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O2 jets on combustion instabilities and NOx emissions in lean-premixed methane flames
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Full Length Article
Effects of annular N2/O2 and CO2/O2 jets on combustion instabilities and NOx emissions in lean-premixed methane flames ⁎
Hao Zhou , Chengfei Tao Zhejiang University, Institute for Thermal Power Engineering, State Key Laboratory of Clean Energy Utilization, Hangzhou 310027, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Combustion instability Lean-premixed flames Annular injection NOx emissions Oxy-fuel
Experiments on the effects of annular N2/O2 and CO2/O2 jets on combustion instabilities and NOx emissions in lean-premixed methane flames were conducted. Two variables were investigated to validate the effectiveness of suppressing combustion instabilities and NOx emissions synchronously—the flow rate and the volume fraction of O2. Results indicate that the amplitude-damped ratio of combustion instability can reach 79.53% under CO2/O2 injection, sound pressure reduced from 30.3 Pa to 6 Pa. Damped ratio of NOx emissions achieve 45% under the case of CO2/O2 injection, NOx emission reduced from 22.5 ppm to 12 ppm, but the N2/O2 injection case cannot significantly reduce NOx emissions. Mode shifting of flame heat release rate is observed during the process of annular injection, the amplitude of flame heat release rate increases to two times than that of the uncontrolled flames during the N2/O2 injection case, but the amplitude maintains a relatively stable level in the case of CO2/ O2 injection, under both N2/O2 and CO2/O2 jets, oscillation frequency of flame heat release rate jumps from 265.5 Hz to approximately 125 Hz. The CO2/O2 jets lead to a more uniform temperature field than the N2/O2 jets, thus contribute to lower NOx emissions. The application of CO2/O2 injection improves combustion velocity and provides better mixing, the flame length becomes shorter and compact. This study realized the reduction of combustion instabilities and NOx emissions simultaneously, which could be conducive to the prevention of combustion instabilities or pollutant discharge in industrial gas turbine burners.
1. Introduction Lean-premixed combustion technologies were extensively used in the land-based power generation and aviation or astronautics industries [1]. With the application of a lean-premixed combustion technique in gas turbines, the local flame temperature is decreased and NOx formation is reduced exponentially [2]. However, the lean-premixed combustion technique is susceptible to combustion instabilities [3], emerging of combustion instability, or say thermoacoustic instability is an undesired phenomenon that can cause serious damages. Combustion instability is a generalized definition of unsteady flame dynamics, it not only contains unstable combustion dynamics such as extinction limits, flashback limits, limit cycles, bifurcation, and hysteresis, but also contains self-excited acoustic motion in combustor chamber, flame surface variation, equivalence ratio fluctuation, and vortex shedding due to hydrodynamic instability [1–3]. To eliminate thermoacoustic instabilities, methods of suppressing thermoacoustic instability are extensively explored. Active control techniques utilize external excitations (such as acoustic forcing and fuel modulation) to attenuate combustion oscillations. While effective in suppressing the instabilities,
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active control requires high-speed actuators and adds significant complexity to the design of the combustor. For this reason, the potential for suppressing thermoacoustic instability through passive control such as injecting medium near the flame anchoring zone with minimal complexity and lower cost are desirable [4–7]. Suppression methods for eliminating lean-premixed combustion instability have been extensively explored in the past [1–2,4–7], but they tend to partially focused on the effectiveness of combustion instability and neglect the negative side effects on pollutant emissions during control, thus may cause increased pollutant emissions. The recent development in novel combustion techniques demonstrates the promising prospect of oxy-fuel combustion technology in eliminating combustion stability and NOx emissions of gas turbine combustors. Kangyeop Lee [8] demonstrated that the CO2 dilution rate results in a decrease in NOx emission and combustion oscillation frequency in lean premixed flames. Baolu Shi [9–11] explored the effects of N2 and CO2 dilution on flames and found that the flame temperature, NOx emissions, flame structure were different under the N2 or CO2 atmosphere. Ashwani K. Gupta [12–14] investigated the role of CO2 on flame acoustic and heat release signatures and concluded that most
Corresponding author at: State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China. E-mail address: [email protected] (H. Zhou).
https://doi.org/10.1016/j.fuel.2019.116709 Received 19 September 2019; Received in revised form 23 October 2019; Accepted 20 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Hao Zhou and Chengfei Tao, Fuel, https://doi.org/10.1016/j.fuel.2019.116709
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Nomenclature Φ FFT S Z PMT Φ kW Hz
T f φ dh dn CH4 CH* Pa °C
equivalence ratio fast Fourier Transform swirl number flame length photomultiplier tube inner diameter thermal power frequency
flame fluctuations observed at low frequencies in Oxy-CO2-methane mixtures. Mario Ditaranto [15] studied combustion instabilities in sudden expansion oxy-fuel flames and found that the heat release and pressure fluctuations are linked with the O2 mass-flow rate. Mustafa Ilbas [16] found the oxy-fuel combustion performances of the coalderived syngases are better than that of the air-fuel combustion. Martin de la Torre [17] compared radiative heat release from premixed oxysyngas and oxy-methane flames and revealed that the presence of CO2 in the flames becomes more dominant over the effect of flame temperature as the CO2 flow rate exceeds 6%. C.Y. Liu [18] use GRI Mech 3.0 to study thermodynamic and chemical kinetic of oxy-fuel combustion in gas turbines and point out the existence of a window for optimal oxygen/diluent ratio in the oxidizer flow streams for oxy-fuel combustion. Direct numerical simulation was carried out to explore the role of CO2 and H2O in premixed turbulent oxy-fuel combustion [19]. Sebastian Bürkle [20] investigated the flue gas thermochemical composition of an oxy-fuel swirl burner, oxy-fuel atmospheres were shown to increase the temperature gradients in the axial direction, due to radiative heat loss. With the Raman measurements, Peter Kutne [21] revealed a significant difference between the temperature measured in the outer recirculation zone and the adiabatic flame temperature, the
flame temperature cutoff frequency swirler vane angle inner tube diameter vane cross-section diameter methane chemiluminescence sound pressure centigrade
larger heat capacity of CO2 compared to N2 that leads to lower combustion temperatures for the same equivalence ratio and the lower flame speed. A comparison between the stabilization of premixed swirling CO2 diluted methane oxy-flames and methane-air flames was conducted [22–25] and indicate that the presence of CO2 in the oxidant has a profound effect on the flame structure. Paul Jourdaine [26] studied the difference of stabilization of CH4/CO2/O2 and CH4/N2/O2 flames and found that CO2-diluted flames are however less stable than N2-diluted flames near blow off-limits. Although there are many studies for suppressing combustion instability with annular air injection or fuel injection around the flame [4–7], to date, there is no research on the application of CO2/O2 oxidizer for suppressing of lean-premixed combustion instability and NOx emissions simultaneously. As the oxy-fuel combustion technique could significantly lower the NOx emission and improve combustion efficiency [8], the combination of oxy-fuel combustion with annular jets around the flame anchoring zone can be used to control combustion instability and NOx emissions synchronously. Besides, mode shifting characteristics of flame heat release rate during air injection (N2/O2) and oxy-fuel injection (CO2/O2) has never been investigated before. Moreover, a flame structure such as length and temperature
Fig. 1. Schematic figure of the model combustor. 2
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distribution variation tendency has not been specifically explained in terms of different injection medium as the oxidizer. The purpose of the present work is to explore the effects of annular N2/O2 and CO2/O2 jets on combustion instabilities and NOx emissions in lean-premixed methane flames, thus verify the feasibility of damping lean-premixed combustion instability and NOx emissions synchronously in a model gas turbine combustor and illuminate the mechanism behind. Systematic experimental research is proposed to evaluate the impact of CO2/O2 fraction and injection flow rates on the effectiveness of coordinated control. This study can promote a further understanding of thermoacoustic instability passive control with methods of micropore jets around the flame nozzle. Identically, providing a useful tool for a better combustion instability and NOx emissions suppression of industrial gas turbine burners.
S =
2 ⎛ 1 - (dh/ dn )3 ⎞ 2 tan φ ≈ tan φ 3 ⎝ 1 - (dh/ dn )2 ⎠ 3 ⎜
⎟
(1)
where φ is the swirler vane angle, dh and dn are the diameters of the inner tube and the vane cross-section [1]. With equation (1), the swirl numbers of the combustor are calculated as 1.153. The length of the optically accessible quartz combustion chamber is 300 mm, with two holes symmetrically placed at the downside of quartz for pressure measuring and ignition, the inner diameter of the two glass holes is 10 mm. The downstream of the quartz combustor is connected to a stainless exhaust outlet with a height of 315 mm, temperature measuring holes are reserved. 2.2. CO2/O2 and N2/O2 injection section design In Fig. 2, an annular stainless air injection section with eight annular holes is installed between the outlet of the burner nozzle and the quartz combustion chamber, the height of the air injection section is 50 mm and four holes were used for micropore injection in this article, with an inner diameter of 1.0 mm. The external diameter of the injection section is 120 mm, eight air injection holes with an inner diameter of 4 mm were evenly weld around the section surface, the height of these weld holes is 25 mm. Four holes were selected every other one from the eight holes in the injection section, the CO2/O2 and N2/O2 injection medium were transferred to the four injection nozzles and then to the root of the flame, four jet nozzles with 1.0 mm inner diameter were inserted in the injection section holes, and the four jet nozzles were arranged evenly at a 90-degree angle toward the radical center of the combustion chamber, the outer of the annular stainless tube is connected with Teflon tubes. With the geometry parameters of the combustor, the cut-off frequency of the lean-premixed combustor was calculated as f = 1504 Hz, this value was bigger than the excitation frequency of flame-acoustic resonance in this study.
