GB2210446A - Fuel distribution system - Google Patents

Description

1 nú.21044U" FUEL DISTRIBUTION SYSTEM The United States Government has

rights in this invention in accordance with contract number DAAE07-84-C-RO83 awarded by the Department of the Army.

Background of the Invention

The present invention relates to fuel distribution systems for gas turbine engine combustors.

Fuel distribution systems for gas turbine engines are called upon to distribute fuel from a common distributor or manifold uniformly to a plurality of fuel injectors or nozzles positioned at various locations in the engine combustor. This uniform fuel distribution must be effected over a wide range of engine operating conditions from light-off to maximum power, which represents a considerable variation in fuel flow rates.

In a typical engine configuration, the fuel injectors are arrayed in a: vertical plane circumferentially about the periphery of an annular 1 3LN-1 758 -2- combustor. Consequently, the injectors are positioned at relatively different heights and thus have correspondingly different fluid heads associated with their positions. The fuel distribution system design therefore must also take into account.the maximum fluid head existing between the uppermost and lowermost positioned injectors if uniform fuel distribution to all injectors is to be achieved.

Each injector fuel line should thus incorporate sufficient pressure loss to overcome this maximum fluid head differential to ensure that the lowermost injector does not receive more fuel than the uppermost injector. This problem is most pronounced at low fuel rates since this maximum fluid head differential then becomes significant relative to the fuel flow driving pressure developed by the fuel pump. Thus, it is necessary to design the fuel distribution system such that the pressure losses in the injector fuel line are sufficiently high at low fuel flow rates to handle this differential fluid head consideration, and yet are not so great at high fuel flow rates as to require unduly high fuel pressurization. It is also important that the pressure loss in each of the plural injector fuel lines be uniform over the entire engine operating range to avoid fuel maldistribution and as low as possible for maximum fuel distribution efficiency.

Heretofore these design considerations have been meet through the utilization of rather complicated and relatively expensive flow dividing and fuel metering 1 3LN-1 758 -3- valves to achieve uniform fuel distribution throughout the engine operating range. Since one of these valves is incorporated in each injector fuel line and a typical gas turbine engine will utilize a plurality of fuel injectors, e.g., twelve or more, these valves represent a significant expense item particularly in the case of small gas turbine engines, i.e., less than 3000 horsepower. These valves, which may be of the mechanical or fluidic type, typically operate automatically in response to fuel pressure to impose the requisite variable impedances to fuel flow, i.e., pressure losses, in the injector fuel lines calculated to achieve uniform fuel distribution throughout the engine operating range.

Furthermore, the reliability of the flow dividing and fuel metering operations of these valves may be affected by any contaminants in the fuel. Thus, these valves typically require periodic servicing and in some instances replacement.

Summary of the Invention

It has been discovered that if a predetermined laminar fuel flow condition is established in each of the plural injector fuel lines of a fuel distribution system for a gas turbine engine combustor, an appropriate flow impedance or pressure loss versus fuel flow rate relationship can be achieved to ensure uniform fuel distribution not only at high fuel flow rates, but also, and more significantly, at low fuel flow rates without resort to flow dividing and fuel metering valves. Thus in accordance with one embodiment of the present invention, there is provided a fuel distribution system 1 3LN- 1758 -4- including a distributor or manifold into which pressurized fuel is introduced. In separate fluid communication with the distributor are a plurality of fuel lines, each leading to a different one of a plurality of fuel injectors or nozzles arrayed in a vertical plane about the periphery of the gas turbine combustor. Into each of these fuel lines is incorporated a laminar fuel flow establishing element, which in the preferred embodiment of the invention is simply a tube of predetermined diameter calculated to establish and sustain laminar flow of the fuel conveyed therethrough over a range of flow rates from light- off to at least approaching maximum power. Then by selecting appropriate lengths for these laminar flow tubes, the desired pressure loss versus flow rate characteristic may be established in each fuel line to ensure uniform fuel distribution over the entire operating range. Consequently, these laminar flow elements or tubes effectively serve the flow dividing and metering functions heretofore performed by expensive mechanical and fluidic valves.

