Diagnostics of D2, D2+H2 and D2+He supersonic jets by Raman spectroscopy

Abstract

An experimental study of supersonic jets of pure D2, and of its mixtures with H2 and He, is presented. Supersonic flows of hydrogen isotopes (H2 and D2) are difficult to model by using either continuous gas dynamics, even with empirical ad hoc ¿molecular¿ corrections, or numerical Monte Carlo simulations. This is due to [1]: 1. their low molecular mass and moment of inertia (large energy gap between rotational levels) leads to the occurrence of non-negligible quantum effects, and a marked decoupling between translational and rotational motions. 2. normal hydrogen isotopes behave like a mixture of two non-interconvertible species (ortho and para), due to nuclear spin statistics.

Raman (inelastic light) scattering is a non-intrusive and powerful diagnostics technique for molecular gas flows [2]: it is able to map, with a high spatial resolution (few microns), number densities and rotational populations (temperatures) of molecular gases, even at the high gradients occurring in shock waves [3-5]. In this work, a number of supersonic microjets of pure D2, and mixed with H2 and He with different mixing ratios, have been quantitatively characterized in terms of number densities n(z) and rotational populations PJ(z) along the jet axis (z) by means of vibrational Raman spectra with rotational resolution. The circular nozzle had an exit diameter of 350 um, stagnation pressure ranged from 0.25 to 3 bar, and stagnation temperature from 293 to 363 K. Rotational populations PJ were obtained from the relative intensities of the rotational Q(J) lines of the vibrational Q branches at ~4150 cm¿1 (H2) and ~2990 cm¿1 (D2). Number densities n(z) were obtained by comparing the integrated intensity of each full Q branch with that from static samples at known densities. Translational temperatures can be obtained from number densities and rotational populations by conservation of mass, momentum, and energy along the jet streamlines [6]. Special attention was paid to an accurate sensitivity calibration of the CCD detector, and to the high gradients present in the flow field close to the nozzle exit. Experimental data, like those gathered in the present work, are expected to be of great value to improve our understanding of such flows at molecular scale.

REFERENCES [1] S. Montero, J. Pérez-Ríos, Journal of Chemical Physics 141, 114301 (2014) [2] S. Montero, B. Maté, G. Tejeda, J.M. Fernández, A. Ramos, in Atomic and Molecular Beams. The State of the Art 2000, Springer, Berlin, 2001, pp. 295-306 [3] A. Ramos, B. Maté, G. Tejeda, J.M. Fernández, S. Montero, Physical Review E 62, 4940 (2000) [4] I. Graur, T. Elizarova, A. Ramos, G. Tejeda, J.M. Fernández, S. Montero, Journal of Fluid Mechanics 504, 239 (2004) [5] A. Ramos, G. Tejeda, J.M. Fernández, S. Montero, Journal of Physical Chemistry A 114, 7761 (2010) [6] B. Maté, G. Tejeda, S. Montero, Journal of Chemical Physics 108, 2676 (1998)