Experimental and computational study of combustion characteristic of

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Experimental and computational study of combustion characteristic of Dual-stage Lean Premixed flame Wenhua Zhao, Li Liu, Wenkai Shen, Yajin Lyu, and Penghua Qiu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04492 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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Energy & Fuels

Experimental and computational study of combustion characteristic of Dual-stage Lean Premixed flame Wenhua Zhao, Li Liu, Wenkai Shen, Yajin Lyu, Penghua Qiu* School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, People’s Republic of China

ABSTRACT: A new type of flame organization, Dual-stage Lean Premixed (DLP) combustion, is proposed. It constructs lean premixed flame with non-uniform equivalence ratio in the combustion zone, which is aimed at improving the load adaptability and ensuring low NOx emission of lean premixed combustor. The combustion characteristics of DLP flame were further studied through both experimental and calculation methods. The results showed that the laminarSMOKE code with detail reaction mechanism can accurately simulate the combustion state of DLP flame. Thermocouple measurement has a certain deviation from actual situation but still reflects the interaction inside DLP flame. Diffusion transport of CH4 has an important influence on the combustion of secondary flame. NOx production in DLP flame can be significantly limited because of the abundant intermediates present in combustion zone, and the NO generated is mostly converted into NO2.

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KEYWORDS: Staged combustion; Dual-stage Lean Premixed flame; LaminarSMOKE; Flame structure

1. INTRODUCTION The increasing environmental pressure has attracted more attention in recent years. With environmental regulations becoming more stringent, low emission combustion shows an irreplaceable importance1. The lean premixed (LP) combustion is considered a promising technological solution capable of achieving ultralow NOx emissions from industrial equipments2,3. The lean fuel-air mixtures in LP combustors may burn in a lower temperatures to prevent the generation of thermal NOx4-6. However, many problems have also appeared during its application7. To obtain a better environmental performance, the combustion temperature need to be controlled within a special range, generally from 1670K to 1900K, so the equivalence ratio of premixed gases entering the combustion chamber also corresponds to an optimum value which is usually around 0.658,9. But for lean premixed flames, lower equivalence ratio also means low adaptability to the external disturbances. And during the start-up and shutdown processes, or in other low-load conditions, which have deviated from the rated operation, if it still maintains a lower equivalence ratio, the flameout trend in combustion zone would grow stronger, and flame stability decreases drastically10. To avoid the occurrence of lean blow-off, flashback, thermo-acoustic oscillation and other unstable combustion, the equivalence ratio sometimes must be increased to ensure stable operation. But at the same time, it will seriously affect the environmental performance of combustors. Therefore, increasing the load adaptability of LP combustor under the premise of low NOx emission is becoming a key issue in the development of lean premixed combustion technology.


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To improve the performance of lean premixed flame the idea of Dual-stage Lean Premixed (DLP) combustion has been conceived. It belongs to a development of grading lean premixed combustion. As shown in Figure 1, there are firstly two types of premixed gases with different equivalence ratio introduced into the combustion zone. They are both lean premixed, but the primary flame in central has a high equivalence ratio, which can help stabilize overall combustion. On the contrary, the outside secondary flame has a low equivalence ratio which is even lower than the lean flammability limit of fuel. Because the secondary premixed gas is so lean that its ignition must be with the supports from primary flame, but it can introduce more air to combustion zone to help control the temperature, which is beneficial to reducing the generation of NOx. Through the cooperation of the both flames sited reasonably, the DLP flame can be stabilized at a low global equivalence ratio with low NOx emissions. Moreover, DLP flame is a dual-channel combustion mode, which means it can provide more adjusting methods for load change. But due to the relatively weak combustion of secondary flame, there may be some uncompleted combustion substances in the combustion products. So a stream of reburning fuel is introduced in downstream position. When it is mixed with the upstream flue gas, a combustion environment with high temperature and low oxygen will be built. This is the condition under which Moderate or Intense Low-oxygen Dilution (MILD) combustion occurs. So another combustion zone is formed and the reaction will be completed. The MILD combustion is characterized by low pollutant emissions and enhanced combustion stability. The amount of NOx generated in this section can be controlled within a minimum range. Reburning Fuel Exhaust Primary Gas