The preferred embodiment provides a fuel distribution system which avoids the need for a flow dividing and metering valve in each of the injector fuel lines.

An embodiment of the present invention may provide a fuel distribution system wherein uniform fuel distribution is achieved over the entire range of engine operating conditions while minimizing the energy or pressure loss in the individual injector fuel lines.

An embodiment may provide a fuel distribution system wherein pressure loss in the individual injector fuel lines is equalized over the range of engine operating conditions from light-off to maximum power.

An embodiment may provide a fuel distribution system which is relatively immune to being plugged by contaminants in the fuel.

An embodiment may provide a fuel distribution system which is inexpensive to implement, efficient in operation and reliable over a long service life.

An embodiment of the present invention, given by way of example, will now be described with reference to the accompanying drawings, in which:

1 3LN-1 758 -6- FIGURE 1 is a schematic diagram of a gas turbine combustor fuel distribution system constructed in accordance with one embodiment of the present invention; and 5 FIGURE 2 is a schematic diagram of a fuel distribution system constructed in accordance with another embodiment of the invention.

Detailed Description of the Invention

Referring to the fuel distribution system embodiment of the invention seen in FIGURE 1, fuel pressurized by a suitable pump (not shown) is introduced, as indicated diagrammatically at 10, into a manifold or distributor block 12 for parallel distribution via a plurality of fuel lines, generally indicated at 14, to a corresponding plurality of conventional fuel injectors or nozzles 16 arrayed in an essentially vertical plane about the periphery of an annular combustor 18 in a gas turbine engine. It will be appreciated that, in actuality, the number of fuel lines and injectors is considerably greater than the four illustrated in FIGURE 1. Each fuel line includes a hose 20 and a tube 22 connected end-to-end in fluid dommunicating relation. Tubes 22 are preferably all of uniform length and inner diameter', and thus hoses 20, which may bei conventional aircraft hose, are necessarily of varying lengths to make up the varying distances from the various injectors 16 to distributor block 12.

13LN-1 758 -7- Embodying the nresent invention, the diameter of tubes 22 is selected such as to establish laminar flow of the fuel being conveyed through each of the fuel lines 14. As is well known in the fluid mechanics art, fluid flow through a conduit of circular cross section, for example, is either laminar or turbulent depending on the ratio of the inertia to viscous forces acting in the fluid. This ratio, which reduces to a dimensionless number traditionally known as the Reynolds number (R), is expressed as follows:

p dV p where p is the fluid density, d is the conduit inner diameter, V is the mean axial fluid velocity, andiiis the fluid viscosity. For Reynolds numbers below approximately 2300, fluid flow is laminar, whereas above approximately 2300, fluid flow is turbulent.

Thus, by taking into account the density and viscosity of the fuel, including the variations in these quantities due to temperature, and the desired maximum fuel flow rate, the inner diameter of tubes 22 is selected such that the Reynolds number does not exceed 2300 over a range of engine operating conditions at least approaching maximum power. These laminar flow tubes should have a reasonably smooth bore to ensure laminar flow and may be in the form of drawn tubing of a suitable metal such as stainless steel or INCO 625 or of a suitable plastic such as Teflon. The inner diameter of 13LN-1 758 the hoses 20 is selected to be of a sufficiently large dimension, e.g., four or more times larger than the diameter of the laminar flow tubes 22, to impose negligible pressure loss on fuel flow therethrough even at maximum fuel flow rates.

According to the Hagen-Poiseuille formula, pressure loss (AP) in a laminar flow tube is expressed as follows:

AP 128 jp LQ 7rd 4 where pis the fluid viscosity, L is the tube length, Q is the fluid flow rate, and d is the tube inner diameter.

