Patent Application
20180356292
The present invention relates generally to thermal measurements and, more particularly, to high temperature thermal measurements.
Measurements of gas-phase temperature are critical for understanding fluid transport, heat transfer, and chemical reactions in power generation and propulsion systems (such as ground-based gas turbines and aircraft engines). Typically, combustion-based power generation systems operate at pressures and temperatures significantly above ambient conditions and require specialty high-temperature materials for confining the combusting flow. Thermally-insulating materials, such as ceramic coatings and (more recently) ceramic-matrix composites, are used to extend the high temperature operational limit of these systems. In fact, ceramic materials are commonly used in high-temperature, industrial chemical reactors and furnaces. In each of these applications, accurate gas-phase temperature measurements are required to optimize the thermochemical processes and to provide monitoring for process control and feedback.
Two primary issues limit the effectiveness of thermometry techniques under these conditions: (1) probe-based measurements are physically invasive and (2) non-invasive, optical measurements require significant optical access. Probe-based measurement techniques require materials comprising the probe to survive harsh high-temperature environments, and such probes can significantly impact the thermochemical process. Examples of such conventional probes include ceramic-insulated thermocouples, gas-dynamic probes, and species-based extractive sampling probes. Optical-based measurement techniques lead to heat loss through the optical access or provide spatially integrated measurements. Examples of optical probes include coherent anti-Stokes Raman scattering, laser-induced grating spectroscopy, Rayleigh scattering, tunable diode laser absorption spectroscopy, and IR thermal imaging.
As such, there remains a need for improved devices and methods of measuring the temperature of high temperature gas systems.
The present invention overcomes the foregoing problems and other shortcomings, drawbacks, and challenges of conventional devices and methods for measuring temperature of high temperature gas systems. While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. To the contrary, this invention includes all alternatives, modifications, and equivalents as may be included within the spirit and scope of the present invention.
According to an embodiment of the present invention, a method of measuring a temperature of a thermally-insulated, high temperature system that includes directing a first electromagnetic energy into the high temperature system so that the first electromagnetic energy may cause multi-photon ionization of a molecular or atomic species within the high temperature system. A second electromagnetic energy resulting from the multi-photon ionization is detected through a thermally-insulating wall of the high temperature system. The detected second electromagnetic energy is related to a temperature within the high temperature system.
Other embodiments of the present invention are directed to a method of measuring a temperature of a thermally-insulated, high temperature system that includes directing a first electromagnetic energy into the high temperature system so that the first electromagnetic energy may cause multi-photon ionization of a molecular or atomic species at a first position within the high temperature system. A second electromagnetic energy resulting from the multi-photon ionization at the first position is detected through a thermally-insulating wall. The first electromagnetic energy is then directed into the high temperature system so that the first electromagnetic energy may cause multi-photon ionization of a molecular or atomic species at a second position within the high temperature system. A third electromagnetic energy resulting from the multi-photon ionization at the second position is detected through a thermally-insulating wall. The detected second and third electromagnetic energies are related to temperatures at first and second positions, respectively within the high temperature system.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention.
It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to facilitate visualization and clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration.
Embodiments of the present invention overcome the limitations imposed by conventional gas-phase thermometry methods while enabling highly accurate, spatially localized measurements with minimal optical access requirements and, thus, are suitable for application in ground-based gas turbines, aircraft engines, industrial furnaces, and high-temperature chemical reactors. Generally, laser-based resonance-enhanced multi-photon ionization (“REMPI”) is used to probe N2, O2, other combustion-relevant species, or combinations thereof with species-specific ultraviolet excitation through a small access port (less than about 5 mm diameter). Unlike conventional methods requiring optical access to collect emitted light from the multi-photon ionization process (e.g., laser-induced breakdown spectroscopy with greater than 50 mm access requirements), electron density of the REMPI process may be probed using microwave radiation (“radar”). The primary advantage of using radar to probe the REMPI process is that microwaves can transmit through optically-opaque ceramic insulators and ceramic-matrix composites with minimal attenuation. Such characteristics enable high signal-to-noise ratio measurements in highly confined combustors and reactors that would otherwise not accommodate conventional optical thermometry techniques. The radar-based REMPI signal may then be related to temperature through the Boltzmann distribution on energy states with high accuracy. While radar-based REMPI has previously been demonstrated for gas-phase temperature measurements, the unique aspect of embodiments of the present invention is an ability to make the measurement through ceramic walls in a non-invasive nature by radar detection without sacrificing accuracy, precision, or spatial resolution.
