High temperature measurements: Porous burner
Combustion in solid sponges
Flame Stability
Stationary flames can be categorized according to the fuel/oxidizer mixing condition,
as being either premixed flames or diffusion flames. In premixed flame, the fuel
and the oxidizer are mixed at molecular level prior to the occurrence of any significant
chemical reaction. In diffusion flame, the reactants are initially separated, and the
reaction takes place only at the interface between the fuel and the oxidizer, where
mixing and reaction both take place.
The laminar burning velocity is a very important characteristic of premixed flames. It
is defined in the case of a freely propagating flame with the coordinate system fixed
to the propagating wave as the velocity of the approaching unburned mixture. The
laminar burning velocity is a characteristic of the fuel/oxidizer mixture for the given
process parameter (T, p). In order to discuss the stability of the premixed flames, a
flow of the flammable mixture through a pipe is assumed. After the ignition of this
mixture the flame will propagate against the flow direction with the certain velocity.
Assuming in this hypothetical experiment that the velocity profile in the pipe flow is
planar a planar flame will be built as shown in the Fig. 1. For the stationary conditions
the burning velocity must be equal to the velocity of the incoming gas mixture Sl=vmix.
In this case the flame is considered to be stable. If the burning velocity is higher than
the velocity of the gas flow the flame will propagate against the flow direction Sl>vmix.
This phenomenon is called flash back and for the safeguard concerns must be
prohibited since it could cause an explosion. If the velocity of the gas flow is higher
than the burning velocity the flame will migrate to the burner exit Sl<vmix. This
instable state of the flame is difficult to be controlled and for that reason also
prohibited in combustion systems.
Figure 1: Sketch of planar flame front
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High temperature measurements: Porous burner
Porous Burner technology
Figure 2: Ceramic sponge
Solid sponges made of ceramic, metal or
polymers are highly porous, monolithic
materials
with
fascinating
properties
resulting from their overall fluid permeable
structure. Their open cell structure consists
of stiff, interconnected struts building a
continuous network (Fig. 2). Although
sponge–like materials are overall permeable
for fluid flow, the resulting flow field in such
structures is very complex influencing all the
heat and mass transport processes,
accordingly.
Premixed flames exhibit the potential of
significantly lower NOx emissions in
comparison to diffusive flames offering a possibility to meet the challenge of
increasingly stringent regulations for limiting pollutant emissions. However, these
flames are strongly connected with the instability risks such as flashback or blow off.
A possible solution to ensure stable burning of the premixed flame is to provide flame
stabilisation within a porous inert material. Employing the porous burner concept,
combustion takes place within an inert solid matrix exhibiting significantly higher heat
transport properties in comparison to the combustible gas mixture alone. The ceramic
porous body in the reaction zone provides highly efficient heat transport by means of
solid radiation and conduction resulting in a stronger preheating of the incoming gas
mixture and, thus, increasing the burning velocity.
Beside the excellent emission and flame stability characteristics, the porous burner
concept offers further advantages resulting from the three dimensional solid matrix
located in the combustion zone. In this manner, the solid body exhibiting high
temperatures permits a very effective performance as an infra red radiator.
Therefore, the porous burner technology opens new fields of application in industrial
drying and heating. A benefit of infra red heating is that it can provide much faster
heating times than with convective heat transfer alone. These advantages applied to
industrial applications can allow reduced oven lengths, increased conveyor speeds,
and improved surface finish of products.
In order to demonstrate the basic principle of the flame stabilization in a porous
burner, the temperature profiles of the gas and the sponge as well as concentration
profiles of some important species calculated using a 1-dimensional model of the
combustion process in porous inert media (PIM) are shown at Fig. 3. In the reaction
zone the gas temperature is higher than that of the sponge and the existing
temperature difference causes heat transfer from the gas to the solid phase. The
heat is transported effectively from the gas to the solid due to the high specific
surface of PIM. This heat transfer results in a temperature gradient in the sponge.
The superior heat transport properties of the solid phase lead to an intense upstream
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High temperature measurements: Porous burner
heat flow due to the present temperature gradient by means of solid radiation and
conduction. As a result of the upstream heat flow, in the preheating zone the
temperature of the sponge is higher than the gas temperature. Thus, the heat is
transferred from the sponge to the gas resulting in preheating of the combustible
mixture. This internal heat recirculation results in an increase of the local
combustion temperature above the equilibrium value which in turn, leads to higher
burning velocities and an extension of the stable operating range compared with that
of a laminar flame.
Figure 3: Temperature and concentration profiles calculated
using a 1D model of combustion in PIM (air excess ratio 1.3)
Experimental Setup
Figure 4 depicts a schematic diagram of the experimental set up used in this work.
Natural gas flowing through the fuel injection system enters the static mixer. After
mixing with the combustion air the combustible gas mixture passes through the flow
homogenizer (steel wool) and enters the small pore sponge in the
premixing/preheating zone. The small pore sponge (zirconia partially stabilized with
magnesia; 45 PPI and D=160 mm) has a function of the flame arrestor. The small
pore PIM samples have a height of 25 mm requiring two samples to be piled up in
premix/preheating zone. The large pore sponge (D=160 mm) is placed within the
combustion zone. This zone is insulated using firebrick and fibre mat and is water
cooled to prevent damaging of the steel containment through the high combustion
temperatures.