DLP Flame

MILD Combustion

Secondary Gas


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Figure 1. Schematic diagram of DLP combustion. In fact, the upstream DLP flame can be seen as a pre-combustion section of MILD combustion. Combustion products generated by primary and secondary flames provides conditions for the occurrence of MILD combustion in downstream position. However, previous studies has reported that in axially multi-stage combustor of MILD combustion, the amount of NOx formed in initial segment often dominates the combustor exit NOx level11,12. So for the DLP-MILD system as shown in Figure 1, the DLP flame need to ensure low NOx emissions while providing suitable flue gas for downstream combustion. Meanwhile the combustion efficiency is not its main focus. In the study of DLP combustion, what need firstly be clarified is the combustion characteristics of DLP flame. Its most important feature is the multi-scale distribution of input equivalence ratio. Many studies have reported that the flames propagating in varying equivalence ratio zones have clear differences with equivalent homogeneous flames. Kyritsis13 studied experimentally the propagation of a laminar flame in premixed mixture along the gradient of equivalence ratio, which was named stratified flame. It was found that the flame propagates faster than the one in homogeneous mixture with the same local equivalence ratio. More importantly, it can propagate in the gas mixture with equivalence ratio far below the lean flammability limit of fuel. Lecordier14 have found the same phenomenon in their researches. Zhang15 numerically studied the differences of flame speed and temperature in stratified flame from that of homogeneous flame at the same equivalence ratio. The numerical results showed that the stratified flame has a thinner flame zone and sharper gradients. As a result, the diffusion rate of species and heat increases resulting in increased flame speed. Ye16 carried out simulations of laminar stratified syngas/air flames propagation with different concentration and temperature distribution. By


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comparing with corresponding homogeneous flame, it was concluded that the H radical concentration in stratified flames burned gas has a much significant effect on the acceleration of flame propagation. Shi17 conducted numerical study on stratified flames of three hydrocarbon fuels. The flame front propagation speeds of all three fuels were found accelerated due to enhanced total heat release rate. But for some fuels with lower H/C ratio the reduced level of key radicals consumed by combustion intermediate products might slow fuel consumption speeds compared to homogeneous flame. Richardson18 investigated the turbulent methane-air stratified flame propagation using Direct Numerical Simulation. The effect of equivalence ratio stratification on flame behavior which had been observed in laminar flame previously was found also affects on turbulent combustion. Bartolucci19 used a LES approach to investigate the main driving mechanisms of partially stratified charge combustion. Lipatnikov4 summarized recent investigation in the influence of mixture inhomogeneities on flame propagation. Triple flame is another kind of inhomogeneously combustion having equivalence ratio gradient perpendicular to the direction of flame propagation. It was first observed by Philips20 and the propagation speed was found as much as five times faster than the laminar stoichiometric premixed flame. Owston21 compared the structure and propagation behavior between premixed and triple flames through a numerical study. It was concluded that the triple flame has a greater width, which means greater stability. And the transport of intermediate radical in combustion zone may extend the flammability limits. Kim22,23 experimentally proved that the gradient of the premixed equivalence ratio has a direct effect on the combustion state of triple flame. Grib24 and Renfo25 utilized a slot burner to study the bulk propagation speed of multiple edge flames as a function of concentration gradients and stoichiometric separation distance of two interacting triple flames. Interaction between flames was found to play an essential role resulting in even


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larger propagation speeds than a single triple flame with similar equivalence ratio gradients. Aggarwal26,27 reported a computational study on NOx and soot emissions of triple flames in an opposed-jet configuration. Various fuel injection scenarios with different strategies of blending n-heptane and methane were evaluated. The research on both stratified flame and triple flame has shown that the interaction in combustion zone which is derived from the inhomogeneity of premixed gases can significantly change the combustion performance. The extra heat and active species transferred from fuel-rich flame have an important influence on the fuel-lean flame, which can accelerate its propagating speed and make it capable of burning even outside the lean flammability limit. The interaction in combustion zone is the key determining the combustion state of inhomogeneous flame. However, both stratified flame and triple flame are different from the combustion state of DLP flame. But their common ground is the interactions between flames with different equivalence ratios. Studying the combustion characteristics of DLP flames is the primary issue of current work. It should be noted that although most flames in industrial combustors are turbulent28, the research should start from laminar conditions. This is for the purpose of avoiding influence from turbulent fluctuation and capture more accurately the burning information. Previous study on DLP flame has already shown that the multi-scale distribution of equivalence ratio can significantly extend the extinction limit of premixed flame, which provides a possibility to improve the combustion performance of lean premixed flame in industrial combustor29. This paper conducts a further study on DLP flame through both experimental and simulation methods. The distributions of temperature and concentration around the combustion zone, as well as the NOx emission characteristic, are compared and investigated to get a better understanding on the


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DLP flame structure, which can provide theoretical support for the application of DLP combustion.

