Journal of Thermal Science Vol.18, No.3 (2009) 284−288

Received: March 2009 Fan Jiang: Associate Professor

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DOI: 10.1007/s11630-009-0284-1 Article ID: 1003-2169(2009)03-0284-05

Visual Flame Monitoring System Based on Two-Color Method

Fan Jiang1*, Shi Liu2, Shiqiang Liang1, Zhihong Li2, Xueyao Wang1, Gang Lu 3

1. Key Laboratory of Advanced Energy and Power, Chinese Academy of Sciences (Institute of Engineering Thermo-physics), P. O. Box 2706, Beijing 100190, China 2. School of Energy and Power Engineering, North China Electric Power University, Beijing 102206, China 3. Department of Electronics, University of Kent, UK

Monitoring and control of combustion flames in utility boilers are required in order to optimize combustion con-ditions. This paper presents an instrumentation system for the concurrent measurement of the temperature distri-bution and soot concentration of flames developed on the two-color principle. This system consists of an endo-scope, an optical assembly with optical filters, a CCD camera, a frame grabber and associated image processing software. Experiments are performed on a methane-air combustor and the temperature fields and the soot concen-trations corresponding to the flame images are obtained. The results have demonstrated that the system is capable of performing on-line measurement of flame and temperature distribution, providing temporal and spatial charac-terization of the combustion process. In addition, the combination of advanced optical sensing and digital image processing technique can help to define the threshold by the analysis of the background noise. Furthermore, the utilization of the filter technique can enhance the image presentation effect to an extent.

Keywords: Two-color method, CCD camera, Flame temperature fields, Digital image processing

Introduction

With the rapid development of clean coal technologies, there is an increasing need for developing advanced combustion diagnose and dynamic control system to in-crease combustion efficiency and reduce pollutant emis-sions. Since the flame is the central reaction zone of combustion process and the flame structure, temperature distribution and fluid dynamic characteristics in a boiler are closely related to combustion efficiency, pollutant emissions and operation safety, one of the most effective means is to characterize the flame.

A number of methods for flame temperature meas-urements have been studied in the past. At present, the most widely applied methods use physical probes, such as thermocouples or gas-sampling probes, with obvious disadvantages including intrusive nature, degradation in

harsh environments, and single point measurement. With the advent of optical sensing, image processing and computing techniques, a number of studies have been carried out on the monitoring and characterization of flames. Advanced flame and temperature measurement techniques include Laser Raman (LR)/Laser Rayleigh Scattering (RS), Laser-Induced Fluorescence (LIF), Fou-rier Transform Infrared (FTIR) Spectroscopy, schlieren photography, interferometry and so on. In addition, trac-ers such as smoke, small particles, gas streams and bub-bles have also been used to visualize combustion phe-nomena. All these methods are non-intrusive and each technique has its particular advantages and disadvantages. The two-color method based on the flame radiation prin-ciple has the advantages of simple operation, quick re- sponse, thus having wide applications to the flame meas-urement.

Fan Jiang et al. Visual Flame Monitoring System Based on Two-Color Method 285

Nomenclature ελ monochromatic emissivity C1 the first radiation constant λ wavelength C2 the second radiation constant

Eλ(T) monochromatic intensity of radiation K absorption coefficient per unit flame thicknessT(K) temperature L geometric flame thickness

Ta apparent temperature α absorption coefficient In China, most existing flame monitoring devices are

largely limited to indicate whether the flame is presented or out purely for safety reasons. The simplest traditional method of recording flames is direct photography, which can be used to explore the flame behavior. Another method is based on the inspection of the limited points in the boiler. But the two methods can not provide the on-line, stable and quantitative flame temperature fields. Therefore in China, the improvement, development or invention of flame visualization techniques is very nec-essary for the understanding of combustion phenomena.

This paper presents an intelligent vision system for the monitoring of combustion flames by combining optical sensing and digital image processing technique.

Measurement principle

The two-color method relies on the measurement of the radiation intensity from incandescent soot particles generated during combustion. The radiation intensity is measured at two wavelengths generated by a bifurcation where the light emitted by the soot particles is split into two, and from these measurements the flame temperature and the soot concentration can be estimated.

By definition, the monochromatic emissivity ελ of the body at wavelength λ is given by

,

( )( )b

E TE T

λλ

λε = (1)

where Eλ(T) is the monochromatic intensity of radiation at a wavelength of λ(m) emitted by a body at a tempera-ture of T(K). Eb,λ(T) is the monochromatic intensity of radiation at a wavelength of λ(m) emitted by a blackbody at a temperature of T(K).

