Helmholtz combustor

Pulse combustors may be categorized into three distinct classes according to the specific acoustic system on which their operation depends. These are the Quarter-Wave (or Schmidt) combustor, the Helmholtz combustor, and the Rijke-type combustor. 

Pulse combustors may be categorized into three distinct classes according to the specific acoustic system on which their operation depends (i) the Quarter-wave (or Schmidt) combustor (ii) the Helmholtz combustor and (iii) the Rijke-type combustor. In contrast to the Rijke combustor, which operates with solid fuels, both the Schmidt and Helmholtz combustors accept liquid and gaseous fuels. The Helmholtz combustor is preferred for drying applications because the larger volume of the combustion chamber and the smaller (but longer) tailpipe allows for multivalve assembly. Detailed information on these types of combustor is available in articles by Zinn (1985) and Kudra and Mujumdar (2009). Some combustors also exploit the resonance phenomenon these are referred to as frequency-tunable pulse combustors. 

To determine the origin of the instability frequency acoustic analysis was performed which revealed that both the quarter-wave mode of the inlet and the Helmholtz mode of the combustor-inlet system occurred at 35 Hz. The phase... 

Although this regime is considered as a stable case some acoustic activity exists in the burner two acoustic modes are found experimentally around 300 Hz and 570 Hz and the overall sound level inside the combustor reaches 500 Pa in the LES (more than 140 dB). To identify the nature of these modes, the Helmholtz solver was used with the average temperature field given by LES to identify acoustic eigenmodes with combustion. Table 9.1 confirms that the two frequencies observed in experiments are the two first acoustic modes of the combustor. In LES, a single frequency is observed at 520 Hz, which is close to the second acoustic mode of Table 9.1. To check that the 520 Hz mode found by LES is indeed acoustic, the field of unsteady pressure given by LES is compared to the modal structure... 

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... 

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. 

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... 

Liu, X. D Cao, C. W., Lang, Z. H 2001. Heat transfer between materials and unsteady airflow from a HelmholtZ-type combustor. Drying Technol, 19(8) 1939-1948. 

Wu, Z., Wu, L, Li, Z., Mujumdar, A. S., 2012. Atomization and drying characteristics of sewage sludge inside a Helmholtz pulse combustor. Drying Technol. 30(10) 1105-1112. 

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