Patent Application
20130323660
The present invention relates to a combustion head for a liquid fuel burner, and particularly suited for low NOx emission.
In liquid fuel burners, the combustion reaction between the liquid fuel and the comburent is known to occur by means of a combustion head. The comburent is conveyed through the combustion head into a combustion chamber, where it is mixed with the liquid fuel which is atomized by means of a nozzle. Within the combustion chamber, close to and downstream of the combustion head, an ignition device is arranged, adapted to trigger the mixture of liquid fuel and comburent so as to start the combustion process.
There is an increasing need to reduce the nitrogen oxides NOx which are generated during the combustion process and which cause pollution.
When designing combustion heads, a first solution considers that the related studies have shown that the nitrogen oxides NOx are especially generated when the flame temperature is high.
For this reason, burners have been fine-tuned, which are equipped with combustion heads in which the abatement of the flame temperature occurs by recirculating a part of the fumes generated by the combustion into the combustion head and at the flame itself. In order to recirculate the fumes inside the flame, the high outlet speed of the air from the burner head is exploited, which causes a phenomenon notoriously known in technical language as “recirculation”. Due to this phenomenon, the fumes in the combustion chamber are recalled into the flame and as they do not participate in the combustion reaction, they absorb heat by cooling the flame itself, thus decreasing the nitrogen oxide NOx emissions.
Patent application EP-A1-1705424 describes a combustion head for liquid fuel burners, which comprises a central duct fed with a liquid fuel, has a longitudinal symmetry axis and is provided, at one end, with a nozzle for atomizing said liquid fuel into a combustion chamber. The combustion head comprises a first body with cylindrical extension and coaxial to the longitudinal symmetry axis, which is arranged to receive, at the inlet, a primary flow of comburent and has a frontal wall provided with a plurality of peripheral indentations for producing a swirl in said primary flow of comburent. The combustion head comprises a second body with cylindrical extension and coaxial to the longitudinal symmetry axis, which is fitted on the central duct and on which the first body with cylindrical extension is coaxially arranged, is adapted to receive, at the inlet, a secondary flow of comburent and has a plurality of openings which are adapted to produce a swirl in the secondary flow of comburent. The combustion head then comprises means for regulating the swirl produced both in the primary flow and in the secondary flow of comburent, which comprise a duct coaxial to the longitudinal symmetry axis and external to the second body with the interposition of an intermediate body, the latter being also coaxial to the longitudinal symmetry axis. Between the duct and the intermediate body a channel is defined, which is adapted to feed a tertiary flow of comburent into the combustion chamber.
The combustion process carried out through the combustion head described in EP-A1-1705424 generates the overall effect of curbing the formation of thermal NOx due to reduced flame temperatures. The reduction flame temperatures is obtained by means of a side leak of a portion of the comburent flow. In particular, such a portion of comburent leaks through a plurality of radial holes which are obtained in the first body with cylindrical extension. Thereby, the flame does not exclusively develop from the frontal surface of the nozzle, but it evenly spreads inside the combustion chamber.
However, it has been noted that zones of primary combustion are established close to the nozzle, which result in the formation of thermal NOx.
Moreover, the above-described combustion head for liquid fuel burners has no application in small boilers, in particular for household and residential use, as combustion flames with a high axial extension are generated, which are to be developed inside boilers of large volume.
Instead, document U.S. Pat. No. 4,798,330 describes a combustion head for a burner which is fed with a fuel comprising a plurality of coaxial bodies fitted onto one another. The combustion head is provided so as to keep the frontal surface of the combustion head clean and to keep the combustion flame stable. However, the combustion head provided according to the dictates of U.S. Pat. No. 4,798,330 does not curb the formation of thermal NOx.
Therefore, the object of the present invention is to provide a combustion head which allows the formation of NOx to be minimized during the combustion process, which may also be applied to small boilers, in particular for household and residential use, while being easy and cost-effective to be provided.
According to the present invention, a combustion head for a burner for liquid fuels is provided as described and claimed.
The present invention will now be described with reference to the accompanying drawings, which show a non-limiting embodiment thereof, in which:
In
The combustion head 1 is fitted onto the central duct 3 and comprises a plurality of components assembled with one another, which are coaxial to one another and to the longitudinal symmetry axis X.
