Multipoint fuel injection device

The fuel injection conduits in a multipoint device surrounding a so-called pilot central injection device include tubes of circumferential orientation. By separating the injection conduits from each other, it is possible to attribute to them different head losses which compensate the differences in length that the fuel has to travel: a uniform flow of fuel may be hoped for, for each of the injection holes. The tubes are individual but joined to form a crown that is unitary or composed of two almost symmetrical unitary portions, which lends itself well to manufacture by addition of material.

This application claims priority from French Patent Application No. 18 56773, filed Jul. 20, 2018, the entire contents of which is hereby incorporated by reference herein.

The subject matter of the invention is a multipoint fuel injection device, designed for an aircraft engine.

Such devices offer the perspective of reducing the polluting emissions discharged by the engine, which depend both on the thermodynamic cycle and on the configuration of the combustion chamber. These polluting emissions include nitrogen oxides NOx, carbon monoxide CO, unburned fuel CHx, smoke emissions and fine particle emissions which are or will be regulated by standards.

Among the solutions making it possible to reduce polluting emissions, it is possible to resort to an injection including both a so-called pilot injection, used over the whole of the flight, and an injection used beyond idling speeds and for example during take-off, typically from 30% of the maximum speed, where a lean or sub-stoichiometric combustion takes place. The temperature of the flame is then low, which limits the formation of nitrogen oxides at these high speeds. The fuel injection system then typically includes a central region assigned to the pilot injection, and a peripheral injection surrounding the preceding assigned to this combustion beyond idling speeds. The peripheral injection into the combustion chamber takes place through a series of holes distributed on a crown, which explains why it is called “multipoint”. These holes, which may be ten or so or several tens or so in number, are supplied in known designs by an annular cavity going completely round the crown and supplied by a supply conduit coming from the arm of the injector.

Such a device is described in the document FR 2996287 A1; the document FR 2919898 A1 may also be consulted, also conforming to the preceding description, but which has the particularity that the crown for conveying the fuel of the multipoint injection is surrounded by a cavity in which flows the fuel of the pilot injection. The risks of coking of the fuel of the multipoint injection while it is inactive are thereby avoided, thanks to a constant cooling procured by the fuel of the pilot injection.

Engines are being developed in which the injection system is of smaller volume, which would impose a reduction in section of the cavity of the crown in which flows the fuel intended for the multipoint injection. Increased head losses are then feared, which risk making the flow more difficult, deteriorating the uniformity of the injection flow rate through the different points, thus decreasing the combustion efficiency, potentially making it unstable and increasing the production of new polluting emissions.

It has thus been sought to improve injection systems including a multipoint injection, in order to better guarantee uniformity of flow rate through the injection holes, and thus a regular distribution of the fuel in the azimuth direction of the crown, including for small volume injection systems.

This fuel injection device for a combustion chamber of an aircraft engine includes a first, central, so-called pilot injection part, a second so-called multipoint injection part surrounding the pilot part, the multipoint part being connected to a fuel supply conduit and including holes for injecting fuel into the combustion chamber, characterised in that the multipoint part includes, between said injection holes and said supply conduit, a plurality of conduits parallel with each other and each connected to a respective injection hole, the conduits extending circumferentially around the pilot part over different angular sectors; a part of the parallel conduits being composed of joined and rigid tubes forming a unitary portion and manufactured by a material addition method.

By dividing in this way in an early manner the fuel into flow rates individually supplying each of the injection holes, upstream of the path where head losses can occur, this makes it possible to adjust the head losses undergone by the fuel towards each of the injection orifices, and thus the distribution of the total flow rate towards each of the conduits, so as to obtain uniformity of injection. But given the large number of lengths encountered in multipoint devices, it was difficult to easily find tubes having all the suitable characteristics. An aspect of the invention is thus that the manufacture of the conduits takes place by an additive method, the conduits then forming, at least over a part of the length, a unitary structure. Additive manufacturing has the great advantage of making it possible to construct without difficulty complex bundles of conduits of different lengths and which can also be different by their section or any other geometric or other parameter.