Additional detail regarding certain embodiments of the present invention is provided in Y. W U et al., “Spatially localized, see-through-wall temperature measurements in a flow reactor using radar REMPI,” Optics Letters, Vol. 42 (2017) 53-56. The details of this paper are incorporated herein by reference, in its entirety.
Referring now to the figures, and in particular to
As shown in
The ceramic reactor 22 also includes a heater 28, which is illustrated here as a plasma lighting coil; however, other heaters may also be used. One or more thermocouples 32, 34 may be operably coupled the ceramic reactor 22 so to as measure, monitor, or adjust a temperature within the reaction chamber 26.
The radar-based REMPI system of
A size of a measured volume is generally significantly smaller than an internal volume of the reaction chamber 22 in which the temperature is to be measured. Minimizing the measured volume enables temperature and species measurements in both radial and axial directions. The axial and radial spatial resolution is estimated as 2 mm and 100 respectively, and may be varied by changing the focal length of the lens 38 or the diameter of the laser beam 36 on the lens 38.
The radar-based REMPI system 20 further comprises a microwave horn 40, such as a homodyne detection system, that is configured to transmit and receive microwaves. In particular, the microwave horn 40 should be configured to receive microwaves scattered from ionized reactants within the reactor chamber 22 without need for optical access thereto. Details of a suitable homodyne detection system would be understood by those skilled in the art and are not necessarily illustrated herein. Briefly, and by way of example, a tunable microwave source (such as a 12 dBm, HP 8350B sweep oscillator) may be separated into two channels: a first channel that is configured to radiate plasma using the microwave horn (WR75, 15 dB gain), which may also be used to collect microwaves scattered by a plasma generated within the reaction chamber 22; and a second channel configured as a local oscillator (a reference signal) for the frequency mixer. Microwave scatter received by the horn 40 from within the reaction chamber 22 transmits through the ceramic walls 24 and may be, optionally, passed through a microwave circulator to amplify the signal using a preamplifier (for example, by 30 dB at about 10 GHz or may be converted down in the mixer, two other amplifiers with bandwidth of 2.5 kHz to 1.0 GHz amplified the signal by 60 dB). The resultant signal may be optionally monitored, recorded, or both, such as by using an oscilloscope. Additionally, or alternatively, the oscilloscope may be used to monitor the laser beam.
An orientation of the microwave horn 40 may be selected to maximize signal reception of the signal from the plasma within the reaction chamber 22 while accounting for the geometry of dipole radiation.
Referring still to
wherein Bν is the rotational energy constant, Dν is the distortional energy constant, J is angular momentum, and L is the electronic orbital angular momentum.
An excited state of molecular oxygen, that is after a multiphoton excitation process within the plasma, O2(C3Π(ν=2)), follows Hund's case (a)—that is having strong electrostatic coupling, intermediate spin-orbit coupling, and weak rotational coupling. Hyperfine splitting results in energy levels given by Equation 2:
F
1(Ω=0)=n01+Beff1J(J+1)−Dν1J2(J+1)2
F
2(Ω=1)=n01+Beff2J(J+1)−Dν2J2(J+1)2
F
3(Ω=2)=n03+Beff3J(J+1)−Dν3J2(J+1)2 Equation 2
The constants n0, Beff, and Dν of Equation 2 are available in literature.
Because of the hyperfine structure of radar-based REMPI spectra, each branch (that is, the O, P, Q, R and S branches corresponding to various excited states of the molecule under investigation) may have multiple lines. Thus, a two-photon transition line strength, Sfg(2), between the excited state, C3Π, and the ground state, X3Σ, may be given by:
where βk(2) is the polarization coefficient, J is the rotational quantum number, and the brackets, [ . . . ], denote the Wigner 3-j symbol. For the ground state, X3Σ, Λ1=0, and Σ′=±1,0; for the excited state, C3Π, Λ=1, Σ=±1,0, and Ω=0,1,2. For linearly polarized light: βk(2)=√{square root over (10)}/3, and for circular polarization: βk(2)=√{square root over (5)}. Furthermore, for circular polarization only the k=2 term contributes.
