GB2390675A

GB2390675A - Flame characteristic monitor using digitising image camera

Info

Publication number: GB2390675A
Application number: GB0215985A
Authority: GB
Inventors: Yong Yan; Gang Lu
Original assignee: University of Greenwich
Current assignee: University of Greenwich
Priority date: 2002-07-10
Filing date: 2002-07-10
Publication date: 2004-01-14
Also published as: GB0215985D0

Abstract

The apparatus includes an optical probe (4) for transmitting light from a flame (1) into an imaging device e.g. a monochromatic CCD camera (8), a frame grabber for capturing and digitising images of the flame, and a computer (9), to which the digitised images are conveyed for processing. The flame parameters are then presented graphically or numerically on a computer screen. A set of characteristic parameters such as ignition region or area (11, figure 2), ignition point, and spreading angle (18, figure 2) may be identified from the precessing of the images of the camera. Hence, flame parameters may be quantified by calculating statistical variation from which the total stability of flame may also be calculated by combining the individual parameters of the flame. The camera may be provided with zoom lens(7),and filters (5), whereby flame images of specific wavelengths may be captured.

Description

DIGITAL IMAGING BASED MONITORING OF FLAME STABILITY

This invention concerns monitoring methodologies of flame stability.

Flame stability monitoring is becoming increasingly important to achieve improved plant safety, increased combustion efficiency and reduced pollutant emissions from fossil fuel and waste-fired combustion processes. Current practice 8 of flame monitoring uses a simple optical detector called flame-eye which indicates only whether the flame is present or out for safety reasons. However, the use of fossil fuels from a variety of sources and blends of various fuels in recent years has shown problems of flame stability, often resulting in poor 12 combustion efficiency, high emissions and operational problems. Meanwhile, co-

firing of other fuels such as biomass and waste is also becoming an area of concern in respect of plant safety, fuel rangeability, operating flexibility and maintainability. It is for these reasons that costeffective techniques for the on 16 line monitoring of flame stability have become highly desirable. No techniques are currently available to provide a quantitative measurement of flame stability.

The present invention is concerned with such a technique based upon two-

dimensional optical sensing and image processing algorithms.

An object of the present invention is to provide an improved method of flame characterization. 24 According to a first aspect of the present invention there is provided a method of flame characterization including the steps of visualising the ignition region of a flame using an imaging device to generate images of the flame, digitising the images so generated, feeding the digitised images to a computer memory in the 28 form of image matrices, determining characteristic parameters of the flame from the image matrices, and presenting the parameters graphically and/or numerically on a computer screen.

The primary feature of the method is that it combines digital imaging and image processing techniques. The ignition region of the flame is visualised using a suitable imaging device such as a CCD camera. A set of characteristic 4 parameters of the flame is then determined from the digital images, including ignition point, ignition area and spreading angle. The stability of the flame is quantified by calculating and combining the statistical variations of these parameters. According to a second aspect of the invention there is provided apparatus for flame characterization including an optical probe for transmitting light from a flame into an imaging device, a frame grabber for capturing and digitising 12 images of the flame, and a computer to which in use digitised images are conveyed for processing to generate data for characterizing the flame.

The imaging device may be a monochromatic CCD camera which may be 16 provided with a zoom lens whereby the image size may be adjusted. One or more filters may be provided in association with the camera whereby flame images of specific wavelengths may be captured.

20 By way of example one embodiment of the invention will now be described with reference to accompanying drawings in which: Figure 1 shows a schematic diagram of the invention; Figure 2 illustrates the definition of the relevant parameters of the flame.

28 Figure 1 shows a typical configuration of the imaging system for the monitoring of flame stability. An optical probe 4 is deployed to transmit the flame light 1 into a suitable imaging device 8, such as a monochromatic CCD camera. A water-cooled jacket may be used to prevent the probe from excessive heat from

the furnace 2. The objective lens of the probe is cooled and kept dustfree by a purging airflow. The camera lens 7 is capable of zooming so that the size of the image is adjustable. A short-wave pass filter 5 and a neutral filter 6 are fitted in 4 front of the camera enabling flame images at specific wavelengths to be captured as well as preventing the imaging system from saturation. The video signal from the imaging device 8 is transmitted via a screened cable to the microcomputer 9.