2. EXPERIMENTAL AND SIMULATION METHODS A schematic of the experimental setup of laminar dual-stage combustion used is shown in Figure 2. It is a coflow jet burner with 4 mass flow meters controlling the CH4 and air into the system. The primary premixed gas was supplied through the central tube with an inner diameter of 4 mm and an outer diameter of 6 mm. There was another annular tube arranged coaxially, whose inner diameter was 29 mm. The secondary stream entered combustion zone through the ring channel formed by the two tubes. Near the outlet of secondary channel a sintered metal foam was installed to ensure a uniform flow. The mass flowmeters controlling gas flow rates have the measurement inaccuracy below 1%. Composition of combustion products were analyzed online using Fourier transform infrared spectroscopy (accuracy in 0.01%). A more detailed introduction to the experimental system and method can be found in the previously published article of Ref 29. FT-IR

Exhaust

Sampling tube

Sightglass

Camera Canon 60D EF 35mm

DLP flame

Flame shell Central tube Secondary gas

Annular tube

Water bath 300K

MMC

MMC

IDcentral : 4 mm ODcentral: 6 mm IDannular: 27 mm

MMC

Mass flowmeter

MMC

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Primary gas

Combustor

Methane

Air

Reacting gas Supply system


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Figure 2. Schematic diagram of experimental system. LaminarSMOKE is a framework for the numerical simulation of gases mixture with chemical reactions. It can deal with detailed kinetic schemes and solves the conservation equations of total mass, momentum, individual species mass fractions and mixture energy in laminar conditions with an operator-splitting technique. The mass and thermal diffusion are both taken into account. An optically thin radiation model is also included in the simulation. CH4, H2O, CO2 and CO are assumed to be the only significant radiating species to evaluate the Planck mean absorption coefficient. The kinetic scheme, together with the thermodynamic and transport properties of the species, can be taken from the Chemkin database30,31. At present, laminarSMOKE has been widely used in laminar flame research. In this paper DLP flames were numerically simulated with laminarSMOKE code. GRI-Mech 3.0 mechanism was selected as the reaction mechanism. It contains 53 species and 325 reactions and has been widely used in the study of CH4 combustion. Various field distributions around the combustion zone were calculated. Previously, the DLP flames were experimentally organized in the form of coflow jet. Due to the cylindrical symmetry of the whole system, a 2D axysymmetric mesh was adopted in the calculation, which corresponded to a wedge combustion region with an angle of 5 degree. The computational domain was 24 mm (r) × 115 mm (h). It could ensure that the reaction completed in the calculation area. The inlet boundaries were placed at 15 mm below the burner exit to match the actual experimental situation. The primary and secondary gases enter from the inlet boundaries at their own uniform rate. Zero gradient conditions were imposed at the outlet. The centerline was specified as symmetric boundary condition and slip condition was used at the outer boundary in the radial direction. In addition, the grid quality has great influence on the numerical results. A preferable grid structure was expected to acquire to get accurate


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simulation as well as reduce the computational time. The calculation contained 75 (r) × 230 (h) control volumes. Very fine grids were constructed with a uniform grid resolution of 0.2 mm in the radial direction between 0 and 3 mm. Non-uniform grids were placed in the region outside the central tube with a cell expansion ratio of 2.5, which means that the outermost grid in rdirection had the largest size of 0.5 mm. In the height direction, non-uniform grids were used entirely. They had the smallest size of 0.2 mm around the burner port, and became coarser downstream and upstream with the same cell expansion ratio of 4. In general, the calculation area with the strongest combustion had the finest grids, and the flow-based area had a larger grid size. The grid independence study was conducted by comparing the temperature distributions along the central axis simulated by various grid cell numbers, which had the same inlet boundary conditions. It was found that further refinement of grid did not have significant improvement on the result. They got the nearly coincident temperature curves. Accordingly, the current meshing method was confirmed as the final scheme.

3. RESULTS AND DISCUSSION 3.1. Comparison of experimental and calculated results. Before investigating the combustion characteristics of DLP flame through the data obtained by laminarSMOKE, it is firstly necessary to compare the results from calculation and experiment to verify the accuracy of numerical simulation. During experiment, the flame shapes under various experimental condition were recorded by a Canon EOS 60D Digital SLR Camera equipped with 35 mm prime lens. Its resolution in the combustion zone was about 30 pixels/mm. In the image, the secondary flame is a visualized representation of its combustion state, which is determined by the field distributions around combustion zone. Among the various parameters in calculation results of laminarSMOKE, OH is the one that best reflects the location of combustion reactions in a flame.


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It is one of the most important intermediates in combustion and widely distributed in the reaction zone. So the OH concentration is often used to predict the severity of the combustion reaction and exotherm, and to define the combustion zone of premixed flame. The results of DLP flames, which were obtained respectively from experiment and calculation, are compared and analyzed in the following. As shown in Figure 3, the left half is the experimentally obtained flame image. The brightness in the picture reflects the intensity of combustion reaction. The pseudo-color image on the right side, which is extracted from the calculation result of laminarSMOKE, shows the distribution of OH concentration in the combustion zone. In general, the higher the OH concentration, the combustion reaction is stronger here. The DLP flames on both sides have the same premixed parameters. The images have also been set to the same scale and placed in the same position. As can be seen from the figure, the distributions of secondary flame fronts on both sides are basically coincident. After the burner nozzle they first expand outwards, then shrink to center with the flow developing downstream. The radii of secondary flames at different heights also basically match. However, it should be noted that the root of DLP flame obtained numerically is somewhat different from the actual situation. In experiment there was a "dark space" existing adjacent to the burner port edge, which was caused by the cooling effect of burner port32. No combustion reaction occurred in the dark space. But the quenching effect of the burner wall was not considered in calculation. So this phenomenon has not been reproduced, and the flame was adjacent to the burner port. However, this article is more concerned with the field distribution characteristics in the combustion zone, not the flame stability manner, so this difference will not have a substantial impact. And the agreement between the two methods on the flame shape confirms the accuracy of calculation.