When the radar REMPI system 20 is used for O2 rotational temperature measurements of the illustrative embodiment, the Sit branch may be suitable for temperatures less than 700 K. The wavelengths, rotational ground state energies, and line strengths calculated using Equations 1-3 for the S21 branch lines are provided in Table 1, below.
The radar-based REMPI microwave signal, EMW, resulting from the scattering of microwaves on the plasma is proportional to electron density, Ne, which is related to the number of molecules completing the 2+1 ionization process. This proportionality is expressed in Equation 4:
wherein I is the intensity of the exciting pulse (here, the UV laser beam), σi is the ionization cross-section from the excited state, Eg is the ground state energy, T is temperature, and kB is the Boltzmann constant. Because multiple emission lines are available, Equation 4 may be arranged such that temperature may be determined from a linear plot, with the assumption that Sfg(2) is constant over the scanning wavelength range:
Many ceramic materials commonly used for the ceramic reactor 22 (such as for insulation or high-temperature combustor components) may exhibit high transmission within the range of microwave frequencies of the various embodiments of the present invention. To account for such high transmission, Beer's law may be used to describe the loss of power due to absorption of an electromagnetic wave traveling through a medium:
P(z)=P0e−kδz Equation 6
where k is the wavenumber for the electromagnetic wave in the medium and δ is the argument of a loss tangent, which is a ratio of the real and complex parts of the dielectric constant for the medium. The loss tangent is generally dependent on material thickness, microwave frequency, metallic content, and temperature.
If the loss tangent is small, then a small angle approximation allows δ to be taken as approximately equal to the loss tangent. Thus, the details of
The detected microwave scatter through a 0.5 inch thick sample of the alumina ceramic used in the ceramic reactor 22 is shown in
The following examples illustrate particular properties and advantages of some of the embodiments of the present invention. Furthermore, these are examples of reduction to practice of the present invention and confirmation that the principles described in the present invention are therefore valid but should not be construed as in any way limiting the scope of the invention.
A reactor 22, similar to the embodiment illustrated in
Air having a temperature of 298 K radially entered the reactor 20 by way of the reactant inlet 28, was heated by convection, and radially exited the flow reactor (flow being from right to left). A laser beam 36 propagated in a direction that opposed the air flow direction (thus, from left to right). Air flow rate was set at 2 standard liters per minute (“SLPM”) and was simulated under laminar conditions.
Temperature was measured at six axial positions, which are illustrated on the simulation, and which correspond to distances from an entrance window 42: 215.9 mm, 190.5 mm, 165.1 mm, 139.7 mm, 114.3 mm, and 63.5 mm. A computed variation between adjacent positions was about 20 K. These positions were selected to test the temperature sensitivity of the technique within 20 K, which is comparable to temperature differences of interest in chemical flow reactors.
Temperature measurements were also performed in a radial direction at axial position 2 so as to test sensitivity of spatial resolution for capturing temperature gradients.
Measured positions was changed in both the axial and radial directions by moving the focusing lens 38 to ensure a consistent optical geometry.
During the experiment, the microwave frequency was about 10 GHz, but was adjusted to maximize the signal prior to each data collection. Because microwave radiation at 10 GHz is only slightly attenuated by SiO2, the microwave transmission and detection was performed through the ceramic reactor 22 without the need for optical accessibility. This significantly reduces experimental complexities associated with windows in combustion environments and minimizes heat loss in the reactor 22.
A numerical simulation of the system was also performed and temperature distribution of the flow reactor was prepared using Pro-E software. Consistent with measurements by the thermocouples 32, 34 on the quartz flow reactor, the heating components 30 were modeled with constant temperature boundary conditions of 675 K. Results are illustrated in
Experimental Radar REMPI spectra of molecular oxygen in the reactor 22 are shown in
Conventional, well-stirred reactors (“WSR”) 50, an example of which is illustrated in
Briefly, and in use, vaporized liquid or gaseous fuels are premixed with air in the jet ring manifold. To ensure all fuel is vaporized, and to minimize potential thermal decomposition of the fuel before combustion in the reactor 50, the temperature of the mixture is held at about 505 K. The mixture is injected into the toroidal reactor 60 through the plurality of 1 mm diameter jets 58 with a Mach number of about 0.8. A bulk residence time for non-reacting flow is about 28 ms and about 6 ms when the reactor 50 is operating because of gas expansion.