2. Experimental setup 2.1. Model gas turbine combustor Experiments were performed on a laboratory-scale, swirl-stabilized premixed combustor, Fig. 1 shows a schematic parameter of the lean premixed model gas turbine combustor. The combustor consists of four parts, an air-fuel mixture chamber, a swirl burner nozzle, an optically accessible quartz combustion chamber, and an exhaust gas discharge duct. Fuel and air are mixed in the air-fuel mixture chamber of the combustor, the length of the mixing chamber is 400 mm with a diameter of 114 mm, two honeycomb plate is placed in the air-fuel mixture chamber for better gas mixing thus reduce equivalence ratio fluctuation of flames. Fig. 1 also shows the geometry design of the burner. The inner diameter of the whole combustor is fixed at 114 mm for the quartz combustion chamber, exhaust gas outlet section, and the mixing chamber. The axial swirler has 16 vanes with a 60° vane angle, the height of the burner nozzle section is 100 mm, a bluff body with an external diameter of 6 mm is inserted in the middle of the swirler, the corresponding swirl number can be written as equation (1).
2.3. Experimental condition Fig. 3 shows the layout of the measuring instruments of the
Fig. 2. The geometry of the annular stainless tube with eight air injection holes. 3
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direction of the flame. When the flame figure is recorded, the measuring scale is equal to the magnification scale of the flame, and the practical length of the unit pixel can be obtained. Let the number of N pixels corresponding to the measuring scale of y, then the actual size of the unit pixel is Δy = y/N . The actual length of the flame is Lk ∗ Δy . 4th. Compare the length of Lk ∗ Δy from each edge point one by one, find the maximum value, and get the corresponding coordinates at its maximum value ( x max , ymax ). In Figs. 1 and 3, air as an oxidizer is conveyed through two radially opposed apertures at the upstream of the burner. The air is supplied with an industrial fan (YASHIBA-HG-1100, Output: 300 W, Pressuremax: 15 kPa) and the flow rate is controlled by a glass rotameter (2% FS). Research level methane (CH4, gas purity: 99.995%) is delivered through Teflon pipes to the mixing chamber and then mixed with air completely. In Fig. 3, the volume flow rate of the fuel (methane) is adjusted with a mass flow controller (Alicat MC-series, 0.2% FS) and the thermal power of the combustor is fixed at 3.0 kW, with the methane flow rate keeps constant, during the experiments, the global equivalence ratio of the lean-premixed flame is selected as 0.90. CH4 is provided with fuel storage tanks (10 MPa, 40L), N2, CO2, and O2 is provided with storage tanks (12 MPa, 40L) and mixing in the gas mixer, then converted with Teflon tubes. During experiments, the combustor is operated at atmospheric pressure and ambient temperature. The experimental conditions explored are summarized in Table 1.
Fig. 3. The Layout of related measuring instruments for the experiments.
experiment. Sound pressure oscillations of the combustion chamber are collected with dynamic pressure transducers (Sensitivity: 1 mV/Pa, bandwidth: 0–20 kHz, rise time: 5 μs). In Fig. 3, the dynamic pressure transducer is inserted in the holes of the quartz combustion chamber at the position. Flame CH*(430 ± 10 nm) chemiluminescence signal implies the fluctuation of the heat release rate of premixed flames during thermoacoustic instability. CH* chemiluminescence signal is measured with a photomultiplier tube (PMT, Hamamatsu H10722 series), the sound pressure and CH* chemiluminescence signal is recorded simultaneously using a multi-channel signal recorder (National Instruments, USB-6210) at a 4 k Hz sampling rate. The above experimental data is recorded and processed by using Labview 2012 and Matlab 2019a commercial software. In Fig. 3, along with the PMT, the lean-premixed flame images during experiments are recorded synchronously using a high-speed CCD camera (AOS S-PRI plus) through a signal synchronizer (MODEL DG535). Flame temperature is recorded by a K-type thermocouple (TM-902C, −50–1300 °C). NOx emission concentration is measured with a Testo flue gas analyzer (Testo 350) with a sampling rate of 1 Hz. The visible length of the flame was calculated with the ‘Canny edge detection algorithm’. To identify the edge of the flame and extract the edge of the flame picture for calculation of flame length, the canny edge detection method is not susceptible to noise and can detect true weak edges of flame, the strong and weak flame edges are detected separately using two different thresholds, and the weak edges are included in the output image when the weak and strong edges are connected. The Canny edge detection algorithm contains four steps. Step1: smoothing the image with a Gaussian filter. Step2: Calculate the magnitude and direction of the gradient using the finite-difference of the first-order partial derivative. Step3: Non-maximum suppression of gradient amplitude. Step4: Detect and join edges with a double threshold algorithm. After obtained the canny edge with high-speed flame photography and Matlab code, flame length calculation can be done with the following four steps: 1st. Starting from the root of the flame and calculate the number of boundaries of each line, and find the middle coordinate of the flame root ( x root , yroot ). 2nd. Then scan the boundary image point by point and calculate the pixel distance between each boundary point and the starting point one by one: Lk = (x i − x root )2 + (yi − yroot )2 . Point Lk was the pixel distance of the kth point ( x i , yi ) from the flame root ( x root , yroot ), the coordinates of the flame boundary ( x i , yi ) can be obtained at the same time. 3rd. The above calculation is the pixel length, to obtain the actual flame length, the measuring scale is placed parallel to the burning
3. Results and discussions When the thermal power of the model gas turbine combustor is fixed at 3 kW, the equivalence ratio of the combustor is fixed at 0.90, the combustor exhibits self-excited combustion instability at an oscillation frequency of 265.5 Hz and 30.06 Pa without control. Fig. 4(a) and Fig. 4(b) shows the FFT (Fast Fourier Transform) results for the resonance frequency of sound pressure and flame heat release rate with conditions set in Table 1, the maximum pressure recorded was 30.06 Pa without control, the maximum relative amplitude of flame heat release rate recorded was 0.004. Two parametric variables were investigated—the annular medium injection flow rate and the volume fraction of O2 in oxidizer, the results of CO2/O2 injection are compared with N2/O2 injection. With these conditions, the suppression effects of combustion instability and NOx emissions are experimentally studied in this article. Since annular air injection not only eliminates sound pressure but also triggers frequency and amplitude shifting of flame heat release rate, the dynamic characteristics of the flame heat release rate before or after control is studied. Furthermore, temperature distribution and flame length are analyzed and compared before or after combustion instability control simultaneously. To ensure the uniformity of jets from all the eight jet nozzles before fitting the injection tube, the jet velocity was measured using a hot wire anemometer at the exit of each eight jet nozzles. Fig. 5 shows the velocity of jets from each jet nozzle obtained for a maximum volume flow rate of 5 L/min through the manifold. The average velocity of 5.31, 10.61, 15.89, 21.23, 26.54 m/s is represented by a dotted line. The MSE Table 1 Experimental condition.
4
Parameters
Values
Fuel type Thermal power (kW) Equivalence ratio (Φ) Swirl number (S) Injection tubes diameter (mm) Injection flow rate (L/min) Jet nozzle numbers The Volume fraction of O2/(O2 + CO2)
CH4, gas purity: 99.995% 3.0 0.90 1.153 1.0 1, 2, 3, 4, 5 4 21%, 26%, 31%, 36%, 41%
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gradually from 0 to 5 L/min in Fig. 6, the corresponding sound pressure is recorded with the dynamic pressure transducer and then transmitted to the National Instruments USB-6210 signal acquisition card. From the control effects demonstrated in Fig 0.6, the variation tendency demonstrates that the sound pressure amplitude decreases as the injection flow rate increase. In Fig. 6, the original sound pressure amplitude in the gas turbine chamber was 30.06 Pa. At the initial stage, when the air injection flow rate is relatively small (0–2 L/min) compared to the main output of the air fan (50 L/min), the pressure amplitude declines rapidly. As the injection flow rate increases from 2 to 5 L/min, in this transitional period, the pressure amplitude declines slowly as the air injection rate increases. Damping effects of the annular micropore jets reaches a maximum point when the injection flow rate arrives 5 L/min, in all six cases, the sound pressure vibration intensity after control was reduced to 10–15% compared to uncontrolled case. After the injection flow rate gradually over 5 L/min, the flame was perturbed and ultimately blowout. Fig 0.6 also compares the combustion instability damping effectiveness of different oxygen volume fraction. Effectiveness of combustion instability control is in proportion to the volume fraction of O2, when the injection flow rate was between 0 and 3 L/min, damping effect of the oxy-fuel injection was better than the air injection case, but this was on the contrary when the injection flow rate was between 3 and 5 L/min. The amplitude-damped ratio of the above six cases is investigated in Fig. 7, it shows that in all six cases, the amplitudedamped ratio approaches 79.53% when the injection flow rate arrives 5 L/min, the oxy-fuel injection case have three transitional regions during control, rapid growth (0–2 L/min), slow growth (2–5 L/min) and blowout region. However, for the air injection case, suppression effectiveness during the transitional region (0–5 L/min)grows rapidly from 0 to 79.53% and finally blowout. In Fig. 7, the effects of CO2/O2 and N2/O2 on lean-premixed combustion instability were similar, but the blowout region of CO2-diluted flames (0–5 L/min) are however narrow than N2-diluted flames (0–7 L/min) near blow off-limits, these experimental results are consistent with findings in [26]. To test this further, Fig. 8 shows FFT (Fast Fourier Transform) of the pressure oscillations before and after control with the above six cases, the selected air injection flow rate is 5 L/min. On the whole, the main peak of thermoacoustic instability at 265.5 Hz was almost eliminated after control, this is consistent with the experimental results presented in Fig. 7. From the results demonstrated in Fig. 6, Figs. 7 and 8, with methods of annular micropore jets, self-excited thermoacoustic oscillation in the model gas turbine combustor can be largely suppressed.
Fig. 4. (a) The resonance frequency of sound pressure under combustion instability without control, (b) The resonance frequency of flame heat release rate under combustion instability without control. Thermal power = 3 kW, equivalence ratio = 0.90.
Fig. 5. Variation of velocity for each injection jet nozzles under different injection flow rates.
