THERMO-ACOUSTICAL PHENOMENA IN WOOD POWDER
BURNERS
BURAK GOKTEPE*, RIKARD GEBART
Luleå University of Technology
Corresponding author. Tel.: +46-920-49-1046
E-mail address: burak.goktepe@ltu.se
*
EXTENDED ABSTRACT
In industrial and domestic boilers, unacceptable noise may be emitted during
combustion. The strong noise emissions arise mostly from combustion instabilities
leading to self sustained, high amplitude flow oscillations, known as thermo-acoustic
instabilities. The thermo-acoustic instabilities occur as a consequence of a coupling
between the unsteady heat release and acoustic waves (Rayleigh, 1945).The high
amplitude pressure oscillations associated with enhanced heat transfer may lead to
flame blow-off, increased NOx emissions and serious fatigue problems which lower the
reliability and efficiency of the boiler. Therefore, academic and industrial efforts have
been focussed on predicting the onset and feedback mechanisms of the combustion
oscillations.
Unsteady heat release is the main source of the noise during the combustion. The
unacceptable noise associated with high amplitude flow oscillations is generated by
feedback loop among the flow/mixture perturbations, unsteady heat release and acoustic
waves. According to the feedback loop, the unsteady heat release rate generates acoustic
waves inside the boiler. The acoustic waves propagating inside the boiler are then
reflected from the walls and boundaries of the boiler. Finally, these reflected acoustic
waves cause flow perturbations at the upstream of the flame. There are a large variety of
mechanisms that lead to unsteady heat release rate fluctuations: air-fuel ratio
fluctuations (Lieuwen, Zinn, 1998), variations in flame front area (Schuller, Durox,
Candel, 2003), turbulent intensity (Schuermans et al. 2004) and the interactions between
vortex shedding and flame (Poinsot, Trouve, Veynante, Candel, Esposito, 1987). A key
tool used for the prediction of thermo-acoustic instabilities is the flame transfer
function. The flame transfer function relates the oscillations in relative heat release rate
to the oscillations in relative flow velocity at the burner exit (Kornilov, Schreel, Goey,
2005). Determining flame transfer function, it is possible to predict the acoustic (in)stability of the boiler/burner system.
In this paper, transfer function of a 150 kW swirl stabilized powder burner and a diesel
oil burner mounted in a combustion test furnace are determined experimentally. Due to
the complexity of experimental set-up, the direct measurement of flame transfer
function is very difficult at that moment. Instead, the transfer function of the burner,
which is exposed to pressure fluctuations, is measured. The air flow, which is injected at
the exhaust pipe as a counter flow to exhaust gas, is perturbed by a direct-drive-valve.
The sinusoidal signal driving the valve is generated by a function generator. The water-
cooled pressure transducers are mounted onto the furnace to obtain the pressure
fluctuation intensity inside the combustion chamber. They are connected to a 3 channel
signal conditioner prior to data acquisition board. Simultaneously, unsteady heat release
rate is measured by tracking the OH chemo-luminescence emission along the flame as
OH radicals are good indicators of heat release rate over a range of lean equivalence
ratios (Morrel et al. 2001). A photomultiplier with an appropriate UV light filter is used
to detect OH chemo-luminescence from the flame. The signal from the pressure
transducers and photomultiplier are digitised and recorded by a digital data acquisition
system. The acquired signals are then analyzed spectrally over a range of forcing
frequencies (frequencies that are used for the excitation of acoustic resonances in the
test furnace) and amplitude using Fast Fourier transform technique. The transfer
function of the burner is determined by obtaining power spectral densities and cross
spectral densities during the post-processing of the OH chemo-luminescence and
pressure fluctuation signals.
The focus of this paper is if the theoretical concept of a flame transfer function can be
used to predict and control thermo-acoustic phenomena in experimental systems and
large scale power boilers.
Keywords: Thermo-acoustic, Combustion, Flame Transfer Function, Diesel Burner
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