Characterization of ethane jet from sub-critical to super-critical conditions through visible light and X-ray imaging

International audienceThe injection of fuel in a high-pressure gaseous environment, for automotive, aeronautical or rocket applications leads to thermodynamic conditions where pressure exceeds the critical pressure of working fluids and thus, the supercritical state of matter is reached. Providing reliable experimental results under these particular conditions is still nowadays a challenge, but it is of great importance for the validation of numerical codes. Indeed, at such a high pressure, the distinction between gaseous and liquid phases becomes blurred as surface tension decreases and the interface disappears completely. For such special conditions, experimental data are scarce and need to be consolidated. As an example, the modification of the local refractive index induced by density gradient makes the visible-light imaging technique to be used with care. The REFINE testbench (Real-gas Effect on Fluid Injection: a Numerical and Experimental study) has been designed at CORIA Lab to study the non-reactive injection of Ethane and Propane under sub-and supercritical conditions. The ambient gas pressure can be raised up to 6 MPa and warmed up to 573 K to scan sub-and trans-critical injection conditions. The chamber is equipped with two perpendicular accesses allowing different simultaneous diagnostics to be applied to the jet. Experimental data are collected from shadowgraph, diffused backlight illumination techniques and X-Ray. Quantitative measurements of jet spreading angle, breakup length and density maps are compared to literature results. Introduction The study of high pressure injection is a major topic of research in the transport industry (cars, aircrafts, rockets) because it conditions the combustion performance and therefore the formation of pollutants. Under certain conditions of pressure and temperature the liquid injected into the chamber does not behave as usual and the atom-ization process is replaced by a diffusion one [1, 2]. The experimental study of this transition is a challenge because it requires the development of a robust, precise and well-controlled experiment as well as adapted diagnostics. The number of experiments dealing with supercritical injection is therefore small and the amount of experimental data that can be used for modeling validation [3, 4, 5] is all the more scarce [6]. In particular, the high ambient pressure locally affects the refractive index gradient making the use of laser-based diagnostics questionable [7, 6] to provide quantitative local experimental data. Nevertheless, classical diagnostics such as shadowgraphy or schlieren techniques have been used up to now to observe the transition from a system composed of distinct liquid and gaseous phases [6, 8, 9] to a system where dense and light fluids mix together because the surface tension and the heat of vaporization diminish [10, 11]. Under supercritical conditions, a dense and dark core is visible at the injector exit on shadowgraph [12] and the analysis of the spreading angle shows that supercritical jet has a behavior closer to a gaseous jet than to a liquid jet [13, 14]. In this study, the first results coming from the research program REFINE (Real-gas Effect on Fluid Injection: a Numerical and Experimental study) are provided. It consists in an injection of Ethane into an environment of Nitrogen or Helium at high pressure and moderate temperature. The objective is to deliver a set of quantitative data for the dark core length and the spreading angle of a non-assisted jet, from subcritical to supercritical pressure with respect to the liquid injected, and for various levels of ambiant temperature. A tentative to deliver the value of density on the jet axis through the radiography of the liquid jet is also presented. Radiography is an X-ray imaging technique that has been used for the study of diesel jets [15, 16] or cryogenic injections under supercritical considerations [17]. The physical principle of radiography is the absorption of X-rays by the dense fluid. The transmission of X-rays thus depends on the mixture composition, i.e. the injected fluid and its surrounding. An advantage of X-ray absorption technique is that X-rays are not subject to deflection due to optical index gradients, contrary to laser-based techniques, and the attenuation of the transmitted light is proportional to the mass of fluid crossed by the X-ray beam