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(a)

(b)

Figure 3. Flame shapes obtained respectively from experiment and calculation with φpri=0.9, vsec=0.3 m/s: (a) vpri=0.6 m/s, φsec=0.3, (b) vpri=0.7 m/s, φsec=0.4. Temperature is an important parameter reflecting the characteristic of flame combustion. The temperature distribution of DLP flame was measured with a Pt-6%Rh/Pt-30%Rh thermocouple, whose wire diameter was 0.1 mm and bead diameter was about 0.2 mm. The thermal radiation loss from thermocouple bead has been corrected in post-processing. In the correction the convective heat transfer coefficient was calculated through the Nusselt-Reynolds correlations33. Figure 4 is a comparison between the temperature distributions of a DLP flame obtained respectively from experiment and calculation. In general, they both have the same temperature distribution characteristics. Due to the influence of heat dissipation, the temperature will decrease gradually as it goes downstream and outside. Whereas, at the same time there also exists a little quantitative difference between experimental and calculated results. It can be clearly seen from the figure that the measured flame temperatures at each point are lower than the calculated ones.


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2000 1800 1600 Temperature (K)

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1400 1200 1000 800

Experiments

600

h=5mm h=7mm h=9mm

400 200

0

1

2

Calculations h=5mm h=7mm h=9mm 3

4

5

6

r (mm)

Figure 4. Experimental and computed temperature distribution of a DLP flame with vpri=0.7 m/s, φpri=0.95, vsec=0.1 m/s, φsec=0.2. In fact, the accuracy of thermocouple measurement is affected by many factors. The measurement error can be divided into spatial error and heat exchange error. Although the thermal radiation error can be corrected, there are still many ones that cannot be accurately quantified. Such as the influence of thermocouple insertion on the flame and the effect of soot layer covering thermocouple surface, which is formed in flame, on the thermal resistance. All these can cause the measured temperature to be lower than the actual flame temperature34. Therefore, it is difficult to get the exact flame temperature by contact measurement. However, the influence of premixed parameters on combustion can be accurately reflected in the temperature change, so the trend of measured temperature still provides an important reference for qualitatively studying the flame characteristics. And the agreement of their trend with the calculated results also proves the accuracy of the calculation to some extent.

3.2. Maximum temperature of the DLP flame. For a DLP flame, its primary flame is the core to ensure overall stability. The combustion state of primary flame has an important


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effect on the entire flame. At the same time, the primary flame is also affected by each premixed parameter. Since the secondary flame is burning with the aid of heat transferred from center, and it was found in the experiment that the maximum temperature was generally located at the top of primary cone flame, so the maximum temperature can be considered a concentrated reflection of the combustion state of DLP flame. In the following, the measured maximum temperature of DLP flame is analyzed as a function of the premixing parameters, so as to further obtain the combustion characteristic of DLP flame. Table 1 lists the maximum temperatures of DLP flames under various combinations of secondary premixing parameters. From the two sets of data it is obvious that the secondary premixed parameters has a significant effect on the maximum temperature of DLP flame. First, when there is only the primary premixed gas involved in combustion, which means both vsec and φsec are 0, the flames have relatively low maximum temperatures. But the temperatures are further reduced when air is introduced from the annual tube. This is because the outside air flow can promote heat dissipation from center. Moreover the faster the flow rate, the lower the maximum temperature. However, when the secondary premixed gas is introduced and burns around the primary flame, as can be seen from the table, the maximum temperature of flames will increase significantly. In fact, this is also due to the impact on heat dissipation process in central area. In Figure 4, the temperature curves near secondary flame are all monotonic. This indicates that the heat release of secondary flame itself is not enough to support its independent combustion. The heat from primary flame helps the secondary premixed gas reach ignition condition. However, the heat released by secondary flame can reduce the temperature gradient in the radial direction, slowing heat loss from central region. Therefore, the maximum temperatures of DLP flames are higher than the cases without secondary flames. Moreover, the secondary


