Experiments and numerical simulation of mixing under supercritical conditions

Abstract

Supercritical pressure conditions designate a situation where the working fluid pressure is above the critical point. Among these conditions, it is interesting to identify a transcritical range which corresponds to cases where the pressure is above the critical point, but the injection temperature is below the critical value. This situation is of special interest because it raises fundamental issues which have technological relevance in the analysis of flows in liquid rocket engines. This situation is here envisaged by analyzing the behavior of a nitrogen shear coaxial jet comprising an inner stream injected at temperatures close to the critical temperature and a coaxial flow at a higher temperature. Experiments are carried out both in the absence of external modulation and by imposing a large amplitude transverse acoustic field. Real gas large eddy simulations are performed for selected experiments. The combination of experiments and calculations is used to evaluate effects of injector geometry and operating parameters. Calculations retrieve what is observed experimentally when the momentum flux ratio of the outer to the inner stream \documentclass[12pt]{minimal}\begin{document}$J= (\rho _eu_e^2)/(\rho _iu_i^2)$\end{document}J=(ρeue2)/(ρiui2) is varied. Results exhibit the change in flow structure and the development of a recirculation region when this parameter exceeds a critical value. The instantaneous flow patterns for different momentum flux ratios are used in a second stage to characterize the dynamical behavior of the flow in terms of power spectral density of velocity and density fluctuations. Results obtained under acoustic modulation provide insight into mixing enhancement of coaxial streams with a view of its possible consequences in high frequency combustion instabilities. It is shown in particular that the presence of strong acoustic modulations notably reduces the high density jet core length, indicating an increased mixing efficiency. This behavior is more pronounced when the jet is placed at the location of maximum transverse velocity fluctuations.

Supercritical Mixing and Combustion of Liquid-Oxygen/ Kerosene Bi-Swirl Injectors

“…Substantial experimental [8][9][10][11][12] and numerical [13][14][15] efforts on the characterization of propellant injection, mixing, and combustion processes at near-and supercritical conditions have recently led to improved understanding of the mechanisms involved. Most of these studies, however, have focused on shear coaxial injectors with light fluids, such as hydrogen and methane, as fuel.…”

Numerical simulations of mixing under supercritical pressures of a shear coaxial injector using a high-order method: effect of outer jet temperature

“…On the other hand, quite large temperature §uctuations occur in Case 2 at the shear layer between the outer H 2 jet at 52 K and the chamber N 2 §uid at 810 K. This may be simply due to the large temperature di¨erence of -T ≈ 750 K between the two §uids, which is quite di¨erent from -T ≈ 45 K between the inner and outer jets or those of Case 1. Figure 11 shows a comparison Figure 11 Comparison of nondimensional densecore length in terms of momentum §ux ratio of outer jet to inner jet with earlier data: 1 ¡ N2/N2, experiment [4]; 2 ¡ N2/H2, experiment [3]; 3 ¡ N2/N2, computations [13]; and 4 ¡ N2/H2, present study of dense-core length for experimental and computational data selected from earlier studies for coaxial injectors under supercritical pressures. The dense-core length is scaled with an inner diameter of d 1 = 2.0 mm and the momentum §ux ratio is de¦ned as J = (ρu 2 ) o /(ρu 2 ) i .…”

“…(6) using the range of 0.7 < ρ * < 0.9. The computational data from Schmitt et al [13] show a characteristic dense-core length, which is estimated by a position where the density takes a value of ρ = 0.5(ρ i − ρ o ) + ρ o , which is somewhat di¨erent from Eq. (6) and, moreover, it is also noted that their injector had a recess.…”

“…Masquelet et al [12] made a comparison between experimental results [5] and those obtained using LES, and reasonable agreement was found in experimental time-averaged statistics and distribution of LOx core lengths. Schmitt et al [13] investigated the structure of transand supercritical nitrogen jets formed by a coaxial injector operating under high pressure using LES and compared their theoretical results with the experimental ones. They showed that when the momentum §ux ratio exceeds a critical value, the inner jet is abruptly terminated, which is linked to the formation of a recirculation region near the injection section.…”

Cryogenic oxygen jet response to transverse acoustic excitation with the first transverse and the first combined longitudinal-transverse modes

“…The peak response, with the most shortening, was for St values from 0.2 to 0.3, above which the jet length increased again. Schmitt et al [34] compared St values for unexcited and excited cryogenic nitrogen jets. Natural jet instability frequencies were measured from large-eddy simulations, and the corresponding St values compared to those de¦ned by the acoustic forcing in an accompanying experiment.…”