Pulse with flapper valves

Fig. 2.1 Operation of a pulse combustor with flapper valves.

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. 

The performance characteristics of a flapper valve pulse combustor fed with various fuels are listed in Tab. 2.1 (Wu, 2007). 

Liu et al. (2001) placed a 10 mm brass sphere in the flow field inside the tailpipe of a Helmholtz-type pulse combustor with an external flapper valve to determine the convective heat transfer coefficient under various oscillation frequencies. Figure 2.11 presents the results of measurements of sphere temperature and heat transfer coefflcient in a gas stream oscillating at 75 Hz. Based on the results of these experiments, Liu et al. (2001) determined the following correlation between the Nusselt number (Nu) and the frequency of oscillations (f) over the temperature range from 300 to 900 °C ... 

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... 

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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