FR3017415A1 - COOLING A MAIN CHANNEL IN A FUEL SYSTEM WITH MULTIPOINT INJECTION - Google Patents

Abstract

The invention relates to a fuel system (1) for a turbomachine, adapted to inject fuel into a combustion chamber (5) of the turbomachine, comprising: - a pilot circuit (10) adapted to inject fuel into the fuel chamber; combustion (5) via a pilot line (14), - a main circuit (20) adapted to inject fuel into the combustion chamber (5) via a main line (24) , the main pipe (24) being housed at least partially in the pilot line (14).

Description

FIELD OF THE INVENTION The present invention relates to the field of fuel systems for turbomachines, in particular aircraft, and more particularly relates to fuel injection systems.

More specifically, the invention relates to injection systems with a dual fuel injection circuit, which comprise a central injector, commonly called a pilot injector, delivering an optimized permanent fuel flow for low speeds, as well as a peripheral injector, sometimes referred to as the main injector, which delivers an optimized intermittent fuel flow for high speeds. These injection systems have been developed to allow a better adaptation of the fuel injection to the different operating conditions of the combustion chambers of these turbomachines, in order to reduce their emission of pollutants such as nitrogen oxides and fumes.

BACKGROUND ART The fuel system may comprise a set of fuel injectors arranged in the combustion chamber, a fuel pump for pressurizing fuel from the fuel tank, a fuel metering unit FMU (acronym Fuel Metering Unit) for metering the fuel quantity to the injectors, and a fuel supply circuit fluidly connecting the fuel metering unit to the fuel injectors. Combustion chambers with low NOx emissions need to be able to distribute the fuel injected between at least two injection lines, a pilot line and a main line, in order to be able to adjust the flow of the injectors according to the phase of flight and improve the homogeneity of the air / fuel mixture and thus the combustion, which makes it possible to reduce the pollutant level.

For this purpose, the fuel system may comprise multiple flow paths, typically two sets of injectors (main and pilot), pipelines for each series and a flow split valve (also known as "split"). valve ") arranged downstream of the dosing unit. In such systems, the fuel is delivered to the pilot injectors and to the main injectors according to turbomachine operability laws. For example, during startup of the turbomachine, the fuel is initially supplied only to the pilot injectors. Since the main line is intermittent, the fuel it contains can remain stagnant for long periods, typically twenty minutes to one hour. During these periods, the stagnant fuel is therefore subjected to a severe environment, especially in temperature, which results in the increase of the temperature of the stagnant fuel in the main pipeline and therefore a risk of coke formation, which can obstruct the injectors. the main ones downstream of these pipes. In order to prevent coke formation in the main lines, air could be drawn upstream of the fuel system to cool the main lines. However, it turns out that this solution has an impact on the performance of the turbomachine. SUMMARY OF THE INVENTION An object of the invention is to propose a fuel system, in particular for an aircraft turbomachine, comprising at least two fuel supply lines configured to inject fuel into the combustion chamber as a function of the fuel system. turbomachine regime, to limit or even avoid, in an efficient and simple management, the coking fuel likely to stagnate in the injection lines. For this, the invention proposes a fuel system for a turbomachine, adapted to inject fuel into a combustion chamber of the turbomachine, comprising: a pilot circuit adapted to inject fuel into the combustion chamber via a pilot line; a main circuit adapted to inject fuel into the combustion chamber via a main line, the fuel system being characterized in that the main line is housed at least partially in the pilot line; .

Some preferred but non-limiting characteristics of the fuel system described above are the following: the main pipe is coaxial with the pilot pipe, the main pipe is eccentric with respect to the pilot pipe, the main pipe is housed in the pipe. pilot the entire length of said pilot line, and - the fuel system further comprises at least one stiffener, preferably several, extending between the main line and the pilot line to maintain a minimum distance between the main line and the main line. pilot pipeline. The invention also proposes a turbomachine comprising a fuel system as described above. BRIEF DESCRIPTION OF THE DRAWINGS Other features, objects and advantages of the present invention will appear better on reading the detailed description which follows, and with reference to the appended drawings given as non-limiting examples and in which: FIG. 1 a partial view of an embodiment of a main pipeline housed in a pilot line of a fuel system according to the invention, in which the pilot line has been partially cut to allow visualization of the main line, and Figure 2 schematically shows an example of a fuel system according to the invention. DETAILED DESCRIPTION OF AN EMBODIMENT A fuel system 1 comprises, from upstream to downstream in the direction of the fuel flow, a fuel tank 2, a high pressure pump 3 adapted to pressurize fuel. of the tank 2, and - a fuel metering unit 4, supplied with fuel by the high-pressure pump 3 and adapted to dose the quantity of fuel to the combustion chamber 5 via a fuel supply circuit .