A subsidiary but important aspect of the invention relates to the ejection of fuel out of the conduits. In many cases, the parallel conduits will be connected to the holes by connecting tubes that will be oriented in a direction essentially identical to a central axis of the crown, and will thus form a more or less straight angle with the main portions of the conduits. It is then recommended that these connecting tubes have identical length over diameter ratios

LD,
in order to obtain penetration depths of the fuel jets in the combustion chamber that are identical, here again to favour regularity of injection and combustion.

The parallel conduits envisaged here may typically have considerable lengths, which are however variable depending on the distance of their injection hole from the supply conduit outlet: the longest parallel conduits may have an extension of 135° at least, or even around 180° in the case, preferred for the invention, where the parallel conduits form two groups of which each extends into a respective angular half of the crown; all the circumference (apart from a small angular portion corresponding to a distribution chamber at the outlet of the supply conduit) may thus be occupied by these conduits.

The aim of the invention is attained if the parallel conduits are subject to identical head losses. Yet, head losses depend notably on the length of each of the conduits, but also their diameter. This may be attained by making the parallel conduits different to each other in such a way that their linear head loss (per unit of length) is inversely proportional to their length. This can be achieved in various ways by making the flow a little more difficult in the short conduits, for example if the parallel conduits have different sections, increasing for increasing values of the angular sectors; and notably with identical length over diameter ratios

LD
or instead, in plausible embodiments where the parallel conduits are connected to the holes by connecting tubes which are oriented in a direction essentially identical to a central axis of the crown, if the connecting tubes have identical length over diameter ratios.

The crown could be problematic to manufacture with conventional material removal methods, but it can be manufactured very well by an additive method. The crown may be included in another crown assigned to the supply with fuel of the pilot part, in order to be bathed by the fuel intended for the pilot part to maintain the fuel of the multipoint part below coking.

The injection device is established through a wall1of a combustion chamber25.FIG.5schematically represents the combustion chamber25surrounded by the gas flow path28of the turbomachine of which a part surrounds the combustion chamber25. The injection device emerges at the bottom of the combustion chamber25, opposite the outlet29of the combustion chamber25. A supply conduit2is at the centre of an injection arm3, and it supplies with fuel a so-called multipoint injection part including a multipoint injection crown6. The latter is arranged in a cavity7of an outer crown8which envelopes it and which is the seat of a permanent flow of fuel during flight, itself also supplied by the supply conduit2, which supplies a so-called pilot injection part, including a pilot injection conduit9which ends in a pilot injection jet11at the centre of the device, directed towards the inner volume12of the combustion chamber. The injection device is supported by an envelope13that surrounds the crowns6and8, is connected to the wall1and flares out into a lodge14towards the inner combustion volume12. The pilot injection conduit9has a shape bent at 180° and extends into a region diametrically opposite to the supply conduit2with respect to a central axis X of the multipoint injection crown6and the outer crown8, passing through the pilot injection jet11and horizontal inFIG.1. This central axis X conventionally makes it possible to define axial, radial and tangential (or angular) directions, which will be used to describe the device.

The crown6is composed of two unitary portions, of which one is represented in a clearly visible manner inFIG.2. Each unitary portion includes a group of juxtaposed and joined tubes15forming parallel conduits, which extend in the tangential direction of the injection device. The tubes15of the portion have however different lengths and stop at locations regularly distributed on the circumference of the multipoint injection crown6and which comprise multipoint injection holes16. In the embodiment represented, this half of the multipoint injection crown6includes six tubes15and as many multipoint injection holes16, of which one of them, noted16a, is diametrically opposite to the supply conduit2and depends on a tube, noted15a, of which the length—nearly 180° of angular extension—is greater. In this embodiment, the multipoint injection part includes eleven multipoint injection holes16in total, regularly distributed over the complete circumference of the multipoint injection crown6, and the other half of the crown6may be virtually symmetrical thereto, except that there is no symmetric tube of the longest tube15a(FIG.3). The five tubes (in this embodiment) of the group of tubes of the unitary portion, not represented, of the multipoint injection crown6may have angular lengths respectively similar to those of the tubes15represented (the tube15abeing excluded), or not, a greater dissymmetry of the halves of the multipoint injection crown6then being accepted. It is thereby possible to obtain a device including an even or odd number of multipoint injection holes16.