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flame with higher premixed equivalence ratio can release more heat, so the temperature of corresponding DLP flames will also increase. However, the flow rate of secondary premixed gas does not show a regular effect on the maximum temperature of DLP flame. It is considered that when the flow rate of secondary premixed gas increases, more heat is taken away on the one hand, but on the other hand more fuel can take part in combustion. The both processes have opposite effects on the flame temperature, so this is the case. Table 1. The maximum temperature of DLP flame with different secondary premixing parameters (a) vpri=0.6m/s, φpri=1 vsec

φsec

m/s 0

0.2

0.3

0.4

0.5

--

--

--

0

1873 --

0.1

1855 1914 1945 1960 2010

0.2

1841 1893 1919 1951 1997

0.3

1824 1876 1904 1943 1982

(b) vpri=0.7m/s, φpri=0.95 vsec

φsec

m/s 0

0.2

0.3

0.4

0.5

--

--

--

0

1897 --

0.1

1896 1930 1965 1977 2008

0.2

1880 1919 1945 1971 2015

0.3

1882 1915 1942 1972 2016

The maximum temperatures of DLP flames under various combination of primary premixing parameters are listed in Table 2. The primary flame has a higher combustion temperature to


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transfer heat outward to ensure the ignition of secondary flame. So the maximum temperature of DLP flame is actually more related to the combustion state of primary flame. Hence, the effect of primary equivalence ratio on the maximum temperature is more pronounced, as is shown in Table 2. In addition, as the primary velocity increases, more premixed gas enters the combustion zone and more heat is generated. So the corresponding maximum temperature will increase too. Table 2. The maximum temperature of DLP flame with different primary premixing parameters (a) vsec=0.3m/s, φsec=0.3 vpri

φpri

m/s 0.9

0.95

1

0.6

1809 1878 1904

0.7

1882 1942 1971

0.8

1880 1935 1992

(b) vsec=0.2m/s, φsec=0.4 vpri

φpri

m/s 0.9

0.95

1

0.6

1840 1925 1951

0.7

1904 1971 2004

0.8

1892 1977 2032

In a DLP flame, it is evident that the secondary flame with ultra-lean premixed equivalence ratio must be burned under the support from primary flame. In return, the presence of secondary flame can also reduce the heat dissipation from central zone, enhancing the stability of primary flame. The temperature gradient formed in the radial direction is also a key parameter for heat transfer. The maximum flame reflects the interaction between the primary and secondary flames.


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3.3. Structure of DLP flame. The combustion manner of secondary flame has been analyzed in previous study. As shown in Figure 3, the primary flames belong to the Bunsen flame, where combustion occurs when premixed gas flows through the flame front. But for the secondary flame it is a little different. When the secondary stream exits the annular nozzle, its velocity is parallel to the central axis. But as flowing downstream, the streamlines will deflect outward due to the volume expansion caused by combustion in center. However, in Figure 3, it can be seen clearly that the secondary flame has a long tail which keeps parallel or shrinks towards center. Different from the primary flame and the upstream part of secondary flame, the downstream secondary flame cannot obtain reactants from outside stream through convection transport. However, the above analysis is mainly based on the inference of flame images, which has not been confirmed. So in the following, the combustion characteristic of DLP flame will be further studied through the calculation results. Figure 5 is the streamline chart of a DLP flame, which is extracted from the calculated data. The result is basically consistent with the previous inference. The combustion zone can be identified by the concentration of OH. As analyzed earlier, the volume of premixed gas expands after passing through the flame front. But then the heat dissipation causes temperature to drop, and the gas volume will shrink accordingly. As a result, the streamlines deflect toward center. To study the combustion of secondary flame in more detail, the flow track of premixed gas close to the outer wall of center tube is analyzed. After flowing out of nozzle, the secondary stream first enters combustion zone from the outside of secondary flame front. Here the combustion state is similar to the primary flame, fuel being transported convectively to the front. The only difference is that it needs heat assistance from center to meet the ignition requirements. Subsequently, the burned gas continues to flow downstream and eventually crosses the secondary flame front again


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to the outside. However, reaching the flame front for the first time, the secondary gas is a fresh premixed gas carrying a certain concentration of fuel. So the combustion can occur at the right place. But when the burnt gas passes through the flame again from the inside out again, no fuel is contained in the stream. The calculated result also shows that in the zone enclosed by the secondary flame the CH4 has been exhausted. Therefore, there must be other ways to obtain fuel for the secondary flame in downstream position to maintain the combustion reaction.

Figure 5. Streamline chart of a DLP flame with vpri=0.7 m/s, φpri=0.9, vsec=0.3 m/s, φsec=0.3. The calculated temperature and CH4 mass fraction distributions of DLP flame are shown in Figure 6. The OH mass fraction distribution is also displayed to show the flame position. By comparison, it can be seen that the high-temperature combustion products and the fresh secondary premixed gas are on both sides of secondary flame respectively. It is obvious that the gradients of temperature and CH4 concentration will be formed in the radial direction. Because the primary and secondary premixed gases are both lean, which means that the entire combustion zone is in a state of sufficient air. So the conditions of temperature and fuel concentration have become the keys for secondary flame to ignite. Meanwhile the mass diffusion and heat conduction due to parametric gradients can provide more possibilities for combustion. In


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summary, it can be inferred that the position of secondary flame whose equivalence ratio is lower than the lean flammability limit of CH4 is actually determined by factors such as temperature and fuel concentration. But in different parts of secondary flame, the fuel concentration is affected by different processes. Convection and diffusion transport play the major role respectively in the upstream and downstream positions.

