Technical background
[0001] The invention concerns power, transport and chemical industry and can also be used
in the gas turbine systems.
State of the art
[0002] From EP 0 747 635 a premix module for gas turbine is known which incorporates a fuel tube with fuel dispersing
holes which are located in various distances along the length of the tube. The dispensing
holes are aligned in two rows and are located 120° apart and orientated such that
they are positioned sixty degrees for and aft the radius which extends from the center
of the radial swirler to the fuel tube.
[0003] WO03/
056241 A 1 presents a burner which incorporates a fuel tube with fuel dispersing holes
and around the first part of the fuel tube a conical sheet with fuel distribution
holes is arranged.
[0004] US 5,408,830 presents a tube for air passage which ends with a tip with discharge orifices and
at the beginning just following the swirler a radially orientated body with holes
for fuel discharge is arranged.
[0005] Further there is a known method for fuel combustion used in burners and this consists
of dividing the fuel stream into more smaller streams or groups of streams, which
subsequently one after another fall into an air flow and are mixed in that flow during
the period T
mix, resulting in the so-called "poor" homogeneous mixture, and this mixture is conveyed
to a partly closed combustion tube (with only one opened end) where hot combustion
products release the heat. This known burner in which the method is realized consists
of a cylindrical casing with the diameter D, an air swirler arranged coaxially with
the casing and the vanes are located in the casing where a cylindrical pre-chamber
is created. In this place there is a beginning of the distributing tube for fuel,
which is installed inside and along the axis in the external casing. The fuel distributing
tube is provided with openings as a fuel distributing equipment to feed the fuel into
the pre-chamber. The existence of the time delay (retarder) between the moment of
fuel feeding into the air flow and the moment of releasing of the thermal energy within
the time range of T
mix and T
comb results in a certain phase-shift between the oscillation of the released pressure
and releasing of heat, at the end of which pressure pulsing may occur in the flow.
The disadvantage of this known method and system seems to be a relatively low stability
of the combustion process, reflected in the pressure pulsing which often reaches dangerous
levels, which results in a reduced service lifetime and mechanical damages of the
whole fuel combustion system.
[0006] Such type of the burner is known from GB 2348484 A. The burner has a
duct with the fuel tube inside and from this fuel tube radially protruding small tubes
with distribution orifices are arranged and this is in various places along the tube.
This is a close state of the art to the presented invention and an analogical solution
is on the Fig.3. So
each burner is provided with fuel injector(s) located in the duct at various distances
to balance the oscillation.
[0007] The known method of combustion is based on the fact that the fuel flow is divided
into three smaller streams (or groups of streams) conveyed in various sectors to the
air flow and on the route from the sector where the first fuel stream falls (or a
group of streams) to the section where the last stream falls (or a group of streams)
during the period T
feed a "poor" homogeneous mixture of air and fuel is created and this mixture is fed to
the half-limited combustion tube (with one opened end) filled with the hot combustion
gases where during the period T
comb combustion products are created and heat is released.
[0008] In the known combustion apparatus for the application of the above method, the fuel
combustion burner contains an external cylindrical casing with the diameter D, coaxial
to the casing there is a swirler and this swirler together with the external casing
creates a cylindrical pre-chamber at the inlet of which there is a fuel distributing
tube with three smaller openings (or three groups of openings) to feed the fuel into
the pre-chamber. The openings (groups of openings) are located in a relative distance
from each other in the burner axis and the axis distances from the closest to the
most distant opening (group of openings) to the outlet from the pre-chamber correspond
to the distances L1 and L2. When splitting the fuel stream to the individual streams
(groups of streams) and their subsequent feeding in various sections to the air flow,
each stream (group of streams) is provided with its own time delay between the fuel
feeding moment and the power releasing moment, at the end of which a number of pulsing
processes with various frequencies may be generated. Nevertheless, the pulsing pressure
amplitudes of the individual frequencies will be substantially lower in an embodiment
according to the invention than by fuel combustion taking place in the known system
and the heat energy released by pulsing limited by the temperature of fuel combustion
will be divided amongst all the pulsing processes in the series. The disadvantage
of known burners seems to be an insufficient stability of the combustion process.
This is shown by the fact that the likelihood of the occurrence of the pressure pulses
with individual frequencies showing dangerous amplitudes remains substantially high
due to the fact that in some relations (ratios) of T
feed, T
mix and T
comb in the aforementioned series of pulsing processes the frequencies of some pulsing
may coincide with harmonious components (multiple frequencies) of other pulsing, resulting
in a response.
[0009] Besides, given relatively low T
comb values, this fuel combustion method approaches the known methods and its combustion
stability is low, too. The known fuel combustion burner shows a certain disadvantage
- it does not ensure a high combustion process stability. The geometrical characteristics
of the burner D, L
1 and L
2 determine (given a constant flow) the characteristic time intervals T
feed, T
mix and T
comb of the fuel combustion method. And thus, by analogy, given some relations in the
presented geometrical characteristics, the combustion process system will not be sufficient.
The fuel is brought to the air stream along the cylindrical pre-chamber on points
in a distance from each other (Fig 3.) where the radially distribution elements are arranged.
[0010] The aim of a new fuel combustion method is to increase the stability of the combustion
process while excluding the possibility of pressure pulsing with high amplitudes.
The purpose of the presented system is to realize the proposed fuel combustion process,
to be specific, the design of such a burner, that would exclude pressure pulsing with
high magnitudes.
Summary of the invention
[0011] The above shortcomings are eliminated to a great extent by the method of fuel combustion
with the use of the combustion apparatus according to the invention, where the task
is handled in the following way:
Method for fuel combustion where the streams or groups of streams of the fuel is brought
to the air stream in such a manner that the following relation:

