Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
1Three-dimensional Visualization of Flame in Gas TurbineCombustor
ICHIYU FUJIO*1SHINSUKE TAJIRI*2

3ATSUSHI MORIWAKI* HIROTAKA OKA*4

TAKURO IKEDA*5TATSUYUKI NISHIMIYA*2

Mitsubishi Heavy Industries, Ltd. (MHI) has been developing a Gas Turbine Combined
Cycle (GTCC), which is more efficient and operable than conventional thermal power plants. The
combustor of a large gas turbine has technological issues such as suppressing combustion
instability and reducing NOx emissions, and it is important to evaluate the state of the flame in thecombustor in order to solve these issues. In this report, we introduce "three-dimensionalvisualization of flame in gas turbine combustor", one of the measurement technologies we havedeveloped to understand the flame distribution in a combustor.

|1. Introduction
Large gas turbines apply a lean premixed combustion method to suppress the increase inNOx emissions caused by higher combustion temperatures for the purpose of higher efficiency.
However, this method has a higher risk of combustion instability and flashback due to unstablecombustion conditions compared to the diffusion combustion method, which is a technologicalissue. Combustion instability are caused by the interaction between pressure fluctuations and heatrelease rate fluctuations, and are considered to be closely related to the heat release rate distribution
in the combustor. NOx emissions are also considered to be closely related to the heat release rate
distribution because they are strongly affected by the local combustion gas temperature. Therefore,
in order to achieve both stable combustion and low NOx emissions, it is important to evaluate theheat release rate distribution in the combustor.
OH radical chemiluminescence measurement is a simple measurement method for evaluating
the distribution of heat release rate in a combustor. This method obtains the relative distribution ofheat release rate in a flame based on the fact that the intensity distribution of chemiluminescence ofOH radicals (OH*) excited by combustion is correlated with the heat release rate distribution. For
example, the blue flame of a gas stove is also derived from excited chemiluminescence of CH
radicals (CH*), and visualization of this chemiluminescence intensity distribution is equivalent tovisualization of the flame shape.
We have evaluated the relationship between heat release rate fluctuations and combustion
instability using the OH radical chemiluminescence measurement and the OH Planar Laser-Induced
Fluorescence (OH-PLIF) method (1)~(3). However, the conventional OH radical chemiluminescencemeasurement can only measure the integrated value in the optical path direction and cannot
evaluate the three-dimensional heat release rate distribution. The OH-PLIF method is a planar
distribution measurement and cannot evaluate three-dimensional heat release rate distribution aswell. Therefore, we have developed a measurement technology to visualize the three-dimensional
relative heat release rate distribution in a combustor by combining OH radical chemiluminescencemeasurement with the computed tomography (CT) method, which is a three-dimensional
distribution reconstruction method. This report presents an overview of the technology and the*1

Combustion Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2Team Manager, Combustion Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*3

Team Manager, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*4EPI Department, Digital Innovation Headquarters, Mitsubishi Heavy Industries, Ltd.

*5Service Engineering Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
2results of three-dimensional visualization of a flame in an actual combustor.

|2. Development and validation
reconstruction programofchemiluminescence

CTIn order to apply chemiluminescence CT measurement to an actual combustor, it is necessary
to introduce an appropriate CT processing method for the flame of the combustor to be measured.
In this chapter, we describe the development of a reconstruction program for CT processing and theresults of its validation.
First, the Ordered Subset Expectation Maximization (OS-EM) method (4), one of thesuccessive approximation methods, was used as the basic algorithm for the reconstruction program.
The OS-EM method is a reconstruction method that produces reconstruction results with fewer
artifacts (processing errors that occur when reconstructing images), and is a faster version of theMaximum Likelihood Expectation Maximization (ML-EM) method, which is also used in medical
CT reconstruction. Figure 1 shows an overview of the CT process for the chemiluminescence
distribution of a flame in a combustor.

Figure 1Overview of CT processing of flame chemiluminescence in combustor

To perform CT reconstruction processing, images taken from multiple directions arerequired. For example, X-ray CT used in the medical field acquires images taken from multiple
directions by rotating the X-ray tube and detector. However, this assumes that the object does notchange over time, making it difficult to apply it to flames in a combustor, which may change overtime. Therefore, the measurement system presented in this report uses 12 cameras for simultaneous
imaging to eliminate the influence of time-related changes. Since OH radical chemiluminescence isultraviolet, special bandpass filters and cameras were used to measure the ultraviolet
chemiluminescence. The 12 chemiluminescence images were reconstructed by the OS-EM method
described above to obtain the three-dimensional distribution of the flame.
In the development of this measurement technology, the validity of the CT processing
program was confirmed using several simple flames (Bunsen burners). When multi-directional
imaging is used for the purpose of CT measurement, it is desirable that there are no structures in thefield of view that would create blind spots for flame imaging. However, when this method isapplied to an actual combustor such as a gas turbine combustor, a pillar is structurally necessary to
hold the optical window, which creates a blind spot in the field of view. Therefore, we developed amethod to reconstruct the image without the effect of these structures. When conventional CT
process method is applied, the data of blind spot area in the image is processed as valid data. For
this reason, the CT process treats the area as if there is no flame, and the intensity of the flame inMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
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the blind spot is evaluated low in the reconstructed image. On the other hand, in our newly
developed CT process method, blind spot areas are identified in advance, and the data of blind spot
areas in the image can be processed as invalid data in the reconstruction process. For example,
when a certain area of flame enters the blind spot in images taken from three directions among theimages taken from twelve directions, the chemiluminescence distribution in that area isreconstructed using constraint conditions based on data obtained from the images taken from theremaining nine directions.
The CT processing program that took into account blind spots was validation-tested toconfirm its validity. Figure 2 shows an overview of the test simulating blind spot areas. The
measurement objects were four Bunsen burners and structures forming blind spot areas were placedaround the objects. In this test, a single camera was revolved around the burners to capture images
from multiple directions. Based on the captured images, the three-dimensional distribution wasreconstructed using the CT processing program.

