环形燃烧器中OH-PLIF检测方案(CCD相机)

cogentengineeringWang et al.， Cogent Engineering (2019)， 6：1593074https：//doi.org/10.1080/23311916.2019.1593074CIVIL & ENVIRONMENTAL ENGINEERING RESEARCH ARTICLE cogent-engineeringWang et al.， Cogent Engineering (2019)， 6：1593074https：//doi.org/10.1080/23311916.2019.1593074 Experimental study on premixed flamecombustion of annular burner with CO2 dilutionbased on OH-PLIF technology Zhonghua Wang1， Qiang Tang1*， Zhongqing Yang and Pengao Hel Abstract： In view of the combustion characteristics of annular chamber of microgas turbine， a planar laser-induced fluorescence (PLIF) technique was used toinvestigate the methane/air premixed turbulent multiple flames in the annularcombustor and the effect of CO2 on the characteristic parameters of flame structurewas further discussed. The results show that the distribution of the OH radicalsexhibits a ring structure with flame lift and bifurcation phenomena. The instanta-neous and average image of OH-PLIF reveals that the multiple flames have thecommon characteristics of turbulent and laminar premixed flame structure. Thequantitative analysis of the characteristic parameters of flame structure shows thatthe increase of CO2 concentration， the flame premixed conical structure isstretched， the overall flame chemistry reaction area is reduced and the intensity oflocal combustion is weakened； The Coanda effect causes the flame to bend towardsthe inner wall， and the relative bending of the outer flame increases as well as theflame angle. Subjects： Environment & Resources； Energy & Fuels； Bio Energy； Renewable Energy；Energyefficiency Keywords： micro gas turbine； annular combustion chamber； OH-PLIF； premixed turbulentflame； flame structure； carbon dioxide ABOUT THE AUTHOR PUBLIC INTEREST STATEMENT Zhonghua Wang is a postgraduate and Tang Micro gas turbine is one of the most efficientQiang is an associate professor in the College of technology with great development potential.Energy and Power Engineering at Chongqing Flame reaction zone parameters is the mostUniversity. Zhongqing Yang， associate professor， concerned area in the study of combustionis mainly engaged in scientific research of clean mechanism and industrial application. The studyenergy conversion and environmental protec- on the structure of the reaction zone of flame cantion. Pengao He is Qiang Tang's student. They provide help for the design of nozzles and com-are mainly engaged in energy utilization and bustors with high stability， high efficiency and lowconversion， system energy saving， Combustion emission in industry. PLIF technology can wellPollutant Control and so on. Qiang Tang has present the combustion state in the combustor.authored and co-authored over 50 peer- In this paper， the changing process of multi-flamereviewed scholarly articles within his research interaction structure is obtained by PLIF andinterest. reveals the characteristics of annular turbulentflame. Besides， research shows the relationshipbetween flame parameters and CO2 dilutionquantitatively rather than qualitatively by settingcharacteristic parameters of flame structure. 1. Introduction It is known that fossil energy reserves are limited and will cause serious pollution problems， sosubstitute energy is needed urgently. Micro gas turbine has the characteristics of high fuel adapt-ability， low emission， high power density and low maintenance cost (Waitz， Gauba， & Al Gautam，1998). As a key part， the internal flow field changes and combustion situation of combustionchamber directly affect combustion stability， energy efficiency and pollutant emissions. The flamereaction zone is the key to the combustion mechanism research and industrial application of microgas turbine (Liu， 2016). The study on the structure of the flame reaction zone can provide guidancefor the optimization design of the combustion chamber (Bidabadi， Ghashghaei Nejad， Rasam，Sadeghi， & Shabani， 2018； Bo， Costa， Li， Alden， & IBai， 2017； Santis， Ingham， Ma， &Pourkashanian， 2016；Zornek，Mosbach， & Aigner，2018). The annular combustor has been widely used in the micro gas turbine because of its smallpressure loss， strong adaptability to different working conditions and no need for the interconnec-tor. The special design of the premix nozzle increases the interaction between the flames andmakes it easier to meet the ignition requirements. Worth and Dawson (2013) studied the globalflame dynamics of a model annular gas turbine combustor undergoing strong self-excited circum-ferential instabilities is presented. Wang et al. (Chao， Liu， Liu， Jiang， & Lin， 2015) had experimentalinvestigation on detonation combustion patterns of hydrogen/vitiated air within annular combus-tor. Li， Peng， and Liu (2006) simulated the annular combustion chamber and studied the influenceof combustion and cooling on the annular structure of the flame. It was found that any diluentinjection would cause local temperature higher near the diluent hole. Philip et al. (Sheehan， 2016)simulated the ignition process in the annular burner and compared it with the experimental data. Planar Laser Induced Fluorescence (PLIF) combustion diagnostic technology represents thereaction region of the flame in the flow field by measuring the concentration of the hydroxylradicals (it is also defined as OH radicals) (Haber， Gebretsadik， Bockhorn， & Zarzalis， 2015； Kutne，Meier， Boxx， Slabaugh， & Lucht， 2015； Zhu et al.