Pulsed Laval Nozzle

We use a de Laval nozzle to achieve an isentropic molecular beam that is uniform (free from shocks), cold and in thermodynamic equilibrium. This gives us a stable environment for our experiment, where the physical properties can be found as a function of the Mach number (ratio between the flow velocity and the speed of sound) of the beam.

A gas mixture is prepared by heavily diluting reactant gases in an inert carrier gas, such as Ne, using mass flow controllers. To achieve the correct mixing ratios, one or both reactant gases may be prepared in a premix tank diluted with the carrier gas. The concentrations of these gases are chosen to give pseudo first order kinetics.

45K Ar nozzleThe beam is generated by "pulsing" the premix into a stagnation region before expanding though the nozzle. By using a piezo stack to drive the valve, we are able to get a high repetition rate. Our switch time to switch between full open and full closed at about 10 microseconds with pulse widths of approximately 100 microseconds.

 

Using a script developed by our group, Laval nozzles are designed using the method of characteristics1,2,3 to create a flow of parallel lines at the nozzle exit. By varying the design parameters, we are able to get temperatures between 20K and 70K. The performance of an individual nozzle is determined by pressure impact measurements using a Pitot sensor inserted into the flow.  We then can verify the flow temperature through measuring the rotational distribution of molecules in the flow using our CP-FTMW spectrometer.

(1) Atkinson, D. B.; Smith, M. a. Rev. Sci. Instrum. 1995, 66 (9), 4434.
(2) Cohen, B. C. B.; Reshotko, E. L. I. The compressible laminar boundary layer with heat transfer and arbitrary pressure gradient report 1294; 1956.
(3) Anderson, J. D. Fundamentals of Aerodynamics, Second.; Corrigan, J. J., Castellano, E., Eds.; McGraw-Hill, 1991.