1. Introduction
Monitoring of a flame’s state, including the ignition, combustion, and blowout, in combustors with simple, low-cost, and high-reliability flame sensors, is essential to the security and performance of thermal power systems such as industrial boilers, vehicle engines, and ground and aviation gas turbines [
1,
2]. The primary applications of flame sensors are ignition detection and blowout alert, which prevent the combustor from the explosion caused by fuel accumulation in the combustor [
3,
4,
5,
6]. Moreover, flame sensors are routinely used to monitor and avert abnormal combustions in order to achieve the operation requirement of stable, high-efficiency, and low-emission combustion [
7,
8,
9].
Various attempts were performed to precisely and quickly detect the normal ignition, abnormal combustion, and accidental blowout in different combustors. Accompanying these attempts, flame sensing technologies based on flame characteristics, including heat release, pressure fluctuation, luminescence, composition variation, and production of ions and electrons, have been proposed [
2]. Meanwhile, flame sensors based on thermoelectricity, piezo-electricity, photoelectricity, and electrochemistry were developed [
1,
10,
11].
The most prevailing flame sensors are optical emission and absorption sensors. For example, ultraviolet flame sensors have been widely used in ground gas turbine combustors to monitor flame states. Docquier et al. adopted an intensified CCD camera to investigate the chemiluminescence of OH*, CH*, and C2* radicals in premixed methane/air flames and observed that the OH* and CH*/C2* radicals were suitable for monitoring lean and rich flames, respectively [
12]. Ding et al. investigated the relationship between equivalence ratios and chemiluminescence intensities in CH
4/air flames and verified that the CH*/OH* intensity ratio could be an equivalence indicator [
9,
13]. Hariharan and Mishra proposed a wavelet technique with the CH signature to sense dynamic flame stability, including a stable flame, flame liftoff, and main flame extinction, which could be applied in active control systems to avert lean flame blowout [
14]. Tsai and Young used ultraviolet and multiband infrared technology to develop a multisensory-based fire alarm system [
15]. Deguchi et al. used laser diagnostics, including laser-induced fluorescence (LIF), laser-induced breakdown spectroscopy (LIBS), and tunable diode laser absorption spectroscopy (TDLAS) to monitor the flame temperature and species concentration for controlling secondary air allocation and achieving a higher combustion efficiency [
16]. Liu et al. developed a fan-beam tomographic sensor using TDLAS to monitor the temperature and gas concentration, and validation of the sensor exhibited good applicability for flame monitoring [
17]. Later, Liu et al. developed an online and highly spatially resolved imaging system based on TDLAS tomography to monitor the dynamic behavior of swirling flames, which helped to better understand the lean blowout (LBO) mechanism [
18,
19].
Optical sensors are non-intrusive and have the merits of a fast response and high resolution. However, the complicated structure of the sensor composed of many elaborate optical components reduces the sensor’s flexibility, reliability, and maintainability, and the optical windows opened on the combustor reduce the reliability of the combustor structure. Thus, most of the optical flame sensors are usually applied in fundamental research in laboratory, rather than practical flame monitoring in industrial combustors, especially in aero-engines.
For the flame monitoring of industrial combustors working under high pressure, high temperature, and drastic vibration, intrusive flame sensors, such as thermocouples and pressure transducers, are widely used. K-type thermocouples, which are broadly used in ground and aviation gas turbines, are mounted at the turbine exit to monitor the exhaust gas temperature, from which the flame state could be inferred. High frequency dynamic pressure transducers are installed on the afterburners of some turbojet engines, for detecting flame ignition. Intrusive sensors are also often used in lab-scaled experiments. For instance, Gardiner examined the potential of using exhaust gas temperature thermocouples combined with an electronic signal processing method for alarming flame-out. Results from a GE J-85 combustor verified that the proposed thermocouple flame sensor could identify flame-outs and the response time was less than 100 ms [
20]. Rolando et al. detected soot and nanoparticles in a diffusion ethylene flame by transient-thermocouple-based measurements. Their results demonstrated that the method was potent in detecting the particulate volume fraction [
21]. Muruganandam et al. used a microphone to analyze the acoustic emission in a premixed swirl-stabilized combustor and observed the short duration, localized extinguishing, and re-ignition events as the flame approached blowout [
22]. Nair and Lieuwen observed that low-frequency spectrum and intermittent fluctuation of the acoustic signal could be the precursor of a lean premixed flame in combustors with pilot, swirl, and bluff-body stabilizers, and proposed the acoustic method to monitor the precursor of LBO [
23,
24].
