Rotary valves, pulse combustors

Fig. 7.81 Schematic representation of the PCS rotary-valved pulse combustor/atomizer. Expianations see text (courtesy Pulse Combustion Systems, San Rafael, CA, USA).

The heart of this drying system is a rotary-valved pulse combustor. Referring to Fig. 7.81, combustion air (1) is pumped at low pressure into the pulse combustor s outer shell and flows through an unidirectional air valve (2) into a tuned combustion chamber [ Helmholtz Resonator (3)] where fuel (4) is added. The air valve closes. The fuel-air mixture is ignited by a pilot (5) and explodes, creating hot air which is pressurized to approx. 0.2 bar above combustion air fan pressure. The hot gas exits the chamber through a pipe (6) towards the atomizer area (7). Just above the atomizer, quench air (8) is blended in to achieve the desired process temperature. The orifice releases the liquid... 

Figure 23.3 shows a pulse combustor with rotary valves, which is a modification of membrane combustors (Smucerowicz, 1999). It consists of two valve plates with identical orifices. One plate is static, while the other one rotates. The air flows into the combustion chamber when slots in the two plates overlap otherwise, the valve is closed. The rotation of the valves has to fix the oscillations frequency. The valves must be open when underpressure occurs in the chamber to get engine into acoustic resonance. 

Based on the manner in which fuel and air charge the combustion chamber, pulse combustors are divided into two general categories those with mechanical valves and those with aerodynamic valves (also called valveless combustors). Mechanical valves can be further divided into three types flapper valves, reed valves, and rotary valves. 

Figure 14.4 Design principle of pulse combustor with rotary valve. (From Lock-wood, 1987.)...

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. 

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. 

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