WO1997025609A1

WO1997025609A1 - Optical probe for in-situ detection of hydrocarbon concentration

Info

Publication number: WO1997025609A1
Application number: PCT/US1997/000389
Authority: WO
Inventors: Robert W. Dibble; Rajiv K. Mongia; Quang-Viet Nguyen
Original assignee: University of California Berkeley; University of California San Diego UCSD
Current assignee: University of California Berkeley; University of California San Diego UCSD
Priority date: 1996-01-12
Filing date: 1997-01-09
Publication date: 1997-07-17
Anticipated expiration: 1998-07-12
Also published as: AU1825797A

Abstract

A probe (22) with two light pipes (46, 48) and two prisms (52, 54) affixed to the ends of the light pipes (46, 48). A light beam having a wavelength in the infrared range is combined with a light beam having a wavelength in the visible range to form a combined beam (36). The detection apparatus directs the combined beam (36) through a probe volume (24) between the faces of the prisms (52, 54) to detect the concentration of the hydrocarbon vapor.

Description

OPTICAL PROBE FOR IN-SITU DETECTION OF HYDROCARBON CONCENTRATION

Background of the Invention

The present invention relates to the detection of the hydrocarbon vapors, and, more particularly, to optical probes to detect the absorption of infrared radiation and measure the concentration of hydrocarbon vapors in a combustion environment.

Many combustion devices inject a hydrocarbon fuel into an oxygen-containing atmosphere to form a mixture of air and fuel . The hydrocarbon fuel is traditionally stored in a liquid state, and may be evaporated to form a gaseous fuel vapor or sprayed into the oxygen-containing atmosphere as a fine mist of liquid fuel droplets. The mixture of air and fuel (whether droplets or vapor) is then ignited.

Knowledge of the fuel vapor distribution, i.e., the concentration, as a function of time, of gaseous hydrocarbon fuel at various locations in the engine, is essential to model the fuel efficiency, performance, and pollutant emissions of an engine. The mixing of the fuel with the oxygen-containing atmosphere is a turbulent process, and the concentration of any fuel vapor may be spatially and temporally non-uniform. Therefore, there is a need for a detector which can measure the fluctuating concentration of hydrocarbon vapors in an engine environment . One method of measuring the fuel vapor concentration is to insert a hypodermic or hollow needle into a portion of the engine, e.g., a fuel line, and suction off a sample of the gas. The gas sample is fed into a gas chromatograph, mass spectrometer, or some similar instrument, and the hydrocarbon concentration is measured. The needle may be manipulated, e.g., pushed or pulled through the fuel line, in order to measure the fuel concentration at different locations. It takes about one second to draw a sufficient amount of gas through the needle to form a sample. Because most engines fire more than twenty times per second, the single sample represents the average concentration, over at least twenty cycles, of fuel at the tip of the needle. Another method of measuring the concentration of fuel vapor is to detect the absorption of light by the hydrocarbon gas. Most hydrocarbon molecules that can be used as fuel, such as methane, ethane, propane, n- pentane, n-hexane, n-heptane, n-octane, methanol, ethanol, butanol, acetone, and iso-octane, have absorption lines in the infrared range. The peak and width of an absorption line depends upon the type of hydrocarbon molecule, but most hydrocarbon molecules have a strong absorption line around 3.39 microns. This absorption line is due to the vibrational transition (v₃P7 for methane) of the carbon-hydrogen bond in the hydrocarbon molecule.

