ACOUSTICALLY FORCED EXTERNAL PULSED COMBUSTION
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Pulsed combustion is one of the most energy-efficient and less polluting ways to produce a hot gas stream for heating purposes. It is usually limited, however, by the need to confine the combustion within a resonant tube. In the experiments presented here, pulsed combustion was achieved in the open air by means of upstream acoustic forcing of a porous matrix burner. Using metal matrices at least 20 mm thick, with pore densities of 60 pores per inch, a stoichiometric mixture of air and natural gas at a constant rate of 217.5 standard cubic feet per hour was forced by a speaker mounted on the plenum upstream of the matrix by means of sinusoidal waves of variable amplitude, at 10 to 150 Hz. Instantaneous surface temperature, as well as pressures upstream and downstream of the matrix were acquired. Phase-locked photographs of the combustion zone were acquired by means of a mechanical stroboscope synchronized to the forcing signal. It was found that, as the forcing amplitude increases, the mode of combustion switches from continuous to oscillating flame, to pulsed combustion consisting of a series of ignitions and extinctions. The change of combustion mode was accompanied by substantial changes in matrix surface temperature, with the pulsed mode reaching temperatures 170°C higher than steady-state combustion and 70% increase in radiant heat flux emitted. This was usually accompanied by a contraction in the size of the hot part of the matrix, resulting in an overall increase in radiant heat output of 5%, for our experiments. Maximum surface temperature was reached for a 35 mm thick matrix, which is hereby recommended for practical radiant heaters using this process. The mechanism hypothesized earlier was nearly proved with pending confirmation from reverse velocity measurements.