(Mean square error) of the injection velocity variation of all eight jet nozzles was below 3.5%, which indicates that the flow uniformity of the jet nozzles is very small [5]. 3.1. CO2/O2 and N2/O2 on combustion instability Fig. 6. Sound Pressure amplitude after annular injection control with 1.0 mm jet nozzles, equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
Fig. 6 shows the damping effects of combustion instability with different conditions of micropore jets. The injection flow rate increases 5
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Fig. 7. Amplitude-damped ratio after annular injection control with 1.0 mm jet nozzles, Equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
Fig. 9. CH* chemiluminescence intensity after annular injection control. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
Fig. 9 shows the CH* chemiluminescence intensity of flames recorded after annular injection control with 1.0 mm jet nozzles, equivalence ratio = 0.90. It can be inferred from Fig. 9 that the average CH* chemiluminescence intensity of flames under air injection control is two times that of the oxy-fuel injection. During the process of thermoacoustic suppressing, frequency shifting of CH* chemiluminescence intensity was observed in Fig. 10. The oscillation frequency shifted from 265.5 Hz to approximately 125 Hz as the injection flow rate increases. When the thermoacoustic instability was finally damped, the mode of CH* chemiluminescence intensity was altered at the same time. The shifting frequency varies among different types of O2 volume fraction, both O2 volume fraction and injection flow rates affect the initial shifting point of CH* chemiluminescence intensity. As the frequency shifting is connected with the process of thermoacoustic damping, mechanisms behind this phenomena may attribute to the micropore injection that changes the flow field and vortex shedding of the flame, which triggered a new heat release rate oscillation between 100 and 140 Hz. Compared with heat release signatures for swirl assisted distributed combustion in [12–14], it can be concluded that CO2/O2 injection reduced heat release fluctuations significantly.
Combined variation trend in Fig. 9 with Fig. 10, the amplitude of heat release rate increases while the air injection flow rate increases, this was different in the cases of oxy-fuel injection, the amplitude of heat release rate gradually declines while the air injection flow rate increases, this may attribute to the inert effects of carbon dioxide, results in reference [8] also indicates that with more CO2 added, the amplitude of flame heat release rate was lower than no CO2 added condition. This strange dynamic characteristic signifies that although thermoacoustic coupling was broken by annular injection control, the variation tendency of flame heat release rate was different with different injection medium. Flame heat release rate experienced a more complicated evolution during the process of oxy-fuel coordinated control. Dynamic characteristics of the CH* chemiluminescence intensity in Fig. 9 may stem from the CO2/O2 injection that increases the equivalence ratio of combustion, but an inert CO2 atmosphere which could reduce the temperature in the combustion chamber. The combined effect of oxygen and carbon dioxide changes the mode of flame heat release rate. Compared with the N2/O2 injection case, as the amounts of CO2 increased, the total CH* chemiluminescence intensity in all CO2/O2
Fig. 8. Pressure oscillation frequency after annular injection control with 1.0 mm jet nozzles, Equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection. 6
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burners, suppression of combustion instability and NOx emissions can be achieved by lower cost. Owing to CO2′s thermal effects of high heat capacity and radiation loss, through replacing the N2 by CO2, the NOx emission can be reduced under CO2/O2 injection. The effect of CO2 injection yields a reduction in the temperature field that eventually results in thermal NOx reduction, and the temperature distribution in the combustion chamber was more uniform than air injection or no injection cases. Besides, compared with the air injection and no injection cases, the N2 in the annular injection flow was replaced by CO2, thus the whole volume of N2 in the combustion chamber decreased which contributes to more thermal NOx reduction than the CO2/O2 injection cases. Also, as the volume of oxygen increases, the average equivalence ratio declines in both CO2/ O2 and N2/O2 injection cases compared to no injection cases, thus leads to a more lean-premixed combustion condition which brings lower NOx emissions. The temperature distribution changes along with the flame shortening process. To make an in-depth investigation of this associated relation, the temperature distribution along the radial direction is measured at the axial position of Z = 48 mm, as indicated by the arrows at the bottom of Fig. 2. With the same experimental conditions in Table.1, the flame temperature is recorded with a thermocouple before and after control. The geometrical benchmark of temperature measuring set at Z = 0 is parallel to the airflow direction, the core temperature of flame recorded at Z = 48 mm. There are 27 gauging points in the radius direction within the combustion chamber, for the reconstruction of temperature distribution before or after thermoacoustic control. The 27 measurement points in the combustion chamber were located evenly along the radial direction of the combustion chamber at the radical height of Z = 48 mm, there was a measurement point in the middle of the combustion chamber, and two measurement points at the inner wall of the combustion chamber, other 24 measurement points were located at a scale of 5 mm along the axial direction. Because the turbulent lean-premixed swirl-stabilized flames have the highest temperature at the height of Z = 48 mm in this research, so Z = 48 mm was selected as the reference position to explore the change of temperature distribution under annular injection. Four jet nozzles with inner diameter equal 1.0 mm were chosen to study temperature distribution during thermoacoustic suppressing, air injection flow rate fixed at 5 L/min. When measuring the flame temperature, the quartz glass tube of the combustion chamber was wrapped with high-temperature resistant quartz wool for thermal insulation, the thermal conductivity of the quartz wool was 1.38 W.M−1.K−1, maximum resistance temperature of the quartz wool was 1350 °C. The flame
Fig. 10. Frequency shifting of lean-premixed methane flames after annular injection control. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
injection cases decreased and reached a relatively stable level. Mechanism behind the transition of CH* chemiluminescence intensity in CO2/O2 injection modes may be fostered by the difference of density between CO2/O2 and N2/O2 injection lead to different density gradient and precessing vortex core across the reaction zone, thus the average temperature distribution was more uniform in CO2/O2 injection cases than in N2/O2 injection cases, as more uniform temperature distribution will lead to stable combustion which may reduce the CH* chemiluminescence intensity [8]. Besides, as more CO2 was injected into the reaction zone (contains higher carbon particles), the CO2/O2 injection may result in different flame emissivity and flame heat release dynamic characteristics than the N2/O2 injection cases [14]. In addition, annular jets penetrated into the flame not only changes the local equivalence ratio but also alters the flame structure, the outer recirculation zone (ORZ) and the inner recirculation zone (IRZ), CO2/O2 and N2/O2 injection may lead to different flame-vortex interaction mechanism thus changes the CH* chemiluminescence intensity [6]. 3.2. Nox emissions and temperature profiles Pollutant emission of NOx is also examined for the lean-premixed flames with air (N2/O2) and oxy-fuel (CO2/O2) injection, respectively. With CO2 addition, the flame temperature and NOx emission are reduced, this may primarily be attributed to its thermal effects of high heat capacity and radiation loss, and chemical effects of participation in reaction. In the present study, the NOx concentration is measured far downstream of the flame zone. The results are illustrated in Fig. 11. In the case of air (N2/O2) injection, the NOx concentration slowly decreases with raising injection flow rates, and finally achieved a reduction of NOx emissions near 10%, drops from 23.5 ppm to 21.5 ppm. By replacing the N2/O2 with CO2/O2, the NOx emission significantly reduces, owing to CO2′s thermal effects of high heat capacity and radiation loss than N2, injection of CO2/O2 achieved a reduction of NOx emissions near 45%, drops from 23.5 ppm to 13 ppm. As indicated in Fig. 11, under oxy-fuel (CO2/O2) injection cases, NOx emissions were approximately inversely proportional to the CO2/O2 injection flow rates during 1–5 L/min. Even when the oxygen content is increased to 0.41, the measured NOx concentration is less than 15 ppm. It indicates that by replacing part of N2 with CO2 will be beneficial to reduce the NOx emission. Although with oxy-fuel combustion technology, the combustion condition and NOx emissions reduction can be improved [8–11,18–21], however, the cost of oxy-fuel combustion and CO2 recycling is large because huge consumption of oxygen and power, but with the application of annular micropore injection in gas turbine
Fig. 11. NOx emissions after annular injection control with 1.0 mm jet nozzles, equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. 7
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shows a W shape, it may signify that the annular jets break the inner and outer circulation zone of the flame and improves turbulent combustion velocity, thus eliminating the coupling of thermoacoustic instability. As the air injection flow rate increases, thermoacoustic instability was suppressed and the flame becomes more stable and shorter, the swirling intensity after injection was intense than the no controlled cases. When annular jets injected into the core of flame, it alters the local equivalence ratio and hence the heat release dynamics simultaneously. Consider together with Figs. 9 and 13, the luminosity of N2/O2 injection flames is intense than that of CO2/O2 injection flames. Fig. 14 shows the calculated flame length with a method of the ‘Canny edge detection algorithm’ before and after the annular injection control, the detailed description of this method is in section.2. The original length of flame without air injection is 115 mm. Fig. 14 compared the length of flame and the injection flow rate, and the result was a strong negative relationship between both variables. For all experimental cases presented in Fig. 14, the flame length is inversely proportional to the air injection flow rate, the variation tendency of flame length was similar to each other, with 41% O2 injection, the flame length declines to 100 mm. The degree of length reduction from N2/O2 injection flames is intense than that of CO2/O2 injection flames. The degree of length reduction is inversely proportional to the O2 volume fraction in CO2/O2 injection flames. The mechanism behind these characteristics may attribute to the flame structure variation, better airfuel mixing, and inert CO2 atmosphere effects. Comparing all injection cases, the flame length is short in N2/O2 injection conditions. As presented in Fig. 13, the internal and external recirculation zone of the flame was changed by the annular micropore jets, which improves combustion efficiency and reaction rate with more oxidizer added and better mixing. In Fig. 14, the length of flame was reduced when air injection flow rose up, the shorter flame length may attribute to the increased reaction rate of turbulent combustion, which improves turbulent combustion velocity. Then the turbulent swirling combustion velocity of the gaseous mixture significantly increased and finally shortens the tail of flame. At the same time, cross injection CO2/O2 enhances the swirling intensity of the flame and creates an inert CO2 atmosphere which could reduce the temperature in the combustion chamber, make the temperature field more uniform than annular N2/O2 injection.