Connected to the central duct 3 is a so-called primary swirl body 4, with cylindrical symmetry and which is hollow inside and coaxial to the longitudinal symmetry axis X. In particular, body 4 comprises an external, cylindrical lateral wall 5 which is coaxial to the longitudinal symmetry axis X, and a frontal wall 6 which is provided with a through hole 7 to allow the central duct 3 and nozzle 2 to be inserted. The frontal wall 6 of body 4 has a plurality of indentations 8 which are evenly spaced about the longitudinal symmetry axis X; each pair of reciprocally adjacent indentations 8 defines a partition 9 in which the frontal wall 6 is divided. In particular, according to the embodiment shown in
According to a preferred variant, angle α is between 42° and 48°, and the angle is preferably equal to 45°. Indentations 8 are in direct communication with the through hole 7.
Assembled on body 4 is a so-called secondary swirl body 13 (shown in
Each opening 17 is defined by a base wall 18 and by a pair of lateral walls 19, which face each other and are inclined by an angle β with respect to the plane defined by a rear flat surface of the rear cylindrical portion 15. According to a preferred variant, angle β is between 42° and 48°, and angle β is preferably equal to 45°. The openings 17 are not in direct communication with the through hole of body 13.
An intermediate body 20 is fixed in turn on body 13, which has cylindrical symmetry, is hollow inside and coaxial to the longitudinal symmetry axis X. In particular, the intermediate body 20 comprises an internal cylindrical surface 21 coaxial to the longitudinal symmetry axis X which defines a through opening within which body 13 is accommodated. The diameter of the internal cylindrical surface 21 is substantially approximate to an external volume diameter of the rear portion 15 of body 13.
The intermediate body 20 is also divided into a rear portion 22 and a front portion 23. The rear portion 22 has an external cylindrical surface, which has an overall external volume diameter which is greater than the overall external volume diameter of the front portion 23. Moreover, the rear portion 22 has a rear truncated cone-shaped surface 24 and a frontal truncated cone-shaped surface 24**, which are both coaxial to the longitudinal symmetry axis X. As shown in greater detail in
It is worth noting that the intermediate body 20 acts as a supporting element for primary swirl body 4, secondary swirl body 13 and central duct 3.
It is also worth noting that a frontal surface (defined in this case by the annular portion 23** of frontal wall) of the intermediate body 20 substantially lies on the plane defined by the frontal wall 6 of the primary swirl body 4. Nozzle 2 is placed directly facing the combustion chamber CC and, in use, the liquid fuel is directly atomized into the combustion chamber CC.
Duct 25 is fixed in turn to the intermediate body 20, which has a substantially cylindrical symmetry, is hollow inside and coaxial to the longitudinal symmetry axis X.
Duct 25 is divided into a rear cylindrical portion 26 and a front portion 27. The rear cylindrical portion 26 has an external cylindrical surface and an internal cylindrical surface 28, which are both coaxial to the longitudinal symmetry axis X. In addition, the front portion 27 has a truncated cone extension which is coaxial to the longitudinal symmetry axis X and is tapered towards the free end facing the combustion chamber CC. Furthermore, a plurality of indentations 27* are obtained on the front portion 27 for the flame probe and the ignition electrodes to pass.
Duct 25 then comprises a plurality of projections 29 which are connected to the internal cylindrical surface 28 of the rear portion 26 in a position close to the front portion 27 and extend inwards from duct 25. The projections 29 are evenly spaced about the longitudinal symmetry axis X, extend in the longitudinal direction over a section of the rear portion 26. According to a preferred variant, duct 25 comprises six projections 29 spaced 60° apart from one another. Duct is adapted to translate in one of the two longitudinal directions indicated by arrow P. In order to allow the translation of duct 25, a control (manually actuated or by means of an actuator of known type) is provided; the projections 29 rest with contact and, in use, slide on the external surface of the intermediate body 20 to allow the movement of duct 25.
It is worth noting that such a compact geometry of combustion head 1 allows the size of the combustion flame to be contained, as better described below.
In use, a blower (of known type and not shown) provides a flow F of comburent, such as air, which is conveyed into duct 25 which indeed encloses the whole combustion head 1, and from here it is then divided into a number of comburent flows.
In particular, as shown in detail in
The primary comburent flow F1 flows into the primary swirl body 4 in the longitudinal direction. When the primary comburent flow F1 meets indentations 8, the flow lines F1 take n a helical and no longer longitudinal flow, and the speed at which flow F1 exits the primary swirl body 4 has a high tangential motion (swirl) component. The indentations 8 are hence arranged to produce a swirl in the primary comburent flow F1. The primary comburent flow F1 is then further exclusively divided into a primary swirled comburent flow which exits the indentations 8, and an axial primary comburent flow which exits the section left free from the nozzle into the through hole 7.