Coincidentally, a helium-neon laser emits light at a wavelength almost exactly equal to the peak of the hydrocarbon absorption line, i.e., at about 3.39 microns. For example, 2947.909 cm^"1 is a working value for the wavenumber (the inverse of the wavelength) of the peak of the helium-neon laser emission, whereas 2947.912 cm ¹ is a working value for the wavenumber of the peak of the methane absorption. The emission line for the helium- neon laser is very narrow compared to the absorption line of the hydrocarbon molecules; the broad absorption line overlaps the narrow emission line. Thus, the concentration of hydrocarbon gas can be measured by firing a helium-neon laser through the mixture of air and fuel and detecting the absorption of laser light. Unfortunately, such measuring systems are expensive and generally unable to measure hydrocarbon vapor concentration in situ, i.e., inside a combustor such as a furnace, incinerator or engine. In view of the foregoing, there is a need for an device which can measure the concentration of hydrocarbon vapors in a combustion environment. Such a device should detect the absorption by hydrocarbon molecules of light having a wavelength around 3.39 microns. In addition, such a device should have a probe which measures the hydrocarbon concentration in situ. Furthermore, such a device should have a high spatial resolution, e.g., a few millimeters, and a high temporal resolution, e.g., less than fifty milliseconds, and be cost-effective and reliable.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized by means of the instrumentalities and combinations particularly pointed out in the claims.

Summary of the Invention An apparatus for detecting the concentration of hydrocarbon vapor uses a first light source to generate a first light beam having a wavelength of in the infrared range. A second light source generates a second light beam having a wavelength in the visible range. The first and second light beams are combined into a third light beam by a beam combiner. A beam splitter splits the third light beam into fourth and fifth light beams. An infrared light detector is positioned in the path of the fourth light beam, and a visible light detector is positioned in the path of the fifth light beam. A probe, which includes a two substantially parallel and adjacent light pipes, and two prisms attached to the ends of the light pipes, is positioned in the optical path of the third light beam. The two prisms have substantially parallel opposing faces which are separated by a gap. A first optical fiber couples one light pipe to the first beam splitter, and a second optical fiber couples another light pipe to the second beam splitter.

There may also be an optical element to focus the third light beam into the first optical fiber. Each light pipe may comprise a sapphire rod, and each prism may comprise a polished end of the sapphire rod. Each light pipe may comprise a metal tube with one of the optical fibers inserted into the tube, and each prism may comprise a sapphire prism attached to an end of the metal tube.

Brief Description of the Drawing(s) The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. FIG. 1 is a schematic cross-sectional view of an optical probe inserted into an inlet of a combustion device.

FIG. 2 is a schematic graph of the emission spectrum of a laser and the absorption spectrum of a hydrocarbon molecules a function of the wavenumber.

FIG. 3 is a schematic diagram of a hydrocarbon detector according to the present invention. FIG. 4 is a schematic perspective view of the detector of FIG. 3 in which the probe includes optical fibers inserted into metal tubes.

FIG. 5 is a schematic perspective view of the detector of FIG. 3 in which the probe includes sapphire rods.

Description of the Preferred Embodiment (s) Referring to FIG. 1, a detector 20 is provided to measure the concentration of hydrocarbon vapor in a fuel injection system 10 for a combustion device. In injection system 10, an oxygen-containing atmosphere, e.g., air, flows from a port 12 through a line 14 and into a combustion chamber 18 having an exhaust line 19. Some means for distributing liquid or gaseous fuel projects into line 14. For example, a pipe 16 may project across the diameter of the line, and fuel may flow out of a plurality of holes in the inward facing side of the pipe. If the fuel is in a liquid state, then it may be sprayed as a mist of droplets into line 14. In addition, liquid fuel will begin evaporating as it flows through the line. The air from port 12 and fuel from pipe 16 mix turbulently as they are drawn through line 14 toward combustion chamber 18. Due to this turbulent mixture, the concentration of hydrocarbon vapor in line 14 will vary.

In order to measure the concentration of the hydrocarbon vapor, a probe 22 of detector 20 is inserted into line 14 so that the air and fuel mixture pass through a probe volume 24 located at the tip of the probe. The detector 20 may measure the concentration of hydrocarbon vapor in probe volume 24 in real time, with a sampling rate of at least one hundred hertz, and more preferably about five to ten kilohertz. In addition, the probe can be moved across the diameter of the line in order to measure the spatial distribution of the hydrocarbon vapor.

