EXPERIMENTAL AND NUMERICAL CHARACTERIZATION OF A PULSED SUPERSONIC UNIFORM FLOW FOR KINETICS AND SPECTROSCOPY

%begin{wrapfigure}{r}{0pt}_x000d_ %includegraphics[scale=0.1]{UF_v2.eps}_x000d_ %end{wrapfigure}_x000d_ _x000d_ The current CPUFfootnote{J. M. Oldham, C. Abeysekera, J. Joalland, L. N. Zack, K. Prozument, I. R. Sims, G. Barrat Park, R. W. Filed and A. G. Suits, J. Chem. Phys. 141, 154202, (2014).} (Chirped Pulse Uniform Flow) and the new UF-CRDS (Uniform Flow Cavity Ring-Down Spectroscopy) setups relie mostly on the production of a good quality supersonic uniform flow._x000d_ _x000d_ A supersonic uniform flow is produced by expanding a gas through a Laval nozzle - similar to the nozzles used in aeronautics - linked to a vacuum chamber._x000d_ The expansion is characterized by an isentropic core where constant very low kinetic temperature (down to 20K) and constant density are observed. The relatively large diameter of the isentropic core associated with homogeneous thermodynamic conditions makes it a relevant tool for low temperature spectroscopy. On the other hand, the length along the axis of the flow of this core (could be longer than 50cm) allows kinetic studies which is one of the main interest of this setup (CRESU techniquefootnote{I. Sims, J. L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, P. Bocherel, B. Rowe, I. W. Smith, J. Chem. Phys. 100, 4229-4241, (1994).})._x000d_ _x000d_ The formation of a uniform flow requires an extreme accuracy in the design of the shape of the nozzle for a set of defined temperature/density. The design is based on a Matlab program which retrieves the shape of the isentropic core according to the method of characteristics prior to calculate the thickness of the boundary layerfootnote{D. B. Atkinson and M. A. Smith, Rev. Sci. Instrum. 66, 4434, (1995).}._x000d_ _x000d_ Two different approaches are used to test the viability of a new nozzle derived from the program. First, a computational fluid dynamic software (OpenFOAM) models the distribution of the thermodynamic properties of the expansion. Then, fabricated nozzles using 3-D printing are tested based on Pitot measurements and spectroscopic analysesfootnote{N. Suas-David, V. Kulkarni, A. Benidar, S. Kassi and R. Georges, Chem. Phys. Lett. 659, 209-215, (2016)}. _x000d_ I will present comparisons of simulation and measured performance for a range of nozzles. We will see how the high level of accuracy of numerical simulations provides a deeper knowledge of the experimental conditions