Pulsed combustors

Steam superheater and an air heater installed on the pulse combustor flue gas (D13938S, p. 9-2)... 

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... 

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. 

A pulse combustor for a conventional atmospheric burner (e.g., Maxon and Eclipse) and ducting for the hot air from the burner to the drying 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. 

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] ... 

Furthermore, some pulse combustors are self-aspirating so they do not require a separate fan to supply combustion air. Becanse of the high temperature and large volume of the combustion products, there is in general no need for a blower. 

Industrial applications of pulse combustion were limited so far to space and water heating mainly becanse of the difficulty of resonating the process volume at a fixed frequency if its acoustic characteristics vary with operating conditions. However, the progress in tunable-freqnency pulse combustors... 

The operation of a pulse combustor is controlled by a complex interaction between an oscillatory combustion process and acoustic waves that are excited inside the combustor (Ligure 21.13). Ignition of the fuel-air mixture by a spark plug... 

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. 

To excite large-amplitude pulsations in a drying chamber with a reasonable power input into the pulse combustor, the combustor must operate at a frequency that equals one of the acoustic frequencies characteristic of that chamber. Then, the pressure pulsations inside the combustor and the pressure pulsations generated inside a drying chamber are in resonance [31]. In practice, the acoustic mode of industrial dryers depends on the chamber geometry and material... 

FIGURE 21.15 Valveless pulse combustor. (From Kentfield, J. A.C., Proceedings of the International Symposium on Pulsating Combustion, Monterey, CA, Paper A-6, 1991. With permission.)... 

In practical applications, the pulse combustor can be used alone or in combination with a conventional burner. The primary function of the pulse combustor is then to serve as a speaker that excites resonant pulsations in a drying chamber, thereby improving the dryer performance. In such a case, the pulse combustor supplies from 10% to 30% of the process energy. The total amount of energy supplied to the burner and pulse combustor by a fuel is equal to or lower than the amount of energy supplied to the dryer with a conventional burner alone [26]. 

A deterrent that prevents broad application of pulse combustors is, in some cases, their unacceptable noise level. In pulse combustors, there are three main sources of this noise detonating character of combustion (this is an intrinsic feature of the combustor operation), vibrations of metal walls of the combustor, and velocity difference between gases flowing from the combustor and ambient air. The methods to efficiently decrease the noise level are coupling of two combustors so that they work in counter-phase, application of ejectors at the combustion inlet and outlet, and shielding the space between the ejectors, inlet and outlet. 

FIGURE 23.2 A mechanically valved pulse combustor with membrane valves. 

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. 

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). 

The valveless pulse combustor is characterized by simple construction with no movable parts. 

Pulse combustors become attractive because of their enhanced combustion efficiency, economic use of fuels, and environmentally friendly operation. Recently, the area of applications of pulse combustors has increased significantly. Pulse combustion applications can be divided into two general classes those that use the pulse combustors as a heater and those that utilize the pulsating flow inside and/or outside the pulse combustor to improve processes such as drying or calcining (Zinn, 1992). 

Besides the applications of pulse combustors as residential, space heater and propulsion procedure, etc., a more active area is that of drying. Ellman et al. (1966) first carried out experimental studies of lignite drying using a 205 kW pulse... 

Benali and Legros (2004) carried out an experimental study on thermal processing of particulate solids on a valved pulse combustion unit. The test bench consists of a 60 kW natural-gas-fired, valved (flappers) pulse combustor having a 4.63 X 10" m combustion chamber, horizontal tailpipe with variable geometry, and a cylindrical drum. The particulate solid used was clean sand (311 pm and 2646 kg/m ), which flows within the tailpipe and was collected by the cylindrical drum located at its end. The sand flow rate was varied from... 

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