Nozzle Laval

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

Convergent/Divergent Nozzles (De Laval Nozzles) During frictionless adiabatic one-dimensional flow with changing cross-sectional area A the following relations are obeyed ... 

Rowe and co-workers are developing a so-called diffusion technique to extend the temperature and pressure range. The technique will use the conversion of the initial kinetic energy (per unit volume) of the jet into a pressure increase downstream of the mass spectrometer, when the flow is brought from a supersonic to a subsonic regime through suitably shaped tubing. Also, it has been shown that the use of pulsed Laval nozzles reduces the appreciable amounts of gas that are consumed in the continuous flow CRESU apparatus [55]. 

The high temperature gas generated in the combustion chamber of a rocket engine is expanded through a De Laval nozzle to convert a major portion of the enthalpy to mechanical thrust. The thrust F of a rocket engine is defined by the following conservation of momentum equation ... 

Near-equilibrium flow conditions generally yield the maximum thrust for rocket propulsion, because partial recombination of the dissociated atoms, as the temperature falls, releases additional kinetic energy. On the other hand, when the rocket engine is considered for high temperature chemical processing, it is invariably desirable to freeze the composition attained in the combustion chamber. From both theoretical and practical standpoints, it is not always possible to predetermine the flow conditions in the De Laval nozzle as the foregoing discussion indicates,... 

The T dependence of (ib) was unexpected as it implies that there are strong attractive forces acting between 02(A ) and I at relatively large internuclear separations. To explore this phenomena in more detail, and to test the reliability of Eqs. (20) and (21), we examined reaction (lb) at a temperature of 150 K. ° These measurements were carried out using a Laval nozzle to provide low temperature gas flows. Traces of I2 were entrained in He/02 mixtures, and pulsed laser photolysis was used to generate I by exciting just above the 12(B) dissociation limit. Near threshold photodissociation was used to ensure that I was produced with a translational temperature in equilibrium with the local conditions. I decay kinetics were... 

Fig. 9. LIF scans over the I 5p 6s( P3/2) hp ( Pi/2) transition. I was generated by 498 nm photolysis of I2 in a Laval nozzle expansion at 150 K. The upper trace shows peak intensities that reflect the nascent hyperfine level populations. The lower trace shows the effect of relaxing this distribution by collisions with 02(X).

Figure 8.3 is a plot of AM versus Ji for steady, one-dimensional, frictionless, adiabatic flow of a perfect gas. It is, in effect, a design guide for a supersonic nozzle If we wish the Mach number to increase linearly with distance in steady, isentropic flow of a perfect gas, then the cross-sectional area-distance relation must be exactly the curve in Fig. 8.3, This is the diagram for a converging-diverging nozzle, a type of nozzle commonly referred to as a de Laval nozzle after its inventor, who used it in the first practical steam turbine [11]-...

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