Multispectral Mid-Infrared Camera System for Accurate Stand-Off Temperature and Column Density Measurements on Flames.

Juan Meléndez,Guillermo Guarnizo

doi:10.3390/s21248395

Abstract

Accurate measurement of temperature in flames is a challenging problem that has been successfully addressed by hyperspectral imaging. This technique is able to provide maps of not only temperature T (K) but also of column density Q (ppmm) of the main chemical species. Industrial applications, however, require cheaper instrumentation and faster and simpler data analysis. In this work, the feasibility and performance of multispectral imaging for the retrieval of T and Q in flames are studied. Both the hyperspectral and multispectral measurement methods are described and applied to a standard flame, with known T and Q, and to an ordinary Bunsen flame. Hyperspectral results, based on emission spectra with cm resolution, were found in previous works to be highly accurate, and are thus considered as the ground truth to compare with multispectral measurements of a mid-IR camera (3 to 5 m) with a six interference filter wheel. Maps of T and Q obtained by both methods show that, for regions with T K, the average of relative errors in multispectral measurements is ∼5% for T (and can be reduced to ∼2.5% with a correction based on a linear regression) and ∼20% for Q. Results obtained with four filters are very similar; results with two filters are also similar for T but worse for Q.

Highlights

Combustion processes are of paramount importance in many economic sectors
Since in previous works hyperspectral temperature measurements in the National Physical Laboratory (NPL) standard flame have proven very accurate, as explained in the Introduction, we will assume throughout this article that the results of the hyperspectral method are our “ground truth”’, and use them to validate the multispectral temperature maps
Values are qualitatively very similar in the flame region, they show important differences where the gases are colder. This is to be expected since results are unreliable outside the central zone, because radiance, and signal-to-noise ratio (SNR), decrease sharply outside of the burner area, which translates into erroneous estimates of T and Q

Summary

Introduction

Combustion processes are of paramount importance in many economic sectors. Since temperature is the key factor that determines chemical reaction rates, an accurate control of it is essential to optimize these processes: monitoring of temperature improves consistency and energy efficiency in manufacturing, reduces wastage and pollution, increases reliability, and extends intervals of maintenance tasks. To measure physical parameters in a combustion process is a challenging problem, because intrusive probes can be damaged by the harsh environment, and even the toughest ones, such as thermocouples, induce perturbations in the flame, and provide readings with important systematic errors [1]. Active laser-based techniques, such as CARS (Coherent Anti-Stokes Raman Spectroscopy) or LIGS (Laser Induced Grating Spectroscopy) require complex laboratory setups that are difficult to install and operate in industrial environments, but still have a relatively large uncertainty, generally not better than 5% [2]. A promising alternative is passive techniques such as gas emission spectroscopy, whose setups are simpler since no excitation of the flame is required. The main chemical species in a flame have very specific emission profiles in the mid-IR band, that make possible their identification, quantization and, in principle, the measurement of their temperature

Objectives

Methods

Results