temperature is recorded by a K-type thermocouple (TM-902C, −50—1300 °C, ± 2.5 °C), which brings measurement uncertainty of ± 2.5 °C. The experiment was conducted under a room temperature of 298 K and 50–55% relative humidity. In each measurement, the temperature is tested at least five times, and the MSE (Mean square error) of the temperature of all 27 measurement points was below 2.33%, which is less than 5%. The average temperature value of flame before or after control is plotted in Fig. 12. Fig. 12 signifies that before annular injection, the flame has inverted 'V shape' temperature distribution, the temperature drops rapidly from the inner flame region to the wall of combustion chamber, the reduction tendency was slower under N2/O2 injection with similar inverted 'V shape', however, this was different in CO2/O2 injection cases which have a 'Bell shape' temperature distribution along the axial direction. At position of Z = 48 mm, close to the inner wall of the quartz combustion chamber (radius = 57 mm), the gas temperature is relatively low with 623 K; the temperature gradually increases to its peak around the flame front by approaching the center (radius = 0 mm), the temperature reaches the highest value of 1328 K. After annular injection control with CO2/O2, the average temperature close to the inner wall of the quartz combustion chamber was 700 K, which is higher than the uncontrolled case. However, the average temperature close to the combustion chamber center was 1155 K, which was lower than the uncontrolled case. Along the direction of axial, during the region of 0–25 mm, the overall temperature of the uncontrolled case was greater than the CO2/O2 injection cases. But during the region of 25–57 mm, the overall temperature of the uncontrolled case was less than the CO2/ O2 injection cases. The average temperature value of N2/O2 injection cases was less than the CO2/O2 injection cases, and the CO2/O2 injection cases have a more uniform temperature distribution, the drop of overall temperature in the combustion chamber and uniform temperature distribution contributes to the reduction of NOx emissions significantly. The main reason why the air injection cases has lower temperature distribution beyond the distance 25 mm point may attribute to the variation of injection momentum between two jets in cross-flow. In Fig. 12, beyond the range of 25 mm, the temperature in the outer recirculation zone of the N2/O2 injection flame was lower than that of the CO2/O2 injection flames, this may be owing to the heavier density of CO2/O2 injection (with higher momentum) compared to N2/O2 injection (with lower momentum) across the reaction zone, as the heavier density injection penetrates the shear-layer enveloping of the inner recirculation zone (IRZ), then wraps back around the outer recirculation zone (ORZ), thus makes the temperature more uniform in the outer recirculation zone. However, the N2/O2 injection with lower density injection momentum does not wrap back around the outer recirculation zone, but proceeds to spread further downstream, the decrease in temperature probably owning to entrained gas from the outer (outside the flame front) unburned cold reactant.
4. Conclusion Effects of annular N2/O2 and CO2/O2 jets on combustion
3.3. Variations of flame structure According to the coordinated control cases of combustion instability and NOx emissions in the above sections, the corresponding appearance and length of the lean-premixed combustion flame were recorded at a specific point of 0, 1, 2, 3, 4 and 5 L/min, the inner diameter of these jet nozzles is 1.0 mm. In Fig. 13, lean-premixed flame images during experiments are recorded synchronously using a high-speed CCD camera through a signal synchronizer. The length of the flame shortens as the injection flow rate increases. At the initial stage, the tails of the flame change slightly with air injection. At the 5 L/min cases, flame tails almost invisible, the appearance of the flame becomes dispersed and bright, this was different from the no controlled cases, where the flame is stretched and claret-colored. Fig. 13 also signifies that before annular injection control, the flame shows a typical reversed V shape, but as the injection flow rate increases, the flame length gradually shortens and
Fig. 12. Flame temperature distribution at a height of 48 mm. Equivalence ratio = 0.90, number of jet nozzles = 4, Φ = 1.0 mm. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection. 8
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effectively. The combustion instability damped ratio achieved near 80% compared to the original uncontrolled cases. The rapid growth region and slow growth region of annular injection control were also identified. The phenomenon of Mode shifting appeared in flame heat release, and the flame heat release intensity of CO2/O2 injection was low than N2/O2 injection. During the process of annular injection control, a new mode of flame heat release rate was triggered. 2) The CO2 atmosphere has a significant influence on NOx emissions, temperature field, heat release intensity and turbulent flame combustion speed of lean premixed combustion of CH4. While NOx emission is reduced to an extent of 45% with methods of CO2/O2 injection. However, N2/O2 injection cannot significantly reduce NOx emission (only 10%). Due to the inert effects of CO2, the overall temperature of the gas turbine combustion chamber decreases and becomes more uniform, which is conducive to NOx emission reduction. 3) The shape, length, and brightness of the flame were transformed after annular micropore injection. After CO2/O2 injection, the flame length becomes shorter, the mixing of the combustion medium becomes more uniform, the turbulent combustion speed increases, thus the flame stability improves, but the CO2/O2 injection flame is easier to blow out than the N2/O2 injection flame. This study indicates that in such a lean-premixed combustion system it is possible to stabilize flames and reduce NOx emissions with annular CO2/O2 injection without overhaul modification of the injector design. Declaration of Competing Interest Fig. 13. The appearance of flame before and after coordinated control with four jet nozzles, air injection flow rate varies from 0 to 5 L/min, equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by The National Science Fund for Distinguished Young Scholars of China (51825605). References: [1] Huang Ying, Yang Vigor. Dynamics and stability of lean-premixed swirl-stabilized combustion. Prog Energy Combust Sci 2009;35:293–364. https://doi.org/10.1016/ j.pecs.2009.01.002. [2] Park Seik, Choi Gyung Min, Tanahashi Mamoru. Demonstration of a gas turbine combustion-tuning method and sensitivity analysis of the combustion-tuning parameters with regard to NOx emissions. Fuel 2019;239:1134–42. https://doi.org/ 10.1016/j.fuel.2018.11.021. [3] Kim Jae Hyeon, Kim Seul Gi, Lee Kee Man, Park Jeong. An experimental study on thermoacoustic instabilities in syngas-air premixed impinging jet flames. Fuel 2019;257:115921https://doi.org/10.1016/j.fuel.2019.115921. [4] Labry Zachary A, Shanbhogue Santosh J, Speth Raymond L, Ghoniem Ahmed F. Flow structures in a lean-premixed swirl-stabilized combustor with microjet air injection. Proc Combust Inst 2011;33:1575–81. https://doi.org/10.1016/j.proci. 2010.06.092. [5] Deshmukh Nilaj N, Sharma SD. Suppression of thermo-acoustic instability using air injection in horizontal Rijke tube. J Energy Inst 2017;90:485–95. https://doi.org/ 10.1016/j.joei.2016.03.001. [6] Murat Altay H, Hudgins Duane E, Speth Raymond L, Annaswamy Anuradha M, Ghoniem Ahmed F. Mitigation of thermoacoustic instability utilizing steady air injection near the flame anchoring zone. Combust Flame 2010;157:686–700. https://doi.org/10.1016/j.combustflame.2010.01.012. [7] Uhm Jong Ho, Acharya Sumanta. Role of low-bandwidth open-loop control of combustion instability using a high-momentum air jet—mechanistic details. Combust Flame 2006;147:22–31. https://doi.org/10.1016/j.combustflame.2006. 08.002. [8] Lee Kangyeop, Kim Hyungmo, Park Poomin, Yang Sooseok, Ko Youngsung. CO2 radiation heat loss effects on NOx emissions and combustion instabilities in lean premixed flames. Fuel 2013;106:682–9. https://doi.org/10.1016/j.fuel.2012.12. 048. [9] Shi Baolu, Jie Hu, Ishizuka Satoru. Carbon dioxide diluted methane/oxygen combustion in a rapidly mixed tubular flame burner. Combust Flame 2015;162:420–30.
Fig. 14. Variation tendency of flame length before and after coordinated control with four jet nozzles, air injection flow rate varies from 0 to 5 L/min, equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
instabilities and NOx emissions in lean-premixed methane flames were demonstrated in this article. The feasibility of simultaneous control of lean-premixed combustion instability and NOx emissions by annular oxy-fuel jets is experimentally validated. 1) The results obtained demonstrate that the self-excited thermoacoustic oscillations can be suppressed by annular injection 9
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2011.08.004. [19] Zhong Shenghui, Zhang Fan, Peng Zhijun, Bai Fuqiang, Qing Du. Roles of CO2 and H2O in premixed turbulent oxy-fuel combustion. Fuel 2018;234:1044–54. https:// doi.org/10.1016/j.fuel.2018.07.135. [20] Bürkle Sebastian, Becker Lukas Georg, Dreizler Andreas, Wagner Steven. Experimental investigation of the flue gas thermochemical composition of an oxyfuel swirl burner. Fuel 2018;231:61–72. https://doi.org/10.1016/j.fuel.2018.05. 039. [21] Kutne P, Kapadia BK, Meier W, Aigner M. Experimental analysis of the combustion behaviour of oxyfuel flames in a gas turbine model combustor. Proc Combust Inst 2011;33:3383–90. https://doi.org/10.1016/j.proci.2010.07.008. [22] Seepana Sivaji, Jayanti Sreenivas. Flame structure investigations of oxy-fuel combustion. Fuel 2012;93:52–8. https://doi.org/10.1016/j.fuel.2011.07.033. [23] Chakroun NW, Shanbhogue SJ, Dagan Y, Ghoniem AF. Flamelet structure in turbulent premixed swirling oxy-combustion of methane. Proc Combust Inst 2019;37(4):4579–86. https://doi.org/10.1016/j.proci.2018.06.181. [24] Watanabe Hirotatsu, Shanbhogue Santosh J, Taamallah Soufien, Chakroun Nadim Walid, Ghoniem Ahmed F. The structure of swirl-stabilized turbulent premixed CH4/air and CH4/O2/CO2 flames and mechanisms of intense burning of oxy-flames. Combust Flame 2016;174:111–9. https://doi.org/10.1016/j.combustflame.2016. 09.015. [25] Kim HK, Kim Y, Lee SM. Studies on combustion characteristics and flame length of turbulent oxy fuel flames. Energy Fuels 2007;21:1459–67. https://doi.org/10. 1021/ef060346g. [26] Jourdaine Paul, Mirat Clément, Caudal Jean, Lo Amath, Schuller Thierry. A comparison between the stabilization of premixed swirling CO2-diluted methane oxyflames and methane/air flames. Fuel 2017;201:156–64. https://doi.org/10.1016/j. fuel.2016.11.017.
