Supersonic limitations

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. 

It might be thought that the small number of molecules in a typical supersonic jet or beam would seriously limit the sensitivity of observation of the spectra. Flowever, the severe rotational cooling which may be produced results in a collapsing of the overall intensity of a band into many fewer rotational transitions. Vibrational cooling, which greatly increases the population of the zero-point level, concentrates the intensity in few vibrational transitions, and these two effects tend to compensate for the small number of molecules. 

Further reductions in reservoir pressure move the shock front downstream until it reaches the outlet of the no22le E. If the reservoir pressure is reduced further, the shock front is displaced to the end of the tube, and is replaced by an obflque shock, F, no pressure change, G, or an expansion fan, H, at the tube exit. Flow is now thermodynamically reversible all the way to the tube exit and is supersonic in the tube. In practice, frictional losses limit the length of the tube in which supersonic flow can be obtained to no more than 100 pipe diameters. 

Newer high velocity thermal spray coating processes produce coatings in compression rather than tension because of the shot peening effect of the supersonic particles on impact. This has permitted coating as thick as 12,500 p.m without delamination as compared to older processes limited to 1,250 p.m. The reduced residence time of particles at temperature minimises decomposition of carbides present in conventional d-c plasma. This improves wear and hardness (qv) properties. 

The first commercial supersonic transport, the Concorde, operates on Jet A1 kerosene but produces unacceptable noise and exhaust emissions. Moreover, it is limited in capacity to 100 passengers and to about 3000 miles in range. At supersonic speed of Mach 2, the surfaces of the aircraft are heated by ram air. These surfaces can raise the temperature of fuel held in the tanks to 80 °C. Since fuel is the coolant for airframe and engine subsystems, fuel to the engine can reach 150°C (26). An HSCT operated at Mach 3 would place much greater thermal stress on fuel. To minimize the formation of thermal oxidation deposits, it is likely that fuel deflvered to the HSCT would have to be deoxygenated. 

There are certain limitations on the range of usefulness of pitot tubes. With gases, the differential is very small at low velocities e.g., at 4.6 m/s (15.1 ft/s) the differential is only about 1.30 mm (0.051 in) of water (20°C) for air at 1 atm (20°C), which represents a lower hmit for 1 percent error even when one uses a micromanometer with a precision of 0.0254 mm (0.001 in) of water. Equation does not apply for Mach numbers greater than 0.7 because of the interference of shock waves. For supersonic flow, local Mac-h numbers can be calculated from a knowledge of the dynamic and true static pressures. The free stream Mach number (MJ) is defined as the ratio of the speed of the stream (V ) to the speed of sound in the free stream ... 

Detection limits in the lOOfg range can be obtained with a tuneable UV laser working at a wavelength of maximum absorption for the compounds of interest. Continuous supersonic beams require high gas loads and combination with a pulsed ionisation technique (e.g. REMPI) is unfavourable in terms of sensitivity. Pulsed valves are a better approach for a GC-UV-MS interface [1021]. 

This report deals primarily with the origins and effects of ozone and other photochemical oxidants. It is limited, more or less, to the problem of urban pollution and to such closely related topics as natural background in the earth s boundary layer. No consideration is given to the stratospheric ozone layer and the effects produced by supersonic transport (sst) emission or halocarbons. 

Figure 8. Scheme of the experimental setup of the CIS experiment. Two Fourier-transform-limited nanosecond laser pulses with different frequencies are interacting with cold molecules or van der Waals complexes in a skimmed supersonic molecular beam. 

Many of the problems in combustion research stem from the difficulty of burning in combustors at high speeds and altitudes. The advances in this field have been tremendous. The jet-turbine combustor will release 500 times the energy for a given space as the stationary boiler furnace. Its operating problems have been overcome (with the usual partial aid of fundamental combustion research) so that the limiting operational altitudes of aircraft are well above 50,000 feet and the speeds are supersonic. 

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