Publicado 6 números por año
ISSN Imprimir: 2150-766X
ISSN En Línea: 2150-7678
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DEVELOPMENT OF A BROADBAND MICROWAVE INTERFEROMETER FOR DIAGNOSTIC MEASUREMENTS OF DETONATIONS
SINOPSIS
This paper describes the use of a novel coaxial waveguide configuration to perform detonation velocity measurements using the microwave Doppler interferometry technique. With this non-intrusive method, continuous velocity histories of detonation waves propagating in explosive gaseous mixtures can be obtained. Existing microwave diagnostic techniques are mostly based on the use of the detonation tube as a hollow waveguide. Consequently, the phase and group velocity of electromagnetic waves propagating in the waveguide have a strong frequency dependency, rendering swept frequency and other broadband techniques impractical. The principle advantage of using coaxial configuration is the possibility of operating in the TEM (transverse electric-magnetic) mode, where the phase and group velocities are frequency independent in a lossless medium, permitting the use of broadband remote sensing radar techniques.
In the present study, the feasibility of a TEM mode coaxial system is examined and demonstrated under single-frequency conditions. To facilitate the interpretation of the microwave interferogram, digital signal processing techniques are used. This is done using computer software to extract the velocity information from the microwave Doppler interference signals.
The detonation tube used in the present microwave interferometer consisted of a copper tube 38.4 mm in diameter and 3.5 m long to 13 m long, with a thin wire stretched along the centre axis acting as a centre conductor for the coaxial configuration. The system was tested at microwave frequencies of 6.70 GHz and 9.21 GHz by performing a number of detonation experiments in gaseous mixtures of C2H2+2.5O2 and C3H8+5O2 at low initial pressures (0.53 kPa to 10.66 kPa). Average velocity measurements obtained by the microwave method agreed within 2% with independent photodetector measurements.
The present technique has been used to explore unstable detonations propagating in near-limit conditions. The results demonstrate that the improved microwave Doppler interferometer is particularly well suited for studying unstable detonations where large-scale velocity fluctuations must be monitored continuously over long distances. It may be concluded that the present coaxial microwave Doppler interferometry technique shows promise as a useful diagnostic tool for studying unstable detonations and may provide insight into characterizing near-limit behaviour.