， 2016). Because of its high sensitivity， shortresponse time and low disturbance of flow field， it has been widely used in the practice ofcombustion diagnosis. There have been a lot of studies using PLIF technology to study thecombustion characteristics of wide range of syngas compustions， such as different ratios of CH4and different dilutions (Yang， Shen， Hai， Wu， & Lu， 2015； Yong， Jeon， & Chang， 2006). Pu et al.(2018) studied the premix combustion characteristics of methane syngas using OH-PLIF technol-ogy. The results showed that the increase of methane content and equivalent ratio weakened OHsignal strength， and the increase of Vch4 also made the flame height higher， which inhibited theflame propagation speed. Liao and Hermanson (2018) studied the structure of the flame reactionzone near the nozzle of the strong pulsed jet under the action of swirling flow based on PLIFtechnology. Data show that： In the early stage of combustion with swirl fuel injection， its tipdiscontinuity of the instantaneous image is not obvious. Jin， Tang， Wu， and Huang (2014； Jin et al.，2013) studied the relationship between combustion velocity， structure and fuel composition ofpremixed flame of syngas， and found that preferential diffusion and flow field stretch acted on thechange of flame structure. Wu et al. (2018) extracted the conical flame contour from PLIF imagesof Bunsen burner flame and calculated the flame propagation rate. The adiabatic temperature of premixed flame is high and the NOx emission is large. CO2 dilutioncan inhibit the generation of pollutants and reduce the flame temperature to affect the combus-tion process. Besides， biomass gas has high carbon dioxide content Therefore， it is extremelyimportant to reveal the combustion characteristics and laws under the condition of CO2 dilution.Zhen et al. (2017) studied the effects of varies diluents on temperature and concentration fields ofOH radicals by using PLIF thermometry and bi-directional PLIF，and drew the conclusion that CO2has excellent performance in reducing local high temperature of flame. Jourdaine， Mirat， Caudal，Lo， and Schuller (2017) compared the stability of methane air and methane oxygen combustionunder CO2 dilution premixed rotatory flow. The position of flame front and combustion gas wasobtained by PLIF technology， and the shape of flame was deduced. The structure of single flame， fuel composition and other influencing factors have been analyzedin detail in the above studies， however， the complex combustion chamber experiments withmultiple flames interactions and the effect of CO2 on the size of flame structure are rarely involved.Previous experiment methods are difficult to capture the changing process of multi-flame inter-action structure. In addition， numerical simulation is mostly used to study the combustion char-acteristics of multi-beam flame interaction in annular combustor compared to experiment. In this paper， PLIF technique is used to measure the OH radical distribution of lean combustionpremixed turbulent multiple flames under CO2 dilution in a annular combustion chamber. Thechanging process of multi-flame interaction structure inside the combustion chamber are studiedand mainly analyzes the effects of CO2 dilution on multiple flames interaction structure. Thesework provide support for optimizing annular combustion chamber structure and achieving lowemission combustion. 2. Experiment 2.1. Experimental system The experimental system consists of OH-PLIF laser diagnostic system， air supply system andcombustor， as shown in Figure 1. The air supply system consists of a high-pressure cylinder，a rotor flowmeter， a gas mixing chamber and a one-way valve. The gases of each componentare fully premixed in the mixing chamber before entering the burner nozzle. In the process of theexperiment， the Reynolds Numbers range from 800 to 2000， the equivalent ratio is 0.6 to 0.9 andthe CO2 dilution is 4%， 8%， 12%， and 16%. 2.2. Combustor structure The annular combustor is used according to the combustor structure of Capstone C65 micro gasturbine， and the size is determined by the principle of modeling. The material is made of quartzwith good light transmittance and treated by dihydroxylation. The outer diameter of annularcombustion chamber is 80 mm， the inner diameter is 40 mm， the ring width is 20 mm， and theoverall height is 180 mm. The four nozzles are in the same plane， 30 mm away from the bottom ofthe chamber. The inner diameter d of the nozzle is 6 mm， and the wall thickness is 2 mm. Themodel is shown in Figure 2. The combustion chamber is of annular structure， and four nozzles are evenly arranged along thecircumference. Methane/air premixed gas enters the annular chamber from the nozzles for ignitionand combustion. 