The fuel supply circuit here comprises a type of injector comprising two fuel circuits 10, 20 and thus having two fuel inlets 12, 22, each associated with supply lines 14, 24. More specifically, the supply circuit comprises a pilot circuit 10, adapted to continuously inject fuel into the combustion chamber 5 via pilot injectors 12, and a main circuit 20, adapted to inject fuel intermittently. in the combustion chamber 5 via main injectors 22. The flow of the pilot circuit 10 and main 20 is controlled by means of a flow-dividing valve 6, arranged between the dosing unit 4 and the pilot injectors 12 and main 22. The flow-dividing valve 6 adjusts the flow distribution between each supply circuit 10, 20 according to the operating laws of the turbomachine.

In order to limit, or even prevent, the coking of the fuel likely to stagnate in the pipes 24 of the main injectors 22, the invention proposes to use the pilot line 14 as a thermal protection barrier. For this purpose, the invention proposes to house at least partially the main pipe 24 in the pilot line 14.

Indeed, the pilot line 14 is still debiting, while fuel is likely to stagnate in the main pipe 24, whose flow is intermittent. Thus, the main pipe 24 will be protected from the external environment by the pilot line 14, the permanent flow of which thermally isolates the main pipe 24 by absorbing the heat transmitted by the external environment to the injection system 1 and directing them towards the combustion chamber 5.

It will be understood that the size of the main pipe 24 and the pilot line 14 are chosen so as to allow partial housing of the main pipe 24 in the pilot line 14, while allowing sufficient flow rates therein. In general, the cross section (in a plane normal to the flow direction of the fuel in the pipes) of the pilot line 14 is therefore greater than the cross section of the main pipe 24. Thus, for tubular pipes, the internal diameter of the pilot line 14 must be greater than the outer diameter of the main line 24.

Furthermore, sufficient space between the main pipe 24 and the pilot line 14 is formed to prevent them from colliding when they are subjected to strong vibrations, which could damage them. This space must be large enough to allow the pilot flow to circulate without having too many losses in the circuit 10. Thus, in one embodiment, the main pipe 24 may for example be kept away from the pilot line 14 to using stiffeners 7 or any equivalent capable of keeping in vibration and maintaining the spacing between the two pipes 14, 24.

In a first embodiment, illustrated in FIG. 1, the main pipe 24 and the pilot pipe 14 may be coaxial, the main pipe 24 extending centrally inside the pilot pipe 14. As a variant, the main pipe 24 may extend eccentrically with respect to a central axis of the pilot line 14.

The main pipe 24 can be housed in the pilot line 14 over the entire length of the pilot line 14 which extends between the flow-dividing valve 6 and the combustion chamber 5. Alternatively, in order not to modify the connection between the pipes 14, 24 and the valve 6 and the combustion chamber 5 which extend upstream and downstream thereof, the main pipe 24 may be housed only over part of the length of the pilot line 14, preferably in the fuel system zone which undergoes a severe temperature environment (upstream of the combustion chamber 5). When both the main line 24 and the pilot line 14 are flowable, for example when the turbomachine is in cruise mode, the fuel temperature in the main line 24 and in the pilot line 14 are homogeneous and equal. On the other hand, when the main pipe 24 is not flow-generating, for example during a start-up phase of the turbomachine, the heat flows at the main pipe 24 and the pilot pipe 14 are as follows: Convective flow szikv from the outside air to the pilot line 14: 0Cv (Outdoor T p lote) x S outside where a is the outside air exchange coefficient, Outside is the outside air temperature, Toote is the temperature of the outside air the pilot line 14 and outer is the exchange surface between the pilot line 14 and the outside air. Conductive flow 0 of the duct -rinci-ale 24 to the pilot line 14: (Tpn - Pilot key) x S wall pe contact30 where A is the thermal conductivity of the material constituting the wall of the main pipe 24, Tprincipale is the temperature of the main pipe 24, Scontact and eparoi are respectively the surface and the thickness of the wall of the main pipe 24 housed in the pilot line 14 (and immersed in the fuel flowing in the pilot line 14). The sum of the conductive flow om p of the main line 24 to the pilot line 14 and the convective flow Oct, gives the total flow 0 of heat to the pilot line. Conductive flow 0. of the line -ilote 14 to the main line 24: op m _ (pilot pilotT) x Scontact e wall Op m is the total heat flow received by the main pipe 24. The thermal flow received by the pipe pilot 14 will heat the fuel passing through it. Thus, the fuel contained in the pilot line 14 will heat up according to the following formula (the conductive flow Om p being negligible in front of the convective flow Oc ') mass x heat. where At is a time during which the fuel is exposed to heat flow.