(a)

(b)

Figure 6. Temperature and CH4 mass fraction distributions of a DLP flame with vpri=0.7 m/s, φpri=0.9, vsec=0.3 m/s, φsec=0.3. Another problem reflected by secondary flame is the ignition condition of ultra-lean premixed gas. In general, the flammability ranges of fuels has been studied for many years and a large number of techniques have also been proposed to determine the limits. Research on the flammability ranges of fuels is of great practical significance, but these techniques are all based on the condition of normal temperature. While for the secondary flame, it is burning with the assistance from primary flame and has a ultra-lean equivalence ratio. So this part of content needs to be re-studied.


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Firstly, the combustion starting position of secondary flame needs to be determined. However, the secondary flame burns slowly and has a relatively thick reaction sheet. This makes it difficult to accurately determine the flame sheet. But the fresh unburned premixed gas is on the outside of secondary flame, so it can be considered that the combustion begins there too. Moreover, the main research object is the relationship between parameters around the flame, so the flame boundary should be defined from the variation of fuel concentration. As previously mentioned, the mass diffusion plays an important role in the fuel transport of secondary flame. The CH4 molecules are transported from outside to inside, but due to thermal decomposition the diffusion flux of CH4 will decrease gradually near the flame. And the location having the fastest reduction in diffusion flux means that the CH4 is consumed the most quickly there. So in the following, the positions at each height where the diffusion flux in radial direction decreases the fastest is defined as secondary flame boundary and selected to analyze the ignition characteristic of secondary flame. Figure 7 shows the distributions of temperature and CH4 mass fraction along the boundary of secondary flames with different premixing parameters. It can be seen that the temperature does not change monotonically with the height, but overall is decreasing. This is because the central temperature is gradually decreasing with the flow developing downstream due to heat dissipation. On the other hand, errors introduced by factors such as grid precision and flame boundary determination will also affect the calculating result. So the temperature and fuel concentration do not show regular changes. But the temperature only fluctuates within about 50K, which is very small relative to the flame temperature. Another obvious feature shown in Figure 7 is the strict complementarity of temperature and CH4 concentration. When one of them changes with height, the other will show an opposite trend. The fuel consumption will inevitably lead to an increase in


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Energy & Fuels

temperature, but there must be some dimensionless conservation of temperature and CH4 concentration around the secondary flame, which is worth further study. However, as shown in Figure7(d), the CH4 mass fraction on the curve is much higher than the other three conditions, and the main difference in combustion conditions from others is the parameters of secondary premixed gas, which means that the conservation is also affected by some other factors. Although the current results are not enough for a deep analysis, it is certain that the ignition of secondary flame is determined by temperature, concentration and some other parameters, and these factors have some complementary relationship around the flame front. 1.40

1.35 16

vpri= 0.7 m/s

14

vsec= 0.3 m/s

10

14 -4

sec= 0.3

3

Temperature (10 K)

-4

12

1.30

CH4 mass fraction (10 )

sec= 0.3

3

Temperature (10 K)

vsec= 0.3 m/s

1.35

16

pri= 0.8

pri= 0.9


vpri= 0.7 m/s

12

1.30 10 8

8 6 2

4

6

8

10

6 12

1.25

2

4

6

h (mm)

8

10

(b)

1.35

22

1.35 12

1.30 vpri= 0.6 m/s

8

pri= 0.9

vsec= 0.3 m/s 4

16

1.30 vpri= 0.7 m/s

6

sec= 0.3

2

18

3

3

10

Temperature (10 K)

-4


20

6

8

10

12

pri= 0.9

14

vsec= 0.2 m/s

12

sec= 0.4

1.25

2

h (mm)

(c)

-4

(a)

1.25

12

h (mm)


1.25

Temperature (10 K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

6

8

10

10 12

h (mm)

(d)


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Energy & Fuels

Figure 7. Temperature and CH4 mass fraction distributions along the boundary of secondary flames at different heights.