The secondary comburent flow F2 flows into the secondary swirl body 13 in the longitudinal direction. The flow rate of the secondary comburent flow F2 is determined by the number and section of the openings 17 made in the rear cylindrical portion 15, and is usually greater than the primary comburent flow F1. Also in this case, due to the passage of the secondary comburent flow F2 through the openings 17, the flow lines F2 take on a helical and no longer longitudinal flow, and the speed at which flow F2 exits the secondary swirl body 13 has a high tangential motion (swirl) component. The openings are hence arranged to produce a swirl in the secondary comburent flow F2.
The truncated cone-shaped profile of the rear surface 24 of the intermediate body 20 allows both the average speed of flow F1 at the inlet of the primary swirl body 4 and the average speed of flow F2 at the inlet of the secondary swirl body 13 to be increased. The increase the aforesaid average speeds results in an increase of the tangential motion (swirl) components of both the speed at which flow F2 exits the secondary swirl body 13 and the speed at which flow F1 exits the primary swirl body 4.
On the other hand, the tertiary comburent flow F3 is transported through a channel C1 defined between duct 25 and intermediate body 20. It is apparent that the final flow rate of the tertiary comburent flow F3 is determined by the distance between the front, truncated cone-shaped portion 27 of duct 25 and the intermediate body 20, which can vary due to the translation motion of duct 25.
The tertiary comburent flow F3 flows in channel C1 along a direction parallel to the longitudinal axis X.
At an end section thereof, close to the combustion chamber CC, channel C1 has a tapered profile which converges towards the primary swirl body 4 and towards the secondary swirl body 13. In particular, the end section has a truncated cone-shaped profile defined by the front portion 27 of duct 25, being the same as that of the rear portion 22 of intermediate body 20.
The profile of channel C1 is obtained so as to accelerate the tertiary comburent flow F3 before being fed into the combustion chamber and so as to direct the tertiary comburent flow F3 directly towards the primary comburent flow F1 and towards the secondary comburent flow F2 to limit the spatial area downstream of the combustion head 1, there the combustion flame develops.
Duct 25 is movable between a maximal closure position, corresponding to a tertiary comburent flow F3 with minimum flow rate, preferably equal to zero, and a minimum closure position corresponding to a tertiary comburent flow F3 with maximum flow rate.
In the minimum closure position, the axial component of the tertiary comburent flow F3 directly directed towards the primary comburent flow F1 and towards the secondary comburent flow F2 opposes the swirl generated by primary swirl body 4 and secondary swirl body 13; moreover, in this case, the profile of the intermediate body 20 on which the tertiary comburent flow F3 runs adherent due to the “Coanda effect” allows the combustion flame to have a prevalently axial flow.
In the maximal closure position, the tertiary comburent flow F3 does not influence the swirl generated by primary swirl body 4 and secondary swirl body 13; in this case, the combustion flame has a significantly small axial development.
The combustion process sequentially includes the following steps:
It is known from literature that the intensity of the swirl obtainable with a combustion head 1 has relevant effects on the polluting emissions of a combustion process. Moreover, it has been verified that the intensity of the swirl is quantifiable through the number of swirls and a combustion process with low levels of NOx emissions can be obtained for a swirl number greater than 1.
In essence, the combustion head 1 described hereto comprises four bodies indicated with numerals 4, 13, 20 and 25, respectively, which are assembled together and may be fitted onto any duct 3 with axial symmetry.
It is also worth noting that duct 25 and intermediate body 20 which determine the tertiary comburent flow F3 also influence the primary comburent flow F1 and the secondary comburent flow F2 due to the dynamic characteristics (i.e. due to flow rate and speed range) of the tertiary comburent flow F3.
In greater detail:
Thereby, the formation of thermal NOx can be curbed by means controlling the axial and tangential components of the comburent flows F1, F2 and F3.
It has been experimentally verified that the number of swirls obtainable by means of the combustion head 1 described hereto (with a primary swirl 4 and a secondary swirl 13) is substantially high, in the order of 5.45, and such as to ensure a low level of NOx emissions.
Moreover, the opportunity to control the intended swirl level allows a yellow-blue colouring of the flame to be kept, which is easily detected by an optical sensor with a light dependent resistor and which is more reliable and less costly than those with ultraviolet radiation usually used in applications of this kind.