At a given wavelength λ, we can define a blackbody apparent temperature Ta as the temperature of a black-body which will emit the same radiation intensity as a non-blackbody at temperature T. Combining the defini-tion for monochromatic emissivity (Equation 1) with the definition of apparent temperature gives

,

,

( )( )

b a

b

E TE T

λλ

λε = (2)

where Eb,λ(Ta) is the monochromatic intensity of radia-tion at a wavelength of λ emitted by a blackbody at a temperature of Ta.

According to the Planck’s Law, that is, the intensity of radiation from a blackbody is expressed as a function of temperature and wavelength, it follows

2

51

, ( ) 1b C TC

Eeλ λ

λ −

=−

(3)

where C1 and C2 are the first and second radiation con-stants (C1=3.7418×10-16 W·m2 and C2=1.4388×10−2 m·K).

Substituting Equation (3) into Equation (2) yields 2

2

1

1a

CT

CT

e

e

λ

λ

λ

ε −=

−

(4)

According to the Hottel and Broughton empirical equations,

1KL

eαλ

λε−

= − (5) where K is the absorption coefficient per unit flame thickness (m−2), and L is the geometric flame thickness along the light axis of the flame detection system. α is the absorption coefficient.

Substituting Equation (5) into Equation (4) 2

2

1ln 1

1a

CT

CT

eKL

e

λα

λ

λ⎛ ⎞⎜ ⎟−

= − −⎜ ⎟⎜ ⎟

−⎝ ⎠

(6)

By re-writing the above equation for two specific wavelengths, λ1 and λ2:

1 21 2

2 2

1 2

2 2

1 1 2 2

1 11 1

1 1a a

C CT T

C CT T

e e

e e

α αλ λ

λ λ

λ λ

⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟− −

− = −⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟− −⎝ ⎠ ⎝ ⎠

(7)

where T is the actual flame temperature. The apparent blackbody temperature Ta1 and Ta2 at the wavelength of λ1 and λ2 respectively can be obtained by calibration. Therefore, from Equation (7), the flame actual tempera-ture can be obtained. And replacing T in Equation (6), the soot concentration KL can be computed.

Experimental system

Fig. 1 shows a schematic diagram of combustion test- rig and flame monitoring system. Methane gas is purged through a decompression valve, a regulation valve, a

286 J. Therm. Sci., Vol.18, No.3, 2009

flowmeter, a nozzle of 8.2 mm diameter, and then spurted into an ejection device. Primary air from 9 pores with a diameter of 4 mm enters into the ejection device, where it is mixed with methane gas. The methane/air mixture is ignited when leaving a 51 mm spay nozzle. Second air is entrained into the flame for complete combustion.

The flame monitoring system consists of an endoscope, an optical assembly, a CCD camera, a frame grabber and image processing software. Besides, the system also in-cludes connecting lens, gas/water cooling and sweeping assembly etc. The characteristics of the constituent ele-ments are shown as follows.

Fig. 1 Schematic diagram of the system

Endoscope The endoscope is capable of giving images with high

resolution and low light energy loss. The ocular angle of the objective tube of the endoscope is 90°, which means a wider field of view. Also, the focus is adjustable, so it is easy to get a clear image. The total length of the endo-scope is about 1.5 m, suitable for the application to both power plants and experiments.

Optical Assembly Light passes through the lens is not only from a flame

but also from the back-wall of the combustion chamber and other sources. Furthermore, the intensity of the light from a flame is different from region to region. The light with exceedingly strong intensity may cause a saturation problem in the image signal. To prevent the ambient light from entering the lens and to suppress the amount of strong intensity light, the use of suitable optical filters is necessary. The filters should cut out unwanted incident light as much as possible without the loss of the impor-tant information of a flame. A particular requirement for the filters is that they should be suitable for a wide range of combustion conditions. In addition, the bandwidths of the two filters should be as narrow as possible to acquire the ideal single-wavelength radiation. Compromising the

factors addressed above has led to the selection of wave-length centered at 632 nm and 700 nm with a bandwidth of 40 nm.