https://doi.org/10.1016/j.combustflame.2014.07.022. [10] Zhao Xiaoyao, Shi Baolu, Peng Weikang, Cao Qing, Xie Dingjiang, Dong Wei, et al. Effects of N2 and CO2 dilution on the combustion characteristics of C3H8/O2 mixture in a swirl tubular flame burner. Exp Therm Fluid Sci 2019;100:251–8. https:// doi.org/10.1016/j.expthermflusci.2018.09.009. [11] Li Bo, Shi Baolu, Zhao Xiaoyao, Ma Kang, Xie Dingjiang, Zhao Dan, et al. Oxy-fuel combustion of methane in a swirl tubular flame burner under various oxygen contents: operation limits and combustion instability. Exp Therm Fluid Sci 2018;90:115–24. https://doi.org/10.1016/j.expthermflusci.2017.09.001. [12] Khalil AEE, Gupta AK. The role of CO2 on oxy-colorless distributed combustion. Appl Energy 2017;188:466–74. https://doi.org/10.1016/j.apenergy.2016.12.048. [13] Khalil AEE, Gupta AK. Acoustic and heat release signatures for swirl assisted distributed combustion. Appl Energy 2017;193:125–38. https://doi.org/10.1016/j. apenergy.2017.02.030. [14] Khalil AEE, Gupta AK. Flame fluctuations in Oxy-CO2 -methane mixtures in swirl assisted distributed combustion. Appl Energy 2017;204:303–17. https://doi.org/ 10.1016/j.apenergy.2017.07.037. [15] Ditaranto M, Hals J. Combustion instabilities in sudden expansion oxy–fuel flames. Combust Flame 2006;146:493–512. https://doi.org/10.1016/j.combustflame.2006. 04.015. [16] Ilbas Mustafa, Bektas Abdulkadir, Karyeyen Serhat. Effect of oxy-fuel combustion on flame characteristics of low calorific value coal gases in a small burner and combustor. Fuel 2018;226:350–64. https://doi.org/10.1016/j.fuel.2018.04.023. [17] De la Torre M, Arifur Chowdhury ASM, Love N, Choudhuri A. Radiative heat release from premixed oxy-syngas and oxy-methane flames. Fuel 2016;166:567–73. https://doi.org/10.1016/j.fuel.2015.11.024. [18] Liu CY, Chen G, Sipöcz N, Assadi M, Bai XS. Characteristics of oxy-fuel combustion in gas turbines. Appl Energy 2012;89:387–94. https://doi.org/10.1016/j.apenergy.
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Full Length Article
Effects of annular N2/O2 and CO2/O2 jets on combustion instabilities and NOx emissions in lean-premixed methane flames ⁎
Hao Zhou , Chengfei Tao Zhejiang University, Institute for Thermal Power Engineering, State Key Laboratory of Clean Energy Utilization, Hangzhou 310027, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Combustion instability Lean-premixed flames Annular injection NOx emissions Oxy-fuel
Experiments on the effects of annular N2/O2 and CO2/O2 jets on combustion instabilities and NOx emissions in lean-premixed methane flames were conducted. Two variables were investigated to validate the effectiveness of suppressing combustion instabilities and NOx emissions synchronously—the flow rate and the volume fraction of O2. Results indicate that the amplitude-damped ratio of combustion instability can reach 79.53% under CO2/O2 injection, sound pressure reduced from 30.3 Pa to 6 Pa. Damped ratio of NOx emissions achieve 45% under the case of CO2/O2 injection, NOx emission reduced from 22.5 ppm to 12 ppm, but the N2/O2 injection case cannot significantly reduce NOx emissions. Mode shifting of flame heat release rate is observed during the process of annular injection, the amplitude of flame heat release rate increases to two times than that of the uncontrolled flames during the N2/O2 injection case, but the amplitude maintains a relatively stable level in the case of CO2/ O2 injection, under both N2/O2 and CO2/O2 jets, oscillation frequency of flame heat release rate jumps from 265.5 Hz to approximately 125 Hz. The CO2/O2 jets lead to a more uniform temperature field than the N2/O2 jets, thus contribute to lower NOx emissions. The application of CO2/O2 injection improves combustion velocity and provides better mixing, the flame length becomes shorter and compact. This study realized the reduction of combustion instabilities and NOx emissions simultaneously, which could be conducive to the prevention of combustion instabilities or pollutant discharge in industrial gas turbine burners.
1. Introduction Lean-premixed combustion technologies were extensively used in the land-based power generation and aviation or astronautics industries [1]. With the application of a lean-premixed combustion technique in gas turbines, the local flame temperature is decreased and NOx formation is reduced exponentially [2]. However, the lean-premixed combustion technique is susceptible to combustion instabilities [3], emerging of combustion instability, or say thermoacoustic instability is an undesired phenomenon that can cause serious damages. Combustion instability is a generalized definition of unsteady flame dynamics, it not only contains unstable combustion dynamics such as extinction limits, flashback limits, limit cycles, bifurcation, and hysteresis, but also contains self-excited acoustic motion in combustor chamber, flame surface variation, equivalence ratio fluctuation, and vortex shedding due to hydrodynamic instability [1–3]. To eliminate thermoacoustic instabilities, methods of suppressing thermoacoustic instability are extensively explored. Active control techniques utilize external excitations (such as acoustic forcing and fuel modulation) to attenuate combustion oscillations. While effective in suppressing the instabilities,
⁎
active control requires high-speed actuators and adds significant complexity to the design of the combustor. For this reason, the potential for suppressing thermoacoustic instability through passive control such as injecting medium near the flame anchoring zone with minimal complexity and lower cost are desirable [4–7]. Suppression methods for eliminating lean-premixed combustion instability have been extensively explored in the past [1–2,4–7], but they tend to partially focused on the effectiveness of combustion instability and neglect the negative side effects on pollutant emissions during control, thus may cause increased pollutant emissions. The recent development in novel combustion techniques demonstrates the promising prospect of oxy-fuel combustion technology in eliminating combustion stability and NOx emissions of gas turbine combustors. Kangyeop Lee [8] demonstrated that the CO2 dilution rate results in a decrease in NOx emission and combustion oscillation frequency in lean premixed flames. Baolu Shi [9–11] explored the effects of N2 and CO2 dilution on flames and found that the flame temperature, NOx emissions, flame structure were different under the N2 or CO2 atmosphere. Ashwani K. Gupta [12–14] investigated the role of CO2 on flame acoustic and heat release signatures and concluded that most
Corresponding author at: State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China. E-mail address: [email protected] (H. Zhou).
https://doi.org/10.1016/j.fuel.2019.116709 Received 19 September 2019; Received in revised form 23 October 2019; Accepted 20 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Hao Zhou and Chengfei Tao, Fuel, https://doi.org/10.1016/j.fuel.2019.116709
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Nomenclature Φ FFT S Z PMT Φ kW Hz
T f φ dh dn CH4 CH* Pa °C
equivalence ratio fast Fourier Transform swirl number flame length photomultiplier tube inner diameter thermal power frequency
flame fluctuations observed at low frequencies in Oxy-CO2-methane mixtures. Mario Ditaranto [15] studied combustion instabilities in sudden expansion oxy-fuel flames and found that the heat release and pressure fluctuations are linked with the O2 mass-flow rate. Mustafa Ilbas [16] found the oxy-fuel combustion performances of the coalderived syngases are better than that of the air-fuel combustion. Martin de la Torre [17] compared radiative heat release from premixed oxysyngas and oxy-methane flames and revealed that the presence of CO2 in the flames becomes more dominant over the effect of flame temperature as the CO2 flow rate exceeds 6%. C.Y. Liu [18] use GRI Mech 3.0 to study thermodynamic and chemical kinetic of oxy-fuel combustion in gas turbines and point out the existence of a window for optimal oxygen/diluent ratio in the oxidizer flow streams for oxy-fuel combustion. Direct numerical simulation was carried out to explore the role of CO2 and H2O in premixed turbulent oxy-fuel combustion [19]. Sebastian Bürkle [20] investigated the flue gas thermochemical composition of an oxy-fuel swirl burner, oxy-fuel atmospheres were shown to increase the temperature gradients in the axial direction, due to radiative heat loss. With the Raman measurements, Peter Kutne [21] revealed a significant difference between the temperature measured in the outer recirculation zone and the adiabatic flame temperature, the
flame temperature cutoff frequency swirler vane angle inner tube diameter vane cross-section diameter methane chemiluminescence sound pressure centigrade
larger heat capacity of CO2 compared to N2 that leads to lower combustion temperatures for the same equivalence ratio and the lower flame speed. A comparison between the stabilization of premixed swirling CO2 diluted methane oxy-flames and methane-air flames was conducted [22–25] and indicate that the presence of CO2 in the oxidant has a profound effect on the flame structure. Paul Jourdaine [26] studied the difference of stabilization of CH4/CO2/O2 and CH4/N2/O2 flames and found that CO2-diluted flames are however less stable than N2-diluted flames near blow off-limits. Although there are many studies for suppressing combustion instability with annular air injection or fuel injection around the flame [4–7], to date, there is no research on the application of CO2/O2 oxidizer for suppressing of lean-premixed combustion instability and NOx emissions simultaneously. As the oxy-fuel combustion technique could significantly lower the NOx emission and improve combustion efficiency [8], the combination of oxy-fuel combustion with annular jets around the flame anchoring zone can be used to control combustion instability and NOx emissions synchronously. Besides, mode shifting characteristics of flame heat release rate during air injection (N2/O2) and oxy-fuel injection (CO2/O2) has never been investigated before. Moreover, a flame structure such as length and temperature
Fig. 1. Schematic figure of the model combustor. 2
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distribution variation tendency has not been specifically explained in terms of different injection medium as the oxidizer. The purpose of the present work is to explore the effects of annular N2/O2 and CO2/O2 jets on combustion instabilities and NOx emissions in lean-premixed methane flames, thus verify the feasibility of damping lean-premixed combustion instability and NOx emissions synchronously in a model gas turbine combustor and illuminate the mechanism behind. Systematic experimental research is proposed to evaluate the impact of CO2/O2 fraction and injection flow rates on the effectiveness of coordinated control. This study can promote a further understanding of thermoacoustic instability passive control with methods of micropore jets around the flame nozzle. Identically, providing a useful tool for a better combustion instability and NOx emissions suppression of industrial gas turbine burners.
