Two-phase jet releases and droplet dispersion : scaled and large-scale experiments, droplet-size correlation development and model validation (original) (raw)

Two-phase jet releases and droplet dispersion: rainout experiments and model validation

2011

Many accidents involve two-phase releases of hazardous chemicals into the atmosphere. This paper describes the results of the fourth phase of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion. The objective of Phase IV of the JIP was to generate experimental rainout data for non-flashing experiments (water and xylene), and to develop recommendations for the best methodology to predict rainout [total rainout mass and its spatial distribution ('distributed' rainout)].

Flashing liquid jets and two-phase droplet dispersion

Journal of Hazardous Materials, 2007

The large-scale release of a liquid contained at upstream conditions above its local atmospheric boiling point is a scenario often given consideration in process industry risk analysis. Current-hazard quantification software often employs simplistic equilibrium two-phase approaches.

Spray droplet modeling: 2. An interactive Eulerian-Lagrangian model of evaporating spray droplets

Journal of Geophysical Research: Oceans, 1996

This paper describes an imeractive Eulerian-Lagrangian model of the turbulent transport of evaporating droplets. A k-e (where k is turbulem kinetic energy and • is its rate of dissipation) turbulence closure model is used to accurately simulate stable, near-neutral, and unstable boundary layers within the large air-sea interaction tunnel at the Institut de M6canique Statistique de la Turbulence (IMST), Luminy, France. These results are then used with the Lagrangian model described in part 1 [Edson and Fairall, 1994]. The coupled model is shown to give excelleto agreemere with droplet dispersion measuremeres made during the 1988 Couche Limite Unidimensionelle Stationnaire d'Embrums (CLUSE, a French acronym that translates to one-dimensional stationary droplet boundary layer) campaign. Additionally, this paper describes how the coupled model can now be used to investigate the imeraction between the evaporating droplets and the turbulent fields of temperature and humidity. The investigation shows that although the influence of the droplets is small under the conditions simulated at IMST, the potential for substamial modification of the surface energy budget exists for high-wind conditions over the ocean. 1. Introduction This paper describes an interactive Eulerian-Lagrangian model of the turbulent transport of evaporating spray droplets. The model has been developed to address some of the limitations described by Edson and Fairall [1994] (hereinafter referred to as part 1), and to allow the use of the model in more complicated flows. The model development involved the integration of the Lagrangian model described in part 1 with an Eulerian model of turbulent flows that uses prognostic equations for the evolution of the turbulent kinetic energy k and its rate of dissipation e. The integrated code has been christened Gwaihir, and we shall refer to the model as such in the following discussion. The initial tests of the k-e model are conducted through simulations of developing boundary layers using a two-dimensional version of the code. The model nms are initialized and compared with measurements taken within the large air-water interaction simulation tunnel at the Institut de M6canique Statistique de la Turbulence (IMST), Luminy, France, during the 1988 Couche Limite Unidimensionelle Stationnaire d•rnbm (CLUSE, a French acronym that translates to onedimensional stationary droplet boundary layer) campaign [Mestayer et al., 1990]. These simulations have provided a means to test the various droplet dispersion aspects of Gwaihir, as well as the performance of the Eulerian code in simulations of the marine atmospheric surface layer. The paper describes in some detail both the physical model and the numerical procedure used in our approach. It also addresses some of the advantages of this combined (Eulerian plus Lagrangian) approach over separate approaches (Eulerian or Lagrangian) in simulations of the turbulent transport of heavy particles. It then concludes with the results from the interactive model for simulations of droplet dispersion in both a laboratory and marine atmospheric surface layer.

Modeling the evolution of droplet size distribution in two-phase flows

International Journal of Multiphase Flow, 2007

A theoretical model is developed in the present study to simulate droplet motion and the evolution of droplet size distribution (DSD) in two-phase air/dispersed water spray flows. The model takes into account several processes which influence DSD and droplet trajectory: droplet collision and coalescence, evaporation and cooling, gravitational settling, and turbulent dispersion of dispersed phase. The DSDs determined by the model at different locations in a two-phase flow are evaluated by comparing them to experimental observations obtained in an icing wind tunnel. The satisfactory coincidence between simulation and experimental results proves that the model is reliable when modeling two-phase flows under icing conditions. The model is applied for two particular examples in which the modification of DSD is calculated in twophase flows under conditions describing in-cloud icing and freezing drizzle.

Prediction of the droplet size and velocity joint distribution for sprays

Fuel, 2001

This work addresses the development of a mathematical model to predict the joint distribution for both size and velocity of the droplets in sprays, based on the maximum entropy formalism. Using this joint distribution, models to obtain separated distributions for size and velocity of sprays are also presented. Correlations for the average velocity for both pressure jet and airblast atomisers, based on assumed pro®les in the atomiser gun, are obtained as a function of easily measurable parameters. Several distributions for different types of atomisers are then predicted. Agreement between available data for the velocity distribution and the corresponding predictions is satisfactory.