The acquisition and digitization of the images are achieved using a standard 8 frame grabber. The digitised images are then transmitted into in the host memory in the form of image matrices. A set of the characteristic parameters of the flame is determined from the image matrices. The stability of the flame is then quantified from the flame parameters and finally presented graphically and 12 numerically on the computer screen.

The definitions of the relevant parameters are illustrated in Figure 2. Logical steps have to be followed to obtain these parameters.

1) Ignition points (Ip,,,ol, Ipm,n and IpoVC) represent the maximum, minimum and average distances between the burner outlet and the illuminated points where the flame is ignited. To determine the ignition points, the boundary of the flame front 20 has to be identified. This is achieved by searching the illuminated points between points 14 and 15, as shown in Figure 2, line by line horizontally over the flame image, starting from the burner side of the image. To reduce random noise, the gray-level of the pixel being processed is replaced by the averaged graylevel of 24 the nearest 3x3 adjacent pixels. The ratio between the graylevels of two adjacent pixels is compared with a preset threshold. If the ratio exceeds the threshold the pixel is then considered as the ignition point at the corresponding line. This procedure is repeated until all the points between points 14 and 15, which form 28 the boundary of the flame front 10, are identified. The distance between the burner outlet 24 and the furthest illuminated point is defined as Ip,,,," 13 whilst the distance from the burner outlet to the nearest illuminated point is defined as Id,,,,

12. IpaVC is determined by averaging the distances between the burner outlet and all the illuminated points on the boundary of flame front 10.

4 2) Spreading angle (Sang/c) is defined as the angle formed between the two straight lines 19 and 20 scribing through the outer edges of the flame, as shown in Figure 2. The locations of lines 19 and 20 are determined according to the following procedure: 8 (a) Let line 19 scanning the image area clockwise about the point 15, starting from the vertical position at an appropriate pace; (b) Calculate the averaged graylevel of the pixels along the line 19 for each scanning; 12 (c) The averaged gray-level obtained in (b) will be higher when line 19 crosses: the flame region than that when it is outside the flame. By introducing an appropriate threshold, the angle 16 between line 19 and the burner axis 21 is determined; 16 (d) Similarly, the angle 17 between line 20 and burner axis 21 is obtained by repeating steps (a) to (c) but scanning line 20 counterclockwise about the point 14; 9 The spreading angle 18 (Sang/e) is then the sum of angles 16 and 17.

3) Ignition area (la) is a measure of the area 11 (shaded area in Figure 2) encompassed between burner outlet 24, flame front 10, lines 19 and 20. The ignition area 11 is regarded as the total number of pixels within the area, which 24 are obtained by counting the pixels line by line within the area, and then normalised to the area of the entire viewing field.

The flame parameters defined above are computed continuously in the system software. Due to the inherent dynamic nature and instability of the flame, these parameters vary with time. The variability of each parameter is quantified by 4 introducing a new term entitled the instability, which is defined as: fix = X xioO% (1) X where Cal and X are the standard deviation and full-scale value of the parameter lx) respectively, and is the resulting instability of the parameter.

Finally, the instability of the flame is represented by combining the instabilities of all the flame parameters: 12 =(WI6Ipmax) +(W231pmm) + (W3ilpave) +(W431a) (W535angle) (2) where w2 = I (3) =! 16 and w, (i = l, 2,..., 5) is the weight fraction of each flame parameter.

The method described represents a typical example of the embodiments within the present invention. It will be understood that the invention may be embodied 20 otherwise without departing from such principles. All such ways are intended to be included in the scope of the claims.

Claims

CLAIMS:

4 1. A method of flame characterization including the steps of visualising the ignition region of a flame using an imaging device to generate images of the flame, digitising the images so generated, feeding the digitised images to a computer memory in the form of image matrices, determining characteristic 8 parameters of the flame from the image matrices, and presenting the parameters graphically and/or numerically on a computer screen.
2. A method according to Claim 1 in which the ignition region of the flame is

12 visualised using a CCD camera.
3. A method according to Claim 1 or 2 in which a set of characteristic parameters of the flame is determined from the digital images, including 16 ignition point, ignition area and spreading angle.
4. A method according to Claim 3 in which the stabilities of the flame parameters are quantified by calculating the statistical variations of these 20 characteristic parameters.
5. A method according to Claim 2 in which the stability of the flame is quantified by calculating and combining the stabilities of the individual 24 parameters of the flame.
6. A method of flame characterization substantially as hereinbefore described with reference to the accompanying drawings.
7. Apparatus for flame characterization including an optical probe for transmitting light from a flame into an imaging device, a frame grabber for capturing and digitising images of the flame, and a computer to which in use

digitised images are conveyed for processing to generate data for characterizing the flame.