2.3. Method of measurement OH-PLIF system is mainly composed of Nd：YAG laser (Quanta-Ray，Spectra)， dye laser (CobraStretch Series， Sirah Lasertechnik， GmbH)， CCD camera (Imager ProX) and receiver. The Nd：YAGlaser generates an initial wavelength of 1064 nm， which is divided into 1064 nm and 532 nm bya 1/2 glass slide， then， becomes 355 nm after stacking. The dye laser uses coumarin153-ethanoldiluent as dye medium. The laser from YAG laser is tuned by the grating in the dye laser， dyed and Figure 1. PLIF experimentalsystem diagram. Figure 2. Model diagram ofannular combustor. converted into a laser with a wavelength of 283.5 nm after passing through the octave crystal.Equipment make the laser stay above 8 mJ to excite the OH radicals base transition in order toproduce the fluorescence signal. The fluorescence signal is received by the CCD camera， which is equipped with OH filter (LaVision，VZ14-0353) and ultraviolet lens (UV-60 mm， F3.5 s) in front of the lens to filter the interferencesignal. The CCD camera has a gate width of 100 ns and a gain of 70 ns to minimize the impact ofambient light. In each working condition， 200 transient OH-PLIF photos are continuously recordedwith a frequency of 200 Hz and a period of 1 s. The recorded images are processed by systemsoftware Davis8 (LaVision， GmbH)， besides， the background and noise were eliminated. The measurement error is mainly affected by temperature， laser parameters and the collectionsystem. The corresponding values for the uncertainties in the temperature，collisional quenchingcoefficient， and fluorescence signal are 1%， 5%， and 2.2%， respectively. Therefore， it can beconcluded that the mean OH concentration measurements have a maximum uncertainty of~17%. Related settings and contents have been referred to the earlier works by Krishna andRavikrishna (2015). 3. Results and discussions 3.1. Premixed turbulent flame shape Methane and carbon dioxide flow remain unchanged， while air flow increases. The CCD image ofpremixed flame in annular combustion chamber is shown in Figure 3. The total flow rate rangesfrom 800 L/h to 2000 L/h. In the case of low air flow， the flame near the nozzle forms a stablecone-shaped blue premixed flame， which can clearly distinguish the outline of the inner and outerflame and the flame at the four nozzles do not interfere with each other. With the increase of airflow， the flame began to grow longer and fluctuate， and the four airflows began to influence eachother. The flame changed from a stable conical state to a ring state， but it was still in an unstablestate. When the nozzle velocity reaches -4 m/s， the flame at the four nozzles ignites with eachother to make the ring structure tend to be stable and the flame distribution is more uniform. Asthe air flow continues to increase， the turbulence of the flow field increases continuously and theoverall color of the flame becomes dark. 3.2. Effects of the multi-flame interaction structure The instantaneous images of OH-PLIF of premix turbulent flame under different working conditionsare shown in Figure 4.“t”in is the time that the photo is taken in one period. The changing processof multi-flame interaction structure is captured in figures and the flame front represented by OHradical presents a state of folding and curling with the change of time， which mainly appeared in Figure 3. CCD image ofmethane premixed flame inannular combustor. Flow Rate Low High Figure 4. Instantaneous imageof premixed flame OH-PLIF inannular combustor.(a)中=0.7， Vco2=4%；(b)中=0.7，Vco2=12%； (c)中=0.5， Vcoz= 4%. (c) the upstream and downstream of fuel jet. In addition， the premixed conical flame structure alsoproduces distortion phenomenon. With the increase of CO2 dilution， the flame front becomes moreunstable. The distribution area of OH fluorescence signal in the whole annular combustion cham-ber increases， but the combustion intensity in the flame chemical reaction area at the nozzledecreases. The structure of the flame front is closely related to the turbulence intensity. In theexperiment， the turbulence vortex disturb the outer layer of the reaction zone of the flame， but notthe preheating zone of the inner layer. The turbulence scale is approximately the same as thelaminar flame thickness， so the flame retains a certain laminar flame structure and is located inthe flame sheet model region. OH radicals mainly concentrate on the front of premixed flame. Dueto the influence of the annular structure of combustion chamber and air flow disturbance， OHradicals formed in premixed combustion zone gradually spread outward. Under the action of hightemperature， OH radical is slowly consumed and gradually formed the external front of the flame. With the decrease of the equivalent ratio， the discontinuous pulsation of the flame front changeswith time， suggesting the phenomenon of local extinction and reignition of the turbulent front.Moreover， the flame cone structure was stretched， the width of the flame root decreased， and thelength increased. The overall OH radicals fluorescence