EP3252376B1

EP3252376B1 - Combusting head for liquid fuel

Info

Publication number: EP3252376B1
Application number: EP17174109.3A
Authority: EP
Inventors: Flavio Comencini; Roberto Marchetti; Giovanni Scaion
Original assignee: Riello SpA
Current assignee: Riello SpA
Priority date: 2016-06-01
Filing date: 2017-06-01
Publication date: 2020-01-29
Anticipated expiration: 2037-06-01
Also published as: ITUA20164028A1; EP3252376A1

Description

The present invention relates to a combustion head for liquid fuel, in particular to a combustion head for a residential burner.
Similar combustion heads have been described in the following documents:

DE-A1-39 19 797

The main object of the present combustion head is to ensure a stable flame both as regards its geometrical shape in the combustion chamber, and in relation to the guaranteed thermal efficiency, with the aim of maintaining the CO and NOx emissions within the limits of current regulations.
Consequently, according to the present invention a combustion head is made as claimed in claim 1, or in any one of the claims depending, directly or indirectly, on claim 1.
For a better understanding of the present invention, a preferred embodiment will now be described by way of a nonlimiting example and with reference to the appended drawings, wherein:

Figure 1 illustrates a three-dimensional view partially in longitudinal cross-section of the combustion head according to the present invention;
Figure 2 shows an exploded view of the combustion head illustrated in Figure 1;
Figure 3 shows a longitudinal cross section of the combustion head in Figure 1 combined with a combustion chamber;
Figure 4 shows some elements assembled to each other comprised in the combustion head in figure 1;
Figure 5 illustrates a three-dimensional view of a conical diffuser used in the combustion head in Figure 1;
Figure 6 is a cross-section of the conical diffuser in figure 5.
Figure 7 illustrates the cross section of the conical diffuser in figure 6 combined with a flame disc;
Figure 8 shows a three-dimensional view in longitudinal cross-section of an alternative embodiment of a fire tube used in the combustion head in figure 1;
Figure 9 illustrates first flows of exhaust gases recycled on a conical diffuser belonging to the combustion head in figure 1; and
Figure 10 shows second flows of exhaust gases recycled.

In Figures 1, 2 and 3 reference numeral 100 globally denotes a combustion head made according to the teachings of the present invention.
As will be seen more clearly below the combustion head 100 is applied to a combustion chamber 1000 (Figure 3).
The combustion head 100 comprises the following elements:

a fire tube 10 with a substantially cylindrical shape and having a central symmetry axis (X) (figure 3); a number of holes 11 made on the fire tube 10 in the shape of slots; each hole 11 occupies an arch made on the cylindrical extension of said fire tube 10; in the example shown the slot-shaped holes 11 are four in number and are preferably equi-spaced from one another and positioned with an angular separation of 90° relative to one another, and in addition are centred at the angular values 0°, 90°, 180°, 270°; the angular and axial extension of each hole 11 can be made to vary depending on the levels of thermal performance and emission containment to be achieved;
a flame disc 20 (made of a material resistant to the combustion temperature) placed perpendicularly to said central axis of symmetry (X); the flame disc 20 is shaped as a circular flat disc having a central circular hole 21 (again with axis (X)); advantageously, but not necessarily, the flame disc 20 is peripherally welded to the inner surface of the fire tube 10;

a conical diffuser 30 with double conical inclination (see below);
a nozzle holder 40, of a cylindrical shape and with axis (X), to the free end of which a nozzle 41 is fixed suitable to atomize the liquid fuel; and
a pair of electrodes 50 for lighting the flame.

