GB2178850A - Apparatus and method for determining luminosity of hydrocarbon fuels - Google Patents

Abstract

In apparatus for injecting a controlled stream of hydrocarbon liquid into a concurrently flowing stream of gas in a Venturi forming droplets of the hydrocarbon liquid, the droplets are subsequently combusted and the level of radient emissions is measured photoelectrically, optionally at a series of locations along the direction of flow. Results obtained are useful in predicting service life of combustor walls in jet engines and the like. There is reference to use of photographic, photodiode, photomultiplier and pyroelectric detector means.

Description

SPECIFICATION Apparatus and method for determining luminosity of hydrocarbon fuels This invention relates to methods and apparatus for measuring the luminosity of hydrocarbon fuels, particu- larlyjetfuels used in aviation turbine engines.

The luminosity associated with the combustion of aviation turbine fuels can have a significant impact on the performance and long term durability of jet aircraft engines. Radiative heat transfer to the combustorliner walls increases with increasing luminosity, leading to higherwalltemperatures and ultimately to reduced combustor lifetimes due to loss ofstructural integrity. Flame luminosity is also related to the amountofsoot formed from the fuel, an important consideration for exhaust smoke emissions.

Standardized fuel quality specifications have been developed in an attempt to consistently insure proper combustion characteristics related to flame luminosity and soot formation. These specifications are based on ASTM test methods and include smoke point tests (D 1322) and luminometer numbertest (D 1740). However, because these testing procedures are conducted under conditions fardifferentfrom those in real engine combustors, it has often been difficult to correlate these results with the engine performances. As a result, some engine combustors have been modified and instrumented to measuretheflame luminosity and its effect on the linertemperatures as a function of fuel type.Good results can be obtained from such experiments; there are, however, several drawbacks to this approach: 1. The results are highly sensitive to the particular model of the combustor used because of the uniqueness ofthe design, such as spray, cooling method and physical configuration.

2. These combustortest rigs are complex and are expensive to construct and operate. The large fuel samples required often exceed the volume ofthe experimental fuels available.

A primary object of this invention isto provide an improved method for measuring jetfuel luminosity and predicting combustor liner temperature increases. Another object ofthis invention isto provide an apparatus for this measurement which is relatively simple to construct and easy to operate.

Accordingly, the present invention provides an apparatus for measuring the luminosity of a burning hydrocarbon liquid fuel comprising a venturi positioned at or nearthe upper end ofatransparenttubular member, a capillary tube having an open end positioned nearthethroatoftheventuri, a liquid injectorfor injecting a stream of liquid hydrocarbon from the capillary tube intotheventuri and thence intothetubular member, a gas injectorfor injecting a stream of gas into the space defined between theventuritube andthe capillary tube, thereby forming a series of droplets of hydrocarbon liquid, an ignitorator nearthetop ofthe tubular memberfor burning in the presence of excess oxygen a combustible gas and for igniting the droplets of hydrocarbon liquid and a luminometerfor measuring the luminosity ofthe burning droplets of hydrocarbon fuel as they flow through the tubular member.

In another embodiment, the present invention provides a method of determining luminosity of a hydrocarbon fuel by burning it in the above apparatus.

Figure lisa schematic drawing of a portion of the test apparatus of this invention wherein a stream of liquid hydrocarbon fuel droplets is generated.

Figure 2 is a schematic drawing ofthe overall testapparatus.

Figures 3and 4 are charts showing luminosity profilesforseveral jet fuels as droplets of the fuel traversethe apparatus and are combusted.

Figure 5is a chart showing the peakcombustorlinertemperature equivalent to measured luminosity in terms ofvoltage output by a photodiode.

One arrangement of apparatus which can be utilized in the process of this invention is shown in Figure 2. It consists primarily of a droplet generating device 8 communicating with a combustion duct or chamber 6fitted with a photodiode 10 and oscilloscope to measure the luminosity of the burning drops generated.

Figure lisa schematic drawing ofthe droplet generating portion ofthe apparatus. The device basically consists of a vertical capillarytube 2through which a liquid flows, surrounded byan outervertical concentric tube 12through which a gas (e.g., nitrogen) flows. Astream of small, preferably uniformly-sized and uniformly-spaced droplets is produced by inducing premature detachment of incompletelyformed droplets at the tip ofthe capillary. This detachment is accomplished by drag caused by the annularflow of gas (preferably inert) pastthe capillary tip which is positioned in the throat of a venturi 16.The gas is accelerated past the tip of the capillary by the venturi and then decelerated upon its exit. Dropletsthusformed are much smallerin diameter than those that would result from "natural" detachmentwhen theweight of the dropletovercomes the interfacial tension atthe capillary tip. The droplet size, spacing,frequency and initial velocity are controlled by varying the liquid flowthrough the capillary,theflowofgas pastthe capillary tip and the capillary size.