CCD Camera A charge-coupled device (CCD) is a high-sensitivity

solid-state imaging device which produces an analogue signal corresponding to the intensities of light falling on the sensor elements, i.e., CCD array. The camera, having a monochromatic CCD array of 756 (H) ×581 (V) pix-els, is capable of capturing flame images at 50 frames S-1. The exposure time of the camera is fixed at 1/500 s. The choice of the monochromatic camera rather than a color one in this study is because it gives higher resolution of images in a single band. Furthermore, real-time control of the flame also requests concise images information of the flame and fast processing time. The characteristics of the spectral response of the camera has to be considered to ensure that important information of flames can be captured over a desired spectral band. Fig. 2 shows the spectral responses of the CCD array of the camera.

Fig. 2 Spectral characteristics of the CCD array

Frame Grabber Frame grabber digitizes the analogue signal from a

camera and converts it into a series of 2D digital images. A digital image is stored in a computer as an array of bits corresponding to the luminous intensities of a flame. The image array can then be processed using various com-puting techniques. The frame grabber employed in the system is Matrox Meteor-II/multi-channel. A 32-bit bus interface with a transfer rate of 130 MB per second can provide real-time transfer of images to on-board display memory or directly to the host memory.

Calibration The calibration was carried out using the blackbody

furnace (BF-1400) with an emissivity value of 0.996 and a temperature range from 900℃ to 1400℃ of China National Measuring Science Research Institute. The sys-tem is calibrated at different shutter speeds in order to test the influence of dissimilar conditions on the meas-urement. And the relationship between the temperature of the blackbody furnace and the grey-levels of the corre-

Fan Jiang et al. Visual Flame Monitoring System Based on Two-Color Method 287

sponding images at the selected wavelengths is setup.

Experimental results and discussions

Flame Temperature and Soot Concentration Distribu-tions

Fig. 3 shows typical banded images of the flame under the methane gas flow rate of 4.716 m3/h and the air flow rate of 2.323 m/s. Also, the flame temperature fields and soot concentration computed from the two images using the two-color method are also shown in Fig. 3. It can be seen that the results obtained over a range of combustion conditions demonstrate that the system is capable of per-forming on-line measurement of flame temperature and soot concentration distribution, providing temporal and spatial characterization of the combustion process.

Background Noise Analysis Although a CCD array has a high S/N ratio and a wide

dynamic range, there still exists a certain background noise, with a significant effect on the accuracy of meas-urement. In order to distinguish the background noise from the flame effective images, a threshold is introduced. Fig. 4 shows the grey levels of flame images captured at the wavelength of 632 nm. It can be computed that the average grey levels of the background equal to 40.

Flame image at 632 nm Flame image at 700 nm

Flame temperature and soot concentration distribution Fig. 3 Flame images and the temperature and soot concentra-

tion distribution

Fig. 4 Grey levels of flame at 632 nm Therefore, during the image processing, the threshold is defined as 40.

At the center of the image where the grey levels are apparently higher are the effective flame images. Since the apparent temperature of flame can be computed from the grey levels of every pixel according to the calibration data, whole temperature fields can then be obtained from the grey levels of the flame effective images.

Intensification of Flame Image From Fig. 3, it can be seen that the flame images are

somewhat blurred. At the convenience of direct observa-tion and image analysis, digital image processing tech-niques are applied to the intensification of flame images. Fig. 5 shows the image intensifying effects of different filter methods: averaging filter, median filter and self- adaptive filter. It can be seen that the filter techniques can enhance the image display effects to an extent. Further-more, the averaging filter method has more apparently intensified results than the other two. The more in-depth combination of digital image processing technique with the flame inspection needs to be developed in the future.

Conclusions

An instrumentation system has been developed for the on-line continuous measurement of the temperature dis-tribution in a methane-air combustor. The system, in-cluding all optical, mechanical, electronic and computing elements as well as the dedicated application software, has proven reliable and operational in a methane fired combustion environment. Experimental results have demonstrated that this system is capable of characterizing the flame both qualitatively and quantitatively. Analysis of the flame using the image processing technique can help define the threshold and effectively intensify the image display effect. The flame temperature can be quan-tified over a range of combustion conditions, which sets up a solids foundation for combustion analysis.

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Original image at 632 nm Averaging filtering Median filtering Self-adaptive filtering

Fig. 5 Image intensifying effects of filtering methods

Acknowledgement

This work is supported by the National High Tech-nology Research and Development of China (863 Pro-gram) (2006AA05A103) and the National Natural Sci-ence Fund (grant No. 40501017，grant No. 50706053).

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