S =
2 ⎛ 1 - (dh/ dn )3 ⎞ 2 tan φ ≈ tan φ 3 ⎝ 1 - (dh/ dn )2 ⎠ 3 ⎜
⎟
(1)
where φ is the swirler vane angle, dh and dn are the diameters of the inner tube and the vane cross-section [1]. With equation (1), the swirl numbers of the combustor are calculated as 1.153. The length of the optically accessible quartz combustion chamber is 300 mm, with two holes symmetrically placed at the downside of quartz for pressure measuring and ignition, the inner diameter of the two glass holes is 10 mm. The downstream of the quartz combustor is connected to a stainless exhaust outlet with a height of 315 mm, temperature measuring holes are reserved. 2.2. CO2/O2 and N2/O2 injection section design In Fig. 2, an annular stainless air injection section with eight annular holes is installed between the outlet of the burner nozzle and the quartz combustion chamber, the height of the air injection section is 50 mm and four holes were used for micropore injection in this article, with an inner diameter of 1.0 mm. The external diameter of the injection section is 120 mm, eight air injection holes with an inner diameter of 4 mm were evenly weld around the section surface, the height of these weld holes is 25 mm. Four holes were selected every other one from the eight holes in the injection section, the CO2/O2 and N2/O2 injection medium were transferred to the four injection nozzles and then to the root of the flame, four jet nozzles with 1.0 mm inner diameter were inserted in the injection section holes, and the four jet nozzles were arranged evenly at a 90-degree angle toward the radical center of the combustion chamber, the outer of the annular stainless tube is connected with Teflon tubes. With the geometry parameters of the combustor, the cut-off frequency of the lean-premixed combustor was calculated as f = 1504 Hz, this value was bigger than the excitation frequency of flame-acoustic resonance in this study.
2. Experimental setup 2.1. Model gas turbine combustor Experiments were performed on a laboratory-scale, swirl-stabilized premixed combustor, Fig. 1 shows a schematic parameter of the lean premixed model gas turbine combustor. The combustor consists of four parts, an air-fuel mixture chamber, a swirl burner nozzle, an optically accessible quartz combustion chamber, and an exhaust gas discharge duct. Fuel and air are mixed in the air-fuel mixture chamber of the combustor, the length of the mixing chamber is 400 mm with a diameter of 114 mm, two honeycomb plate is placed in the air-fuel mixture chamber for better gas mixing thus reduce equivalence ratio fluctuation of flames. Fig. 1 also shows the geometry design of the burner. The inner diameter of the whole combustor is fixed at 114 mm for the quartz combustion chamber, exhaust gas outlet section, and the mixing chamber. The axial swirler has 16 vanes with a 60° vane angle, the height of the burner nozzle section is 100 mm, a bluff body with an external diameter of 6 mm is inserted in the middle of the swirler, the corresponding swirl number can be written as equation (1).
2.3. Experimental condition Fig. 3 shows the layout of the measuring instruments of the
Fig. 2. The geometry of the annular stainless tube with eight air injection holes. 3
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direction of the flame. When the flame figure is recorded, the measuring scale is equal to the magnification scale of the flame, and the practical length of the unit pixel can be obtained. Let the number of N pixels corresponding to the measuring scale of y, then the actual size of the unit pixel is Δy = y/N . The actual length of the flame is Lk ∗ Δy . 4th. Compare the length of Lk ∗ Δy from each edge point one by one, find the maximum value, and get the corresponding coordinates at its maximum value ( x max , ymax ). In Figs. 1 and 3, air as an oxidizer is conveyed through two radially opposed apertures at the upstream of the burner. The air is supplied with an industrial fan (YASHIBA-HG-1100, Output: 300 W, Pressuremax: 15 kPa) and the flow rate is controlled by a glass rotameter (2% FS). Research level methane (CH4, gas purity: 99.995%) is delivered through Teflon pipes to the mixing chamber and then mixed with air completely. In Fig. 3, the volume flow rate of the fuel (methane) is adjusted with a mass flow controller (Alicat MC-series, 0.2% FS) and the thermal power of the combustor is fixed at 3.0 kW, with the methane flow rate keeps constant, during the experiments, the global equivalence ratio of the lean-premixed flame is selected as 0.90. CH4 is provided with fuel storage tanks (10 MPa, 40L), N2, CO2, and O2 is provided with storage tanks (12 MPa, 40L) and mixing in the gas mixer, then converted with Teflon tubes. During experiments, the combustor is operated at atmospheric pressure and ambient temperature. The experimental conditions explored are summarized in Table 1.
Fig. 3. The Layout of related measuring instruments for the experiments.
experiment. Sound pressure oscillations of the combustion chamber are collected with dynamic pressure transducers (Sensitivity: 1 mV/Pa, bandwidth: 0–20 kHz, rise time: 5 μs). In Fig. 3, the dynamic pressure transducer is inserted in the holes of the quartz combustion chamber at the position. Flame CH*(430 ± 10 nm) chemiluminescence signal implies the fluctuation of the heat release rate of premixed flames during thermoacoustic instability. CH* chemiluminescence signal is measured with a photomultiplier tube (PMT, Hamamatsu H10722 series), the sound pressure and CH* chemiluminescence signal is recorded simultaneously using a multi-channel signal recorder (National Instruments, USB-6210) at a 4 k Hz sampling rate. The above experimental data is recorded and processed by using Labview 2012 and Matlab 2019a commercial software. In Fig. 3, along with the PMT, the lean-premixed flame images during experiments are recorded synchronously using a high-speed CCD camera (AOS S-PRI plus) through a signal synchronizer (MODEL DG535). Flame temperature is recorded by a K-type thermocouple (TM-902C, −50–1300 °C). NOx emission concentration is measured with a Testo flue gas analyzer (Testo 350) with a sampling rate of 1 Hz. The visible length of the flame was calculated with the ‘Canny edge detection algorithm’. To identify the edge of the flame and extract the edge of the flame picture for calculation of flame length, the canny edge detection method is not susceptible to noise and can detect true weak edges of flame, the strong and weak flame edges are detected separately using two different thresholds, and the weak edges are included in the output image when the weak and strong edges are connected. The Canny edge detection algorithm contains four steps. Step1: smoothing the image with a Gaussian filter. Step2: Calculate the magnitude and direction of the gradient using the finite-difference of the first-order partial derivative. Step3: Non-maximum suppression of gradient amplitude. Step4: Detect and join edges with a double threshold algorithm. After obtained the canny edge with high-speed flame photography and Matlab code, flame length calculation can be done with the following four steps: 1st. Starting from the root of the flame and calculate the number of boundaries of each line, and find the middle coordinate of the flame root ( x root , yroot ). 2nd. Then scan the boundary image point by point and calculate the pixel distance between each boundary point and the starting point one by one: Lk = (x i − x root )2 + (yi − yroot )2 . Point Lk was the pixel distance of the kth point ( x i , yi ) from the flame root ( x root , yroot ), the coordinates of the flame boundary ( x i , yi ) can be obtained at the same time. 3rd. The above calculation is the pixel length, to obtain the actual flame length, the measuring scale is placed parallel to the burning
3. Results and discussions When the thermal power of the model gas turbine combustor is fixed at 3 kW, the equivalence ratio of the combustor is fixed at 0.90, the combustor exhibits self-excited combustion instability at an oscillation frequency of 265.5 Hz and 30.06 Pa without control. Fig. 4(a) and Fig. 4(b) shows the FFT (Fast Fourier Transform) results for the resonance frequency of sound pressure and flame heat release rate with conditions set in Table 1, the maximum pressure recorded was 30.06 Pa without control, the maximum relative amplitude of flame heat release rate recorded was 0.004. Two parametric variables were investigated—the annular medium injection flow rate and the volume fraction of O2 in oxidizer, the results of CO2/O2 injection are compared with N2/O2 injection. With these conditions, the suppression effects of combustion instability and NOx emissions are experimentally studied in this article. Since annular air injection not only eliminates sound pressure but also triggers frequency and amplitude shifting of flame heat release rate, the dynamic characteristics of the flame heat release rate before or after control is studied. Furthermore, temperature distribution and flame length are analyzed and compared before or after combustion instability control simultaneously. To ensure the uniformity of jets from all the eight jet nozzles before fitting the injection tube, the jet velocity was measured using a hot wire anemometer at the exit of each eight jet nozzles. Fig. 5 shows the velocity of jets from each jet nozzle obtained for a maximum volume flow rate of 5 L/min through the manifold. The average velocity of 5.31, 10.61, 15.89, 21.23, 26.54 m/s is represented by a dotted line. The MSE Table 1 Experimental condition.