The WSR 50 has been used for measuring ignition probability as a function of residence time and equivalence ratio. In this case, the temperature profile in the WSR 50 before ignition is a critical initial boundary condition for the ignition process. While measurement of the temperature profile may be made by physically scanning a Type-K thermocouple across a diameter of the toroidal reactor 60, the physical probe may perturb the flow and, therefore, the temperature profile. Additionally, thermocouple will measure total temperature of the flow which will deviate from the static temperature of the flow near the middle of the toroidal reactor 60 because of the inlet jets 58. Accurate, non-invasive measurement of the static temperature distribution is critical for providing high-fidelity boundary conditions for modeling of ignition events in the WSR 50.
For this proof-of-principle, a radar REMPI system 82 according to an embodiment of the present invention was used to measure the temperature profile in the WSR 50 for constant temperature inlet conditions. The measurement setup is shown in
Accordingly, the WSR 50 was heated, exclusively, by flowing heated air through the system. Fuel was not used in the characterization experiments.
Data collection via a microwave horn 98 commenced once the system was near thermal equilibrium, as measured by a type-B thermocouple (not shown). The WSR 50 was operated with an air flow rate of 500 SLPM and an inflowing air temperature of 477 K. The temperature of the WSR 50, at thermal equilibrium, was about 450 K.
The computed temperatures are plotted as stars in
Overall, the see-through-wall radar REMPI temperature measurements agreed well with simulated results near an inner surface of the toroidal reactor. At positions closer to the surface of the system, the air temperature (heated by surface conduction and radiation) reached the temperature set point of 675 K. However, at positions near the center of the toroidal reactor, only convection plays a significant role to the heating of air. As a result, the air temperature at the positions near the center of the toroidal reactor were lower than those measured at the surface. This temperature distribution is captured by both the simulation and experimental measurements.
The see-through-wall radar REMPI temperature measurements also agreed very well with the simulated results and exhibited absolute differences of less than 5 K. FIG. 11B highlights the sub-millimeter spatial resolution of the radar REMPI technique. Such resolution is sufficient to capture the about 5 K/mm gradient in the radial direction.
Fairly consistent spectral measurements were obtained for positions in a lower half of the toroidal reactor. Accurate measurements were not achieved in an upper half of the toroidal reactor because of competition between (1) limited optical focal lengths required to avoid breakdown on the upper wall in the toroid and (2) stronger signal of molecular oxygen plasma in the WSR. Breakdown was identified as an abnormal increase in a few points of the spectra, which essentially sets the limits of the focusing lens and laser energy. No ceramic ablation was observed on the WSR after measurements, indicating the breakdown was entirely due to interactions with the heated air.
Normalized signal by rotational line strength and laser intensity is shown as the y-axis in each of
During the experiment, the signal-to-noise ratio increased with height of measurement within the flow reactor. Such correlation is reflected in
The comparison of temperatures obtained from the radar REMPI and thermocouple measurements is summarized in Table 2 and plotted in
Embodiments of the present invention, as describe herein, are directed to devices and methods of coherent microwave scattering from REMPI for rotational temperature measurements through ceramic-walled reactors. Through limited single-ended optical access, a laser beam is focused to generate local ionization of molecular oxygen within a ceramic reactor or combustor. Coherent microwave scattering from the laser-induced plasma, which is transmitted and received through the ceramic walls, is used to acquire rotational spectra of molecular oxygen and determine temperature. Temperature measurements with an accuracy of ±20 K (±3%) may be achieved in operating reactors. Methods according to the various embodiments therein enable non-invasive, high-fidelity quantification of spatially localized temperature in confined reactors and combustors constructed of ceramic materials in which limited or non-existing optical access hinders usage of conventional optical diagnostic measurement techniques. Such methods may be used to improve temperature measurement quality and fidelity in chemical flow reactors, gas-turbine combustors utilizing ceramic composites, and industrial ceramic air heaters.
While the present invention has been illustrated by a description of one or more embodiments thereof and while these embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.
Pursuant to 37 C.F.R. § 1.78(a)(4), this application claims the benefit of and priority to prior filed co-pending Provisional Application Ser. No. 62/516,786, filed 8 Jun. 2017, the disclosure of which is expressly incorporated herein by reference in its entirety.
The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.
| Number | Date | Country | |
|---|---|---|---|
| 62516786 | Jun 2017 | US |