4
8. Apparatus according to Claim 6 in which the imaging device is a monochromatic CCD camera.
9. Apparatus according to Claim 7 in which the camera is provided with a zoom 8 lens.
10. Apparatus according to Claim 8 or 9 in which one or more filters is provided in association with the camera whereby flame images of specific wavelengths 12 are captured.
11. Apparatus for flame characterization substantially as hercinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A method for the monitoring and characterization of the stability of a flame produced by a burner, the method including the steps of 5 visualising the ignition region of a flame using an imaging device to generate images of the flame, digitising the images so generated in the form of image matrices, determining characteristic parameters of the ignition region of the flame from the image matrices, and estimating the stability of the flame by combining 10 any statistical variations of the said parameters.

2. A method according to Claim 1 in which a set of the characteristic parameters of the flame is determined from the digitised images, including ignition point, ignition area and spreading angle.

3. A method according to Claim 2 in which the ignition points of a flame are represented by its maximum, minimum and average values, the average values being determined by searching the illuminated points of the pixels within the ignition region of the 20 flame line by line horizontally over the image matrix of the flame, starting from the burner side of the image.

4. A method according to Claim 3 in which the gray-level in each pixel of the said image matrix is replaced by the averaged grey 25 level of the nearest adjacent pixels, typically 3x3, in order to reduce random noise.

5. A method according to Claim 4 in which the said illuminated point of the flame at a horizontal line is determined by determining the 30 ratio between the gray-levels of two adjacent pixels at that line with predetermined threshold.

6. A method according to Claim 5 in which the distance between the outlet of the burner and the furthest illuminated point is defined as the maximum ignition point whilst the distance from the burner 5 outlet to the nearest illuminated point is defined as the minimum ignition point, and the averaged distance between the burner outlet and all the illuminated points is the mean ignition point.

7. A method according to Claim 2 in which the spreading angle of the 10 flame is the angle between the straight line scribing through the upper outer edge of the flame and a straight line scribing through the lower outer edge of the flame.

8. A method according to Claim 7 in which the location of the said 15 straight line scribing through the outer edge of the flame is determined by averaging the gray-level of the pixels along the said straight line and scanning the said straight line clockwise consequently about a predetermined point at the upper part of the flame image, starting from the vertical position at an appropriate 20 pace until the said gray-level of the pixels along the said straight line exceeds a predetermined threshold.

9. A method according to Claim 7 in which the location of the said lower straight line is determined by averaging the gray-level of the 25 pixels along said straight line and scanning the said straight line counterc] oclcwise consequently about a predetermined point at the lower part of the flame image, starting from the vertical position at an appropriate pace until the said gray-level of the pixels along the said straight line exceeds a predetermined threshold.

to 10. A method according to Claim 2 in which the ignition area of a flame is a measure of the area encompassed between the burner outlet and flame points, which is obtained by counting the pixels line by line within said area over the image matrix, and then 5 normalised to the area of the entire viewing field or a reference

area. 11. A method according to any one of the preceding claims in which the stabilities of the flame parameters are quantified by calculating 10 the statistical variations of these characteristic parameters, which are defined by fix - X X 100%, where and are the standard deviation and full-scale value of the parameter (x) respectively, and is the resulting instability of 15 the parameter.
12. A method according to Claim 11 in which the instability of the flame is determined by combining the instabilities of all the aforesaid flame parameters by using 20 = 4(W'3pmax) + (W23pm.n) + (wJ3pave) + (whoa) + (w535ang,e), where w2 =1 and w, (i = 1, 2,..., 5) is the weight fraction of i=] each flame parameter.
13.A method for the monitoring and characterization of the stability of 25 a flame substantially as hereinbefore described.

i\
14.Apparatus for the monitoring and characterization of the stability of a flame substantially as hereinbefore described with reference to the accompanying drawings.