intensity of the combustion chamber isreduced. Under three working conditions， the distribution area of the OH group in the annularcombustion chamber is the same， besides， the central symmetric premixed turbulent flamestructure is formed outside the four nozzles， and the flame bifurcation phenomenon appears.Compared with the CCD image forming ring flame in Figure 3， it is observed that the premixflame in the instantaneous image of OH-PLIF is not formed at the nozzle， but is held up. The increase in CO2 dilution， premixed flow cannot stabilize fire at the nozzle. Flame at the top ofthe upstream to the downstream flame roots extend， upstream of flame combustion products bydiffusion and combustion instability changed the movement of downstream airflow， underminedits originally can satisfy the cone-shaped flame temperature and speed conditions. Furthermore，the lack of a fixed ignition source also promotes the diffusion of premixed gas to the outside in theannular space where flame lift occurs. Under the same experimental conditions as in Figure 4， the average OH-PLIF image of methanepremix turbulent flame obtained from the superposition of 200 instantaneous images is shown inFigure 5. Because the flame is in a turbulent state， the front of the instantaneous turbulent flameappears as a ruffled brush with a certain randomness. By superimposing and averaging severalinstantaneous images， the range of the fold area on the flame front can be obtained. Througha large number of data and image analysis， the fluorescence intensity of the fold area in thisexperiment ranges from 150 to 500 counts. The concentration of OH radicals in the reaction zoneis significantly higher than that in the chemical equilibrium， which represents the main reactionzone of the flame. It can be seen from Figure 5(a) that the distribution of OH radicals in the combustionchamber forms a continuous ring structure， Besides， the OH radical distribution areas of eachnozzle are connected with each other and maintain an obvious premixed conical flame struc-ture. Figure 5(b) shows that the CO2 dilution increases， the thickness of the flame cone reactionzone decreases as well as the maximum OH group fluorescence intensity， but the overalldistribution of OH radical have slightly change from Figure 5(a). With the increase of dilution，the occurrence of local high temperature is reduced， and the gradient of temperature field inthe flame is slowed down， but it has little influence on the overall combustion intensity of theannular combustion chamber. Figure 5. Average OH-PLIFimage of premixed flame inannular combustor. (a)中=0.7， Vco2 = 4%； (b)o=0.7，Vcoz=12%；(c)中=0.5，Vcoz=4%. (a) (b) (c) The results in Figure 5(c) show that the distribution of OH radicals in the combustion chambercannot maintain a complete annular flame structure when the dilution is the same and theequivalent ratio is reduced. The mixed conical flame structure at the nozzle was stretched， theOH group distribution area and fluorescence intensity were reduced， and the fold degree of flamefront was relatively reduced. The effect of methane concentration diffusion and flow field makesthe flame length longer， but the effective area of combustion reaction zone decreases as a whole. 3.3. Characteristic parameters of flame structure in annular combustor As shown in Figure 6， the bifurcated flame at the nozzle is divided into inner flame and outer flame，and the characteristic parameters of flame structure are defined to characterize the law ofinteraction between turbulence and flame. Where Li and Lo are the chord length of the innerflame and the outer flame respectively， and are the straight length between the root of the flameand the top of the flame； b represents the maximum thickness of the flame； f is the maximumdistance from the middle camber line in the flame to its chord line； relative bending F is thepercentage value of f and L； is the angle between the inner and outer flame chord lines. Figure 7 shows the variation of flame structure characteristic parameters with carbon dioxideconcentration. As can be seen from Figure 7(a，b)， the length of both the inner flame and the outerflame decrease with the increase of CO2 dilution； Figure 7(c，d) shows the variation of the thicknessof inner and outer flame. The variation rules of them are basically the same and both showa downward trend. In addition， when equivalence ratio is relatively low， the influence degree is Figure 6. OH-PLIF schematicdiagram and flame structure. Figure 7. The characteristicparameters of flame structurechange with CO2 concentration. greater. The results of the experiment show that the Vco is improved， the temperature of theadiabatic flame decreases， besides， it reduces the speed and intensity of the fire， and the overallreaction zone of premixed flame shrinks. Furthermore， the chemical reaction retention time ofpremixed flame is very small under different CO2 dilution degree， and the difference becomeslarger with the decrease of equivalent ratio in lean combustion zone. Figure 7(e，f) shows the rule that