In order to record the axial temperature profile 16 thermocouples have been inserted,
each 40 mm deep in both, small and large pore sponge. K-type thermocouples are
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High temperature measurements: Porous burner
applied for the measurement within the premixing / preheating zone. High temperatures in the combustion zone are recorded using 12 S-type thermocouples. Due to
the mechanical limitations, the S-type thermocouples are installed in 3 columns
displaced by angle of 15°. The output signals of all measuring devices (flow,
pressure, temperature measurement) have been recorded and evaluated
continuously with the time using a LabView based program.
Figure 4: Experimental setup
Experimental procedure
In respect to experimental determination of the flame stability limits there are two
possible approaches as depicted in Figure 5. Here, the experimentally determined
flame stability limit is shown, indicating that for the conditions above the line the flame
is instable. The operational points at stability limit can be determined either by
keeping the air excess ratio constant at certain value and successively increasing the
volumetric flow rate (thermal load) or by stepwise increase of the air excess ratio at
constant thermal load.
Stability investigations including the determination of the burning velocity of
natural gas/air flames in the PIM (porous inert media) for particular conditions are
conducted using the letter procedure mentioned above. After ignition at predefined
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High temperature measurements: Porous burner
thermal load and certain air excess ratio the flame was allowed to stabilize for the
given conditions. Keeping the thermal load constant, the air excess ratio is increased
in steps of 0.05 allowing between two steps enough time for the flame to stabilize. An
increase of air excess ratio (λ) results in a decrease of both, laminar burning velocity
and combustion temperature. At a certain λ, the velocity of unburned mixture became
higher than the burning velocity in the sponge for the given conditions and the flame
front migrated slowly to the burner exit indicating flame blow off. Fig. 6 shows
measured temperature profiles at different air excess ratios at constant thermal load
of 20 kW. The dashed line marks an instantaneous position of the instable flame with
the slowly moving flame front.
Figure 6: Measured temperature
Figure 5: Schematic representation of
profiles in
possible approaches for determination of
K04 at 20 kW for the variation of air
flame stability limits
excess ratio
Measurement Technique
As described above the axial temperature profile in the porous burner is recorded
continuously with the time in order to detect the position of the flame and to
determine whether the flame is stable or not. For this purpose several thermocouples
are inserted in the porous media.
In the present experiment the high temperatures in the combustion zone are
recorded with the thermocouples Type S and the lower temperatures in the
preheating zone are recorded with thermocouples type K.
In order to measure the temperature of the hot porous solid in the combustion zone
the quotient pyrometer is applied. The basic principle of the pyrometry is introduced
in following paragraph.
Each material in its aggregate state and at the temperature above the absolute zero
emits the thermal radiation that arises from the vibrations of the atoms or molecules.
This thermal radiation takes place only in certain spectral range of the total
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High temperature measurements: Porous burner
electromagnetic radiation spectrum. The radiation of the material ranges from the
visible spectral range starting from app. 0,5 μm up to infra red range with the
wavelengths higher than 40 μm. This radiation can be used for the experimental
determination of the object temperature. A pyrometer would be any non-contacting
device intercepting and measuring thermal radiation emitted from an object to
determine surface temperature. A radiation thermometer (pyrometer), in very simple
terms, consists of an optical system and detector. The optical system focuses the
energy emitted by the object onto the detector, which is sensitive to the radiation. The
output of the detector is proportional to the amount of energy radiated by the target
object (less the amount absorbed by the optical system), and the response of the
detector to the specific radiation wavelengths. This output can be used to infer the
objects temperature. The output signal of the detector (Temperature T) is related to
the thermal radiation or irradiance j* of the target object through the Stefan–
Boltzmann law:
The constant of proportionality σ is called the Stefan-Boltzmann constant and ε is the
emissivity of the object. The emissivity of the object is an important variable in
converting the detector output into an accurate temperature signal.
1) measurement object
2) Lense
3) Aperture diaphragm
4) Selective beam splitter
5) Detector 1
6) Amplifier
7) Spectral filter
8) Measurement field aperture
9) Detector 2
10) Amplifier 2
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High temperature measurements: Porous burner
The quotient pyrometer measures thermal radiation of two wavelengths at the same
time and gives the ratio of the two signals. In this way, assuming that the emissivity of
the measurement objects ε is the same for both measured wavelengths it is possible
to determine the object temperature also when the object’s emissivity is unknown.
Furthermore, if the emitted rays pass through the glass window and/or another
medium and similar their intensity will be weakened. Using the quotient pyrometer,
this effect of the surrounding will be the same for both wavelengths an can be
suppressed in the built ratio.
Tasks and execution of the experiment
After the ignition of the porous burner the flame shall be allowed to stabilize for the
given air excess ratio. The axial temperature profile at this condition will be recorded
as well as the temperature of the solid sponge. The air excess ratio shall be
increased in the steps of 0.5 and after each changing of the conditions the flame will
be allowed to stabilize and again the axial temperature profile and the solid body
temperature should be determined. This procedure shall be repeated until the flame
becomes instable.
Draw axial temperature profiles for all investigated air excess ratio. From the diagram
determine at which air excess ratio is the flame instable. Comment it. Draw the
dependency of the solid body temperature over the air excess ratio and comment it.
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