Fuel metering circuit and method with compensation for fuel-density variability

Abstract

A fuel metering circuit for a turbomachine includes: a meter; a pump; a control valve configured to return an excess flow of fuel delivered to the meter towards the pump on the basis of a fuel pressure differential at the terminals of the meter; a diaphragm; and a volumetric flow meter. The diaphragm and the volumetric flow meter are mounted parallel to the meter, downstream of the control valve, in order to determine a density of the fuel flowing in the metering circuit.

Claims

1. A fuel metering circuit for a turbomachine comprising: a metering device; a pump configured to circulate a fuel flow rate toward the metering device; a regulating valve configured to regulate the fuel flow rate delivered to the metering device according to a difference in fuel pressure at terminals of the metering device; a diaphragm; and a volume flow meter configured to determine a volume flow rate of fuel passing through the diaphragm, wherein the diaphragm and the volume flow meter are mounted in parallel with the metering device in a bypass duct, downstream of the regulating valve, in order to determine a density of the fuel circulating in the metering circuit.

2. The metering circuit according to claim 1, wherein the volume flow meter is mounted upstream or downstream of the diaphragm.

3. The metering circuit according to claim 1, further comprising an electronic card configured to receive information from the volume flow meter on the volume flow rate of the fuel and adjust a metering device monitoring setpoint by taking into account the fuel density thus determined.

4. The metering circuit according to claim 1, wherein the pump comprises a volumetric pump.

5. A turbomachine comprising a fuel metering circuit according to claim 1.

6. An aircraft comprising the turbomachine of claim 5.

7. A fuel metering method implemented in a fuel metering circuit according to claim 1, the method comprising: determining the difference in fuel pressure at the terminals of the metering device; measuring the volume flow rate of the fuel using the volume flow meter; and calculating, from the difference in fuel pressure, from the volume flow rate and from constants related to the diaphragm, the fuel density.

8. The metering method according to claim 7, further comprising a step during which the flow meter transmits information on the volume flow rate of the fuel to an electronic card and the electronic card adjusts a metering device monitoring setpoint by taking into account the fuel density.

9. The metering method according to claim 7, wherein the fuel flow rate is monitored by recirculating a variable fuel flow rate toward the pump by the regulating valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics, aims and advantages of the present invention will become apparent upon reading the following detailed description, and in relation to the appended drawings given by way of non-limiting examples and in which:

(2) FIG. 1, already described, represents the variation in density of several fuels as a function of temperature.

(3) FIG. 2 schematically represents a metering circuit according to one embodiment of the invention.

(4) FIG. 3 is a flowchart illustrating steps of one exemplary embodiment of a metering method according to the invention.

DETAILED DESCRIPTION OF ONE EMBODIMENT

(5) FIG. 2 represents a fuel metering circuit 1 for a turbomachine, including at least one combustion chamber 2 and one fuel tank 3.

(6) The fuel metering circuit 1 includes a volumetric pump 4, a metering device 6 and a metering device supply line called high-pressure supply line, connecting the outlet of the volumetric pump 4 to the inlet of the metering device 6. The metering device 6 is adapted to deliver a target mass flow rate to the combustion chamber 2 from an initial flow rate which is delivered thereto by the volumetric pump 4 via the high-pressure line.

(7) The metering device 6 comprises a surface, called metering device opening surface, of variable size, which allows the flow of the liquid. The flow rate delivered by the metering device 6 is therefore in particular a function of the opening surface.

(8) The metering device 6 opening surface is variable on the driving of a servovalve, which controls the movement of a movable metering part to gradually obstruct a metering orifice or slot. A position sensor allows knowing the position of the movable part. The position sensor is typically an LDVT (linear variable differential transformer) sensor.

(9) There are different types of metering devices 6, for example with a conventional metering slot, described in document U.S. Pat. No. 7,526,911, or with an exponential slot, described in documents EP 1 231 368 and FR 2 825 120. In the case of an exponential slot, the opening surface exponentially increases with the movement of the movable part, which allows better accuracy at low flow rate.

(10) The metering circuit can further comprise a stop valve 10 or HPSOV (High Pressure Shut-off Valve) configured to authorize or block a fuel injection into the combustion chamber.