can be assured,
where
Tweed - is the time period of the distribution along the distributing tube from the
fall of the first stream or the first group of streams to the fall of the last stream
or the last group of streams
Tmix - is the time period from the fall of the last stream or the last group of streams
into the air flow when the fuel and air are mixed to the entrance to the half-limited
combustion tube (with one open end) filled up with hot combustion gases,
Tcomb - is the period of the combustion of the mixture,
and in that the diameter of the fuel feeding openings gets smaller as their location
is more situated to the outlet from the combusrtion tube, so that the fuel stream
is divided into streams which are conveyed into the air flow with decreasing flow
volumes.
[0012] The technical conclusion to the proposed method consists in increasing the combustion
process stability while excluding the possibility of pressure pulsing with high amplitudes.
[0013] This is achieved by the fuel stream (group of streams) to the air flow effected as
mentioned in the above relation. This is explained below. It is known that deceleration
of time T between the fuel feed moment to the air stream and the moment of its burning
with thermal energy releasing may result in an instability of the combustion processes
expressed by the pressure pulsing with a frequency determined by the following relation:

where
F - is frequency
T - time between the fuel feed moment to the air stream and the moment of its burning.
[0014] The physical mechanism of the aforementioned relation is based on the fact that when
defects occur in the fuel and air mixture stream with a frequency f, phase shift between
oscillation of the flow, pressure and heat releasing related to the decelerated time
T, it may happen that in the air and fuel mixture combustion zone in the oscillation
phase of heat releasing and mixture concentrations coincide. This results in a response
(resonance). In the context of the described mechanism of the occurrence and maintaining
of the pulsing process during time T
comb it is necessary to understand the time interval from the moment of fuel and air mixture
feeding to the half-limited space to the moment when the heat released during combustion
reaches its maximum. In practice, T
comb can be determined mathematically, using the known methods of mathematical modelling
of reacting flows or by experiments. When dividing the fuel stream into streams (groups
of streams) and subsequent feeding through various sections into the air stream, just
like in this invention, each of them may generate pressure pulsing of a defined (determined)
frequency. The amplitudes of the pulsing may be smaller than in the case of one-time
feeding of all fuel to the air stream as the power of the oscillating process which
seems to be a function of the performance of heat releasing during air and fuel mixture
combustion is divided into a number of these pulsing processes. The first stream (group
of streams) falling to the air flow generates pulsing with the lowest frequency:

where
f
min - lowest pulsing frequency
Last - with the highest frequency

where
f
max -highest pulsing frequency
[0015] In general in a number of frequencies generated by corresponding fuel flows in a
range from f
min to f
max there can be frequencies, harmonies (multiple frequencies), which coincide with other
frequencies of the series. Such coincidences may result in dangerous increases in
the amplitudes by pressure pulsing at the relevant frequency and therefore it must
be excluded as soon as possible.
[0016] In this case, this may be achieved by the aforementioned fuel flow feeding to the
air stream while keeping the following relation:

[0017] If the range of frequencies is from f
min to f
max, where the oscillation power is divided into a number of pulsing processes, the result
is quite narrow:

[0018] The analyzed mechanism of increasing the pressure pulsing is little efficient as
the described fuel combustion method approaches known method and takes over its disadvantages,
see above.
[0019] The presented system of the burner, ensures that technical result. The inlets in
the relation (2), the axis distances L1 and L2 from the closest to the most distant
opening (group of openings) to the outlets from the pre-chamber ensuring fuel combusting
can be determined, if we know the basic geometrical dimensions of the burner and the
air flow amount. If the data is used to calculate the medium, axial air speed in the
pre-chamber W
ax.speed, we can determine L1 and L2.