From this formula it is seen that pressure loss is directly proportional to tube length and to flow rate. Thus, having selected the requisite tube inner diameter for laminar flow, it remains to select the preferably uniform length for laminar flow tubes 22 calculated to establish an equivalent relationship between pressure loss and fuel flow rate in each fuel line 14 for performing the flow dividing and fuel meteri ng functions requisite to achieving uniform fuel distribution to the various injectors 16 over the entire range from light-off to maximum power.

An additional benefit gained from using laminar flow tubes 22 is that, as noted above, pressure loss under laminar flow conditions is directly proportional to flow rate to the first power whereas, as is well known in the fluid mechanics art, under turbulent flow conditions, pressure loss is directly proportional to flow rate to 13LN-1 758 -9- the second power, i.e., squared. From this it can be seen that, under laminar flow conditions, large variations in fuel flow can be accommodated with smaller variations in pressure loss. That is, by virtue of the linear relationship between pressure loss and flow rate under laminar flow conditions, a given increase in pressure loss will: support a directly proportionate increase in fuel flow. Consequently, less burden is imposed on the fuel pump to increase power (fuel flow rate), which becomes a significant benefit as gas turbine engines age. Moveover, this feature has the potential of enabling the maximum power of an engine to be increased to a limited extent without having to replace an existing fuel pump.

It is appreciated that the fluid head of each fuel injector 16 is the static fluid pressure of a fluid column of a height equal to the elevation of each injector relative to common reference. Thus the maximum fluid head differential that must be overcome to avoid fuel maldistribution is the difference in elevation between the uppermost and lowermost fuel injectors.

In tailoring a fuel distribution system to a particular gas turbine engine there are two principal design points on the requisite pressure loss versus.fuel flow rate operating curve that must be satisfied. One is a high fuel flow rate design point which corresponds to an upper limit of pressure loss that the fuel pump must be capable of overcoming. The other is a low fuel flow rate design point corresponding to a minimum pressure loss which is nevertheless sufficient to avoid fuel 13LN-1 758 -10maldistribution at low power engine operating conditions, e.g., light-off and idle. By virtue of the laminar fuel flow conditions imposed in each fuel line by laminar flow tubes 22, the operating curve can be readily made to intersect these high and low fuel flow rate design points since it is a straight line (pressure loss being proportional to flow rate to the first power). Absent these laminar flow tubes, the turbulent fuel flow conditions imposed by the fuel injectors and any trim orifices incorporated in the fuel lines would result in an exponential operating curve since, with turbulent flow, pressure loss is proportional to fuel flow rate to the second power. If this exponential operating curve is made to satisfy the high fuel flow.rate design point, the pressure losses produced by all fuel flow rates below this design point are consistently less than the pressure losses produced by correspondingly lower fuel flow rates under laminar flow conditions (flow rate proportional to square root of pressure loss for turbulent flow versus flow rate proportional to pressure loss for laminar flow).

It is thus seen that at low fuel flow rates there is insufficient pressure loss under turbulent fuel flow conditions to overcome the maximum fluid head differential between the uppermost and lowermost fuel injectors, and consequently fuel flow dividing and metering values are required to avoid fuel maldistribution therebetween by controllably imposing the requisite increased pressure loss. In contrast, the laminar flow tubes 22 provide the requisite pressure loss in and of themselves, and thus their incorporation 1 3LN-1 758 in each fuel line is an eminently practical solution to this problem of fuel maldistribution at low fuel flow rates.

For certain engine applicat.ions it may become necessary to resort to some tailoring of the pressure loss versus fuel flow rate relationship in order to achieve uniform fuel distribution to the various injectors while adhering to a predetermined fuel pressure to output power schedule. To this end, it is contemplated that each fuel line 14 may incorporate at least one trim orifice which is illustrated at 24 in FIGURE 1 as being positioned at or immediately adjacent to the entry into each fuel line 14 from distributor block 12. If an additional trim orifice is called for, it may be incorporated in each laminar flow tube 22, as indicated in one instance at 26. It will be appreciated that the presence of trim orifice 26 will create a localized turbulent flow condition, however laminar flow is restored several tube diameters distance downstream therefrom. These illustrated locations of the trim orifices in the fuel lines are illustrative only, as their positions are not critical. Their only significance is the contribution of their pressure loss characteristics to that of the laminar flow tubes in tailoring a fuel flow curve to meet a particular application.