(11) Optionally, the metering circuit 1 can comprise an electronic card 11 to monitor the metering of the fuel. To do so, the electronic card can, for example, communicate with the metering device 6 in both directions: it can send position setpoints to the metering device 6 and recover data on the metering device.

(12) The electronic card 11 can also be connected to a monitoring unit, external to the device. The monitoring unit is typically an electronic regulation module ECU (engine control unit) of a FADEC (Full Authority Digital Engine Control) that is to say of a full authority digital regulation system which monitors the variable geometries (actuators, metering devices, etc.) of the aircraft. The monitoring unit can be located within the aircraft perimeter and therefore cannot be dedicated solely to the regulation of the fuel. Conversely, the electronic card 11 is preferably exclusively dedicated to the metering of the fuel and to the auxiliary functions. As a variant, it is also possible to have an additional monitoring device, in addition to the main monitoring device that can be in particular exclusively reserved for the metering. The connection between the monitoring unit and the electronic card 11 is generally made with a connection harness.

(13) Only the electronic card 11 of the metering circuit 1 is connected to the aircraft monitoring unit (by means of a single harness), the redistribution then being carried out within the metering circuit 1 by the electronic card 11. The metering circuit 1 therefore comprises a single inlet from the monitoring unit to the electronic card 11, which divides this inlet into several outlets, namely in particular the metering device 6.

(14) The fuel metering circuit 1 further includes a regulating valve 5, adapted to regulate the flow rate delivered to the metering device 6. Particularly, the regulating valve 5 is adapted to return an excess fuel flow rate at the inlet of the volumetric pump 4, as a function of the pressure difference at the terminals of the metering device 6. The regulating valve 5 is also used to maintain the fuel pressure differential ΔP constant between the upstream and the downstream of the metering device 6.

(15) Typically, the regulating valve 5 comprises a movable shutter acting against the action of a loaded spring on a predetermined value of the pressure differential ΔP to be maintained. The shutter is generally perforated so as to discharge fuel on a pipe leading to the recirculation loop, according to its position of equilibrium against the action of the spring.

(16) One example of a regulation valve 5 which can be used here has been described in document FR 1655944, filed on Jun. 27, 2016 by the Applicant.

(17) In order to allow accurate setting for the small openings, the metering circuit 1 further comprises a bypass duct 7 placed in parallel with the metering device 6 and comprising a minimum flow rate diaphragm 8 and a volume flow meter 9.

(18) The diaphragm 8 has a fixed section S.sub.d set during preliminary tests carried out on a bench. Typically, the diaphragm may comprise an orifice of fixed dimension and shape.

(19) At the terminals of the diaphragm 8 is applied a pressure difference which, as seen above, is regulated and defined by the regulating valve 5. This pressure difference ΔP is equal to the pressure difference ΔP at the terminals of the metering device 6, since the diaphragm is mounted in parallel with the metering device 6 in the bypass circuit 7.

(20) The pressure difference ΔP can in particular be measured by a differential sensor.

(21) Furthermore, the head loss due to the passage through the diaphragm 8 is determined by the following formula (B):

(22) $\begin{array}{cc}\Delta P=\frac{1}{2}\rho Q^2\frac{\xi }{S_d^2}& \left(B\right)\end{array}$

(23) Where

(24) ρ is the density of the fuel,

(25) ξ is the head loss coefficient of the diaphragm 8, which is a constant,

(26) Q is the volume flow rate passing through the diaphragm 8 of section S.sub.d.

(27) However, the pressure upstream and downstream of the diaphragm 8 is known and defined by the regulating valve 5. It can also be measured using the differential sensor. The section of the diaphragm 8 is determined beforehand by tests carried out on a bench. The volume flow rate is measured using the volume flow meter 9 which is placed in series with the diaphragm 8 (upstream or downstream of the diaphragm 8, in the bypass duct 7). Finally, the head loss coefficient of the diaphragm 8 is a constant: therefore, the ratio

(28) $\frac{\xi }{S_d^2}$
is also constant.

(29) It is deduced that, within the measurement errors, according to the formula (B), the volume flow rate Q varies exclusively as a function of the fuel density.

(30) The diaphragm 8 and the volume flow meter 9 placed in series in the bypass duct therefore form an in-line density meter that allows improving the overall accuracy of the metering circuit 1.