W
ax.speed - medium axial speed of air flow in the pre-chamber, m/s
[0020] The empiric coefficient K ensuring the fuel combustion, may be determined as follows:

[0021] This formula can be also formulated as:
where
Lcomb =
length of the fuel combustion
D =
outer diameter of the burner
Length of the fuel combustion Lcomb is set by use of the physical modelling of the combustion process on a
flame stand and it is sugested that it is equals to the lenght of the visible flame.
[0022] By using the results of L1, L2 and the relation (2) we can obtain relation (1) i.e.
the presented method of fuel combustion.
[0023] The technical solution to the proposed method consists in an increase in the degree
of homogeneity of the fuel and air mixture which is very important for the generation
of low toxic combustion equipment.
[0024] This can be achieved as follows. It is known that the more time is provided for the
fuel and air mixing, the higher the quality of the mixture, the more even and homogeneous
the mixture. If we consider the aforementioned, to achieve the degree of homogeneity
of the fuel and air mixture in this option, the fuel stream is divided into at least
three streams (groups of streams) with uneven flow amounts and the flows flowing later
to the air flow have a lower flow amount. To achieve this result in the presented
burner option, the fuel feeding openings in the pre-chamber located closer to the
outlet are manufactured with smaller diameters.
Brief description of the drawings
[0025] The invention will be presented by means of drawings where Fig.1 is a scheme of the
fuel feeding system according to the state of the art, Fig. 2 is a scheme of the fuel
feeding system
corresponding with the method according to the invention, Fig.3 is a burner and fuel distributing assembly according to the state
of the art with fuel distribution tube with radial pillars with openings, Fig.4 is
the burner and fuel distributing assembly
corresponding with the method according to the invention with a spiral fuel distribution tube.
[0026] The reference signs on the figures are:
Fuel main stream 1, fuel streams 2, air flow 3, combustion products (gases), half-limited
(with one opened end) combustion tube 4, external cylindrical casing 5, vanes 6, swirler
7, fuel inlet tube 8, cylindrical pre-chamber 9, fuel distributing spiral 10, 11 -
openings for fuel feeding to the pre-chamber.
Examples of the embodiments
[0027] The presented method and system according to the state of the art and according to
the invention is compared of Figs.1 and 2. The main fuel stream 1 is divided into
three smaller streams (or groups of streams) 2 and the streams are then conveyed in
various sections of the pre-chamber to the air flow 3 and the fuel is added on the
route from the sector where the first fuel stream falls (or a group of streams) to
the section where the last stream falls (or a group of streams) during the period
T
feed and the fuel and air are mixed in the stream during the period T
mix. The so-called "poor" homogeneous fuel and air mixture is created and this mixture
is conveyed in the hot combustion product streams in the half-limited (with one open
air) combustion tube, where it burns during the period T
comb and combustion products are generated and heat is released. In the embodiment
corresponding with the method according to the invention (Fig.2) the fuel flow is divided into streams (groups of streams) with various flow
volumes wherein these streams (groups of streams) are conveyed into the air stream
with decreasing flow volumes.
[0028] The burner projected for the use of such a method for fuel combustion is presented
also according to the state of the art and
corresponding with the method according to the invention is Figs.3 and 4. The burner contains an external cylindrical casing 5 with the diameter
D, coaxial to the casing 5 there is a swirler 6 with vanes 7 and this swirler 6 together
with the external casing 5 creates a cylindrical pre-chamber 9 at the inlet of which
there is a fuel distributing spiral 10 with three small openings 11 (or three groups
of openings) to feed the fuel to the pre-chamber 9,
[0029] The fuel distributing equipment may have various shapes. As regards the burner according
state of the art shown in Fig. 3, on the inlet of the fuel inlet tube a swirler is
