Three-Dimensional Hybrid RANS/LES Simulations of a Supercritical Liquid Nitrogen Jet

Abstract

Simulations of an uni-element, shear coaxial injector configuration with liquid nitrogen in the inner tube mixing with a co-flowing warm nitrogen gas stream are presented; the chamber has a supercritical pressure of 4.95 MPa with the liquid nitrogen jet temperature mildly supercritical at 129 K. Unsteady solutions on a complete three-dimensional configuration were computed using a hybrid RANS/LES framework and compared with experimental data. Our results reveal a highly unsteady flow field with strong fluctuations in the post region between the inner jet and co-flowing outer jet. This unsteadiness is caused by strong thermodynamics gradients near the critical point, which cause the inner jet to expand out radially as it mixes with the warmer fluid from the outer jet. Analysis of the turbulence field indicates that nominal temperature fluctuations generate large amplitude density fluctuations which in turn increases the Reynolds stress in the liquid jet shear layer. A key finding of this study is that the unsteady mixing is dominated by three-dimensional helical instabilities on the interface of the liquid jet shear layer. In this important respect, a trans-critical/supercritical jet is very different from a gas jet where the jet nearfield is unstable only to axisymmetric instabilities. A comparison of the mean radial temperature distribution indicates reasonable comparison with experimental measurement; the dramatic mixing of the liquid shear layer is captured well but the mixing in the outer gas shear layer is underpredicted. A comparison of the jet mixing core length prediction along the centerline with the dark core length measurements from flow visualization indicates that the numerical results overpredict the mixing and this may be due to a combination of uncertainty in the temperature measurement values as well as numerical errors.