The fire tube 10 is a hollow cylinder open at the two end bases.
In addition, the fire tube 10 is made of a material resistant to the temperatures at which combustion of the liquid fuel takes place in output from the nozzle 41.
Advantageously, but not necessarily, the fire tube 10 may be made in two portions 10A and 10B, having the same diameter, respectively upstream and downstream of the flame disc 20.
In particular, the portion 10A can be a full wall cylinder to the free end of which the flame disc 20 is welded.
The portion 10B is also cylindrical, but on the end facing the portion 10A has the aforesaid four slot-shaped holes 11.
Obviously the welds of the portion 10B to the flame disc 20 are made at the end portions of the portion 10B where the four holes 11 are absent.
In an alternative embodiment of the fire tube 10* shown in figure 8 two cylindrical portions 10A* and 10B* are connected by a circular collar 10C* (on which four holes 11* are made (only two holes 11* are visible in figure 8) followed by a conical connection surface 10D*. In this case the diameter of the portion 10B* is greater than that of the portion 10A*.
Similarly to the first embodiment, a flame disc 20* is located upstream of the holes 11* with respect to the flow direction of the air/fuel mixture (not shown in Figure 8).
As already seen for the fire tube 10, the various components of the fire tube 10* can be assembled by welding, or they can be made in one piece. Advantageously, but not necessarily, the conical diffuser 30 is an axially symmetrical element with an outer conical converging surface 31 (with respect to the flow (F1) of the fuel/primary air mixture - figures 6, 7), and an inner conical central hole 32 having a diverging surface 32A (again with respect to the flow (F1) of the fuel/primary air mixture - figures 6, 7).
The axial extension of the conical diffuser 30 and the angle between the two conical surfaces 31, 32A can be made to vary depending on the desired functionality.
In other words, the angles (α) and (β), respectively, of the outer conical converging surface 31, and the inner conical diverging surface 32A may vary from 0° to 90°, these two values being degenerate.
As shown in Figure 5, the conical diffuser 30 has two through holes 33 for housing the electrodes 50 and may comprise, possibly, through holes for the fastening screws to the flame disc 20; or the conical diffuser 30 may constitute a single body with said flame disc 20 by means of welding or machining of the piece.
In general, the diameter of the initial central opening 30A of the conical diffuser 30 is equal to that of the central circular hole 21 of the flame disc 20 (figure 7).
In addition, the diameter of the central circular hole 21 may depend on the desired functionality of thermal performance and emissions.
The profile of the central circular hole 21 may have a chamfer 21A (Figure 7) according to an angular value which may vary with respect to the axial direction, in the range (-90°, 0°) in the case of chamfering upstream of the disc, or in the range of (0°, 90°) in the case of chamfering downstream of the disc; in particular, if the angular value is 0°, the profile is parallel to the angular direction.
In actual use, the combustion head 100 is inserted (with reference to figure 3, from left to right) in the combustion chamber 1000 of a boiler (not shown in its entirety), and is positioned so that the holes 11 of the fire tube 10 lie entirely inside said combustion chamber 1000.
The flame develops in a zone (ZF) (Figure 3) inside the fire tube 10 and the products of combustion (for example, fumes and emissions) come out of the fire tube 10 through an outlet section (SU) and propagate into the combustion chamber 1000.
It is well known that one of the most widespread methods for limiting the emission values of nitrogen oxides (NOx) is the one that takes into account the recirculation of the combustion products, which consists of returning to the head (through the holes 11 of the fire tube 10) the products emitted by the flame, and which have already entered the combustion chamber 1000 in order to burn the unburnt components.
Figure 9 shows the aforementioned flow (F1) of fuel/primary air mixture coming out from the central hole 21 of the flame disc 20 and from the central hole 32 of the diffuser 30.
From this flow (F1) the flame which develops from the flame disc 20 is generated.
Said primary flow (F1) moves at a much higher speed than the speed of the mass of fumes present in the combustion chamber outside the fire tube 10, so that it generates a pressure gradient between the outer area, at a higher pressure, and that inside the fire tube 10, at a lower pressure.
This gradient generates the flows (F2) of recirculating fumes entering the fire tube 10 through the holes 11 (in figure 9 for the sake of clarity the recirculation from the lateral holes is not shown).
The recirculation flows (F2) lap the primary flow (F1) and thus the peripheral zone of the flame, and this proves sufficient, experimentally, to ensure the combustion of their unburnt components, in order to lower the NOx emissions.
Other flows (F3) (Figure 10), formed by closed trajectories, are established in the area inside the fire tube delimited by the recirculation flows (F2) and by the inner wall of the fire tube 10, due to the speed gradient between the air in contact with the inner wall and the recirculation fumes which have a higher speed.
These flows (F3) have the function of keeping the flame stabilized in the central area of the internal volume of the fire tube 10, preventing its excessive expansion in volume leading to a reduction of the thermal power transmitted by it.
The geometry of the conical diffuser 30, of two conical flaps with opposite conicity, has been studied experimentally and numerically to facilitate its cleaning by the recirculation flows, which lapping in a continuous and constant manner the two surfaces over time (figure 9), remove the carbon deposits that are formed both as a secondary product of the flame and as possible crystallization, due to temperature gradients, of any liquid fuel droplets that come into contact with the metal surface.
Figure 9, a side view of the conical diffuser 30, shows how the flow lines of the recirculation fumes, after having passed through the holes 11 of the fire tube 10, lap the entire outer conical surface 31 of the conical diffuser 30 and then descend along most of the inner conical surface 32A, before being distanced by the fast primary flow (F1) coming from the central hole 32.
The main advantages of the combustion head described above are as follows:

drastic reduction of NOx due to an optimal recirculation of the primary combustion products; and
optimal cleaning of both the outer and inner surface of the conical diffuser thanks to optimization of the flow of the fuel / primary air and recirculation flows of the combustion products.

Claims

A combustion head (100) comprising the following elements:
- a fire tube (10; 10*) with a substantially cylindrical shape and having a central symmetry axis (X) and at least one first hole (11; 11*) for the recirculation of the combustion products;

- a flame disc (20), which is arranged perpendicularly to said central symmetry axis (X) and has a central second hole (21);

- a conical diffuser (30) with a double conical inclination consisting of an axial symmetry element with an outer conical surface (31) converging relative to a flow (F1) of the primary fuel/air mixture, and a central inner conical third hole (32) having a surface (32A) diverging relative to said flow (F1);

- a nozzle (41), which is designed to atomize liquid fuel; and

- fire lighting means (50);
the combustion head (100) is characterized in that said conical diffuser (30) is such that recirculating combustion products having entered the fire tube (10; 10*) through the first hole (11; 11*) lap the outer conical converging surface (31) and the diverging surface (32A) before being distanced by the flow (F1) of the primary fuel/air mixture coming from the second hole (21) and from the third hole (32) .
Combustion head (100), according to claim 1, characterized in that the axial extension of said conical diffuser (30) and the angle between the two conical surfaces (31, 32A) are chosen based on the desired functions.
Combustion head (100), according to claim 2, characterized in that the angles (α) and (β) - of the outer conical converging surface (31) and of the inner conical diverging surface (32A), respectively - vary between 0° and 90°.
Combustion head (100), according to anyone of the preceding claims, characterized in that said first hole (11; 11*) for the recirculation of the combustion products, made on the fire tube (10; 10*), has the shape of a slot, which occupies an arc extending on the cylindrical surface of the fire tube (10; 10*) itself.
Combustion head (100), according to anyone of the preceding claims, characterized in that said fire tube (10) comprises a first portion (10A) and a second portion (10B) having the same diameter, upstream and downstream of said flame disc (20), respectively.
Combustion head (100), according to claim 5, characterized in that said first portion (10A) is a full-wall cylinder, which is welded, at its free end, to said flame disc (20).
Combustion head (100), according to claim 5 or to Claim 6, characterized in that said second portion (10B) is cylindrical and, at its end facing said first portion (10A), has said first hole (11).
Combustion head (100), according to anyone of the claims from 1 to 4, characterized in that said fire tube (10*) is made up of two portions (10A*, 10B*) having different diameters, upstream and downstream of said flame disc (20), respectively; said two portions (10A*, 10B*) being joined to one another by means of at least one conical joining surface (10D*).
Combustion head (100), according to anyone of the preceding claims, characterized in that the second hole (21) is provided with a chamfer (21A).