The outer crown8forms a continuous ring, but the multipoint injection crown6is limited by two walls10which separate its two portions and between which extend a distribution chamber17of low angular extension (at the most an interval between two multipoint injection holes16). The supply conduit2emerges in the distribution chamber17by at least one supply tube4which prolongs it in the arm3, and the tubes15also emerge in the distribution chamber17after having passed through the walls10. Also, the fuel circulates freely from the supply conduit2to the multipoint injection holes16by spreading out in an equitable manner in the tubes15. A valve, not represented, makes it possible however to stop this circulation at will, by closing the supply tube4. The supply conduit2is provided with other supply tubes18, which emerge in the outer crown8outside of the multipoint injection crown6of the two sides of the distribution chamber17and ensure the pilot injection of fuel.

The outer crown8includes an open side in front of an end piece19at the end of the arm3and on its side, and the supply tubes4and18pass through this end piece19. The end piece19has a circular end20corresponding to this open side. When this circular end20is driven into a flange21projecting on the outer edge of the outer crown8towards the arm3, a leak tight adjustment may be obtained. And the circular end20comprises a hole22diametrically opposite to the supply conduit2, in which one end of the pilot injection conduit9is driven in, whereas the opposite end of the pilot injection conduit9, which is bent, is adjusted on the pilot injection jet11. The result is a circulation of fuel without obstacle from the two sides of the outer crown8around the multipoint injection crown6up to the pilot injection jet11. Bathed by external fuel, the multipoint injection crown6escapes coking and the risks of clogging of the tubes15which could have appeared when the fuel that is present therein stagnates while the multipoint injection is stopped. The fuel circulating in the pilot injection and notably around the multipoint injection crown6remains at a temperature of less than 100° C., much less than that of the ambient air, and it thus maintains a sufficient exchange of heat to ensure this protection against coking.

A uniform distribution of fuel may be hoped for in the different tubes15of the two parts of the multipoint injection crown6, in accordance with the objective of the invention which is to ensure an equal flow rate in the different injection holes16thanks to the early division of the multipoint injection flow. It is possible if necessary to construct the tubes15with different geometric characteristics, in order to level out there in head losses and to reinforce this desired flow rate equality: it is thus possible to envisage compensating the effects of the differences in lengths between the tubes15, for example by constructing them with different sections, greater for the longest tubes15, or greater sectional irregularities or other obstacles in the shortest tubes15, since the head losses are proportional to the widths of the conduits, and inversely proportional to their diameter. A great freedom in design is available to do this, which is made possible by manufacturing the multipoint injection crown6by a material addition method, of a part with the outer crown8or separately. Such a manufacturing facility does not exist with conventional construction methods, where tubes would be connected independently to a distribution device, since the choice of the diameters would then be limited, and the distribution device provided with different connections, more complicated to construct. It is to be noted that head losses may also be caused in the shortest tubes15by irregularities of section or other obstacles, but it is simpler and surer to construct them with a judiciously chosen constant section, with identical length over diameter ratios

The tubes15are connected to the multipoint injection holes16by connecting tubes23which extend essentially in the direction of the central axis of the multipoint injection crown6and form a bend with the tubes15.

A dimensioning rule that can favourably be applied thereto is to assign them a ratio

LD
(length, between each multipoint injection hole16and the junction bend to the respective tube15, over diameter:FIG.6) that is identical

(L1D1=L2D2=L3D3),
in order that the penetration heights of the fuel jets in the inner volume12of the combustion chamber25are also identical and that a regular injection is thus obtained. This rule is all the more justified if the tubes5are grouped together into bundles in the outer crown8at different depths under the multipoint injection holes16.

The respect of these length/diameter ratios

LD
also produces identical head losses in the connecting tubes23when they exist.