3.4. NOx emission characteristic of DLP flame. In previous experiment, FT-IR was used to measure the amount of nitrogen oxides in combustion products. NOx emissions from DLP flames and Bunsen flames are compared. It was found that the DLP combustion can effectively reduce NOx emissions. In the system shown in Figure 2, the NOx content in combustion products is affected by many factors such as burner structure, flow rate, heat dissipation, premixed equivalence ratio, etc. When the secondary premixed gas in annular tube is removed or replaced with air, there is only the primary flame burning on the burner. In this case, it is a common Bunsen premixed flame with a uniform equivalence ratio. In the experiment, the primary stream velocity was set from 0.6 to 0.8 m/s, and the velocity of air coflow was from 0 to 0.3 m/s. The measuring results show that under these conditions the NOx emissions of Bunsen stoichiometric flame were not less than 50 ppm. But when a premixed gas was introduced into combustion zone through the annular tube to organize a DLP flame, NOx in the combustion products could be reduced to 10 ppm. It is concluded that the secondary flame burns at a lower temperature and generates a large number of hydrocarbon intermediates, which can react with NOx and reduce them to N2, so the NOx emissions were drastically restrained. However, during the experiment an unusual phenomenon was found. Generally there are many kinds of nitrogen oxides. Yet the main kinds in the combustion products of fossil fuels are NO and NO2. Among them NO accounts for the majority. In measuring the emissions of Bunsen flame, it was found that the NOx in combustion products are mainly NO. No NO2 could be detected for the flame with uniform equivalence ratio. The content of other nitrogen oxides was also negligible. Whereas the result from DLP flame shows a completely opposite situation. The


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combustion products of DLP flames contained NO2 but no NO. In many previous studies, it was usually thought that the NO2 /NO ratio was negligibly small in the combustion products of natural gas. But significant concentration of NO2 have been reported in the exhaust of gas turbines and some other combustion equipments. In some cases the NOx has even been found to be as high as 15 to 20 times the NO levels. Many researchers have studied the conversion between NO and NO2. It was found that in the low-temperature region of flames there is large concentration of HO2 which can react with NO and convert it to NO2. The reaction equation is: NO + HO2→NO2 + OH Although the NO2 formed can be reduced to N2 and O2, or attacked by O and return back to NO, the consuming reactions are relatively slow. So the NO2 can persist under some certain conditions and be detected a large amount in the combustion products. The above content has also been verified in some experiments by mixing NO with the cold-fuel mixtures35,36. Based on the above, it can be concluded that in the DLP flame, due to a condition of low temperature and low equivalence ratio, the secondary flame undergoes an incomplete combustion reaction. A large amount of HO2 is produced there. The HO2 can oxidizes NO generated in central high-temperature zone to NO2. This process has also been reproduced by the calculation. As shown in Figure 8, NO is generated near the primary flame. With flowing downstream, NO meets the HO2 around secondary flame. Then the NO is consumed rapidly and the NO2 concentration rises correspondingly.


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Energy & Fuels

(a)

(b)

(c)

Figure 8. Distributions of NO, HO2 and NO2 in a DLP flame with vpri=0.7 m/s, φpri=0.9, vsec=0.3 m/s, φsec=0.3.

4. CONCLUSION This paper studied the combustion characteristics of DLP flame through both experimental and calculation methods. Experimentally, the flames were organized in the form of coaxial jet. Flame image, temperature and NOx content in combustion products were measured. Numerically, the laminarSMOKE incorporating GRI-Mesh 3.0 was employed to obtain the parameter distributions in the DLP flame. Both experimental and numerical results were compared and analyzed. Based on the above studies, the main conclusions can be drawn as follows: (1) The laminarSMOKE code coupled with GRI-Mech 3.0 mechanism can accurately simulate the combustion state of DLP flame. The agreement between numerical results and experimental


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measurements in regard to chemical reaction distribution demonstrates that the laminarSMOKE can be used as a new approach to study the combustion characteristic of DLP flame. (2) The contact measurement of thermocouple has a large impact on the structure of smallsized flame, making measured temperature usually lower than actual value. However, the result acquired still reflects the interaction within DLP flame that the ultra-lean secondary premixed gas is combusting with heat assistance from primary flame, the secondary flame at the same time reduces heat dissipation from central zone, enhancing the stability of primary flame. (3) Diffusion transport of CH4 plays an important role in the secondary flame, especially for its downstream part. Heat and fuel molecules are transported from both side to the flame front, and combustion occurs where conditions permit. Along the secondary flame front, temperature and fuel concentration have some complementary relationship, which means when one changes, the other will show an opposite trend. (4) Since the low-temperature and low-equivalence ratio condition of secondary flame, a large number of incomplete intermediates are produced during combustion. Their presence reduces the NOx concentration in combustion products. Besides, the HO2 generated around secondary flame can convert NO into NO2, which makes the combustion products of DLP flame contain mainly NO2, while the NO content is negligible. AUTHOR INFORMATION Corresponding Author *Telephone: 0086-451-86413231 ext 804. E-mail: [email protected]. ACKNOWLEDGMENT


