transfer resistivities for sharp vapor-liquid interfaces is derived from a diffuse interface model. Local resistivities within the interface of n-alkane/nitrogen mixtures are predicted for diffuse interfaces by means of classical density functional theory (DFT) in combination with the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. Integration over different domains of the local resistivities yields integral resistivities for sharp, or diffuse, interfaces. This allows computation of temperature jumps projected onto sharp interfaces under non-equilibrium conditions. Using this projection we found that interfacial jumps across the vapor-liquid interface are typically small, even for large temperature differences between droplet and ambient vapor. Second, an evaporation model is considered that includes a diffuse transition layer (Knudsen layer) in the order of a few mean free paths around the droplet, where transport processes are described by kinetic molecular theory. The analysis shows that discontinuities in temperature and chemical potential across the interface are important when heat and mass transfer are dominated by molecular collisions. This may occur not only at low pressures, but also at high pressures and temperatures for conditions sufficiently far from global thermodynamic equilibrium, resulting in large heat and mass fluxes and/or for sufficiently small droplets. On a macroscopic scale, the resulting correction to the boundary conditions for classical diffusion-controlled models may be significant at high evaporation or condensation rates.

We report a comprehensive speed of sound database for multi-component mixing of underexpanded fuel jets with real gas expansion. The paper presents several reference test cases with well-defined experimental conditions providing quantitative data for validation of computational simulations. Two injectant fluids, fundamentally different with respect to their critical properties, are brought to supercritical state and discharged into cold nitrogen at different pressures. The database features a wide range of nozzle pressure ratios covering the regimes that are generally classified as highly and extremely highly underexpanded jets. Further variation is introduced by investigating different injection temperatures. Measurements are obtained along the centerline at different axial positions. In addition, an adiabatic mixing model based on non-ideal thermodynamic mixture properties is used to extract mixture compositions from the experimental speed of sound data. The concentration data obtained are complemented by existing experimental data and represented by an empirical fit.

The electrohydrodynamics of millimetric droplets under the influence of slowly varying electric fields is considered. Strong electric fields applied on liquids induce forces driving fluid motion. This effect can be used, among others, in on-demand droplet generators. In this work, we discuss a convection-conduction model for the simulation of droplet motion in strong electric fields. The model focuses on robustness with respect to topology changes and on dynamic charging effects in liquids. We illustrate the model with the simulation of electrically driven droplet generation. The simulated dynamics for droplets with different conductivities are compared with experiments.

The present paper investigates experimentally the impact dynamics of crown-type splashing for miscible two- and one-component droplet wall–film interactions over a range of Weber numbers and dimensionless film thicknesses. The splashing outcome is parametrised in terms of a set of quantifiable parameters, such as crown height, top and base diameter, wall inclination, number of fingers, and secondary droplet properties. The results show that the outcome of a splashing event is not affected by the choice of similar or dissimilar fluids, provided the dimensionless film thickness is larger than 0.1. Below this threshold, distinctive features of two-component interactions appear, such as hole formation and crown bottom breakdown. The observation of different crown shapes (e.g. V-shaped, cylindrical, and truncated-cone) confirms that vorticity production induces changes in the crown wall inclination, thus affecting the evolution of the crown height and top diameter. The evolution of the crown base diameter, instead, is mainly dependent on the relative importance of liquid inertia and viscous losses in the wall-film. The maximum number of liquid fingers decreases with increasing wall, film thickness, due to the enhanced attenuation of the effect of surface properties on the fingering process. The formation of secondary droplets is also affected by changes in the crown wall inclination. In particular, for truncated-cone shapes the occurrence of crown rim contraction induces a large scatter in the secondary droplet properties. Consequently, empirical models for the maximum number and mean diameter of the secondary droplets are derived for V-shaped crowns, as observed for the hexadecane-Hyspin interactions.

