燃烧H2 / CH4燃料的Burke-Schumann火焰对声激励的响应

     The exact mechanism of combustion instability has not yet been elucidated, but it is
known that the occurrence of the combustion instability depends on the interaction of the
flow perturbation of the reactants, the perturbation of heat release, and the perturbation
due to the acoustic boundary of the combustion chamber. If the above three perturbations
form a positive feedback loop, the probability of combustion instability increases. Conversely, if a negative feedback loop is formed, the probability of combustion instability is lowered. In order to lower the risk of combustion instability, it is necessary to understand the combustion instability conditions as mentioned above. The basis for understanding the conditions which combustion instability occurs is to study the dynamic characteristics of the flame.

       In this study, the dynamic characteristics of Burke-Schumann flame, which is a special
case of non-premixed jet flame, composed of a mixture of methane and hydrogen, was
studied. The dynamic characteristics of the flame were represented by the perturbation of
heat release and flame shape when disturbance was generated from outside. When the flame
was at the open boundary, the heat release and the shape of the flame were measured to
show the response characteristics of the flame to the external disturbance. When the flame
was at the closed boundary, the resonance frequency of the experimental apparatus was
obtained and the effect on the resonance frequency was investigated and the cause of the
self-excited pressure oscillations was analyzed.

       The shape of the flame of the open boundary was three kinds. In the low frequency
region, the flame shape was attached to the nozzle in one periodic wave form, and the flame
was broken in the above region. In the region near 250 Hz, the flame was hardly shaken.
The flame composed of several waves appeared in the remaining region. The flame transfer
function also showed a very low value at the point where the flame did not shake and was
maintained at a constant value above a certain frequency. The gain of the flame transfer
function and the fluctuation of the flame area showed a similar change tendency except for
the region where the flame was broken. The flame of the open boundary showed a linear
relationship in which the heat release perturbation was approximately proportional to the velocity perturbation magnitude, but the influence on the 2nd harmonic frequency of the
excitation frequency occurred to a large extent. Especially, the ratio of the 2nd harmonic
frequency of heat release increased sharply at the excitation frequency or fuel composition,
which shows the low gain of the flame transfer function.
At the closed boundary, the influence of the resonance frequency was analyzed.
Although the closed boundary was formed by the movable nozzle, since the exhaust duct
continued to exist in the longitudinal direction of the combustor, it affected the resonance
phenomenon. Therefore, the orifice was installed to minimize the influence of the exhaust
duct in the longitudinal direction of the combustor. Two resonant frequencies that can affect
the combustion chamber were obtained. The points where these frequencies coincided were
identified and the conditions under which the effect of the resonant frequency was likely to
be strong were identified. In this case, the point where the effect of the resonance frequency
was largest was analyzed by measuring the magnitude of the velocity perturbation. The
conditions which the pressure fluctuation occurred in the combustion chamber when there
was no external disturbance were identified. The cause of this pressure fluctuation under
this condition was analyzed. The conditions of the pressure perturbation coincided with the
overlapping conditions between the resonance frequency of the air supply part, the
intersection of the resonant frequencies affecting the combustor chamber and the flame
transfer function at the open boundary.
It is expected that this study can be used as a preliminary research data for studying
the dynamic characteristics of Burke-Schumann flame as well as the mechanism of
combustion instability of diffusion flame.