MODELLING OF DROPLET HEATING, EVAPORATION AND BREAK-UP: RECENT DEVELOPMENTS

Multiphase, 2006

Several new approaches to the modelling of liquid droplet heating and evaporation by convection and radiation from the surrounding hot gas are reviewed. The finite thermal conductivity of the liquid, recirculation within droplets, time dependence of gas temperature and the convection heat transfer coefficient are taken into account. The relatively small contribution of thermal radiation to droplet heating allows us to describe it by a simplified model, which does not consider the variation of radiation absorption inside the droplets. In the case of stationary droplets a coupled solution of the heat conduction equation for gas and liquid phases is obtained. A transient modification of Newton's law is introduced via a correction to either the gas temperature or convection heat transfer coefficient. The solution is analysed using values of parameters relevant to liquid fuel droplet heating in a diesel engine. Since gas diffusivity in this case is more than an order of magnitude larger than liquid diffusivity, for practical applications in computational fluid dynamics (CFD) codes, this model can be simplified by assuming that droplet surface temperature is fixed. Moreover, if the initial stage of droplet heating (a few μs) can be ignored then the steady-state solution for the gas phase can be applied for the analysis of droplet heating. This solution is described in terms of the steady-state convection heat transfer coefficient. All transient effects in this case are accounted for by liquid phase models. A decomposition technique for the solution of the system of ODEs, based on the geometrical version of the integral manifold method, is described. A comparative analysis of hydrodynamic and kinetic approaches to the problem of diesel fuel droplet evaporation is described. The kinetic approaches are based on a simplified analysis of the Boltzmann equation and its direct numerical solution. Kinetic models predict longer evaporation times and higher droplet temperature compared with the hydrodynamic model. It is recommended that kinetic effects are taken into account when modelling the evaporation process of diesel fuel droplets in realistic internal combustion engines. The preliminary results predicted by deterministic and stochastic models of droplet break-up, both implemented into the KIVA-2 code, are compared with high-speed video images of diesel sprays.

Influence of droplet spatial distribution on spray evaporation

2019

In aero-engines, fuel is injected as a liquid which involves two-phase flow combustion. Consequently, different phenomena such as atomization, droplet dispersion by turbulence or spray evaporation impact combustion processes. In order to study spray combustion, an experimental test rig has been developed at ONERA to partially feature the flow conditions inside the combustion chamber of a turbo-reactor. Experimental campaigns have been conducted in non-reactive and reactive conditions to obtain an experimental database. The present paper deals with an experimental description of the spray spatial distribution based on Mie scattering images. The first part presents the specific image processing algorithm which detects droplets and calculates the nearest-neighbour droplet distance. The second part describes the evolution of the nearest-neighbour according to the inverse square-root of mean spray density under non-reactive conditions.

Improved droplet breakup models for spray applications

International Journal of Heat and Fluid Flow

The current study examines the performance of two zero-dimensional (0D) aerodynamically-induced breakup models, utilized for the prediction of droplet deformation during the breakup process in the bag, multi-mode and sheet-thinning regimes. The first model investigated is an improved version of the widely used Taylor analogy breakup (TAB) model, which compared to other models has the advantage of having an analytic solution. Following, a model based on the modified Navier-Stokes (M-NS) is investigated. The parameters of both models are estimated based upon published experimental data for the bag breakup regime and CFD simulations with Diesel droplets performed as part of this work for the multi-mode and sheet-thinning regimes, for which there is a scarcity of experimental data. Both models show good accuracy in the prediction of the temporal evolution of droplet deformation in the three breakup regimes, compared to the experimental data and the CFD simulations. It is found that the best performance of the two is achieved with the M-NS model. Finally, a unified secondary breakup model is presented, which incorporates various models found in the literature, i.e. TAB, nonlinear TAB (NLTAB), droplet deformation and breakup (DDB) and M-NS, into one equation using adjustable coefficients, allowing to switch among the different models.

Iclass 06-184 Experimental Validation of a Droplet Evaporation Model

2006

The pollutant emissions generated by liquid-fuel fired gas turbine engines are strongly influenced by the fuel preparation process that includes atomization, evaporation and mixing. In order to accurately predict the fuel preparation process, sufficiently precise models of the key thermophysical processes are crucial. In the present paper, the performance of fuel droplet evaporation models are considered as applied to a spray produced by a practical gas turbine fuel injector under actual conditions. Of particular interest are numerically efficient models including the “Distillation Curve” (DC) model. The DC model can account for the behavior of multi-component fuels like diesel fuel #2. Fractional boiling is described by the molar weight as a single process variable. This way, the fractional distillation process during evaporation of droplets is taken into account. In addition, the thermophysical properties of the fuel are supplied as a function of the molar weight. Real gas effects...

An experimental investigation of droplet evaporation and coalescence in a simple jet flow

Experiments in Fluids, 2004

An experimental study of a simple jet flow, which contains a dispersion of fine droplets, has been carried out in order to investigate the effect of turbulence, evaporation and coalescence on the droplet size distributions within the jet. Very little evaporation occurs in the potential core of the jet, while in the far-field, where the potential core has vanished and the droplets disperse more readily, evaporation occurs predominantly in the outer portions of the spray. Evidently, turbulence enhances the evaporation rate of droplets at the edges of the spray, and fresh air entrained from the outer regions increases the evaporative driving force. Coalescence has also been observed within the spray, although this effect is rather subtle compared to the evaporation effect in the dilute jets investigated here. Nevertheless, sufficient measurements have been taken to validate, at least partially, any coalescence models, in addition to any turbulence and evaporation models for dilute poly-disperse sprays.