Precise metering ofthe liquid flow is controlled with an ISCO pump, while a digital mass flow controller supplies a regulated gas flow. Accurate positioning of the capillary tip with respecttotheventuri is possible with an attached micrometer-driven xyztranslation stage 14.

Aschematicdiagram ofthe complete combustion apparatus is shown in Figure 2. A hot combustion environmentforthe burning of liquid droplets is provided bythe post-combustion gases from a lean, premixed (typically CH41O21N2), laminar flat flame supported on an inverted, water-cooled burner 18. The stoichiometry and total flow of the gaseous fuel mixture to the burner is precisely controlled and monitored from an adjacent control panel via needle valves and calibrated flowmeters 20. For cooling purposes, a purge flow of gaseous nitrogen through a shroud surrounding the burner surface is also maintained from the panel.

In the unlikely event of a drop in pressure in any of the gas supply lines ora powerfailure, a combination of pressure switches, relays, and solenoid values serveto immediately shut down the burner and provide a continuous purge of gaseous nitrogen to eliminate any residual, potentially combustible gases from the apparatus and to cool the combustion duct. The internally cooled burner surface and a series ofcheckvalues in the gas lines prevent potential occurrences of flame flashback.

The combustion duct 6 is a transparent, cylindrical quartztube. Its dimensions, for example, can be 70 mm ID x74mm ODx 1 meter long. It is suspended from the burner housing 8via aflange assembly. Combustion gases exit the base ofthe duct into an exhaust collection system (not shown). Temperature profiles ofthe combustion gases within the duct are measured with afinewirethermocouple probe (Pt/6% Rh vs. Pt/30% Rh) corrected for radiation and positioned with a precision XYZtranslation system having 1 m of vertical travel along the axis of the duct.The previously described droplet generator (8) injects a stream of uniformly spaced, mono-sized droplets (approximately 50 to 500 microns initial diameter) down through a hole in the centerof the burnerintothe combustion duct where they spontaneously ignite and burn after a given induction period.

A bi-ocular m icroscope/camera system in tandem with stroboscopic back-illu m ination ofthe droplet stream can be added to facilitate both visual observation and photography of the burning droplets. Quantitative data including droplet size, spacing, and velocity, as well as qualitative information, for example, regarding gas phase sootformation, can be obtained directlyfrom calibrated photographic records. By traversing the length of the combustion duct with the camera system, a detailed record of a droplet's combustion lifetime may be obtained.

For purposes of our invention the transparent cylindrical quartztube is fitted with a photodiode (preferably one with a spectral response of 0.4to 1.1 microns) positioned at multiple locations along the combustion duct.

The photodiode in turn is connected to an oscilloscope. Dual photodiodes are used fordropletvelocity measurements, as shown in Figure 2, as well as for optical triggering of the stroboscopic photography system described earlier.

The unit is started by introducing the fuel gas, air and diluentthroughthe burneratthetop of the apparatus.

The gaseous fuel mixture flows downward and is ignited with a propane torch. The liquid fuel pump is activated to inject the sample through the nozzle at the top of the apparatus to generate droplets. The rate of liquid fuel injection is important in that it affectsthespacing ofthe droplets. The droplets fall down throughthe hot gas, ignite and burn. The photodiode 10 is aimed atthe stream of burning droplets atafixed position.The luminosity of each droplet is detected and displayed on the oscilloscope in a wave form, and the average peak intensity is read and recorded. The photodiode is moved up ordown thetubeto obtain the luminosity profile along the combustion duct. The maximum luminosity is used to characterizethefuel luminosity.

Example In tests with the apparatus, luminosity measurements of individual burning droplets of fuel were made with a photodiode (spectral response, 0.4 to 1.1 microns) positioned at multiple locations along the length of the combustion duct. Intensities were recorded as peak photodiode output voltages as displayed on an oscilloscope. Initial droplet diameters were kept constant at 350 microns, while dropletfrequencies and initial velocities were maintained at 15 droplets per second and 4 meters per second, respectively. The gasflowand thetemperature profile within the combustion duct were also kept constant at valuers corresponding to 3to4 meters/second and 1110 to 2000"K, respectively.These parameters were fixed in order to insure that the only variablefrom runtorunwasfuelcomposition.

The experimental conditions prevalent in the droplet apparatus are also similarto those in an actual gas turbine exceptforthe operating pressure (1 atm vs. 15 atom). For example, droplet size in atypical spray is less than 100 microns, and droplet/gas relative velocities are small. Sprayflametemperatures in the primaryzone are 2500'K,whileturbine inlettemperatures can be 800'wand higher.

