Figure 3. Radial nitrogen oxides concentration and carbon dioxide mole
fraction profiles along the height of the gas fired burner.
The effect of wall thermal conductivity on the wall outer edge
temperature of the gas fired burner is illustrated in Figure 4 along the
length of the burner. The material thermal conductivity affects the
temperature profile within the wall and the possibility of hot spots.
For low-wall-thermal-conductivity materials, significant axial
temperature gradients are observed. Hotspot temperatures in excess of
2000 K can occur, an undesirable situation, as it exceeds the maximum
operating temperatures of most materials of construction [55, 56].
Exceedingly high wall temperatures are characteristic of thermally
stabilized burners [57, 58]. As the wall thermal conductivity
increases, the wall temperature profiles become more uniform and the
wall hot spot is eliminated. Despite the apparent advantages of a higher
wall thermal conductivity for material stability, most materials that
offer high conductance are metals, and therefore would not be inert to
radical quenching. A more reasonable solution would be thicker walls of
a more inert material that may have a lower thermal conductivity. Low
wall thermal conductivities result in large axial wall-temperature
gradients and high maximum temperatures. High wall thermal conductivity
leads to uniform temperature profiles without hotspots. Flame surface
area density is defined as the flame surface per unit volume and serves
as a method of modeling the flame propagation based on the fact that the
flame progression transports, generates, and diffuses the flame surface
area density. The flame propagation is estimated by modeling the flame
generation in such a manner that the flame generation resulting from
laminar flow is inversely proportional to the chemical reaction
characteristic time and proportional to the flame stretch rate. After
combustion begins, the combustion first starts as a laminar flame. The
growth of the flame is expressed by expressing the generation of the
flame surface area density in terms of the growth of the laminar flame.
The generation of the flame surface area density is expressed by
combining the growth resulting from the mode of combustion, namely the
laminar combustion flame growth. The laminar combustion flame growth is
inversely proportional to the chemical reaction characteristic time and
a function of the Reynolds number. The laminar combustion flame growth
is proportional to both the laminar flame speed and to the ratio of the
temperature of a burned portion to the temperature of an unburned
portion. The generation of laminar combustion and the resistance imposed
on the flame by air can be expressed and the flame propagation can be
predicted with good accuracy with respect to combustion modes that are
dominated by laminar combustion. It is possible to predict the flame
propagation at fields where at the beginning of combustion the
turbulence is very weak and laminar combustion dominates due to a very
small Reynolds number. Consequently, it is possible to reproduce a
situation in which the flame speed increases as the turbulence of the
field strengthens and the flame propagation in various fields of weak
turbulence can be predicted.