1. Introduction
Owing to the existential problems of frequent load variation and combustion regulation, as well as the generation of equipment faults, the boiler flame often deviates from the optimum working conditions, failing to ensure a stable and homogeneous flame along with intense and sufficient combustion. The deterioration in the stability of burning not only results in a decrease in thermal efficiency with more pollutants and noise production, but also can cause fire extinguishing or even induce explosions in certain extreme situations. Some theoretical and experimental research on combustion stability has been carried out. Yan et al. studied gas combustion characteristics and flame stability in a cone burner [
1]. It was found that the cone burner was conducive to stable combustion of low calorific value burners. No flameout was observed at higher turbulence intensity. Moore et al. studied the flame stability of methane and oxygen [
2]. Different forms of flame structure were obtained. The relationships between flame morphology and excess air coefficient, oxygen, and gas Reynolds number have been studied. Komarek and Polifke studied the flame dynamics under different swirl numbers [
3]. The effect of flow fluctuation formed by swirls on flame stability were deeply analyzed by using the flame model established by heat release rate pulsation and velocity pulsation. Lilleberg et al. used different chemical reaction mechanisms to analyze the effect of gas equivalent ratio pulsation on combustion stability under different fuel flows [
4]. Voigt and Habisreuther studied the influence of combustion heat release rate pulsation on vortex shedding [
5]. The coupling relationship between vortex shedding and combustion stability in swirl combustion was analyzed and modeled. The stability of combustion has an important impact on the economy, environmental protection, and safety of boilers, thus making the evaluation of combustion stability extremely important. Effective evaluation of flame instability can direct operators to seek better standards as reference for real-time operation, so that the overall synthetic performance of the unit can be improved.
The flame radiates various types of energy signals during the combustion process, such as light, heat, and sound [
6]. Modern flame detection technology can be divided into direct technology and indirect technology. Direct flame detection techniques include differential pressure technology and temperature technology. Although the principles of these technologies are simple, they are very sensitive to the noise on the site. Moreover, the parameters of flame detectors are also difficult to set. Indirect flame detection technologies are currently relatively advanced means of flame detection [
6,
7]. Flame detection based on flame radiation is the method employed most extensively at present but is quite limited in real application of an industrial flame due to the restriction of the theory [
8,
9]. The threshold setting of flame radiation intensity and flicker frequency are difficult to adapt to large range load variation, and detection self-adaptation is restricted as the angle of viewing field of the flame detection probe is in the narrow range [
3]. Furthermore, the flame emission of a single burner is hard to distinguish, owing to the full-filling flame of the furnace, which lead to a serious peep phenomenon, namely the furnace flame is misjudged as the nozzle flame by the flame detection [
10]. Much more, frequency selective filter circuits and relational components in detection devices, which influence the data transmission and the detection effect in the case of slight changes of the radiation signal, often perform poorly [
6]. Besides, transmission quality is affected as the photoelectric detection unit and the signal amplifier are separated [
8]. In the process of sampling the flame radiation signal, the main problem presented is to ensure that the sampling frequency meets the requirements of the data analysis. The flame flicker frequency is generally below 50 Hz. The sampling frequency must be greater than 100 Hz according to the sampling theorem. It is guaranteed that the sampled signal will not be mixed [
8]. Real-time flame detection on site is generally required to take no more than five seconds each time, including signal acquisition, analysis, and judgment.
Researchers and related R&D companies have introduced a series of measurements. Zhou et al. presented a correlation detection method of flame combustion [
11,
12]. Two or more angled flame detection probes were used to detect a single burner flame, and the combustion state was determined based on the correlation analysis. A multi-band detection method was another advanced technique applied by Lv et al., which fused flame radiation signals of multi-bands to determine the flame state [
13]. Detectors of some technical R&D institutions exploit radiation luminance and flicker frequency to identification the flame state. Much research has been made in the area of extracting features of flame radiation signals. A combustion diagnosis method based on combined Fourier transform and mode identification was presented by Ma et al., in which the first lowest 30 signals of each flame’s power spectrum were selected to be used as the neural network’ s input signals [
14]. Li et al. found that the flashing area of the fuel was often fluctuating, and the most prominent location for the flicker frequency was a function of fuel type, load size, fuel flow rate, primary wind speed, and burner diameter [
8]. The flame detector developed by Huang et al. used an optical fiber and photoelectric conversion device to detect the flicker frequency and radiation intensity of visible light emitted by the flame [
11]. Taking into account the installation angle and other issues, the position of the optical fiber was set at the secondary air outlet, avoiding the strong radiation and strong convection of the combustion flame. Ao et al. proposed a new method for distilling the signature by the wavelet algorithm; the signals were subject to a soft threshold value de-noising treatment with the pretreated information serving as a training input to the neural network [
15]. A recent study by Chi et al. showed that four features in time and frequency domains were found to be effective for fuel tracking [
16]. Fuzzy logic inference techniques were applied to combine different features together and infer the type of fuel being burnt. All these studies showed the current research of extraction of features frequently limited in the time domain and frequency domain. Research on the phase domain is very scarce. The phase domain can provide phase information of the digital signal. It can be seen that the traditional combustion evaluation method is limited in data analysis.