The detector 20 operates by measuring the absorption of infrared light, preferably at a wavelength of approximately 3.39 microns. As discussed above, a helium-neon laser emits light, also at a wavelength of approximately 3.39 microns. Referring to FIG. 2, the graph shows the absorption or emission constant, in units of k, the Boltzmann constant, on the y-axis, and the wavenumber (the inverse of the wavelength) on the y-axis. Specifically, the absorption of light by methane is shown by the curve A. Absorption spectrum A shows the absorption of methane with a peak at 2947.912 cm¹. The relative emission of light by a helium neon laser as a function of wavenumber is shown by the curve E. The emission spectrum E has a peak at about 2947.909 cm¹, but is much narrower than the absorption line A. Other hydrocarbon molecules, such as ethane, propane, n- pentane, n-hexane, n-heptane, n-octane, methanol, ethanol, butanol , acetone, and iso-octane, have similar absorption spectra with maxima which overlap the helium neon laser emission line. Oxygen, nitrogen and water do not appreciably absorb light at a wavelength of 3.39 microns; in most combustion devices the only absorptive gas will be the hydrocarbon vapor.

The intensity I of transmitted light through an atmosphere containing absorptive hydrocarbon vapor is given by the Beer-Lambert law:

I/I₀ = e^!-^{α'L Pl} (1) where I₀ is the original intensity of the light, a is the absorption coefficient of the hydrocarbon molecule which is being measured, L is the path length of the light through the hydrocarbon-containing atmosphere, and P is the partial pressure of the hydrocarbon gas. At a wavelength of 3.39 microns, the absorption coefficient of methane is about 10-20 atm^"1 cm¹. Therefore, if the partial pressure of methane is 0.5 atm and the path length is on the order of two millimeters, there will be a 22% reduction from the original intensity I₀. Because a 22% change in light intensity is easily detected by commercially available optical detectors, the typical concentration levels of the hydrocarbon gas in an engine can be measured with a spatial resolution of about two millimeters . Referring to FIG. 3, a detector 20 includes a probe 22 for optically detecting the concentration of hydrocarbon vapor in a probe volume 24. Probe 22 is optically coupled to an infrared light source 26. The infrared light source 26, which may be a helium-neon (HeNe) laser, generates a light beam 28 having a wavelength in the infrared range, i.e., above 0.7 microns, and preferably a wavelength of 3.39 microns. In a breadboard test, a commercially available HeNe laser was utilized for infrared light source 26, but a diode laser or a light emitting diode (LED) having an emission line at 3.39 microns could be used instead.

Probe 22 is also optically coupled to a visible light source 30, which may be a commercially available gas or diode laser. Visible light source 30 generates a light beam 32 having a wavelength in the visible range, i.e., 0.3 to 0.7 microns, preferably at a wavelength of 0.633 microns. Infrared light source 26 and visible light source 30 are carefully aligned so that a beam combiner 34, e.g., an ordinary partial reflector, may combine light beams 28 and 32 into a single multi- wavelength light beam 36. An LED could be used instead of a laser as visible light source 30. In addition, a light source could be designed to emit light in both the infrared and visible ranges. If the light source emits light both the infrared and visible ranges, beam combiner 34 is unnecessary.

Two relatively flexible optical fibers 42 and 44 are connected to probe 22. Optical fiber 42 guides multi-wavelength light beam 36 from beam combiner 34 to probe 22, whereas optical fiber 44 guides the light beam from the probe to a set of detectors. Optical fibers 42 and 44 may be composed of fluoride glass, zinc selenide, or sapphire. Multi-wavelength light beam 36 is focused by reflection off an optical element 40, such as a parabolic mirror, into optical fiber 42. In another configuration, light beam 36 could be focused by transmission through an optical element, such as a lens. The optical element is preferably a mirror because its optical characteristics are independent of the wavelength of the incident light.

Probe 22 includes two substantially parallel, and preferably relatively inflexible, light pipes 46 and 48. The optical fibers 42 and 44 may be coupled to light pipes 46 and 48, respectively, by an optical coupler 50. Each light pipe may be about two inches long. Two prisms 52 and 54 are positioned at the ends of light pipes 46 and 48, respectively. The prisms may be composed of sapphire, and each prism has an angled outer face 56 and a flat inner face 58. The light beam 36 is projected from optical fiber 42 into the base of probe 22. At the tip of probe 22, the multi-wavelength light beam is bent by a prism through a ninety degree angle to travel along a path perpendicular to the length of the light pipe. For example, the light beam may be bent by internally reflecting off angled outer face 56 of prism 52. The light beam exits prism 52 through inner face 58, traverses probe volume or gap 24, and enters prism 54 through flat inner face 58. The light beam 36 is again reflected through a ninety degree angle, this time by angled outer face 56 of prism 52, to pass through light pipe 48.