However, thermocouples are easily oxidized in high temperature flames and respond slowly to flames. Acoustic methods need the impulse line to transmit pressure, which complicates the sensor structure and reduces the pressure accuracy.
An ion current sensor with a simple structure, low cost, fast response, and good maintainability provides an alternative possibility for practical flame monitoring. It has been commonly used for combustion diagnostics in lab-scaled combustors and active combustion control in industrial thermal systems. For example, Strandh et al. used the spark plug as ion current sensor to measure the ion current in homogeneous charge compression ignition (HCCI) combustion and proved that the ion current signal could be an excellent indicator of the combustion timing [
25]. Yoshiyama and Tomita used the spark plug as an ion current probe to detect the combustion quality in a commercial spark ignition (SI) engine [
26]. Chorpening et al. developed a combustion control and diagnostics sensor (CCADS) based on flame electrical properties for lean premixed gas turbine combustors. A CCADS has the capabilities of monitoring the flashback, equivalence ratio, and combustion instability [
27]. Li et al. proposed the ion current sensing method to detect lean blowout in a pulse combustor [
4]. They verified that the ion current sensing method could help to determine the LBO limit and avert an unexpected LBO, and observed that the ion current sensor was more sensitive to the LBO precursor than the pressure sensor. Chang et al. used the ion current signals to detect the LBO events for bluff-body-stabilized flames under a low Reynolds number [
3].
Previous applications in different combustors during recent decades have verified that an ion current sensor with a central electrode can be conveniently and rigidly mounted in a combustor chamber without dramatically altering the chamber structure. The ion current signals can be a fast and reliable indicator for flame state and the relevant operation parameters, and the ion current sensor is a better choice for practical flame sensing provided that the central electrode is reasonably mounted. Nevertheless, ion current sensors have the intrinsic defect of producing weak ion current signals, which can be easily interfered with by other electronic devices and difficult to be collected by the acquisition system. In addition, the weak signals would seriously reduce the accuracy and reliability of the sensor for flame sensing. Therefore, for monitoring flame states, including the ignition, combustion, and extinction of practical flames, the parameters of an ion current sensor, especially the electrode installation positions, should be correctly determined to obtain an intense and stable signal during combustion, and in contrast, very weak signals during extinguishing.
In this paper, we recommend the reasonable sensor parameters for an ion current sensor used in flame monitoring of a Bunsen burner, and a method for further improving the ion current is also proposed. Experiments were performed on a Bunsen burner since most of the practical flames consist of premixed and diffusion flames, and similar in combustion mechanism and flame structure to the flame on a Bunsen burner. Effects of the sensor parameters, including electrode polarity, excitation voltage, electrode area, and electrode radial and vertical positions, on the ion current were investigated first. Then, based on the regularities of the ion current, the reasonable sensor parameters for monitoring the flame’s state of ignition, combustion, and extinction were recommended. Meanwhile, the recommended parameters were testified in a wide operating range, from weak to strong combustion, by altering the propane volumetric flowrate. A method of adding multiple sheet cathodes near the burner exit to further improve the ion current during combustion was proposed, and the effectiveness of the method was validated in a wide operating range.
4. Determination of the Sensor Parameters for Flame Monitoring
4.1. Determination of the Sensor Parameters
The parameters of the ion current sensor must be determined before the sensor is applied to practical engineering. As for the sensor used for flame monitoring, the sensor parameters that are corresponding to the maximum ion current are recommended to be selected. Based on the regularities of the ion current changing with each sensor parameter described in the previous context, the ideal sensor parameters are suggested as follows.
Since I increased with Ue, Ue for the ion current sensor should be set as 120 V, which was the maximum voltage of the DC power supply in the experiment.
In terms of , increased with and independent of . Since the area of the burner rim was larger than that of the single platinum wire, the burner rim should be equivalent to a cathode, whereas the platinum wire should serve as an anode.
In terms of , if the electrode was immersed into the flame, was nearly constant for a different , but decreased with . Thus, making the cathode steadily in contact with the flame with a larger area could improve the magnitude of the ion current. Since the burner rim was very close to the flame base under various throughout, and the area of the burner rim was larger than that of the platinum wire, the burner rim should serve as the cathode, while the platinum wire should act as the anode.