4
Parameters
Values
Fuel type Thermal power (kW) Equivalence ratio (Φ) Swirl number (S) Injection tubes diameter (mm) Injection flow rate (L/min) Jet nozzle numbers The Volume fraction of O2/(O2 + CO2)
CH4, gas purity: 99.995% 3.0 0.90 1.153 1.0 1, 2, 3, 4, 5 4 21%, 26%, 31%, 36%, 41%
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gradually from 0 to 5 L/min in Fig. 6, the corresponding sound pressure is recorded with the dynamic pressure transducer and then transmitted to the National Instruments USB-6210 signal acquisition card. From the control effects demonstrated in Fig 0.6, the variation tendency demonstrates that the sound pressure amplitude decreases as the injection flow rate increase. In Fig. 6, the original sound pressure amplitude in the gas turbine chamber was 30.06 Pa. At the initial stage, when the air injection flow rate is relatively small (0–2 L/min) compared to the main output of the air fan (50 L/min), the pressure amplitude declines rapidly. As the injection flow rate increases from 2 to 5 L/min, in this transitional period, the pressure amplitude declines slowly as the air injection rate increases. Damping effects of the annular micropore jets reaches a maximum point when the injection flow rate arrives 5 L/min, in all six cases, the sound pressure vibration intensity after control was reduced to 10–15% compared to uncontrolled case. After the injection flow rate gradually over 5 L/min, the flame was perturbed and ultimately blowout. Fig 0.6 also compares the combustion instability damping effectiveness of different oxygen volume fraction. Effectiveness of combustion instability control is in proportion to the volume fraction of O2, when the injection flow rate was between 0 and 3 L/min, damping effect of the oxy-fuel injection was better than the air injection case, but this was on the contrary when the injection flow rate was between 3 and 5 L/min. The amplitude-damped ratio of the above six cases is investigated in Fig. 7, it shows that in all six cases, the amplitudedamped ratio approaches 79.53% when the injection flow rate arrives 5 L/min, the oxy-fuel injection case have three transitional regions during control, rapid growth (0–2 L/min), slow growth (2–5 L/min) and blowout region. However, for the air injection case, suppression effectiveness during the transitional region (0–5 L/min)grows rapidly from 0 to 79.53% and finally blowout. In Fig. 7, the effects of CO2/O2 and N2/O2 on lean-premixed combustion instability were similar, but the blowout region of CO2-diluted flames (0–5 L/min) are however narrow than N2-diluted flames (0–7 L/min) near blow off-limits, these experimental results are consistent with findings in [26]. To test this further, Fig. 8 shows FFT (Fast Fourier Transform) of the pressure oscillations before and after control with the above six cases, the selected air injection flow rate is 5 L/min. On the whole, the main peak of thermoacoustic instability at 265.5 Hz was almost eliminated after control, this is consistent with the experimental results presented in Fig. 7. From the results demonstrated in Fig. 6, Figs. 7 and 8, with methods of annular micropore jets, self-excited thermoacoustic oscillation in the model gas turbine combustor can be largely suppressed.
Fig. 4. (a) The resonance frequency of sound pressure under combustion instability without control, (b) The resonance frequency of flame heat release rate under combustion instability without control. Thermal power = 3 kW, equivalence ratio = 0.90.
Fig. 5. Variation of velocity for each injection jet nozzles under different injection flow rates.
(Mean square error) of the injection velocity variation of all eight jet nozzles was below 3.5%, which indicates that the flow uniformity of the jet nozzles is very small [5]. 3.1. CO2/O2 and N2/O2 on combustion instability Fig. 6. Sound Pressure amplitude after annular injection control with 1.0 mm jet nozzles, equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
Fig. 6 shows the damping effects of combustion instability with different conditions of micropore jets. The injection flow rate increases 5
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Fig. 7. Amplitude-damped ratio after annular injection control with 1.0 mm jet nozzles, Equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
Fig. 9. CH* chemiluminescence intensity after annular injection control. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
Fig. 9 shows the CH* chemiluminescence intensity of flames recorded after annular injection control with 1.0 mm jet nozzles, equivalence ratio = 0.90. It can be inferred from Fig. 9 that the average CH* chemiluminescence intensity of flames under air injection control is two times that of the oxy-fuel injection. During the process of thermoacoustic suppressing, frequency shifting of CH* chemiluminescence intensity was observed in Fig. 10. The oscillation frequency shifted from 265.5 Hz to approximately 125 Hz as the injection flow rate increases. When the thermoacoustic instability was finally damped, the mode of CH* chemiluminescence intensity was altered at the same time. The shifting frequency varies among different types of O2 volume fraction, both O2 volume fraction and injection flow rates affect the initial shifting point of CH* chemiluminescence intensity. As the frequency shifting is connected with the process of thermoacoustic damping, mechanisms behind this phenomena may attribute to the micropore injection that changes the flow field and vortex shedding of the flame, which triggered a new heat release rate oscillation between 100 and 140 Hz. Compared with heat release signatures for swirl assisted distributed combustion in [12–14], it can be concluded that CO2/O2 injection reduced heat release fluctuations significantly.
Combined variation trend in Fig. 9 with Fig. 10, the amplitude of heat release rate increases while the air injection flow rate increases, this was different in the cases of oxy-fuel injection, the amplitude of heat release rate gradually declines while the air injection flow rate increases, this may attribute to the inert effects of carbon dioxide, results in reference [8] also indicates that with more CO2 added, the amplitude of flame heat release rate was lower than no CO2 added condition. This strange dynamic characteristic signifies that although thermoacoustic coupling was broken by annular injection control, the variation tendency of flame heat release rate was different with different injection medium. Flame heat release rate experienced a more complicated evolution during the process of oxy-fuel coordinated control. Dynamic characteristics of the CH* chemiluminescence intensity in Fig. 9 may stem from the CO2/O2 injection that increases the equivalence ratio of combustion, but an inert CO2 atmosphere which could reduce the temperature in the combustion chamber. The combined effect of oxygen and carbon dioxide changes the mode of flame heat release rate. Compared with the N2/O2 injection case, as the amounts of CO2 increased, the total CH* chemiluminescence intensity in all CO2/O2
Fig. 8. Pressure oscillation frequency after annular injection control with 1.0 mm jet nozzles, Equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection. 6
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burners, suppression of combustion instability and NOx emissions can be achieved by lower cost. Owing to CO2′s thermal effects of high heat capacity and radiation loss, through replacing the N2 by CO2, the NOx emission can be reduced under CO2/O2 injection. The effect of CO2 injection yields a reduction in the temperature field that eventually results in thermal NOx reduction, and the temperature distribution in the combustion chamber was more uniform than air injection or no injection cases. Besides, compared with the air injection and no injection cases, the N2 in the annular injection flow was replaced by CO2, thus the whole volume of N2 in the combustion chamber decreased which contributes to more thermal NOx reduction than the CO2/O2 injection cases. Also, as the volume of oxygen increases, the average equivalence ratio declines in both CO2/ O2 and N2/O2 injection cases compared to no injection cases, thus leads to a more lean-premixed combustion condition which brings lower NOx emissions. The temperature distribution changes along with the flame shortening process. To make an in-depth investigation of this associated relation, the temperature distribution along the radial direction is measured at the axial position of Z = 48 mm, as indicated by the arrows at the bottom of Fig. 2. With the same experimental conditions in Table.1, the flame temperature is recorded with a thermocouple before and after control. The geometrical benchmark of temperature measuring set at Z = 0 is parallel to the airflow direction, the core temperature of flame recorded at Z = 48 mm. There are 27 gauging points in the radius direction within the combustion chamber, for the reconstruction of temperature distribution before or after thermoacoustic control. The 27 measurement points in the combustion chamber were located evenly along the radial direction of the combustion chamber at the radical height of Z = 48 mm, there was a measurement point in the middle of the combustion chamber, and two measurement points at the inner wall of the combustion chamber, other 24 measurement points were located at a scale of 5 mm along the axial direction. Because the turbulent lean-premixed swirl-stabilized flames have the highest temperature at the height of Z = 48 mm in this research, so Z = 48 mm was selected as the reference position to explore the change of temperature distribution under annular injection. Four jet nozzles with inner diameter equal 1.0 mm were chosen to study temperature distribution during thermoacoustic suppressing, air injection flow rate fixed at 5 L/min. When measuring the flame temperature, the quartz glass tube of the combustion chamber was wrapped with high-temperature resistant quartz wool for thermal insulation, the thermal conductivity of the quartz wool was 1.38 W.M−1.K−1, maximum resistance temperature of the quartz wool was 1350 °C. The flame
Fig. 10. Frequency shifting of lean-premixed methane flames after annular injection control. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
injection cases decreased and reached a relatively stable level. Mechanism behind the transition of CH* chemiluminescence intensity in CO2/O2 injection modes may be fostered by the difference of density between CO2/O2 and N2/O2 injection lead to different density gradient and precessing vortex core across the reaction zone, thus the average temperature distribution was more uniform in CO2/O2 injection cases than in N2/O2 injection cases, as more uniform temperature distribution will lead to stable combustion which may reduce the CH* chemiluminescence intensity [8]. Besides, as more CO2 was injected into the reaction zone (contains higher carbon particles), the CO2/O2 injection may result in different flame emissivity and flame heat release dynamic characteristics than the N2/O2 injection cases [14]. In addition, annular jets penetrated into the flame not only changes the local equivalence ratio but also alters the flame structure, the outer recirculation zone (ORZ) and the inner recirculation zone (IRZ), CO2/O2 and N2/O2 injection may lead to different flame-vortex interaction mechanism thus changes the CH* chemiluminescence intensity [6]. 3.2. Nox emissions and temperature profiles Pollutant emission of NOx is also examined for the lean-premixed flames with air (N2/O2) and oxy-fuel (CO2/O2) injection, respectively. With CO2 addition, the flame temperature and NOx emission are reduced, this may primarily be attributed to its thermal effects of high heat capacity and radiation loss, and chemical effects of participation in reaction. In the present study, the NOx concentration is measured far downstream of the flame zone. The results are illustrated in Fig. 11. In the case of air (N2/O2) injection, the NOx concentration slowly decreases with raising injection flow rates, and finally achieved a reduction of NOx emissions near 10%, drops from 23.5 ppm to 21.5 ppm. By replacing the N2/O2 with CO2/O2, the NOx emission significantly reduces, owing to CO2′s thermal effects of high heat capacity and radiation loss than N2, injection of CO2/O2 achieved a reduction of NOx emissions near 45%, drops from 23.5 ppm to 13 ppm. As indicated in Fig. 11, under oxy-fuel (CO2/O2) injection cases, NOx emissions were approximately inversely proportional to the CO2/O2 injection flow rates during 1–5 L/min. Even when the oxygen content is increased to 0.41, the measured NOx concentration is less than 15 ppm. It indicates that by replacing part of N2 with CO2 will be beneficial to reduce the NOx emission. Although with oxy-fuel combustion technology, the combustion condition and NOx emissions reduction can be improved [8–11,18–21], however, the cost of oxy-fuel combustion and CO2 recycling is large because huge consumption of oxygen and power, but with the application of annular micropore injection in gas turbine
Fig. 11. NOx emissions after annular injection control with 1.0 mm jet nozzles, equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. 7
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shows a W shape, it may signify that the annular jets break the inner and outer circulation zone of the flame and improves turbulent combustion velocity, thus eliminating the coupling of thermoacoustic instability. As the air injection flow rate increases, thermoacoustic instability was suppressed and the flame becomes more stable and shorter, the swirling intensity after injection was intense than the no controlled cases. When annular jets injected into the core of flame, it alters the local equivalence ratio and hence the heat release dynamics simultaneously. Consider together with Figs. 9 and 13, the luminosity of N2/O2 injection flames is intense than that of CO2/O2 injection flames. Fig. 14 shows the calculated flame length with a method of the ‘Canny edge detection algorithm’ before and after the annular injection control, the detailed description of this method is in section.2. The original length of flame without air injection is 115 mm. Fig. 14 compared the length of flame and the injection flow rate, and the result was a strong negative relationship between both variables. For all experimental cases presented in Fig. 14, the flame length is inversely proportional to the air injection flow rate, the variation tendency of flame length was similar to each other, with 41% O2 injection, the flame length declines to 100 mm. The degree of length reduction from N2/O2 injection flames is intense than that of CO2/O2 injection flames. The degree of length reduction is inversely proportional to the O2 volume fraction in CO2/O2 injection flames. The mechanism behind these characteristics may attribute to the flame structure variation, better airfuel mixing, and inert CO2 atmosphere effects. Comparing all injection cases, the flame length is short in N2/O2 injection conditions. As presented in Fig. 13, the internal and external recirculation zone of the flame was changed by the annular micropore jets, which improves combustion efficiency and reaction rate with more oxidizer added and better mixing. In Fig. 14, the length of flame was reduced when air injection flow rose up, the shorter flame length may attribute to the increased reaction rate of turbulent combustion, which improves turbulent combustion velocity. Then the turbulent swirling combustion velocity of the gaseous mixture significantly increased and finally shortens the tail of flame. At the same time, cross injection CO2/O2 enhances the swirling intensity of the flame and creates an inert CO2 atmosphere which could reduce the temperature in the combustion chamber, make the temperature field more uniform than annular N2/O2 injection.