[EN] Single drop impacts on thin liquid layers are of particular interest because of the ejection of secondary droplets, the so-called splashing. Only a few studies handle the deposition/splashing limit for two-component interaction, where the liquid properties of the impacting drop and wall film differ significantly. This study aims at identifying a unified approach for one- and two-component interactions to determine the deposition/splashing limit. Therefore, a large database of both interactions is considered, which includes data from literature for one-component interactions plus the following binary combinations: hyspin-hexadecane, diesel-hexadecane and diesel-motor oil. Furthermore, a systematic study of two-component interactions with several silicon oils and hexadecane is performed. To map the outcomes, the Ohnesorge number Oh and the Reynolds number Re calculated with arithmetically averaged fluid properties between droplet and wall film fluid are chosen. The dimensionsless film thickness δ is added to form a 3D plot, where one- and two-component experiments are combined. Existing correlations from the literature are revised regarding both interactions and their consistency is checked. The investigated range of high viscosity fluids allow us to propose an improvement of the correlation for high Oh. Our results show that the arithmetically averaged fluid properties lead to a good repartition of both one- and twocomponents interactions toward the deposition/splashing limit. They also corroborate the previous findings that an increase of δ inhibits splashing but its influence is decreasing with increasing Oh., The authors kindly acknowledge the financial support of this work by the Deutsche Forschungsgemeinschaft (DFG) in the frame of the International Research Training Group "Droplet Interaction Technologies" (DROPIT)

[EN] Experiments and numerical simulations were carried out in order to contribute to a better understanding and prediction of high-pressure injection into a gaseous environment. Specifically, the focus was put on the phase separation processes of an initially supercritical fluid due to the interaction with its surrounding. N-hexane was injected into a chamber filled with pure nitrogen at 5 MPa and 293 K and three different test cases were selected such that they cover regimes in which the thermodynamic non-idealities, in particular the effects that stem from the potential phase separation, are significant. Simultaneous shadowgraphy and elastic light scattering experiments were conducted to capture both the flow structure as well as the phase separation. In addition, large-eddy simulations with a vaporliquid equilibrium model were performed. Both experimental and numerical results show phase formation for the cases, where the a-priori calculation predicts two-phase flow. Moreover, qualitative characteristics of the formation process agree well between experiments and numerical simulations and the transition behaviour from a dense-gas to a spray-like jet was captured by both, The authors gratefully acknowledge the German Research Foundation (Deutsche Forschungsgemeinschaft) for providing financial support in the framework of SFB/TRR 40. Financial support was also provided by Munich Aerospace (www.munich-aerospace.de). Furthermore, the authors thank the Gauss Centre for Supercomputing e.V. (GCS) (www.gauss-centre.eu) for supporting this project by providing computing time on the GCS Supercomputer SuperMUC at Leibniz Supercomputing Centre (www.lrz.de).

This paper presents a systematic study on flashing atomisation, which includes both standards and retrograde fluids. A novel data reduction method is proposed in terms of the controlling parameters for (bubble) nucleation. The analysis indicates that bubble nucleation is the rate-controlling process for both the transition to fully flashing and for the spray lateral spreading. Specifically, the onset condition coincides with the surmount of the energy barrier to nucleation. The spray lateral spreading, instead, is directly linked to the population of bubble clusters: the larger the population the wider the spray angle. Theoretical aspects of bubble nucleation theory are also reviewed. An interesting conclusion of the analysis is that the experimental trends observed in fully flashing jets are compliant with recent advances in nucleation theory. At very high initial superheat, a complex shock wave structure appears around the flashing jets. The novel aspect of this work is that such shock-systems are observed consistently in both standard and retrograde substances. This similarity indirectly confirms that, far from the critical temperature, the phase transition mechanism is the same for all substances, independently from their degree of retrogradicity.

This paper discusses the feasibility of differential infrared thermography (DIT) for performing quantitative temperature measurements in a flashing jet. In the literature, the technique has been previously applied to map temperature distributions in diesel sprays. Thanks to the differential operation, DIT enables the local determination of the spray emissivity, independently from a detailed knowledge of the drop size distribution. Temperatures can then be determined through application of the Planck’s radiation law. As a first step, we revised DIT constitutive equations in the framework of the theory of radiative heat transfer in particulate media. The objective is twofold: First, to identify the quantity actually measured by DIT and the limits of applicability of the method; Second, all factors, which may hamper quantitative data interpretation, are carefully reviewed. The analysis includes the choice of the background temperature, multiple scattering effects and the spatial resolution of the optical sys...

