Mechanical valves, pulse combustors

FIGURE 23.2 A mechanically valved pulse combustor with membrane valves. 

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). 

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. 

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. 

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... 

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... 

The reed valves normally used in heavy-duty pulse combustors are made from thin-sheet spring-sted (Fig. 2.6b and c), and the spring action of reed valves is such that, when normally shut, the valves are sprung lightly. In order to ensure a vigorous mixing of the fuel and air, the fully open flow area of the inlet reed valves must be considerably smaller than the cross-sectional area of the combustion zone. The major problem often encountered with reed-type mechanical valves is fatigue-based failure. 

Wu and Liu (2002) analyzed the mechanism of the pulse combustion spray drying of salt solution in an oscillating flow field produced by a Helmholtz-type pulse combustor with an external flapper valve. The solution was atomized directly by the pulsating flow in the combustor tailpipe, which reduced the average droplet diameter by about 50% in relation to conventional nozzle atomization. An optical analyzer was applied to measure the droplet size distribution in the spray in order to determine the initial conditions for the CFD model of the process. The results of CFD simulations enabled the average residence time of droplets in the drying chamber (ca. 0.1 s) and the... 

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. 

Aerodynamic valves employ the fluid mechanical properties of specially designed inlets to act as a physical barrier to the backflow of combustion products out of the combustor through the inlet section. The main advantage of aerodynamic conflgurations is lack of valves and moving parts so the risk of mechanical breakdown or failure is eliminated. This is a key consideration for heavy-duty pulse combustion burners where the inlet section undergoes severe operating conditions. 

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