Development and Evaluation of a Tomographic Technique for Volumetric Imaging of Flames

Abstract

With modern day interest to continue the reduction of harmful emissions and improve the efficiency of combustion devices, the need to further improve upon accuracy and ability to describe combustive processes is of high value. Nowadays, multiple capable point (0D) and two-dimensional measurement (2D) techniques exist for flame investigation, however as combustion nearly always is a three-dimensional (3D) process some limitations do exist within many of these techniques. Therefore, the interest to find alternative ways to gather 3D volumetric information with both high temporal and spatial resolution has grown.
This work aims to develop and thereafter evaluate a tomographic technique able to acquire volumetric data from flames and thus put forward a proof of concept that can lay the foundation for future practical applications. Thus, potentially allow for volumetric flame data to be acquired from inside combustion devices with limited optical access.

The tomographic technique was built around the Additive Reconstruction Technique (ART) algorithm and was made to reconstruct both 2D and 3D synthetic flame models, the results were then studied and evaluated. Furthermore, the groundwork for an experimental setup using ten cameras was built to allow for future practical appliance of the tomographic technique in flames. This incorporated the evaluation of different cameras to find the most promising, that in turn would serve for the flame imaging. Finally, a calibration process to map each camera into the same coordinate system was performed to allow for future accurate data gathering.

The results showed that the method proposed in this work for reconstruction of 2D objects was successful, with complex objects of 200-by-200 pixel resolution being reconstructed using ten projection views. Likewise, the modifications made upon the 2D reconstruction, to allow for reconstruction of 3D objects using 2D projections, proved successful in this work. The results show that the interior intensity structure and outer shape was retained in the reconstruction of a 3D synthetic flame model with only minor aberrations present.

In the building of the experimental setup, ten acA1920-40gm-Basler-ace cameras were employed. The model was chosen due to possessing superior signal to noise ratio (SNR) performance than other evaluated models together with a higher resolution that could come of use in future work. The calibration method applied was evaluated to be successful with an individual mean reprojection error of around 0.5 pixels for every camera.

Popular Abstract

Applying a tomographic technique for imaging of flames to pave the way for a cleaner environment.

In this work a tomographic method for three-dimensional combustion studies is developed and evaluated to act as a proof of concept for future use of the technique. Thereafter, a camera selection and groundworks for a laboratory setup able to perform tomographic studies on flames was done and laid out.
With the current development and improvements made in modern society comes a continuing interest to further reduce harmful emissions originating from combustion devices used in various ways such as transportation or power generation. One way to go about this is to find new or improve existing ways of studying combustion processes. Sometimes employing techniques normally used in other scientific research fields and transferring them into one’s own field could bring new solutions to the table.

This work could be seen as such an application, as it builds on the trend of using tomography to allow for full three-dimensional studies of flames and other combustion processes. The technique of tomography is mainly known for its use in medicine where its applied in applications such as Proton Emission Tomography (PET) scans and X-ray measurements that allow for internal imaging of the patient. However, tomography is also applied in other areas such as seismology to look inside mountains or below ground and industrial quality control to search products for internal faults.

The ability of a tomographic technique can be seen as combining multiple different image measurements of an object to give back a result that shows both its internal and external structure. This allows to study internal structures of objects which have proved to be of great use in many fields of research especially in medicine. However, it is important to know that the image measurements used for the tomographic technique must be able to go through the object of interest, that is why x-rays or radio waves usually are used in conjunction with the tomography technique. This allows the measurements to “grab” information on its way through the object and allows for the reconstruction of the interior.

In this work a tomographic technique was developed using an algorithm called ART (Additive Reconstruction Technique) and then applied on two- and three-dimensional synthetic flame objects. This evaluation was performed in a three-dimensional virtual suite where multiple virtual cameras were employed to measure the flame. Because the light captured by each camera comes from the whole volume of the flame object and not just the surface the measurement acquires information about the interior and exterior allowing the technique to compute back the full shape of the measured flame object.

In the end the technique was successful and the interior and exterior of the flame models studied could be acquired. As an example, a tomographic slice result showing the interior of a studied three-dimensional flame object can be seen in the figure. The intensity can be seen to be much higher (yellow) inside of the synthetic flame and lower (blue) in the outer regions.
The initialization of a laboratory setup using ten small machine vision cameras to allow for future practical application of the technique was also started during this work. The cameras used was of the model acA1920-40gm-Basler-ace, that model was selected as the best candidate in an evaluation preformed where multiple different cameras were present.

Because combustion is nearly always a three-dimensional process the goal for this work was to put forward a proof of concept for a technique able to catch three-dimensional aspects of combustion. This work can hopefully pave the way for more tomographic applications for three-dimensional measurements which could lead to improved combustive devices with better efficiency and less harmful emissions.

• author: Sanned, David
• supervisor: Mattias Richter; Karl Åström
• organization: Department of Physics; Combustion Physics
• course: FYSM30 20181
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Physics and Astronomy
• keywords: 3D-Tomography, Additive Reconstruction Technique, Space Carving, Machine Vision Cameras, Imaging of Flame Models
• language: English
• id: 8959672
• date added to LUP: 2018-10-25 10:01:05
• date last changed: 2018-10-25 10:01:05