temperature is recorded by a K-type thermocouple (TM-902C, −50—1300 °C, ± 2.5 °C), which brings measurement uncertainty of ± 2.5 °C. The experiment was conducted under a room temperature of 298 K and 50–55% relative humidity. In each measurement, the temperature is tested at least five times, and the MSE (Mean square error) of the temperature of all 27 measurement points was below 2.33%, which is less than 5%. The average temperature value of flame before or after control is plotted in Fig. 12. Fig. 12 signifies that before annular injection, the flame has inverted 'V shape' temperature distribution, the temperature drops rapidly from the inner flame region to the wall of combustion chamber, the reduction tendency was slower under N2/O2 injection with similar inverted 'V shape', however, this was different in CO2/O2 injection cases which have a 'Bell shape' temperature distribution along the axial direction. At position of Z = 48 mm, close to the inner wall of the quartz combustion chamber (radius = 57 mm), the gas temperature is relatively low with 623 K; the temperature gradually increases to its peak around the flame front by approaching the center (radius = 0 mm), the temperature reaches the highest value of 1328 K. After annular injection control with CO2/O2, the average temperature close to the inner wall of the quartz combustion chamber was 700 K, which is higher than the uncontrolled case. However, the average temperature close to the combustion chamber center was 1155 K, which was lower than the uncontrolled case. Along the direction of axial, during the region of 0–25 mm, the overall temperature of the uncontrolled case was greater than the CO2/O2 injection cases. But during the region of 25–57 mm, the overall temperature of the uncontrolled case was less than the CO2/ O2 injection cases. The average temperature value of N2/O2 injection cases was less than the CO2/O2 injection cases, and the CO2/O2 injection cases have a more uniform temperature distribution, the drop of overall temperature in the combustion chamber and uniform temperature distribution contributes to the reduction of NOx emissions significantly. The main reason why the air injection cases has lower temperature distribution beyond the distance 25 mm point may attribute to the variation of injection momentum between two jets in cross-flow. In Fig. 12, beyond the range of 25 mm, the temperature in the outer recirculation zone of the N2/O2 injection flame was lower than that of the CO2/O2 injection flames, this may be owing to the heavier density of CO2/O2 injection (with higher momentum) compared to N2/O2 injection (with lower momentum) across the reaction zone, as the heavier density injection penetrates the shear-layer enveloping of the inner recirculation zone (IRZ), then wraps back around the outer recirculation zone (ORZ), thus makes the temperature more uniform in the outer recirculation zone. However, the N2/O2 injection with lower density injection momentum does not wrap back around the outer recirculation zone, but proceeds to spread further downstream, the decrease in temperature probably owning to entrained gas from the outer (outside the flame front) unburned cold reactant.
4. Conclusion Effects of annular N2/O2 and CO2/O2 jets on combustion
3.3. Variations of flame structure According to the coordinated control cases of combustion instability and NOx emissions in the above sections, the corresponding appearance and length of the lean-premixed combustion flame were recorded at a specific point of 0, 1, 2, 3, 4 and 5 L/min, the inner diameter of these jet nozzles is 1.0 mm. In Fig. 13, lean-premixed flame images during experiments are recorded synchronously using a high-speed CCD camera through a signal synchronizer. The length of the flame shortens as the injection flow rate increases. At the initial stage, the tails of the flame change slightly with air injection. At the 5 L/min cases, flame tails almost invisible, the appearance of the flame becomes dispersed and bright, this was different from the no controlled cases, where the flame is stretched and claret-colored. Fig. 13 also signifies that before annular injection control, the flame shows a typical reversed V shape, but as the injection flow rate increases, the flame length gradually shortens and
Fig. 12. Flame temperature distribution at a height of 48 mm. Equivalence ratio = 0.90, number of jet nozzles = 4, Φ = 1.0 mm. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection. 8
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effectively. The combustion instability damped ratio achieved near 80% compared to the original uncontrolled cases. The rapid growth region and slow growth region of annular injection control were also identified. The phenomenon of Mode shifting appeared in flame heat release, and the flame heat release intensity of CO2/O2 injection was low than N2/O2 injection. During the process of annular injection control, a new mode of flame heat release rate was triggered. 2) The CO2 atmosphere has a significant influence on NOx emissions, temperature field, heat release intensity and turbulent flame combustion speed of lean premixed combustion of CH4. While NOx emission is reduced to an extent of 45% with methods of CO2/O2 injection. However, N2/O2 injection cannot significantly reduce NOx emission (only 10%). Due to the inert effects of CO2, the overall temperature of the gas turbine combustion chamber decreases and becomes more uniform, which is conducive to NOx emission reduction. 3) The shape, length, and brightness of the flame were transformed after annular micropore injection. After CO2/O2 injection, the flame length becomes shorter, the mixing of the combustion medium becomes more uniform, the turbulent combustion speed increases, thus the flame stability improves, but the CO2/O2 injection flame is easier to blow out than the N2/O2 injection flame. This study indicates that in such a lean-premixed combustion system it is possible to stabilize flames and reduce NOx emissions with annular CO2/O2 injection without overhaul modification of the injector design. Declaration of Competing Interest Fig. 13. The appearance of flame before and after coordinated control with four jet nozzles, air injection flow rate varies from 0 to 5 L/min, equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by The National Science Fund for Distinguished Young Scholars of China (51825605). References: [1] Huang Ying, Yang Vigor. Dynamics and stability of lean-premixed swirl-stabilized combustion. Prog Energy Combust Sci 2009;35:293–364. https://doi.org/10.1016/ j.pecs.2009.01.002. [2] Park Seik, Choi Gyung Min, Tanahashi Mamoru. Demonstration of a gas turbine combustion-tuning method and sensitivity analysis of the combustion-tuning parameters with regard to NOx emissions. Fuel 2019;239:1134–42. https://doi.org/ 10.1016/j.fuel.2018.11.021. [3] Kim Jae Hyeon, Kim Seul Gi, Lee Kee Man, Park Jeong. An experimental study on thermoacoustic instabilities in syngas-air premixed impinging jet flames. Fuel 2019;257:115921https://doi.org/10.1016/j.fuel.2019.115921. [4] Labry Zachary A, Shanbhogue Santosh J, Speth Raymond L, Ghoniem Ahmed F. Flow structures in a lean-premixed swirl-stabilized combustor with microjet air injection. Proc Combust Inst 2011;33:1575–81. https://doi.org/10.1016/j.proci. 2010.06.092. [5] Deshmukh Nilaj N, Sharma SD. Suppression of thermo-acoustic instability using air injection in horizontal Rijke tube. J Energy Inst 2017;90:485–95. https://doi.org/ 10.1016/j.joei.2016.03.001. [6] Murat Altay H, Hudgins Duane E, Speth Raymond L, Annaswamy Anuradha M, Ghoniem Ahmed F. Mitigation of thermoacoustic instability utilizing steady air injection near the flame anchoring zone. Combust Flame 2010;157:686–700. https://doi.org/10.1016/j.combustflame.2010.01.012. [7] Uhm Jong Ho, Acharya Sumanta. Role of low-bandwidth open-loop control of combustion instability using a high-momentum air jet—mechanistic details. Combust Flame 2006;147:22–31. https://doi.org/10.1016/j.combustflame.2006. 08.002. [8] Lee Kangyeop, Kim Hyungmo, Park Poomin, Yang Sooseok, Ko Youngsung. CO2 radiation heat loss effects on NOx emissions and combustion instabilities in lean premixed flames. Fuel 2013;106:682–9. https://doi.org/10.1016/j.fuel.2012.12. 048. [9] Shi Baolu, Jie Hu, Ishizuka Satoru. Carbon dioxide diluted methane/oxygen combustion in a rapidly mixed tubular flame burner. Combust Flame 2015;162:420–30.
Fig. 14. Variation tendency of flame length before and after coordinated control with four jet nozzles, air injection flow rate varies from 0 to 5 L/min, equivalence ratio = 0.90. Volume fraction of O2 = 21%, 26%, 31%, 36%, 41%. Compared with N2/O2 injection.
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