Pulse combustion pressurized-chamber

Although some processes can be carried on in a tailpipe of the pulse combustor, the typical pulse combustion system consists of a combustor and an applicator in which the pressure waves are amplified by acoustic resonance. The frequency of the pressure pulsations is generally close to the frequency of the fundamental acoustic mode of the combustion chamber and the applicator. If properly tuned, the pulsed combustor can excite large-amplitude pulsations in a process (e.g., drying, calcining, or incineration) carried out downstream of its tailpipe. 

An alternative mode of pulse combustion drying was suggested by Zinn et al. (1990), where pulse combustion is mainly applied to generate a large-amplitude pulsation in the drying chamber and, in this condition, provides small portion of heat for moisture evaporation. This kind of pulse combustion dryer consists of a combustor and a dying chamber where the hydro-dynamic action of pressure and velocity waves enhances drying rates. Under certain conditions, these pressure waves can be favorably amplified by an acoustic resonance. To excite... 

The term pulse combustion originates from intermittent (pulse) combustion of solid, liquid, or gaseous fuel in contrast to continuous combustion in conventional burners. Such periodic combustion generates intensive pressure, velocity, and to a certain extent, temperature waves propagated from the combustion chamber via a tailpipe (a diffuser) to the process volume (an applicator) such as a dryer, calciner, or incinerator. Because of the oscillatory nature of the momentum transfer, pulse combustion intensifies the rates of heat and mass transfer. 

Recent studies on pulse combustion indicate that design of the combustion chamber and the geometry of a combustor-dryer system may affect the level of noxious emission. Noticeably lower concentrations of CO, NO, and NOx were obtained for those combinations of the combustor volume and lengths of the tailpipe that provide smoother and sinusoidal pressure fluctuations (Zbicinski et al., 1999). 

The mechanism behind the operation of a pulse combustor is a complex interaction between an intermittent combustion process and pressure/velodty waves that are propagated from the combustor (Fig. 2.1). The process of pulse combustion is initiated when air required for combustion and fuel in the form of a gas jet or a Uquid spray are admitted to the combustion chamber to make-up an explosive mixture, which is ignited by a spark plug and burns instantly in an explosion-like manner. At this moment, the air and fuel inlet ports are closed, either by mechanically operated valves or due to the hydrodynamic action of the rapidly rising pressure. The combustion-generated pressure forces the combustion products to flow out through the tailpipe to the process volume. As the hot flue... 

Fig. 2.21 (a) Pulse combustion pneumatic dtyer and (b) pulse combustion dtyer with pressurized chamber (courtesy of Novadyne Ltd, Hastings, ON, Canada). 

The phenomenon of unstable combustion results from a self-amplifying interaction between combustion processes and the. acoustic oscillations of the gas within the rocket motor. The unexpected appearance of combustion instability in any rocket generally terminates its mission thru motor case rupture from overpressure, disruption of guidance systems by severe vibration, or thrust malalignment. Both axial mode and transverse mode instabilities are observed (Ref 45). In the case of the transverse mode the characteristic wave time is usually that required to travel radially around the proplnt cavity whereas the characteristic time for the axial mode is the time for the wave to travel from end to end in the combustion chamber. Double-base proplnts predominantly are prone to transverse wave instabilities and infrequently to those in the axial mode, while composite proplnts appear to go unstable mostly in the axial mode. In the case of transverse instability chamber pressures have been known o double whereas in axial mode instabilities artificially induced by pulsing the chamber pressure at lOOOpsi, the pressure excursion may reach 300—400psi. A review of recent theoretical combustion modeling for combustion instability has been made by Price (Ref 47)... 

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... 

Typically, pulse combustors oscillate with frequencies that vary from 20 to 150 Hz. Pressure oscillations in the combustion chamber of 10 kPa produce tailpipe velocity oscillations of nominally 100 m/s and the gas jet velocity at the tailpipe exit pulsates from approximately 0 to 100 m/s [27]. The input power for commercially available pulse combustors ranges from 70 to 1000 kW. 

Typically, pulse combustors operate at frequencies from 20 to 250 Hz. Pressure oscillation in the combustion chamber of 10 kPa produces velocity oscillation in the tailpipe of about 100 m/s, so the instantaneous velocity of a gas jet at the tailpipe exit varies from 0 to 100 m/s (Keller et al., 1992). Although pulse combustors deliver flue gases at a higher pressure than the inlet air pressure, the resulting increase in stagnation pressure is relatively small. This restricts practical applications of pulse combustors to the systems where pressure drop is not critical. The amplitude of the pressure rise may vary from 10% (domestic heating applications) to 100% as for heavy-duty pulse combustors for industrial use (Kentfleld, 1993). The output power for commercially available pulse combustors ranges from 70 to 1000 kW. 

The Helmholtz pulse combustor operates under the principle of the standard acoustic Helmholtz resonator in which a short, small-diameter stub (tailpipe) is attached to one of the walls of a large cavity (combustion chamber) and valves are placed at the wall opposite the tailpipe. A Helmholtz resonator operates at a frequency determined by both the volume of the combustion chamber and the length and cross-sectional area of the tailpipe. It is important to note that the pressure within the Helmholtz combustion chamber is considered to be uniform in space while the pressure oscillations become space-dependent once within the tailpipe. 

Typically, pulse combustors oscillate with frequencies that vary from 20 to 150 Hz. Pressure oscillations in the combustion chamber of +10kPa produce tailpipe velocity oscillations of nominally... 

Fig. 2.2 (a) Theoretical and (b) experimental pressure trace in a pulse combustor. A air and fuel enter the combustion chamber B fresh charge ignited, pressure rises as combustion gases heat up, air and fuel inflow are stopped ... 

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