# OH* 3D Concentration Measurement of Non-Axisymmetric Flame via Near-Ultraviolet Volumetric Emission Tomography

## TL;DR

This paper introduces a new method to measure OH* concentration in 3D for non-axisymmetric flames using near-ultraviolet imaging, improving understanding of combustion processes.

## Contribution

A novel NUV volumetric emission tomography method with a threshold-constrained LFBP-SART algorithm for 3D OH* concentration measurement in non-axisymmetric flames.

## Key findings

- The method revealed multiple small flame structures in a non-axisymmetric Bunsen flame.
- Maximum OH* molar concentration was approximately 0.04 mol/m³ with 8.7% relative uncertainty.
- The technique is adaptable to other free radicals due to its straightforward requirements.

## Abstract

Measuring the three-dimensional (3D) concentration of the ubiquitous intermediate OH* across combustion systems, spanning carbon-based fuels to zero-carbon alternatives such as H2 and NH3, provides vital insights into flame topology, reaction pathways, and emission formation mechanisms. Optical imaging methods have attracted vital interests due to non-intrusiveness in the combustion process. However, achieving accurate 3D concentration of OH* via imaging in non-axisymmetric flames remains challenging. This work presents a near-ultraviolet (NUV) volumetric emission tomography-based OH* measuring method that integrates a three-layer OH* imaging model, a calibration procedure utilizing narrow-band NUV radiometry, and a threshold-constrained Local Filtered Back-Projection Simultaneous Algebraic Reconstruction Technique (LFBP-SART) algorithm. When applied to a non-axisymmetric Bunsen flame, the method reveals multiple small flame structures matching the fairing pattern in the reconstructed 3D OH* field, with a maximum OH* molar concentration of approximately 0.04 mol/m3 and an overall relative uncertainty of about 8.7%. Given its straightforward requirements, this technique is considered adaptable to other free radicals.