An experimental and measuring system for detecting flame combustion stability is constructed. It includes an optical sensing device, a transmission device, and a computer. The radiation signal is transmitted to the phototransistors of the probes The optical signal is converted to a voltage signal. The flame signal is obtained via A/D (Analog to Digital Converter) conversion. The data is transferred to the processing computer, and then the characteristic parameters from the time, frequency, and phase domain are extracted, respectively. The model of fuzzy comprehensive evaluation is established, and the index system is also established. As a comprehensive statistical analysis method, the fuzzy comprehensive evaluation method can be effectively used to reveal the flame combustion stability. The results of the fuzzy comprehensive evaluation are presented in the form of vectors, and the evaluation information provided is more abundant than other methods [
17]. The fuzzy comprehensive evaluation method has strong applicability and can be used for comprehensive evaluation of subjective factors and objective factors [
18].
Figure 1 shows the flowchart of flame detection.
3. Experimental Setup and Process
The flame detection system contains a data acquisition system, a gas burner, a fuel gas, a fan, flowmeters, and a DC power source.
3.1. Signal Acquisition System
In this experiment, the circuit system consisted of two parts: a flame detection circuit and a second-order filtering circuit. The optical signal conversion section adopted photosensitive transistors. In addition, the detection of background signals constructed an appropriate working condition for photosensitive conversion components, eliminated the direct current signals caused by background radiation, decreased the interference of the background glare from the outside, stopped the component from coming into the saturation area, and prevented the interference of the background glare from submerging useful signals; therefore, the dynamic extension of detection was broadened and the signal to noise ratio of the signals was increased. This made the quickly changing and effective dynamic flame signals much enhanced. Simultaneously, the reference source designed in the circuit provided the highly steady reference voltage of +7 V, and furthermore, strengthened the detecting effect of slight changes in flame.
Figure 2 shows the signal acquisition system. The probe had a certain distance from the flame. Radiation from the flame did not affect the probe.
3.2. Combustion Experimental System
In the experiment, a suit of gas burning system that could control the flow of gas flux was designed. Air was inducted to the system by an axial flow fan, and then was divided into two parts: one part of the air (one-step air provision) was carried over by the gas flow to form premixed one-step air after the mixing to a certain extent, then entered the burner from the middle bottom of the nozzle, burst forth to burn and form a conform flame. The other part of the air (secondary air) entered directly into the bottom of the nozzle via a flowmeter, and subsequently flowed out from the annular flow pass of the gas vessel, then contributed to the air circumfluence diffusion burning. In the experiment the ration of secondary air was fixed at 2000 L/h, and the ratio of gas and one-step air provision was changed to accomplish a myriad of conditions. Due to the measuring range of the flowmeter, the ration of gas spanned from 40 L/h to 600 L/h, while the ration of one-step air provision spanned from 0 L/h to 1600 L/h. Along with the alteration of gas and one-step air, real-time flame intensity signals were collected and analyzed with MATLAB (Version 7.1.0.246 Service Pack 3, The MathWorks Inc., Natick, MA, USA).
5. Fuzzy Synthetic Evaluation on Flame Instability
The relationship between the combustion stability and the characteristic parameters was studied. However, it was difficult to determine which parameter was more reliable. The fuzzy synthetic evaluation method was used to obtain more accurate results.
Factors that reflect the combustion stability include specific variance, flicker frequency, and bi-spectral phase. In this paper, the factor set was U = (u1, u2, u3) = (specific variance, flicker frequency, bi-spectral phase).