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Energy & Fuels

We acknowledge the support received from the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51421063). Besides, Wenhua Zhao prays that the spring breeze can bring this news and his missing to heaven. ORCID Penghua Qiu: 0000-0003-2095-1965. Notes The authors declare no competing financial interest. REFERENCES (1) Sun, P.; Yuan, Y.; Ge, B.; Tian, Y.; Zhang, Z. Zang, S. Combustion Oscillation Characteristics and Flame Structures in a Lean Premixed Prevaporized Combustor. Energy Fuels 2017, 31, 10060-10067. (2) Park, J.; Nguyen, T. H.; Joung, D.; Huh, K. Y.; Lee, M. C. Prediction of NOx and CO Emissions from an Industrial Lean-Premixed Gas Turbine Combustor Using a Chemical Reactor Network Model. Energy Fuels 2013, 27, 1643-1651. (3) Gamal, A. M.; Ibrahim, A. H.; Alj, E. M.; Elmahallawy, F. M.; Abdelhafez, A.; Nemitallah, M. A.; Rashwan, S. S.; Habib, M. A. Structure and Lean Extinction of Premixed Flames Stabilized on Conductive Perforated Plates. Energy Fuels 2017, 31, 1980-1992. (4) Lipatnikvo, A. N. Stratified turbulent flames: Recent advances in understanding the influence of mixture inhomogeneities on premixed combustion and modeling challenges. Prog. Energy Combust. Sci. 2017, 62, 87-132.


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(12) Hayashi, S.; Yamada, H.; Makida, M. Extending low-NOx operating range of a lean premixed-prevaporized gas turbine combustor by reaction of secondary mixtures injected into primary stage burned gas. Proc. Combust. Inst. 2005, 30, 2903-2911. (13) Kang, T.; Kyritsis, D. C. Methane flame propagation in compositionally stratified gases. Combust. Sci. Tech. 2005, 177, 2191-2210. (14) Balusamy, S.; Cessou, A.; Lecordier, B. Laminar propagation of lean premixed flames ignited in stratified mixture. Combust. Flame 2014, 161, 427-437. (15) Zhang, J.; Abraham, J. A numerical study of laminar flames propagating in stratified mixtures. Combust. Flame 2016, 163, 461-471. (16) Wei, W; Yu, Z.; Zhou, T.; Ye, T. A numerical study of laminar flame speed of stratified syngas/air flames. Int. J. Hydrogen Energy. 2018, 43, 9036-9045. (17) Shi, X; Chen, J.; Chen, Y. Laminar flame speeds of stratified methane, propane, and nheptane flames. Combust. Flame 2017, 176, 38-47. (18) Richardson, E. S.; Chen, J. H. Analysis of turbulent flame propagation in equivalence ratiostratified flow. Proc. Combust. Inst 2017, 36, 1729-1736. (19) Bartolucci, L.; Cordiner, S.; Mulone, V.; Rocco, V. Natural Gas Partially Stratified Lean Combustion: Analysis of the Enhancing Mechanisms into a Constant Volume Combustion Chamber. Fuel 2018, 211, 737-753. (20) Philips, H. Flame in a buoyant methane layer. Symp. Combust. 1965, 10, 1277-1283.


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(21) Owston, R.; Abraham, J. Structure of hydrogen triple flames and premixed flames compared. Combust. Flame 2010, 157, 1552-1565. (22) Kim, N. I.; Seo, J. I.; Oh, K. C.; Shin, H. D. Lift-off characteristics of triple flame with concentration gradient. Proc. Combust. Inst. 2005, 30, 367-374. (23) Kim, N. I.; Seo, J. I.; Guahk, Y. T.; Shin, H. D. The propagation of tribrachial flames in a confined channel. Combust. Flame 2006, 146, 168-179. (24) Grib, S. W.; Renfro, M. W. Propagation speeds for interacting triple flames. Combust. Flame 2018, 187, 230-238. (25) Grib, S. W.; Renfro, M. W. Fuel effects on interacting triple flame propagation velocities. Combust. Flame 2019, 201, 200-207. (26) Han, X.; Aggarwal, S. K.; Brezinsky, K. Effect of Unsaturated Bond on NOx and PAH Formation in n-Heptane and 1-Heptene Triple Flames. Energy Fuels 2013, 27, 537-548. (27) Sabnis, P.; Aggarwal, S. K. A numerical study of NOx and soot emissions in methane/nheptane triple flames. Renewable Energy 2018, 126, 844-854. (28) Liao, Y.; Zhao, X. Plasma-Assisted Stabilization of Lifted Non-premixed Jet Flames. Energy Fuels 2018, 32, 3967-3974. (29) Zhao, W.; Qiu, P.; Liu, L.; Lyu, Y.; Shen, W.; Xing, C. Influence of secondary coflow on the extinction limit of dual-stage lean premixed flame. J. Harbin Inst. Technol. 2018, 50, 59-65.


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Experimental and computational study of combustion characteristic of

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