University of Florida / CNRS ICARE

Experimental work done by Clement Joly at the University of Florida (Gainesville, FL, USA) at the Combustion and Propulsion Lab supervised by Dr. Corin Segal.

This 6-month internship was made possible thanks to a partnership with the CNRS ICARE (National Center on Scientific Research – Institute of Combustion, Aerothermal, Reactivity and Environment) situated in Orléans (France). Praise to Dr. Ivan Fedioun for his implication and interest in my work.

Title of the internship: “Supercritical Jet Disintegration”

Aim: Improve the mixing between the oxydant and the oxydizer to increase, in the end, the combustion efficiency for a supercritical jet.

Dates: 04/19/2012 – 09/14/2012

Definition: a supercritical fluid is a fluid (liquid or gas at ambient conditions of pressure and temperature) that is at a pressure and a temperature higher than its critical pressure and temperature, respectively.

When a fluid is heated up above its critical point it loses/gains features of a liquid and a gas to the point that it is not possible to define it as liquid or gaseous. The major characteristics of a supercritical fluid are the absence of surface tension, the absence of vaporization latent heat and the absence of a clear interface between the supercritical fluid and another fluid. Under this condition the fluid elements leaving the jet cannot be governed by evaporation anymore and are therefore governed by mass diffusion while experiencing a gas/gas behavior with the surrounding.

Supercritical jets can be appreciated in applications like diesel and rockets engines which operate at high pressure and temperature. Some critical pressures and temperatures, respectively, are: 12.8 atm & 33K for hydrogen, 217.7 atm & 647K for water, 33.55 atm & 126K for nitrogen.

Abstract: Experiments were done in order to study the disintegration of an injectant jet to simulate the breakup of hydrogen in aerospace applications under subcritical, transcritical and supercritical conditions in an inert surrounding where the pressure and temperature are controlled. The distinction between subcritical and supercritical fluid was performed through the visual study of experiments results as well as through the analysis of the potential core length and the jet spreading angle. The jet appeared smoother and less wavy under supercritical conditions than under subcritical ones due to the reduction of the surface tension when approaching the critical point of the injectant. The core length was found to evolve differently according to the critical state of the chamber and the injectant while the spreading angle showed a strong dependence with the square root of the injectant-to-chamber density ratio. Moreover a droplet analysis was performed for supercritical injection into a subcritical surrounding to investigate the conditions of formation of these droplets in spite of the supercritical state of the injectant. It was found that the jet can transition locally to subcritical when entering a much cooler environment due to heat transfer according to 2 found parameters. Additionally, the size of the resulting droplets was discovered to evolve as a second order power law with the distance from the injection.

Keywords: Supercritical, jet breakup, disintegration, hydrodynamic instabilities, potential core length, jet spreading angle, mixing, planar laser-induced fluorescence, droplet size

Résumé: Des expériences ont été réalisées afin d’étudier la désintégration d’un jet pour des applications aérospatiales dans des conditions subcritiques, transcritiques et supercritiques dans un gaz inerte à l’intérieur d’une chambre où la pression et la température sont contrôlées. La distinction entre le fluide subcritique et supercritique a été réalisée grâce à l’étude visuelle des résultats des expériences ainsi que par l’analyse de la longueur du cœur potentiel et de l’angle de divergence du jet. A mesure que les conditions expérimentales s’approchent du point critique de l’espèce injectée les tensions de surface disparaissent et le jet supercritique apparaît plus lisse et moins ondulé que le jet subcritique. La longueur du cœur potentiel évolue différemment selon les conditions expérimentales alors que l’angle de divergence a montré une forte dépendance en racine carrée du rapport de densité jet-chambre. En outre, une analyse des gouttelettes a été réalisée pour l’injection supercritique dans un environnement subcritique afin d’enquêter sur les conditions de formation de ces gouttelettes, en dépit de l’état supercritique de l’injectant. Il a été constaté que le jet peut transitionner vers un état subcritique lors de l’injection dans un environnement plus froid en raison du transfert de chaleur. En outre, la taille des gouttelettes résultantes a été trouvée comme évoluant de manière quadratique avec la distance entre la gouttelette et l’injection.

Mots-clés: Supercritique, désintégration de jet, instabilités hydrodynamiques, longueur de cœur potentiel, angle de divergence de jet, mélange, fluorescence par plan laser, taille des gouttelettes

The experiments were realized using PLIF (Planar Laser-Induced Fluorescence) to identify both the boundary and the jet core structure in a high-pressure chamber during the injection of the considered fluid into an inert environment up to 20 jet diameters downstream of the injector location.

Experiments were performed to explore all four possibilities related to supercritical fluids; injection of a subcritical fluid into a sub- and supercritical chamber, injection of a supercritical fluid into a sub- and supercritical chamber. It was seen that under supercritical conditions the jet appeared smoother than the jet injected subcritically which was more wavy and corrugated. Also, due to the reduction and disappearance of surface tension as the critical point of the injected species is approached droplets were less produced and ligaments did not breakup as much.

Below is an example of jet disintegration for a supercritical jet entering a subcritical environment.

Despite the supercriticalness of the fluid when it is injected it can be seen that droplets do form from the jet. This is caused by the local transition (from supercritical to subcritical) of the jet/surrounding interface due to the heat transfer with the cooler chamber. Therefore surface tensions reappear which lead to the formation of droplets that does not occur for a purely supercritical behavior (see example of a supercritical jet injected into a supercritical environment below).

Two parameters were found to determinate whether the formation of droplets will happen according to the test conditions considered. The size distribution and evolution of the resulting droplets were also investigated. The results showed that the size of the average droplet due to the disintegration of the jet evolves as a quadratic function of the distance from the injector

Beside the disintegration structures related to the jet breakup, the potential core length and the jet spreading angle were also investigated. The core length calculation was performed based on the change in density along the centerline of the jet while the spreading angle takes into account the relative angle between the jet boundaries (highest density gradients).

In the end the core length was found to be higher than for a turbulent submerged cold gas jet while following the same trend (constant with the chamber/injectant density ratio). This proves that supercritical fluid injection into a supercritical chamber exhibits a mixing behavior similar to the one of a gas-gas

On the other hand the jet spreading angle was seen to increase with the square root of the density ratio for all the cases done. This result was expected according to the previous researchers’ studies on diesel sprays and supercritical fluids. While there is no clear distinction between liquids and gas above the critical point for the fluid behaves as a mix of the two it was anticipated that the results show a similar result.

Since the mixing efficiency is improved for a high spreading angle and a short liquid core it can be concluded that a higher density ratio leads to a better mixing for a single round injector.

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