Effects of homogeneous nucleation and subsequent droplet growth in compressible flows in humid nitrogen are investigated numerically and experimentally. A Ludwieg tube is employed to produce expansion flows. Corresponding to different configurations, three types of experiment are carried out in such a tube. First, the phase transition in a strong unsteady expansion wave is investigated to demonstrate the mutual interaction between the unsteady flow and the condensation process and also the formation of condensation-induced shock waves. The role of condensation-induced shocks in the gradual transition from a frozen initial structure to an equilibrium structure is explained. Second, the condensing flow in a slender supersonic nozzle G2 is considered. Particular attention is given to condensation-induced oscillations and to the transition from symmetrical mode-1 oscillations to asymmetrical mode-2 oscillations in a starting nozzle flow, as first observed by Adam & Schnerr. The transition is also found numerically, but the amplitude, frequency and transition time are not yet well predicted. Third, a sharp-edged obstacle is placed in the tube to generate a starting vortex. Condensation in the vortex is found. Owing to the release of latent heat of condensation, an increase in the pressure and temperature in the vortex core is observed. Condensation-induced shock waves are found, for a sufficiently high initial saturation ratio, which interact with the starting vortex, resulting in a very complex flow. As time proceeds, a subsonic or transonic free jet is formed downstream of the sharp-edged obstacle, which becomes oscillatory for a relatively high main-flow velocity and for a sufficiently high humidity.

We describe how traditional elastic low-angle light scattering techniques (LAELS) can be employed to determine the evaporation rate of droplet arrays. This objective is achieved by measuring simultaneously the droplet size and spacing (from which the droplet speed can be derived) at different axial positions along the droplet path. The analysis shows that both quantities can be determined with good accuracy. The capability of the LAELS technique in performing reliable particle sizing has been tested by comparing experimental values with theoretical predictions. In addition, the method proved to be a valid tool for studying complex interaction phenomena between vaporising droplets, where competing effects between neighbouring droplets may result in a substantial decrease of droplet vaporisation rate when compared to the isolated droplet case. The temporal evolution of four different droplets arrays has been determined and the influence of interdroplet spacing on the vaporisation process correctly predicted.

We report a detailed experimental characterization of the process of homogeneous condensation in supersonic expanding flow. In our experiments, the supersaturated mixture expands in a Laval nozzle, where, depending on the initial conditions, a steady or periodically oscillating flow may evolve due to the non-linear interaction of nucleation and droplet growth rate with the flow field. Two experimental techniques are utilized: holographic interferometry for the determination of the density field and a time-resolved white-light extinction method. The latter is employed to derive the evolution in time of the droplet cloud (i.e., modal radius, number density, and relative width) and to measure the frequency of oscillations. In combination with the wide-field density data, droplet size measurements provide additional physical insights in the mechanism of interaction in condensing flows and serve as an excellent test case for the critical assessment of nucleation and droplet growth theories. To this purpose, the accuracy of the measurements is carefully reviewed due to the difficulties of characterizing dense sub-micron droplet clouds by means of light-scattering techniques. An important byproduct of this analysis is an evaluation of the applicability of single-scattering approximations, i.e., Lambert-Beer law, for a variety of experimental configurations.

Near-critical jet disintegration studies are performed in a specialised shock tube. Both the injection and test conditions can be varied systematically from subcritical to supercritical values (with respect to the injectant). Three different injection regimes have been identified. Their inception is basically determined by the initial fuel temperature and expansion ratio and has been predicted on basis of a thermodynamic analysis of the injection paths. All three disintegration modes are investigated by means of high-speed Schlieren and Mie imaging for determining the jet lateral spreading and internal structure. The main objective of the present work is to gain a better insight in the physical processes, occurring in the different disintegration regimes and to provide an experimental database for model validation.

This paper describes a double-diaphragm shock tube, designed for the experimental investigation of fundamental processes related to fluid disintegration and mixing at elevated pressures and temperatures, representative for realistic engine conditions. Special features of the shock tube include a variable-area driver section to compensate for shock attenuation and boundary layer effects. In discussing the shock tube operational envelope, particular emphasis is given to the requirements to be fulfilled to attain reproducible and accurate experimental data and to capture the most relevant features of subcritical and supercritical mixing behavior. A review of potential thermodynamic transitions for near-critical jet disintegration phenomena is presented and the main features of each disintegration mode are discussed on the basis of the shock tube experiments.

A shock tube facility to generate a high pressure and temperature test environment has been built and characterised. Shock tube experiments were performed to study the disintegration of liquid hydrocarbon jets at elevated pressures and temperatures. Shadowgraph images of the spray were acquired and analysed. The objective of this paper is to show the feasibility of a shock tube to investigate hydrocarbon spray injection and disintegration processes performed at conditions relevant to different propulsion systems. First results using shadowgraph imaging are presented and compared to literature data.