The determination of weight set was the key to the evaluation. The weight of each factor was given as A = (a1, a2, a3) = (0.6, 0.3, 0.1).
Evaluation level was computed by the fuzzy judgement. The evaluation set was defined as V = (v1, v2, v3) = (stable, transitional stage, unstable). The results of the evaluation model were given as S = (90, 60, 30).
The most important feature of the fuzzy set theory was fixing the membership as continuous closed interval (0, 1) rather than {0, 1}. The membership degrees were applied to solve the contradiction that many other laws are always ignored by the exact demarcation due to the complicated factors. The membership degrees of the judging index were calculated by the membership functions. Based on the analysis of the relationship between the flame instability and the characteristic parameters, the membership degrees of the
ith evaluation grade are shown in
Figure 9. Number 1 presents the stable condition, while number 0 presents the unstable condition. The line ‘stable’ describes the level of approaching the stable condition and the line ‘unstable’ exhibits the probability of being the unstable condition. The threshold (
p1–
p7) of characteristic quantity was set by practical experimental experience. When the numerical value of the characteristic parameters was less-than p1, the state of the flame combustion was considered highly steady and the membership degrees were (1, 0, 0). The possibility of being highly unsteady was not clear until the value reached p2. Two critical points came forth with increases in the value. They illustrated the coequal probability of approaching two closer states. As a transitional state, the line ‘transitional stage’ reached the top in the middle of the threshold. Similarly, the state of the flame combustion was considered highly unsteady and the membership degrees were (0, 0, 1) when the numerical value of the characteristic parameters was more-than p7. By analyzing the measurement database of this experiment, the formula (5.1) – (5.3) manifested the membership degrees of the judging parameters and the thresholds of membership degrees and characteristic parameters, as seen in
Table 1 and
Table 2.
According to the evaluation score calculated, combustion condition class was classified into four grades: highly stable, slightly stable, slightly unstable, and highly unstable.
Table 3 shows the combustion condition class and combustion grades.
According to the former discussion, practical examples were made to examine the feasibility and reliability of the synthetic evaluation method. Some typical combustion tests were used to investigate the model. The judgment matrix of each condition is constructed as follows:
Table 4 gives the flame patterns, their characteristic parameter values, and the evaluation results. The figure presents four different combustion conditions. Their specific variance, flicker frequency, and bi-spectral phase were derived from the data. By computing the evaluation matrix and evaluation score, their combustion grades were calculated. The fuzzy synthetic evaluation of combustion stability based on time–frequency analysis and higher-order statistics was effective.
6. Conclusions
A low-cost experimental device was set up to detect flame combustion. A fuzzy synthetic evaluation method based on time–frequency analysis and higher-order statistics was proposed to evaluate flame instability. The following conclusions were reached.
The appropriate operating parameters of the detection circuit were reasonable and a strict signal acquisition process was set up. The results show that the detection of background signals constructs an appropriate working condition for conversion components. It eliminates the direct current signals caused by background radiation, decreases the interference of the background glare from outside, stops the component from coming into the saturation area, and prevents the interference of the background glare from submerging useful signals. The dynamic extension of detection is broadened and the signal to noise ratio of the signals is increased.
Bi-spectrum calculation shows the non-Gaussian and nonlinear characteristics of different combustion conditions. Three-dimensional figures of the bi-spectrum under various combustion conditions revealed the frequency coupling distribution and phase information. Power spectrum and bi-spectrum information of the phase domain (specific variance, flicker frequency, and bi-spectral phase) were derived. The results showed that the flame radiation frequency domain signal was concentrated in the low frequency range (0 to 10 Hz). With the increasing air/gas ratio, the flickering frequency increased while the specific variance and the bi-spectrum phase decreased.
By settling of the threshold division of membership degrees and the threshold division of characteristic parameters, the mathematical model of fuzzy comprehensive evaluation and the evaluation index system of combustion stability were established. The computation of four different combustion conditions showed that this fuzzy synthetic evaluation method was effective. Compared to existing time–frequency analysis methods, the time, frequency, and phase domain of the combustion radiation signal were analyzed in this experiment. By analyzing the phase domain signal, the analysis method of the flame radiation signal was expanded. The analytical methods need to be further refined to obtain a specific functional relationship between the combustion state and the flame fluctuation information.