Interaction of sound waves with a swirl stabilized wood powder flame and their effects on flame characteristics

Sammanfattning: Swirling flows have been widely used for many years in engineering applications such as; chemical and mechanical mixing devices, separation units, spray drying technologies, turbo machinery and combustion systems. In practical combustion applications, swirl motion has been adopted to the incoming reactant flows in order to enhance the mixing of fuel and oxidizer and to improve flame stabilization and establishment, especially in regions of relatively low velocities, by recirculating hot product gas to the incoming reactants. At critical operating conditions the recirculation zone exhibit high sensitivity to flow disturbances leading to hydrodynamic instabilities. During combustion, these instabilities can interact with flame structures by modulating the rate of heat release, equivalence ratio, flame surface etc. As a result of these interactions, combustion instabilities can form in the systems. At initial state, combustion instabilities can stay unnoticed due to their relatively small amplitudes, but the amplitudes of the instabilities can increase when they couple with acoustical characteristics of any particular system elements. As a result, combustion systems can suffer from high amplitude noise, vibrations, flame flashback, local flame quenching, and even severe damages in system structures. This thesis provides insights into the interaction of acoustic waves with a swirl stabilized wood powder flame and its effects on flame structures. A high speed photography technique has been applied to wood powder flame under external forcing of the secondary air flow pattern to record spontaneous emission of radiant energy from the flame. Simultaneously, dynamic pressure signals were acquired with a data acquisition board in order to relate pressure data with radiant energy which has been assumed to be representative of heat release. In order to investigate the influence of the interactions on combustion, the resulting data were complemented with gas sampling measurements. From digital still images taken without external forcing, the wood powder flame was observed to expand to occupy the entire combustion chamber. In addition, the flame shape and size appears to be unchanged under a wide range of forcing frequencies, with one exception at a particular low frequency for which a resonant behaviour was observed. The critical frequency was 17 Hz independent of amplitude of the forcing frequency and at this forcing frequency dramatic changes in flame size and shape was observed. Instantaneous and phase averaged images have revealed the presence of large scale vortical structures that closely interacted with the flame surface. A fast Fourier transform of the point wise optical signal also shows that the flame is susceptible to instabilities at the acoustical forcing of 17 Hz. The existence of thermo-acoustically induced combustion instability has been investigated by a Rayleigh criterion which states that the amplitude of a sound wave will be amplified when heat is added less than 90 degrees out of phase with its pressure. In this study, the heat release extracted from high speed images recorded at 17 Hz is approximately 40 degrees out of phase with the pressure data which confirms the thermo-acoustic nature of the instability. Finally, from gas sampling measurements it was concluded that the acoustic oscillations at 17 Hz have increased the NOx emission level to around twice the level without forcing.