the opening angle of flame and the relative bending of theouter flame change with the dilution of CO2. When the dilution increased， the opening anglebetween the inner and outer flame also increased； The higher the equivalence ratio， the greater the degree of the angle. The relative bending trend of the outer flame is the same asthat of angle. These two characteristic parameters are closely related to the tensile action offlame. With the increase of CO2 dilution， the local tensile rate of the flame increases. A largeamount of unburned gas in the flow field under the extension of the flow and contact withthe outside air can continue to burn. So that the flame root area becomes narrower and theopening angle increases. The increase of CO dilution increases the Reynolds number to some extent and promotesthe Coanda effect. The Coanda effect is the tendency for a fluid to move away from its originaldirection of flow and instead follow the surface of the bulge (Bardia， Saraf， Maslow，Khabbaz， &Mahmood， 2016). If the D/r ratio (ratio of jet diameter to inner wall radius of curvature) is lessthan 0.5， the Coanda effect is observed. The D/r ratio of the annular combustion chambermodel is 0.3. When the jet flows through the inner annular wall， the Coanda effect induces itsinclination and stretches the frontal structure of the flame. Therefore， the width of the innerflame becomes narrower. Besides， under the combined action of the Coanda effect and theextrusion of the outer wall， the relative bending of the flame increases and the flame angleexpands. 4. Conclusions The OH radical distribution of methane premixed flame in annular combustor under differentdilution conditions of carbon dioxide is experimentally studied by using PLIF technique. Theeffect of carbon dioxide dilution on combustion state was obtained by observing the instan-taneous and average images of OH radical. The variation trend of characteristic parametersof flame structure was quantitatively analyzed， and the following conclusions were drawn： (1) Methane premixed flame presents typical turbulent combustion characteristics such as foldand crimp in the annular combustion chamber， and laminar premixed flame structure is alsoretained at the nozzle， which is located in the folded flame sheet pattern area. The geome-trical structure of the annular combustion chamber keeps the OH radical in annular distribu-tion， and the phenomenon of lift and flame bifurcation occurs. (2) The length and width of inner flame and outer flame decreased with the increase of Vco2，besides， the change trend of inner flame width was the largest， the overall flame reactionregion was reduced， the relative bending of the outer flame is increased.as well as flameangle. The flame premixed conical structure is stretched and the flame combustion speedand intensity are reduced. (3) The Coanda effect causes the flame to bend toward the inner wall， thus affecting therelative bending of the outer flame and the angle. Nomenclature Author details Zhonghua Wang E-mail： henrywangzh@foxmail.com Qiang Tang' E-mail： tangqiang@cqu.edu.cn Zhongqing Yang E-mail： 755241933@qq.com Pengao He' E-mail： 20161002016@cqu.edu.cn Key Laboratory of Low-grade Energy UtilizationTechnologies and Systems， Chongqing University，Ministry of Education， Chongqing 400030， PR China. ( Citation information ) Cite this article as： Experimental study on premixed flamecombustion of annular burner with CO2 dilution based onOH-PLIF technology， Zhonghua Wang， Qiang Tang，. Zhongqing Yang & Pengao He， Cogent Engineering (2019)，6：1593074. ( References ) ( Bardia， A .， Saraf， R.， Maslow， A .， Khabbaz， K . ， & Mahmood， F. (20 16) . The coanda ef f ect. Anesthesia & Analgesia， 1 23(3)， 582. doi： 10 .1213/ AN E.0000 0 0 0 0 0 0 0 0 1474 ) ( Bidabadi， M.， G hashghaei Nejad， P.， Rasam， H.， S a deghi， S.， & Shabani， B. ( 2018) . 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This open access article is distributed under a Creative CommonsAttribution (CC-BY) . license.Page of Page of     In view of the combustion characteristics of annular chamber of micro gas turbine, a planar laser-induced fluorescence (PLIF) technique was used to investigate the methane/air premixed turbulent multiple flames in the annular combustor and the effect of CO2 on the characteristic parameters of flame structure was further discussed. The results show that the distribution of the OH radicals exhibits a ring structure with flame lift and bifurcation phenomena. The instantaneous and average image of OH-PLIF reveals that the multiple flames have the common characteristics of turbulent and laminar premixed flame structure. The quantitative analysis of the characteristic parameters of flame structure shows thatthe increase of CO2 concentration, the flame premixed conical structure is stretched, the overall flame chemistry reaction area is reduced and the intensity of local combustion is weakened; The Coanda effect causes the flame to bend towards the inner wall, and the relative bending of the outer flame increases as well as the flame angle.