Since the pilot line 14 is passing, the fuel contained therein is exposed to the convective flow Ocv only during a period of time. Thus, when the fuel system 1 is stabilized, the temperature of the fuel in the pilot line 14 is as follows: T (t + At) = T (t) + ci, (1) T (to + texpo) = T (t0 ) + vna (-1 outside - T (t. »x S outside i = 1 e mass wall x heat. .massic where T (to) is the temperature of the fuel entering the pilot line 14 at instant to Tai) is the fuel temperature in the pilot line at time ti, with ti = to + i * At and so Tai) = T (to + i * At). In an extreme case, the temperature of the main pipe 24 will reach at most the maximum temperature of the fuel in the pilot line 14, namely the temperature of the fuel contained in the pilot line 14 at the time of its injection into the combustion chamber, either ± -expo, T (to + texpo). However, the temperature Tao t) is much lower than the external temperature Outside. The pilot line 14 thus forms a heat shield for the main pipe 24. Taking into account the mass heats of the various elements as well as the heat flows passing through, the temperature of the stagnant fuel in the main pipe 24 therefore increases much less rapidly. in the case where the main pipe 24 is housed in the pilot line 14 that in the usual case where the pipes 14 and 24 are separated and extend side by side. Indeed, in the case of the invention, a significant portion of the convective heat flow entering the pilot line 14 is discharged directly by the fuel flow 25 in the pilot line 14 to the combustion chamber 5, without being transmitted by conduction to the 24. In the following, an example of a fuel system 1 comprising a main line 24 housed coaxially in a pilot line 14 will be described. In this example, the environment of the fuel system 1 is substantially stabilized with an outside temperature of the order of 193 ° C and a fuel temperature at the entrance to the pilot line 14 of the order of 139 ° C. Moreover, the coefficient of exchange of the outside air a is here of the order of 23 W / m2 / K and the convective thermal flow Oc, from the outside air to the pilot line 14 is 25 W per meter assembly. By taking an outside diameter of 15 mm, a wall thickness of 0.9 mm and a main pipe radius 24 of 4 mm (an external surface area of about 25 × 10 -3 m 2) and a fuel mass heat of the order of 2000J / kg / K, the temperature of the fuel passing through the pilot line 12 is therefore about 2 ° C per second. With a flow rate of the order of 100L / h, the fuel exposure time at the outside temperature of 193 ° C in the pilot line 14 is about 7 seconds.

Thus, in this embodiment, the fuel passing through the pilot line 14 heats up to about 14 ° C between its entry into the pipe 14 and its injection into the combustion chamber, and therefore arrives at a temperature of 153 ° C. 12. It can be deduced from this that the temperature of the fuel contained in the main pipe 24 does not exceed the temperature of 153 ° C., even when it is stagnant for a long time, which makes it possible to avoid the risks. In comparison, when the main line 24 and the pilot line 14 are simply side by side, the main line 24 is thermally insulated if necessary from the remainder of the fuel system 1 (for example at the same time). using an insulating sleeve threaded on the main pipe 24), the entire heat flow coming from the outside, and if necessary passing through the insulating sleeve, e it is directed towards the main pipe 24, which heats the stagnant fuel much more. Typically, under the conditions used for the embodiment described above, the stagnant temperature in the main pipe 24 exceeds 190 ° C for a significant time may exceed thirty minutes, according to the laws of operability of the turbomachine , thus leading to the formation of coke in the main pipe 24.

In one embodiment, the main pipe 24 can also be housed in an intermediate pipe, said intermediate pipe being itself housed in the pilot pipe 14. The intermediate pipe can then be filled with a thermally insulating material or air that forms an additional thermal barrier between the main pipe 24 and the outside and thus reduces the heat flow transmitted to the main pipe 24 through the pilot line 14. The configuration of the fuel system 1 of the invention, consisting of housing the pipe main 24 in the pilot line 14, has the advantage of not adding additional system to perform the purge and eliminates the significant constraints necessary to manage the purged fuel. It also makes it possible to cleverly use the fuel flow of the pilot lines 14 to cool the stagnant flow in the main pipes 24, without adding additional equipment (type heat exchanger).

Claims (6)

REVENDICATIONS1. Fuel system (1) for a turbomachine, adapted to inject fuel into a combustion chamber (5) of the turbomachine, comprising: - a pilot circuit (10) adapted to inject fuel into the combustion chamber (5) via a pilot line (14), - a main circuit (20), adapted to inject fuel into the combustion chamber (5) via a main line (24), the system fuel (1) being characterized in that the main pipe (24) is housed at least partially in the pilot line (14). 15 2. Fuel system (1) according to claim 1, wherein the main pipe (24) is coaxial with the pilot line (14). The fuel system (1) according to claim 1, wherein the main pipeline (24) is eccentric with respect to the pilot line (14). 4. fuel system (1) according to one of claims 1 to 3, wherein the main pipe (24) is housed in the pilot line 25 (14) over the entire length of said pilot line (14). 5. fuel system (1) according to one of claims 1 to 4, further comprising at least one stiffener (7), preferably several, extending between the main pipe (24) and the pilot line (14) 30 to maintain a minimum distance between the main line (24) and the pilot line (14). 6. Turbomachine comprising a combustion chamber and a fuel system (1) according to one of claims 1 to 5.5