Laser flowing chemical

N. L. Rapagnani and S. J. Davis, "Laser Induced Fluorescence Measurements in a Chemical Laser Flow Field," AIAA J. 

The Art- beam was directed into the chemical laser cavity along the optical axis by a focusing optical train. The spot size in the cavity was a fraction of a millimeter, although tighter focusing could have been done, thus increasing the spatial resolution. The Art- beam could be translated in two dimensions, up and down the nozzle face, at a single position in the flow direction, and also downstream from the nozzle face. Hence, the flow field could be visually mapped out. 

Earlier than with pulsed chemical lasers, the first technological breakthrough in chemical lasers occurred for continuous-wave lasers. Almost simultaneously in 1968 two groups successfully operated continuous-wave chemical lasers. One was at the Aerospace Corporation headed by T. A. Jacobs 75>, the other one at Cornell University under T. A. Cool 76>. One of these lasers was an HF laser the other was that is now called a hybrid chemical laser, being pumped by energy transfer rather than by a direct chemical reaction. This laser principle has been described in the context of pulsed chemical lasers in Section 6.5, In addition to these devices, an HF cw laser having millisecond flow duration was also demonstrated in principle in a shock tunnel. The latter employed diffusion of HC1 into a supersonic stream containing F atoms 77>. 

In the final stage of any flowing chemical laser, the processing of the exhaust effluent must be considered. This is usually accomplished by a smooth transition of the cavity shroud into a diffuser designed to raise the pressure and accommodate the low entrance Reynolds numbers. Diffuser efficiency can be considered an integral part of the overall design of the laser system. A common normal-shock diffuser model is usually adequate to optimize the performance of this section. Pumping of this effluent is subsequently accomplished after recovery of the available pressure within the system. 

Several key parameters describing the general performance of chemical lasers are provided in Table I. The mass efficiency am (kJ/kg) describes the laser power achieved per reagent flow rate and is particularly important for space-based applications, where the cost of delivering the fuel to orbit can dominant total system costs. Typical mass efficiencies for HF lasers are 150-300 kJ/kg. By comparison, dynamite (TNT) possesses an energy density of 9 MJ/kg. The nozzle flux parameter represents the laser power achieved per unit cross-sectional area of the nozzle assembly. This is a critical parameter for power scaling and specifies the size of a high-power chemical laser. 

Truesdell, K. A., and Lamberson, S. E. (19 ) SPIE Proceedings International Symposium on 9th Gas Plow Chemical Lasers, p. 476. Truesdell, K. A., Helms, C. A., and Hager, G. D. (19 ) SPIE Proceedings 10th International Symposiun on Gas Flow Chemical Lasers, p. 217. Verdyen, J. T. (1989) Laser Electronics, Prentice-Hall, Englewood Cliffs, NJ. 

K. Shinohara, Y. Sugii, K. Okamoto, H. Madarame, A. Hibara, M. Tokeshi, T. Kitamori, Measurement of pH field of chemically reacting flow in microfluidic device by laser-induced fluorescence. Meas. Sci. Technol, 2004, 35 (5), 955-960. 

Two very different techniques (a flow system with mass spectrometric detection [17] and a measurement of relative gain on HF vs. DF lines in a chemical laser [18]) gave very similar results (a) (b) 1.45 0.03 [17] and 1.42 0.1 [18]. The "prior" statistical expectation would be (a) (b) = 0.88. The large difference could be explained [19] on the basis of toe information theory of Bernstein and Levine. 

Chemical Reactor Flow Characterization by Laser Diffraction... 

The design of present large scale HF(DF) and CO chemical lasers exemplifies the use of gas dynamic techniques for the achievement of a favorable kinetic environment for laser operation. Besides the aforementioned importance of rapid gas dynamic mixing, flow control is essential for other reasons. Supersonic expansion can provide the means to freeze large concentrations of active atoms or radicals initially created by thermal dissociation. Moreover, such a flow can permit precise control of the translational and rotational temperatures within the reaction zone. Supersonic flows also provide a high power per unit cavity volume and minimize flow pumping requirements through exhaust gas pressure recovery. 

Figure 3.3 indicates, schematically, the major components of a transverse flow supersonic DF cw chemical laser. A precombustor is used to thermally dissociate F2 present in excess in an F2/H2 flame substantial dissociation of... 

Figure 3.5. A subsonic transverse flow cw CO chemical laser [W. Q. Jeffers, AppL Phys. Lett., 21, 267 (1972)]. Injectors A and B are used to inject CS2 and, in some cases, an additive gas such as cold CO.

The designs for several small-scale cw HF(DF) chemical lasers have been given in the literature. " These devices deliver power outputs of a few watts when operated with mixtures of H2(D2), SFe, and helium. Operation as an HCl chemical laser is also possible in this type of laser with either the Cl + HI > HCl + I or H + CI2 -> HCl + Cl reactions, albeit with somewhat lower power outputs than the HF(DF) lasers. Figure 3.8 illustrates one such device. An electrical discharge of about 1 kW(DC, RF, or micro-wave) is commonly used for dissociation of SFg to provide a source of F atoms. Hydrogen is injected by means of small orifices in a direction transverse to the primary flow of partially dissociated SFg in a helium diluent. The optical cavity is aligned transversely to the flow direction as shown in Figure 3.8. The output of such lasers usually consists of several P-branch transitions in the 1 ->0, 2-> 1, or 1 ->0, 2-> 1, and 3->2 bands for HF or DF, respectively. Operation as an HCl laser produces P-branch transitions in the... 

Figure 3.10. A high-repetition-rate transverse flow pulsed HF chemical laser used by Lucht/ based on the design of Jacobson et The flow cross section is 13 mm x...

The first of these lasers was the XeBr laser reported by Searles and Hart, following earlier flow tube studies of Velazco and Setser and Golde and Thrush. Although these lasers require an electrical source for initial creation of the Po,2 rare gas metastables, laser pumping is dependent on chemical reactions of the type (3.8) hence these lasers might well be termed chemical lasers. ... 

Chemical lasers will continue to have an inherent fascination because of what they reveal concerning the partitioning of energy in reaction products. The development of chemical lasers has had a crucial influence on fundamental research on reaction dynamics, energy transfer processes, and the gas dynamics of reacting flows. The intellectual climate and knowledge gained from advances in these areas has contributed much to related research in laser-induced chemistry, laser isotope separation, and laser-induced nuclear fusion. 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

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