High-speed shadowgraphy was applied to visualise hydrocarbon spray atomisation at elevated pressures and temperatures. The experiments were performed in the quiescent post-shock region of a double-diaphragm shock tube facility. The latter was recently upgraded with a variable-area driver section to compensate for shock attenuation and is equipped with a heated, fast-response injection system. Test times were in the order of 4.5 ms with reflected-shock pressures ranging from 10-35 bar and test temperatures of 950 K. The fuel (n-dodecane) was injected right after shock reflection. After a transient start-up process, quasi-steady injection was achieved and sustained for about 2.5 ms. Fuel temperatures were varied between 373.15 and 523.15 K. The images attained at 100,000 fps were analysed with respect to the spatial and temporal dispersion of the spray. The influence of the chamber pressure and the fuel temperature on the spray break-up process was investigated. Increasing chamber pressures (chamber densities) and fuel temperatures result in an enhancement of the spray dispersion measured in terms of jet angles and therefore in a better mixing and vaporisation.

In supersonic expansion flows of humid nitrogen, at relatively low cooling rates, self-sustained oscillations may occur due to the combined effects of nucleation, droplet growth, and latent heat release. Different modes of oscillation are found depending on the initial humidity. It is shown numerically that the stability limit and the oscillation frequencies of the different modes strongly depend on the nucleation rate. Numerical results are compared with experiments of Adam and Schnerr and own experiments for nozzles of different shapes. The ICCT nucleation model yields correct results for both series of experiments, when corrected with a factor of the order of 0.01.

A double diaphragm shock tube facility for studying liquid-spray atomization and combustion-related phenomena at elevated pressures and temperatures is described. The present shock tube is specifically intended for the investigation of fundamental processes related to fluid disintegration and mixing under realistic engine conditions. Special features of the facility include a variable-area driver section to compensate for shock attenuation, a square test section to allow flow visualization in the postshock region, a skimmer to dispose part of the boundary layer, a heated, fast-response injector, a fully automated gas-filling system, and a new control system and electronics. Test times of the order of 2-5 ms are possible with reflected shock pressures up to 50 bar and temperatures of 2000 K. Details on the setup design, construction and operation are given. Particular emphasis is placed on the accuracy and reproducibility of the test conditions. To that aim, qualification tests have been performed to assess the shock tube performance in terms of effectiveness of the skimmer concept, the capability to compensate for boundary layer effects and the generation of uniform and reproducible test and injection conditions.

or gaining deeper insight in the process of fuel evaporation in dense fuel sprays, under- standing the fluid disintegration mechanisms at high chamber pressures and tempera- tures is of particular importance. This paper describes a newly commissioned double-diaphragm shock tube (DDST) facility, developed at the Institute of Aerospace Thermodynamics (ITLR) of the Universitat Stutt- gart. A shock tube is an ideal tool to investigate disintegration and evaporation processes, as it can provide a uniform and well-defined thermodynamic state over a wide range of pressures and temperatures. The double-diaphragm shock tube at ITLR consists of five major components: driver, buf- fer, driven section, test section, and dump tank. Driver, buffer, and driven section have a cylindrical inner diameter. The introduction of the short buffer between the driver and the driven section reduces the initial pressure load on each of the two diaphragms and improves the controllability and triggering of the experiments. The test section has a square cross section to allow the application of flat fused-silica windows for optical access. The obser- vable length within the test section is 100 mm. In order to partially dispose the boundary layer and to accomplish the transition from the cylindrical driven section to the square test section, a skimmer is mounted at the beginning of the test section realising a semi-direct connection between these two parts. The driver section is 3 m long, the driven section is about 9.5 m long leading to an overall facility length of about 12.5 m. The new shock tube facility was characterised for several representative conditions, using a helium/argon mixture as the driver gas and argon as the test gas. Pressure levels of up to 50 bar have been attained and temperatures greater than 2000 K were achieved. These values are suitable to investigate most hydrocarbon fuels of interest (e.g. n-dodecane as a kerosene substitute) over a wide range of different thermodynamic regimes (sub,- trans- and supercritical). In the endwall of the shock tube, a fast-acting high-pressure injection system is mounted to rapidly introduce the fuel into the test section, as test-times are limited to about 4-5 ms. First spray characterisation experiments are currently ongoing.