provided and the distributing equipment of the inlet tube is in the form of radial
pillars located in various distances from the outlet of the pre-chamber and is provided
with three fuel distributing openings each. As regards the burner shown in Fig.4,
the fuel distributing equipment is shaped as a spatial spiral distribution tube installed
in front of the swirler and is provided with openings distributing the fuel, each
of which is located in its place from the outlet from the pre-chamber and the diameter
of the fuel feeding openings gets smaller as their location is more situated to the
outlet from the pre-chamber. The openings are located in a relative distance from
each other in the burner axis and the distance from the outlet of the pre-chamber
to the closest stream (or groups of streams is L1 and the distance from the outlet
of the pre-chamber to the most distant stream (or groups of streams) is L2
[0030] The openings (or groups of openings) for fuel feeding into the pre-chamber are located
according to the following relation:
where
L2 - axis distance from the most distant opening (or most distant group of openings)
from the outlet from the pre-chamber,
K - empiric coefficient
D - diameter of the external cylindrical casing,
L 1 - axis distance from the closest opening (or closest group of openings) to the
outlet from the pre-chamber.
[0031] The burner works as follows:
The fuel stream in the main inlet tube 8 is conveyed to the fuel distributing equipment
in the form of spiral distributing tube 10 and using openings 11 the main stream is
divided into streams (or groups of streams) and then falls in various sections of
the pre-chamber 9 into the air flow. The fuel and air are then mixed by means of the
swirler 6 and a "poor" homogenous mixture of fuel and air is created and the mixture
is conveyed from the pre-chamber 9 to a zone filled up with hot combustion products
which is a half-limited combustion tube (with one open end) 4, where it burns and
combustion products are generated and heat is released. The walls defining this filled
up space (not shown in the pictures only schematically on the Fig.1 and 2) are usually
made of a steel tube.
[0032] The
burner is not limited only on the presented embodiment and that widely available elements
and pieces of equipment, such as pipes, cylindrical and conical casings at the air
inlet, swirler, fuel distribution tubes, fuel pillars or steel tubes.
1. Verfahren zur Kraftstoffvebrennung bei welchem der Kraftstoffstrom in drei kleinere
Ströme, oder die Gruppen von Strömen, geteilt wird, die nachstehend durch die verschiedene
Sektionen in den Luftstrom in der Brennerhaube fallen, wobei der Zeit der Kraftstoffdistribution
längst der Distributionsrohr ab dem Fall von dem ersten Strom, oder Gruppe von Strömen,
zu dem Fall des letzten Stromes, oder Gruppe von Strömen, die Zeitperiode T
feed darstellt, dann sind der Kraftstoff und die Luft in einem Strom während der Zeitperiode
T
mix gemischt, wobei eine "arme" homogene Luft/Kraftstoff-mischung entsteht und diese
Mischung fällt dann in die heisse Verbrennugsprodukte in einer Verbrennugsrohr mit
einem geöffneten Ende, wo die Mischung während die Zeitperiode T
comb verbrennt und
dadurch die Verbrennugsprodukte entstehen und die Hitze losgelassen wird,
daduch gekennzeichnet,
dass die Ströme, oder die Gruppen von Strömen, von dem Kraftstoff in den Luftstrom in
so einer Weise zugeleitet sind, dass die folgende Relation

beibehalten werden kann
wo
T
feed - die Zeitperiode der Kraftstoffdistribution längst der Distributionsrohr ab dem
Fall von dem ersten Strom, oder Gruppe von Strömen, zu dem Fall des letzten Stromes
(oder Gruppe von Strömen, ist
T
mix - die Zeitperiode ab dem Fall von dem letzten Strom, oder Gruppe von Strömen, in
den Luftstrom, wenn der Kraftstoff und die Luft gemischt sind, zu dem Eintritt in
die Verbrennugsrohr mit einem geöffneten Ende (mit einem geoffneten Ende, die mit
heissen Verbrennugsgasen gefüllt wird, ist
T
comb- die Zeitperiode der Verbrennung von der Mischung ist
und dass der Durchmesser der Öffnungen in der Kraftstoffdistributionsrohr desto kleiner
sind, wie ihre Lage näher zu dem Austritt aus der Vebrennungsrohr ist, und
dadurch ist der Kraftstoffstrom in Ströme geteilt, welche in den Luftstrom mit verkleinerten
Strommengen zugeleitet sind.