In terms of , if was more than 6 mm, I fluctuated with but was independent with . Thus, I was sensitive to but insensitive to . Since the distance between the burner rim and the flame base was nearly stable under various , the burner rim should be a cathode, while the single platinum should serve as an anode.
In summary, the burner rim should be selected as the cathode, while the single platinum should be selected as the anode.
When the anode radial position was 0 mm, the anode was adequately in contact with the flame fronts and was stronger and more stable. Therefore, is recommended to be set as 0 mm.
I increased with = 1–6 mm, and kept constant at = 6–80 mm. Consequently, to ensure that I is high and stable, should be selected as 6–80 mm. In this paper, was set as 6 mm.
4.2. Sensor Verification for Flame Monitoring
To verify the legitimacy of the sensor parameters determined in
Section 4.1, the ion current sensor with the determined parameters was used for flame monitoring (ignition, combustion, and extinction) on a Bunsen burner. The determined sensor structure is depicted in
Figure 16 and the
signals collected under
= 50–150 L/h are displayed in
Figure 17. In order to verify the accuracy of the ion current sensor for flame monitoring, an optical flame sensor with an ultraviolet phototube was used to synchronously monitor the flame with the ion current sensor. The signal intensity from the optical sensor could correctly reflect the flame state. The signal intensity of 0 V and 6 V referred to the flame extinction and combustion, respectively.
From
Figure 17, the tendency of the ion current signal is consistent with that of the ultraviolet sensor signal. It confirms that the ion current sensor could correctly identify the flame ignition and extinction, and the sensor parameters determined in
Section 4.1 are reasonable. Shapes of the ion current signals are similar and the typical shape can be divided into 3 sections: the section of the horizontal signal that corresponds to flame combustion, and the two sections of the step signals that refer to the flame ignition and extinction, respectively. For instance, at
= 100 L/h,
stepped from 0 μA to 4.09 μA when the flame was ignited successfully; then
fluctuated around 4.09 μA along with flame combustion; finally,
dropped from 4.09 μA to 0 μA as the flame extinction.
In addition, the average ion current during flame combustion under
= 50–230 L/h were acquired, and the results are displayed in
Figure 18. In this Figure, the average
during combustion were all higher than 2 μA but lower than 5 μA.
6. Conclusions
In this work, a method for improving the ion current from the partially premixed flame on a Bunsen burner is proposed, to improve the reliability of the flame monitoring by the ion current sensor. The primary purposes of this work are (1) to investigate the effects of the ion current sensor parameters, including the excitation voltage, electrode area, and the electrode radial and vertical positions on the ion current; (2) to determine the reasonable sensor parameters for achieving a stronger and more stable ion current; and (3) to propose an effective measure to further strengthen the ion current.
The observed results are as follows. The ion current was linearly proportional to the excitation voltage. As for electrode area, if the electrode was inserted into the flame fronts, the ion current rose with the cathode area, whereas being constant with the anode area. As to the electrode radial position, the positions at which the electrode could be immersed into the flame fronts resulted in the higher ion current. As for the electrode vertical position, when the electrode was kept away from the flame base at a distance of at least 6 mm, the ion current was insensitive to the anode vertical position, yet very sensitive to the cathode vertical position. Correspondingly, to achieve a stronger ion current, the parameters of the ion current sensor used for flame monitoring on a Bunsen burner were suggested as follows. The excitation voltage should be set as 120 V, which was the maximum voltage applied in this work. The platinum wire should serve as the anode, while the burner should act as the cathode. The anode radial position should be 0 mm, at which the anode was entirely immersed into the flame fronts. The anode’s vertical position should be 6 mm away from the burner exit, which means that the anode was away from the flame base. A sensor with the recommended parameters was used in this experiment for flame monitoring on a Bunsen burner. The ion current signals from the sensor testified that the ion current could correctly signify the flame ignition, combustion, and extinction. Therefore, the suggested parameters are considered to be reasonable.
In addition, according to the regularities of the ion current varying with the sensor parameters, a method of adding a sheet cathode was proposed to further improve the ion current. Verification of the proposed method was performed during flame monitoring, and the results obtained shows that the proposed method of adding a sheet cathode had no effect on the responses of the ion current sensor to flame ignition and extinction, and significantly strengthened the ion current during combustion. Thus, adding a cathode area is an effective measure to improve the flame ion current.