Pulse combustors, design

Numerous studies on a variety of pulse combustor designs have demonstrated that a pulse combustor can offer the following advantages over the conventional (i.e., continuous) combustion systems [25-27] ... 

Pulse Combustors Design and Operation Tab. 2.1 Operational characteristics of pulse combustor fed with various fuels (Wu, 2007). 

In designing pulse combustor/atomizer drying systems, the pulse intensity as well as the temperature and velocity of the gas at the point of atomization are optimized for each product. A particular advantage of the technology is, that the plant s control system can modify the process conditions such that a variety of dry powder characteristics are met without physically changing the equipment. These characteristics primarily include particle size, flowability, texture, temperature history, residual moisture content, flavor, and ease of reconstitution. 

The aerodynamic valves have a specially designed inlet (e.g., a profiled orifice in the inlet pipe, contoured diffuser, or a shrouding duct) in which the fluid flow characteristics act as a physical barrier (fluid diode) to the backflow of combustion products. Such pulse combustors are termed the valveless combustors. Example of valveless pulse combustor is shown in Figure 23.4 (Putnam et al., 1986). 

Figure 23.15 shows the flow diagram of one prototype pulse combustion dryer designed by Novadyne Ltd (Canada) (Kudra and Mujumdar, 1995, 2002 Kudra et al., 2003). In the flash dryer configuration, the unit consists of a pulse combustor, a pneumatic duct that extends from the tailpipe of the pulse combustor, and devices for materials feed and discharge. The pulse combustor uses a self-actuated flapper valve, and its operating frequency is about 70 Hz. The rated capacity of the combustor equal to 300 kW offers an evaporative capacity of about 275 kg/h of water. Wet particulate materials such as sawdust, hog fuel, corn fibers, or spent coffee grounds are injected directly into the tailpipe... 

The pulse combustor is a combustion chamber with no moving parts operating on either gas or fine coal. The chamber and exit tube are designed in a manner which results in a self-sustaining, periodic combnstion process. The frequency of the resonance varies with chamber size and exit tube length. The exit tnbe of the pulse combustor is immersed in the fluidized bed. The raw coal is pneumatically separated with the coal fines carried to the pulse combustor and the coarse coal to the fluidized-bed combustor via a screw feeder. 

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 only design of the pulse combustion fluid bed dryer appears to be the one patented by Lockwood (1983). To avoid attenuation of the pulsating gas stream by a perforated gas distributor, the flue gases from a pulse combustor enter the bed of particulate material just above the solid floor, which rotates under a plurality of ducting blades adjacent to the floor. The radially spaced blades are fixed to the central hub at one end and to the inner annular baffle at the other one (Figure 14.11). The space between the baffle and dryer wall forms a gas manifold connected to the tailpipe of a pulse combustor. Each... 

The kinetic simulations of the pulse combustor ignition can be carried out under conditions which closely approximate those in a continuously stirred tank reactor (cstr). In those calculations, hot product gases are steadily mixed with cold, unbumed reactants until the mixtures ignite. The reaction mechanisms used are valid for high temperatures, and the most important, sensitive reaction is reaction (3), and the combined influences of chemical kinetics, acoustics, and fluid dynamics can all be incorporated into a coherent practical design model [20]. 

An alternative design of a pulse combustor is a so-called valveless combustor in which the mechanical valves are replaced with an aerodynamic diode in the form of a profiled orifice in the inlet pipe, contoured diffuser, or a shrouding duct [28] (Figure 20.15). Similarly to the combustor with mechanical valves, the high-temperature gases from a combustion chamber start to flow just after a... 

Since mechanical valves provide a physical barrier to the backflow of combustion products through the combustor inlet during the positive-pressure phase of the pulse combustion cycle, the unidirectional flow is the fundamental feature of valved pulse combustors. There are, however, certain problems associated with the design of mechanical valves, such as minimizing valve inertia, protection from corrosion, and resistance to material fatigue due to thermal stress. These specific problems are of major importance in heavy-duty pulse combustors operated at large pressure amplitudes (Kentfield, 1993). 

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. 

Wu (2007) elaborated a CFD model for parametric studies of a pulse combustor with mechanical valves to explain the pulsating flow characteristics. On the basis of numerical results, a small-sized pulse combustor was designed and tested that provided a good agreement vhfh experimental observations. 

The present study was conducted in an effort to better understand ACC mechanisms and to design practical ACC based on pulsed liquid-fuel injection suitable for propulsion devices. The controller utilized a simple fixed phase-delay approach that has been studied previously, but the direct liquid-fuel injection and the novel use of vortex-droplet interaction made the present study unique. The demonstration experiment in a 102-millimeter dump combustor showed that combustion instabilities can be successfully suppressed using properly designed pulsed liquid-fuel injection. 

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