FIGURE 2 illustrates another embodiment of the invention wherein each fuel line leading from distributor block 12 to the individual injectors 16 is constituted entirely by a laminar flow tube 28. For the reasons 1 3LN-1 758 -12- indicated above, each tube is preferably of the same length. The prescribed tube length is determined by the distance between the distributor block and the most remotely positioned injector plus any additional length necessary to establish the desired pressure loss characteristic. The illustrated dash-line portions of the laminar flow tubes leading to less remote injectors are intended to indicate circuitous routings thereof in order to make up this prescribed length. Obviously, these circuitous routings should be accomplished with rather gentle bends so as not to significantly disturb the laminar flow of fuel therethrough.

In addition to the benefits and is advantages. noted above, it is found that the inner diameter of the laminar flow tubes can be quite large, e.g. 0.04 inches, as compared to any trim orifices included in the fuel lines having inner diameters not smaller than.021 inches, and consequently these tubes are virtually immune to being plugged by any contaminants in the fuel. For this reason, it is advantageous to avoid using trim orifices if at all possible.

While the invention has been disclosed in the context of the laminar flow element incorporated in each fuel line being in the form of a tube, it should be understood that this element may take other forms and still provide the desired pressure loss versus flow rate relationship. One such alternative form is a porous media such as a sintered metal block. However, porous media suffers from the obvious drawback of being readily plugged by contaminants in the fuel.

tl It is seen that the illustrated embodiment provides a fuel distribution system which is eminently simple in construction and reliable in its operation of uniformly distributing fuel to a plurality of fuel injectors over a wide range of fuel flow rates in an efficient and practical manner. As will be apparent to those skilled in the art, various changes may be made in the disclosed embodiments without departing from the scope of the invention.

Claims (9)

CLAIMS:

1. A fuel distribution system for a gas turbine engine combustor, said system comprising, in.

combination:

distributor for receiving pressurized fuel; plurality of fuel injectors; corresponding plurality of fuel lines, each said fuel line connecting a different one of said fuel injectors in parallel fluid communicating relation with said distributor; and means incorporated in each said fuel line for establishing laminar flow of fuel therethrough, said means introducing pressure losses in said fuel lines sufficient to ensure uniform fuel flow distribution through said fuel lines.

2. The fuel distribution system defined in Claim 1, which further includes at least one trim orifice incorporated in each said fuel line.

3. The fuel distribution system defined in Claim 1 or Claim 2, wherein at least some of said injectors are stationed at relatively different fluid head positions, and wherein each said laminar flow establishing means is in the form of a laminar fuel flow establishing element, said elements introducing sufficient pressure losses in said fuel lines to overcome the differences in fluid heads of said injectors at low fuel flow rates.

4. The fuel distribution system defined in Claim 3, wherein said laminar flow elements have equivalent pressure loss versus fuel flow rate characteristics.

13LN-1758

5. The fuel distribution system defined in Claim 3 or Claim 4, wherein each said laminar flow element is in the form of a laminar flow tube of circular cross section. 5

6. The fuel distribution system defined in Claim 5, wherein each said fuel line includes one of said laminar flow tubes and a hose connected in end-to-end fluid-coupled relation, said hoses being of a larger 10 inner diameter than said laminar flow tubes.

7. The fuel distribution system defined in Claim 5, wherein each said fuel line is constituted entirely by one of said laminar flow tubes. 15

8. The fuel distribution system defined in any one of Claims 5 to 7, wherein said laminar flow tubes are all of equal length and inner diameter.

9. A fuel distribution system substantially as herein described with reference to the accompanying drawings.

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