Pulse frequency-tunable

The system used to measure the optical fiber signals employs two separate frequency tunable laser light sources operating at about 1320 nm wavelength. One laser acts as a pump laser, whereas the other serves as the probe laser that sends optical pulses down the fiber to interact with the counterpropagating laser light wave pumped into the fiber from its opposite end. 

This uitrasonio-opticai technique (or haif-opticai technique [89]) was aiso a hyphenated technique in terms of energy sources viz. thermai and opticai for phonon and photon production, respectiveiy). Thermai surface phonons restrict practical application of the technique owing to their iow scattering efficiency, which results in overly long data collection times (typicaiiy severai hours for a singie spectrum, even with advanced multipass interferometers). Similar to active Raman spectroscopy, coherent acoustic phonons are assumed to be excited by two narrow-line frequency tunable laser beams at different frequencies or by laser pulses of short duration compared to the acoustic period. 

According to Kentfield (1993), a pulse combustor is a combustion-driven device with self-aspirating feature, and this effect is achieved as a consequence of the internal unsteady flow events. In contrast, a pulsed combustor is a device with cyclic but nonresonant combustion as dictated by wave events. Pulsed combustors usually operate at a much lower than natural frequency, often controlled by an ignition, fuel injection, or a valve sequence. Therefore, valveless or flapper valve combustors fall into category of pulse combustors while mechanically driven valves (e.g., rotary valve) used to control either air or fuel inflow, flue gas outflow, or both should be categorized as pulsed combustors, unless the operation of a mechanical valve is controlled by resonant phenomena in a feedback mode. Such a design is known as a frequency-tunable pulse combustor. 

The frequency-tunable pulse combustor consists of a tube comprised of a combustion zone and an exhaust zone. It is within the combustion zone that the reaction between fuel and air occurs, causing heat to be released and thereby exciting an acoustic wave in the combustor. Separate air and fuel inlets are present to supply the combustion zone with the necessary reactants. Rotary valves are used in the Sonotech system to control fuel and airflows into the combustion chamber (Lockwood, 1987). The resulting hot combustion gases are freely exhausted from the combustor. 

Figure 14.6 Rotary kiln with frequency-tunable pulse combustor. (From Richards, 1994.)...

Richards MK. Demonstration of the Sonotech, Inc., frequency-tunable pulse combustion system (CELLO pulse burner). SITE Program Fact Sheet, U.S. Environmental Protection Agency, Cinciimati, April 1-4, 1994. 

Pulse combustors may be categorized into three distinct classes according to the specific acoustic system on which their operation depends (i) the Quarter-wave (or Schmidt) combustor (ii) the Helmholtz combustor and (iii) the Rijke-type combustor. In contrast to the Rijke combustor, which operates with solid fuels, both the Schmidt and Helmholtz combustors accept liquid and gaseous fuels. The Helmholtz combustor is preferred for drying applications because the larger volume of the combustion chamber and the smaller (but longer) tailpipe allows for multivalve assembly. Detailed information on these types of combustor is available in articles by Zinn (1985) and Kudra and Mujumdar (2009). Some combustors also exploit the resonance phenomenon these are referred to as frequency-tunable pulse combustors. 

One possible design of a frequency-tunable pulse combustor takes advantage of the natural non-longitudinal acoustic modes of the process chamber, such as a dryer or incinerator (Zinn and Daniel, 1988). In order to obtain maximum benefit from the pressure/velocity osdllations in the process chamber, the pulse combustor can be tuned to one (or more) of these acoustic modes by modulating the flow of fuel to the combustion chamber. Such modulation may be accomplished by exciting the acoustic resonance within the fuel line, or via periodic interruptions of the fuel flow, using a rotary valve. 

The foremost of the modem teclmiques is tlie use of lasers as spectroscopic tools. Lasers are extremely versatile light sources. They can be designed with many usetlil properties (not all in the same instmment) such as high intensity, narrow frequency bandwidth with high-frequency stability, tunability over reasonable frequency ranges, low-divergence beams which can be focused into very small spots, or pulsed beams with... 

Pumping is with a flashlamp, as in the case of the ruby laser, and a pulse energy of the order 1 J may be achieved. Frequency doubling (second harmonic generation) can provide tunable radiation in the 360-400 nm region. 

---