Partially premixed combustion
Part of the air is pre-mixed before combustion. A part of the pre-mixed gas-air mixture can be burned as soon as it
exits, while the remaining gas needs to be burned by diffusion and mixing with the surrounding air. This combustion
method is called partially premixed combustion.
Diffusion combustion easily produces soot and the combustion temperature is also quite low. But when a portion of
the air required for combustion is premixed, the flame becomes cleaner, the combustion is strengthened, and the
flame temperature is increased. Therefore, partially premixed combustion has been widely used
>Partially premixed laminar flame
The partially premixed laminar flame structure consists of an inner flame and an outer flame. First, the oxygen in the
primary air reacts with the gas in the inner flame to form a light blue cone-shaped flame that does not emit light.
Since the primary air volume is less than the total air volume required for combustion, only a portion of the
combustion process takes place on the blue cone. The remaining gas is mixed with air outside the inner flame
surface and burned by diffusion. The less air there is at one time, the larger the outer cone will be. If the secondary
air and other conditions meet the requirements, combustion is completed in this zone and carbon dioxide and water
vapor are generated.
>Stabilization of partially premixed laminar flame
In laminar flow, the velocity of the gas along the cross section of the pipe is distributed parabolically. The airflow
velocity is maximum at the center of the nozzle and drops to zero at the tube wall. At the root of the flame, the
airflow velocity near the wall gradually decreases to zero, but the flame will not spread to the burner because the
flame propagation speed there is also reduced due to heat dissipation from the tube wall.
The normal velocity of the air flow at any point on the flame surface is equal to the normal flame propagation
velocity. On the other hand, there is also a tangent velocity at this point, which causes the particle at that point to
move upward. Therefore, the ignition of the lower particles to the upper particles continues on the flame surface.
The existence of the ignition ring is conditional. If the combustion intensity continues to increase, the equilibrium
point where the airflow velocity is equal to the normal flame propagation velocity will gradually approach the fire
hole exit, and the ignition ring will gradually narrow and finally disappear. The flame leaves the burner outlet and
burns at a certain distance. This phenomenon is called flame departure. If the air flow velocity increases again,
the flame will be blown out, which is called defire. If the mixed airflow speed continues to decrease, the blue cone
gets lower and lower, and eventually because the airflow speed is smaller than the flame propagation speed,
the flame will indent the burner and propagate inward, which is called backfire.
As mentioned before, for a gas-air mixture of a certain composition, there must be an upper limit for flame stability
during combustion. If the air flow velocity exceeds this upper limit, defire will occur. This upper limit is called the
defire (speed) limit: On the other hand, when the flow rate of the gas-air mixture is reduced to a certain limit value,
the backfire phenomenon will occur. This limit value is called the backfire (speed) limit. The flame can be stable only
when the velocity of the mixture is between the de-ignition limit and the tempering limit.
When the primary air coefficient is small, carbon particles and soot are formed due to thermal decomposition of
hydrocarbons, which will cause incomplete combustion and pollution. Therefore, the primary air coefficient of
partially premixed combustion should not be too small.
The flame propagation speed and the airflow outlet speed determine whether the flame is stable. The greater
the flame propagation speed of the gas, the higher the position of the defire and backfire curve. Therefore,
artificial gas with a larger flame propagation speed is prone to backfire, while natural gas with a smaller flame
propagation speed is prone to defire. For the same fuel, the primary air coefficient and the flame hole thermal
intensity reflect the changes of the two, and are the main factors affecting flame stability. Under the same fire hole
thermal intensity, when the primary air coefficient = 1, the flame propagation speed reaches the maximum value,
and the tempering limit speed also reaches the maximum value; no matter the primary air coefficient increases
or decreases, the flame propagation speed will decrease, This results in a reduction in the tempering limit speed
As the primary air coefficient increases, the ignition ability of the ignition ring will be weakened, resulting in
a decrease.Under the same primary air coefficient, an increase in the heat intensity of the flame hole will lead to
an increase in airflow velocity and enhanced flameout performance; at the same time, it will cause an increase
in combustion temperature and an increase in flame propagation speed, thereby moving the position of
the backfire and flameout curve upwards.
Flame stability is also affected by the composition of the surrounding air. If the surrounding atmosphere is
polluted by inert gas, since the oxygen content in the air is less than normal, the burning speed of the mixed gas
will be reduced, thereby increasing the possibility of defire.
The flow of air around the flame has an adverse effect on flame stability. The strength of this effect depends on
the speed of the surrounding airflow and the angle between the airflow and the flame.
>Partially premixed turbulent flame
When the combustible mixed gas flow changes from laminar flow to turbulent flow, the flame changes significantly.
Compared with the structure of the laminar flame, the structure of the partially premixed turbulent flame is
significantly reduced in length and the top is rounder. The flame surface changes from smooth to wrinkled.
It can be seen that the flame thickness increases, the total surface area of the flame also increases accordingly,
and the combustion is strengthened. When the turbulence scale is large, the flame surface will be strongly
disturbed, and each gas particle will leave the flame surface and disperse into many burning gas particles.
They will continue to scatter with the flow of the combustible mixture and combustion products, and finally
burn out completely. At this time, the flame surface becomes a combustion layer composed of many combustion
centers, the thickness of which depends on the time required for the mass ignition to burn out at the air flow velocity.
>Stability of partially premixed turbulent flame
Under turbulent flow conditions, the flow rate of combustible premixed gas is greatly improved compared with
laminar flow, and its flow rate is often close to or exceeds the deignition limit of stable combustion. For the same
type of gas, although the flame propagation speed increases as the mixing intensity of the air flow increases,
it is limited compared with the increase in flow velocity. Therefore, for the stability of partially premixed turbulent
flames, the main consideration is how to prevent the occurrence of de-ignition.
In order to stabilize the flame, a balance between air flow velocity and flame propagation speed should be
maintained in the local area. You can start by changing the speed of the airflow and use hydrodynamic methods to
stabilize the flame; you can also start by changing the flame propagation speed and use thermodynamic or chemical
methods to stabilize the flame.
In order to prevent de-ignition, the most common method is to set an ignition source at the burner outlet, which
can continuously ignite the combustible mixed gas. The ignition source may be a continuously acting artificial ignition
device, such as a hot object or a stable auxiliary ignition flame. In addition, a blunt body flame stabilizer can also
be installed in the combustible premixed air flow, causing the hot combustion products to flow back to the root of
the flame to form an ignition source.