CRESU Uniforme

CRESU Cinetique de reaction en ecoulement supersonique uniforme... 

Finally, a low temperature chemical reactor can be obtained using collimated uniform supersonic flows generated by the isentropic expansion of a buffer gas through an axisymmetric convergent-divergent Laval nozzle. Uniform supersonic flows were initially developed in a continuous flow version for the study of ion-molecule reactions. Later, the methodology was adapted to the study of reactions between neutral species in a continuous flow version and also in a pulsed flow version.This technique known as CRESU will be described in detail in Section 2.3. 

The second group of methods relies on supersonic expansion as the source of cooling and the use of supersonic flows as flow reactors. In the early 1980s the group of Rowe, at the rarefied wind tunnel facility, SR3, in the laboratoire d Aerothermique du CNRS in Meudon (France) developed the CRESU technique an ion-molecule reactor that made use of the uniform supersonic flows generated by a Laval nozzle. The first measurements of rate coefficients down to 20 K were reported in 1984 and soon results for temperatures as low as 8 K were obtained.Note that in the CRESU method the temperature is a true temperature, i.e. LTE prevails in the flow. The up-to-date version of this apparatus and the basis of the technique will be described in some detail in the following paragraphs. 

It was realised by Rowe in the early 1980s that the uniform supersonic flows obtained by the correct design of a Laval nozzle and used for decades in rarefied wind tunnels for aerodynamic studies could provide an ideal flow reactor for the study of chemical reactivity at low and very low temperature. This was the cornerstone around which the CRESU technique has been developed. At the exit of the Laval nozzle, as there is no further expansion downstream of the nozzle exit, the flow parameters (i.e. temperature, density, pressure and velocity) do not exhibit any axial and radial variations at least in the centre of the jet (typically 10 to 20 mm in diameter) where the flow is isentropic for several tens of centimetres. The diffusion velocity is always negligible with respect to the bulk velocity therefore avoiding the major problem of condensation associated with the use of cryogenically cooled cells. As a consequence, in such expansions, heavily supersaturated conditions prevail and condensable species such as water, ammonia or even polycyclic aromatic hydrocarbons (PAHs hereafter), can be maintained in the gas phase at very low temperatures. 

Beyond the significant mass flow rates that must be introduced to generate a uniform supersonic flow downstream of a Laval nozzle, it is also important to stress that the inner shape of the divergent part of the Laval nozzle and the temperature of the reservoir completely constrain the flow conditions i.e. nature of the buffer gas, gas flow rate, supersonic temperature and pressure. In other words, for a given Laval nozzle, the temperature in the supersonic flow is not a timeable parameter. Hence, a series of different Laval nozzles are required to match the range of temperature that needs to be explored. The typical temperatures that can be achieved in the present working CRESU apparatuses are usually in the range 15-300 K. This temperature is directly linked to the reservoir temperature by the relation ... 

In this chapter, we describe the current status of theoretical kinetics for chemical reactions at low temperature, i.e., from 1 to 200K. The desire to understand the chemistry of interstellar space and of low temperature planetary atmospheres provides the general motivation for studying chemical kinetics at such temperatures. For example, the chemistry of Titan s atmosphere is currently a topic of considerable interest. This motivation led to the development of novel experimental techniques, such as the CRESU (cinetique de reaction en ecoulement supersonique uniforme) method, which allows for the measurement of rate coefficients at temperatures as low as lOK (see Chapter 2 by Canosa et ai). Such measurements provide important tests for theory and have sparked a renewed interest in theoretical analyses for this temperature range. ... 

Significantly lower temperatures have been achieved by Rowe and co-workers [23, 24] using the CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) technique, in which the gas is expanded through a Laval nozzle and the flow parameters, density, temperature, pressure and velocity in the central 10-20 mm of the resulting uniform supersonic jet are invariant in both axial and radial directions, since the flow is isentropic over a flow distance of tens of centimetres. The jet crosses an electron beam to provide the ions and into a mass spectrometer, which is movable so that the reaction times can be varied. Measurements have been made down to 8 K. Measurements on He + N2 gave... 

Fig. 3.3 Sketch of a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus configured for the study of radical-neutral reactions. In this arrangement, radicals are generated by photolysis of a suitable precursor using radiation from a fixed-frequency pulsed laser operating at one of the three wavelengths, 226, 248, or 193 nm, and are detected by laser-induced fluorescence excited by tuneable radiation from a dye laser or a master oscillator parametric oscillator (MOPO) [56]...

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