The probe volume 24 comprises the space between the opposing and substantially parallel inner faces of the prisms. The probe volume has a gap of width w_:.

Because light beam 36 traverses the probe volume 24, the path length L through the hydrocarbon vapor (used in Equation 1 above) is equal to w_x. The width w_x of probe volume 24 is selected to be sufficiently large that a change in concentration results in a measurable change in light intensity, but sufficiently small that detector 20 has a high spatial resolution. Width _x may be about one to two millimeters.

Referring to FIG. 4, in one configuration of probe 22, each light pipe comprises a portion of an optical fiber which has been inserted into a metal tube 60. The metal tube 60 has a thickness of about one-half millimeter and an inner diameter which substantially matches the diameter of the optical fiber to ensure a tight fit of the fiber inside the tube. Light pipes 46 and 48 are separated by a spacer body 62 and are held rigidly in position so that the gap width w_x does not change. Light pipes 46 and 48 may be attached to spacer body 62 by a glue or an adhesive tape, or some other mechanical means may be provided to hold the light pipes stationary relative to each other. The width of spacer body 62 may be adjusted by a screw 63 to vary the width w₁ of the probe volume. Sapphire prisms 52 and 54 are affixed to the ends of metal tubes 60 by, for example, a commercially available transparent glue. The optical fibers are inserted into the tubes so that the ends of the fibers abut the prisms.

Referring to FIG. 5, in another configuration of probe 22, each light pipe comprises a sapphire rod. The optical fibers 42 and 44 are coupled to light pipes 46 and 48 by an optical coupler 50. The light pipes may be held stationary relative to each other by the optical coupler or by some other mechanism. Prisms 52' and 54' are formed by polishing or cutting angled outer faces 56 into the sapphire rods. Although shown with a rectangular cross-section, the sapphire rods could also have a circular cross-section.

Returning to FIG. 3, optical fiber 44 guides light beam 36 from probe 22 to a beam splitter 64, such as an ordinary partial reflector. Beam splitter 64 splits the light beam into two separate light beams. One light beam is directed through a filter 70 which filters out non- infrared wavelengths to generate an infrared light beam 66. The other light beam is directed through a filter 72 which filters out non-visible wavelengths to generate a visible light beam 68. An infrared light detector 74 is positioned in the path of infrared light beam 66, and a visible light detector 76 is positioned in the path visible light beam 68. Light detectors 74 and 76 may be indium-tin (InSb) photodetectors. In a breadboard test, the detectors 74 and 76 were commercially available liquid-nitrogen cooled InSb photodetectors. However, thermo-electrically cooled InSb detectors, or pyroelectric detectors could also be used. As light beam 36 passes through probe volume 24, the infrared light, but not the visible light, will be absorbed by the hydrocarbon vapor. The higher the concentration of hydrocarbon vapor in the probe volume, the more light will be absorbed. By comparing the intensity I measured by infrared light detector 74 to the original intensity I₀, the partial pressure P of the hydrocarbon gas in probe volume 24 may be calculated from the equation:

-αw_α-P = log(I/I₀) (2) If the hydrocarbon fuel is injected into line 14 (see FIG. 1) in a liquid state, then the air and fuel mixture passing through probe volume 24 may contain liquid hydrocarbon droplets. As light beam 36 passes through the probe volume 24, both the infrared light and the visible light will be scattered by the liquid hydrocarbon droplets, resulting in a reduction in the light intensity measured by both infrared light detector 74 and visible light detector 76. The detector 20 may isolate the reduction in light intensity due to the hydrocarbon vapor in order to accurately measure the hydrocarbon vapor pressure. A correction factor can be determined by comparing fluctuations in the intensity of the visible light to fluctuations in the intensity of the infrared light. If the intensity of the infrared light drops but the intensity of the visible light remains stable, then the fluctuation is due to absorption by the hydrocarbon vapor. On the other hand, if the intensities of the infrared light and visible light drop by the same amount, then the fluctuation is due to a hydrocarbon droplet, or perhaps some other particle. The partial pressure may be calculated from the equation:

where I' is the current intensity measured by the visible light detector and I'₀ is the original intensity, and R is an empirically determined constant.