Figure 2 Validation test simulating blind spot areas caused by structures

Figure 3 shows the results of the validation test. In all cases, four Bunsen burners wereimage-captured and reconstructed. In the case of only quartz tubes without structures, thereconstruction was made with no problem. In the case of the blind spots caused by structures, when
the above-mentioned blind spots are not taken into account, some of the flames were reconstructedwith low luminescence intensity and the flame shape was broken and not correctly reconstructed,
even though the equivalent ratio and flow rate were almost the same as those of the other flames.
On the other hand, when the processing taking into account the blind spots was applied, theresultant chemiluminescence intensities of the four burners were almost equal, which was areasonable result.

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Figure 3 Validation results of CT processing program using simple flames|3. Development of chemiluminescence CT measurement system for
actual-scale combustor
In order to acquire the images necessary for CT processing, we constructed a large image
acquisition system capable of capturing images of an actual combustor flame from multiple
directions. Figure 4 shows the chemiluminescence CT measurement system we developed. The
measurement system is constructed by aluminum frames, and twelve cameras are mounted
circumferentially around the axis of the combustor so as to surround the combustor. Thismeasurement system is self-standing and can be traversed in the direction of the combustor axis.
Figure 5 shows a configuration diagram of the measurement system. Two cameras are controlled
by one camera control PC, and each control PC is controlled by an analysis PC via an Ethernet hub.

Figure 4 Overview of chemiluminescence CT measurement system

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Figure 5 Configuration of chemiluminescence CT measurement system|4. Application to atmospheric pressure combustion test
We visualized flames in our gas turbine combustor using the developed chemiluminescence
CT measurement system. For the validation test, we used our gas turbine combustor atmosphericpressure combustion test rig, and as the measurement target, an actual combustor developed by us(5). In a normal combustion test, the flames inside the combustor are not visible, but in thisvalidation test, the flames inside the combustor were made visible by using a combustor made ofquartz in order to measure the chemiluminescence in the combustor. Figure 6 shows a schematic
diagram and a photograph of the quartz combustor. The quartz combustor is double-tubed, and theinner quartz tube is cooled by air flowing between the inner and outer quartz tubes.

Figure 6

Double-quartz-tube combustor

Figure 7 shows our atmospheric pressure combustion test rig incorporating thechemiluminescence CT measurement system. The chemiluminescence CT measurement system isarranged to surround the combustor described above.

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Figure 7 Our atmospheric pressure combustion test rig incorporatingchemiluminescence CT measurement system

Figure 8 shows the results of chemiluminescence CT measurement under premixed flame
conditions. In the premixed flame, a petal-like flame is formed by the swirling flow, and athree-dimensional flame structure with finely varying distribution in the circumferential direction
can be observed. Such a three-dimensional and complex flame structure has also been observed in
combustion CFD analysis, and it was confirmed that this measurement method is capable ofacquiring complex flame shapes such as those in a gas turbine combustor.

Figure 8

Visualization results of premixed flame in combustor

To confirm that the obtained three-dimensional distribution of relative heat release rate isreasonable, we compared it with the result of combustion CFD analysis. Figure 9 compares theflow-directional distribution of integrated values of heat release rate and chemiluminescenceintensity in the cross-section. The peak positions of the heat release rate in both cross-sections withdifferent phases are coincident. This indicates that the chemiluminescence CT measurement can beused to properly identify the high heat release rate area in the combustor.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
7Figure 9 Comparison of heat release rate distribution obtained by combustion
CFD analysis and chemiluminescence intensity distribution obtained bychemiluminescence CT measurement (Flow-direction distribution ofintegrated values in cross section)|5. Conclusion
This report introduced the three-dimensional visualization of flames in a gas turbine
combustor using chemiluminescence CT measurement. By applying this technology, we were able
to visualize the three-dimensional flame shape of our actual combustor for the first time. Inaddition, we have established a technology that enables us to actually visualize and evaluate theheat release rate distribution of the designed combustor for the development of combustors in thefuture. Furthermore, by using the data obtained by this measurement technology as validation datafor combustion CFD analysis, it is expected to be useful for improving the accuracy of combustion
CFD analysis technology. We will consider applying this technology to combustion evaluation in
the development of hydrogen and ammonia-fired combustors in the future.
(Acknowledgment)
Part of the results presented here were obtained as a result of the High-efficiency Gas
Turbine Technology Demonstration Project (the development of ultra-high efficiency 1700°C-classgas turbines, JPNP16002), which was subsidized by the New Energy and Industrial Technology
Development Organization (NEDO). We would like to express our gratitude for their support.

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