TABLE 1 Measured Luminosity Fuel Photodiode Output, Volts JetA 3.8 JetA 4.2 JP4 2.6 JP5 4.1 *USAFJP8 3.0 USAFJP8-AD1 8.3 USAFJP8-AD2 5.8 USAFJP8-AD3 6.6 USAFJP8-AD4 6.8 ExperimentalJP7 + 6vol% 1MN 1.3 * The USAFfuels were supplied by the Air Force Aero-Propulsion Laboratory and correspond to fuels #2, 6 and 7, respectively, which are described in report #AFAPL-TR-79-2105.

The luminosity data for various fuels were quite reproducible. These data were comparable with thosefrom combustor rigs in quality and have been correlated well with the liner temperature increases.

Atypical luminosity profile obtained underthese conditions is shown in Figure 3, where a commercial JetA fuel was tested. Figure 4 shows luminosity profilesfora series of aromatics-doped JP7 fuels, where 1-MN stands for 1-methylnaphthalene. Peak luminosityvaluesforseveral fuels and fuel blends are shown in Table 1.

Finally, Figure 5 shows peak luminosity data measured in our apparatus plotted versus peaklinertemperature increases reported in USAF report *AFAPL-TR-201 5 fora J79 combustorforseveral fuels.

From this data it is possible, having once determined the luminosity profile ofafuel to predict the liner temperature resulting from combustion ofthe fuel and the corresponding life expectancy of the liner. The combustor linertemperature increase predicted from this data is given by At cruise: T peak, "C = (luminositypeak + 12.2)/.0476 At take off: T peak, "C = (luminositypeak + 26.6)/.0709 while the subsequent estimated combustor liner lifetime is given by Relative Lifetime = 1 0(.283-.o811 luminosity peak) wherethe lifetime is defined relativeto that corresponding to operation of the combustor attake-off conditions on a reference USAF JP4 Fuel (T peak = 425"C, Relative lifetime = 1.0).

The apparatus and method described herein provides several advantages. Only small sample sizes are required. Direct and complete optical access to the combusting droplets is provided. Simple, rapid and reproducible measurements are possible. The effects of spray interaction, aerodynamics and physical configuration ofthe apparatus are eliminated or minimized. Operating temperatures ofthe apparatus are similar to those in actual engines. The droplets generated and the relativeflowfields are commensurate with those in the real engine or engines being used.

Claims (15)

1. An apparatus for measuring the luminosity ofa burning hydrocarbon liquid fuel comprising: (a) a venturi positioned ator nearthe upper end ofa transparenttubularmember; (b) a capillary tube having an open end positioned nearthethroat oftheventuri; (c) a liquid injectorfor injecting a stream of liquid hydrocarbon from the capillary tube into the venturi and thence into the tubular member; (d) a gas injectorfor injecting a stream of gas into the space defined between theventuritube andthe capillary tube, thereby forming a series of droplets of hydrocarbon liquid; (e) an ignitorator nearthetop ofthetubularmemberfor burning in the presence of excess oxygen a combustible gas and for igniting the droplets of hydrocarbon liquid; and (f) a luminometerfor measuring the luminosityofthe burning droplets ofhydrocarbonfuels astheyflow throughthetubularmember.

2. The apparatus of claim 1 wherein the luminometeris photographic.

3. The apparatus of claim 1 wherein the luminometeris a photodiode electronically connected with an oscilloscope.

4. The apparatus of claim 1 wherein the luminometeris a photomultiplier electronically connected with an oscilloscopeand/orsignal averager.

5. The apparatus of claim 1 wherein the luminometeris a pyroelectric detector electronically connected with an oscilloscope and/or signal averager.

6. A method for determining the quality of a liquid hydrocarbon fuel by measuring its luminosity comprising: (a) introducing a stream of linearly spaced droplets ofthe hydrocarbon fuel into a combustion zone; (b) combusting the stream of droplets in the combustion zone; and (c) measuring the intensity of the radiation emitted by the stream of droplets during the combustion.

7. The method of claim 6 wherein the spacing between the linearly spaced droplets is uniform.

8. The method of claim 6 wherein the linearly spaced droplets are of a uniform size.

9. The method of claim 6 wherein the combustion zone comprises a flowing stream of heated gas and airor oxygen.

10. The method of claim 6wherein the combustion zone also contains a diluentgas.

11. The method of claim 6whereinthetemperature in the combustion zone is 1100 and 2000=K.

12. The method of claim 6 wherein the combustion zone contains a hot combustion environment provided by the post-combustion gases fro combusting a lean premixed mixture of methane, oxygen and nitrogen.

13. The method of claim 6wherein the intensity ofthe radiation from the combusting stream of droplets is measured at plurality of locations in the stream.

14. The method of claim 6wherein the diameter ofthe individual droplets is 50to 500 microns.

15. The method of any of claims 6to 14wherein the measured intensity of (c) is utilized to predict liner life of a jet engine.