If probe 22 is inserted into an environment containing a flame or heated material, e.g., an incinerator, the materials in or near the probe volume may emit light. For example, hot soot particles in an exhaust flume may radiate light in the infrared range. The light emitted in or near the probe volume may pass through inner face 58 of prism 54 and be measured by infrared detector, resulting in an underestimation of the hydrocarbon vapor density.

To prevent such a spurious signal, an optical element 80, such as a beam splitter, may be placed in the path of multi-wavelength light beam 36 between beam combiner 34 and optical element 40. Some of the light emitted in or near the probe volume may pass through flat inner face 58 of prism 52 and return along the optical path of light pipe 46 and optical fiber 42. Optical element 80 reflects a portion of light that is returned along the optical train into a light beam 82, and a pyroelectric or infrared optical detector 84 may be placed in the path of light beam 82. The intensity measured by detector 84 may be subtracted from the intensity measured by infrared light detector 74 to generate a corrected intensity measurement.

In summary, the detection apparatus of the present invention includes a probe with two parallel light pipes and two prisms affixed at the ends of the light pipes. A light beam having a wavelength in the infrared range, e.g., 3.39 microns, is combined with a visible light beam to form a combined beam. The detection apparatus directs the combined beam through a probe volume between the parallel faces of the prisms to detect the concentration of hydrocarbon vapor.

Although detector 20 has been described with reference to an inlet port of a combustion device, detector 20 is adaptable to a variety of applications, such as measuring the fuel distribution in the inlet port of a reciprocating engine, in the premixing section of a premixed gas turbine, or in the afterburner of a gas turbine engine. The detection system may used to measure the concentration of hydrocarbons in the exhaust of an incinerator system, furnace, or other processing chamber. In addition, the detection system may be used in various non-combustion environments, such as a building or tunnel, to detect leaks of hydrocarbon vapor.

The present invention has been described in terms of a preferred embodiment. The invention, however, is not limited to the embodiment depicted and described. Rather, the scope of the invention is defined by the appended claims.

What is claimed is:

Claims

1. An apparatus for detecting hydrocarbons comprising: a first light source for generating a first light beam having a wavelength in the infrared range; a second light source for generating a second light beam having a wavelength in the visible range; a beam combiner for combining said first and second light beams into a third light beam; a beam splitter for splitting said third light beam into a fourth and a fifth light beam; an infrared light detector positioned in the path of said fourth light beam; a visible light detector positioned in the path of said fifth light beam; a probe positioned in the optical path of said third light beam, including a first light pipe, a second light pipe substantially parallel to and adjacent said first light pipe, a first prism attached to a second end of said first light pipe, a second prism attached to a first end of said second light pipe, said first and second prisms having substantially parallel opposing faces separated by a gap,- a first optical fiber coupling said first light pipe to said beam combiner; and a second optical fiber coupling said second light pipe to said beam splitter.

2. The apparatus of claim 1 further comprising means for directing said third light beam into said first optical fiber.

3. The apparatus of claim 1 wherein each of said light pipes comprise a sapphire rod.

4. The apparatus of claim 3 wherein said each of said prisms comprise a polished end of said sapphire rod.

5. The apparatus of claim 1 wherein each of said light pipes comprise a metal tube for containing a respective one of said optical fibers.

6. The apparatus of claim 5 wherein each of said prisms comprise a sapphire prism at an end of said metal tube.

7. The apparatus of claim 1 wherein said light pipes are relatively inflexible.

8. The apparatus of claim 7 wherein said probe further includes means for holding said light pipes fixedly relative to each other.

9. The apparatus of claim 8 wherein said probe further includes means for adjusting the gap between said prisms.