(31) Where appropriate, when the metering circuit 1 comprises an electronic card 11, the measurements made by the flow meter 9 are communicated to the electronic card 11 so that the latter deduces the fuel density therefrom. The electronic card 11 can then adjust the metering device 6 monitoring setpoint by taking into account the volume density of the fuel.

(32) As a variant, in the absence of electronic card 11 in the metering circuit 1, the measurements made by the flow meter 9 are communicated directly to the metering device 6 control unit.

(33) In order to estimate the metering accuracy obtained thanks to the diaphragm 8 and to the addition of the flow meter 9, it is necessary to take into account the calibration accuracy obtained beforehand during tests carried out on a test bench and the measurement inaccuracies in normal operation.

(34) The accuracy of a volume flow meter 9 is in the order of +/−0.8% of the measurement. Depending on the measured flow rate, this possible deviation takes into account the entire temperature range. However, in the opposite case, it is possible to measure the temperature in the bypass duct 7 comprising the diaphragm 8 and the flow meter 9 and to apply a patch on the read flow rate, the turbine flow meters being sensitive to the viscosity of the fluid.

(35) In addition, during the preliminary tests carried out on a bench, the electronics are calibrated more finely than on-board electronics. The uncertainty for the characterization (usually of +/−0.5% of the measurement) is therefore lower.

(36) In what follows, from a conservative point of view, identical measurement accuracy in calibration and in operation across all temperatures of +/−0.8% of the measurement will be considered.

(37) Likewise, a differential pressure sensor has an accuracy of +/−0.8% of the full scale.

(38) For the balance sheet, a scale of 5 bars, i.e. accuracy in the order of +/−1% for a measurement of 4 bars (conventional value of regulated pressure difference) will be considered.

(39) Considering the following formula, defining the injected volume flow rate:

(40) $Q=A.Math.S.Math.\sqrt{\frac{\Delta P}{\rho }}$

(41) where:

(42) Q is the flow rate measured in L/h using the volume flow meter 9

(43) ρ is the fuel density in kg/L

(44) S is the fuel passage section, linked to the opening of the metering device 6

(45) A is the opening of the metering slot of the metering device 6 in mm

(46) the impact of the pressure and of the volume flow rate on the measurement of the fuel density ρ is determined:

(47) $Q=A\times S\times \sqrt{\frac{\Delta P}{\rho }}$

(48) Namely:

(49) $\rho =A\times S\times \frac{\Delta P}{Q^2}$$\mathrm{and}$$\frac{d\rho }{\rho }=2\times \frac{dQ}{Q}+\frac{d\Delta P}{\Delta P}$

(50) All these measurement errors are random. Thus, the error on the density ε.sub.ρ will be equal to:
ε.sub.p=√{square root over ((ε.sub.ΔP.sub.calibrationε.sub.ΔP)+2×(ε.sub.Q.sub.calibration+ε.sub.Q))}
ε.sub.p=±√{square root over (1+1+2×(0.8+0.8))}=±2.3%

(51) The error on the flow rate corrected by the density

(52) $\frac{dQ}{Q}=0.5\times \frac{d\rho }{\rho }$
obtained through the diaphragm 8 and the volume flow meter 9 will therefore be

(53) $\frac{2.3\%}{2}=\pm 1.15\%.$

(54) The error on the flow rate corrected by the pressure difference will be

(55) $\frac{0.8\%}{2}=\pm 0.4\%.$

(56) It is noted that in the absence of regulation, the density variation causes a variation in flow rate from −6.4% to +6.1% while with the regulation, the flow rate variation will be comprised in a range of about +/−1.6%, particularly when the metering circuit 1 comprises an electronic card 11. In the absence of the electronic card 11, the flow rate variation can be comprised between −3% and +3%.

(57) The fuel metering using such a fuel metering circuit 1 then comprises the following steps: determining S1 a pressure difference at the terminals of the metering device 6, measuring S2 a fuel volume flow rate using the volume flow meter 9, calculating S3, from the pressure difference, from the volume flow rate and from constants related to the diaphragm 8, the fuel density, determining information on the volume flow rate of the fuel and transmitting S4 this information to the electronic card 11 so that the electronic card 11 adjusts a metering device 6 monitoring setpoint by taking into account the fuel density.

(58) It should be noted that the fuel flow rate is monitored S4 by recirculating a variable fuel flow rate toward the pump 4 by means of the regulating valve 5.