Experimental and Numerical Study on the Combustion Characteristics

Subscriber access provided by University of Sussex Library

Combustion

Experimental and Numerical Study on the Combustion Characteristics of Propane / Air Laminar Premixed Flame at Elevated Pressure Yan-huan Jiang, Guoxiu Li, Hong-meng Li, Lei Li, Li-Li Tian, and Haitao Huang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01468 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 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

Energy & Fuels

Experimental

and

Numerical

Study

on

the

Combustion Characteristics of Propane/air Laminar Premixed Flame at Elevated Pressure

Yan-huan Jiang a,b, Guo-xiu Li a,b*, Hong-meng Li a,b, Lei Li a,b, Li-li Tian a,b , Hai-tao Huang c a

School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing, 100044, China

b

Key Laboratory of Vehicle Advanced Manufacturing, Measuring and Control Technology (Beijing Jiaotong University), Ministry of Education, Beijing, 100044, China

c

Beijing Research Institute of Precise Mechatronics and Controls, Beijing, 100076, China Information of the corresponding author: Prof. Dr.-Ing LI G.X. School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, P.R. China Tel: +86 010 51682047，Fax: +86 010 51682047 Email: [email protected]

ACS Paragon Plus Environment

1

Energy & Fuels 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

Page 2 of 23

Abstract: To investigate the flame propagation characteristics and flame structural characteristics, experimental and numerical methods have been used to study the propane /air mixtures with the equivalence ratio ranging from 0.9 to 1.3 under various initial pressures. The core elemental reactions and active radicals, which had significant effect on the laminar burning velocity, were analyzed and confirmed. In addition, the influence of flame inherent instabilities on the flame structural characteristics was quantitatively studied by extracting the cracks information. The results indicated that the laminar burning velocity increased firstly and then decreased with the increase in equivalence ratio. As the initial pressure increased, the laminar burning velocity decreased. The laminar burning velocity showed the highest sensitivity to the chain branching reaction of H+O2=O+OH. As the equivalence ratio increased, the main chain inhibiting reaction changed from H+O2+H2OHO2+H2O to CH3+H (+M) =CH4 (+M). As the equivalence ratio increased, the hydrodynamic instability increased firstly and then decreased. Meanwhile, the thermal-diffusive instability increased significantly, the flame became more unstable as a whole, while cracks length under the same radius increased significantly. With the increase in initial pressure, the hydrodynamic instability was enhanced, and cracks length increased. KEYWORDS: Propane; Pressure; Equivalence ratio; Flame inherent instability; Crack.

1. Introduction Combustion is the source of impetus for the progress of human society. Throughout the history of human social development, from the beginning of heating, self-defense to all kinds of industrial equipment and power plants, combustion has penetrated into every aspect of human society [1-3]. Although the energy infrastructure has undergone many innovations, fossil fuels are still the main components of energy. In the background of current energy crisis, it is of great significance to further deepen our understanding of combustion to achieve efficient control of combustion and to find alternative energy sources [4, 5]. In the last few decades the automotive industry has directed many efforts towards the liquefied petroleum gas (LPG) to be used as an alternative fuel for Spark Ignition (SI) engine [6-8]. Propane is the main component of LPG, and therefore


2

Page 3 of 23 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

Energy & Fuels

research on the basic combustion of propane is of great significance to further improve the combustion control of SI engine. Laminar burning velocity is an important parameter for studying the chemical reaction dynamics, and is also the basis to study the turbulent premixed combustion [9, 10]. Scholars conducted a systematic study on the laminar burning velocity of propane/ air mixtures. Metghalchi et al. [11] measured the laminar burning velocity of propane/air mixtures using a constant volume chamber, indicating that the laminar burning velocity increased with the increase in initial temperature, while the burning velocity decreased with the increase in initial pressure. Desoky et al. [12] and Razus et al. [13] evaluated the laminar burning velocity of propane/air mixtures with various initial equivalence ratios, pressures and temperatures from pressure measurements in a spherical vessel. Liu et al. [14] compared the laminar burning velocity of propane/air mixtures of different scholars at various equivalence ratios under atmospheric pressure and temperature, and demonstrate that the maximum burning velocity occurred at the equivalence ratio 1.1. To study the effects of nitrogen dilution on the combustion characteristics of propane/air mixtures, Tang et al. [15-16] investigated the laminar burning velocity of propane/air mixtures at atmospheric pressure, temperature and elevated pressures and temperatures. Furthermore, to improve combustion efficiency and reduce the emissions, the effect of addition of hydrogen on the propane/air mixtures was also experimentally investigated by Tang et al. [17, 18]. Li et al. [19] numerically studied the effect of addition of hydrogen on the flame structure of propane/air mixtures using three different mechanisms under atmospheric pressure and temperature, and demonstrated that the reaction H+O2 OH+O promoted the flame speeds. Jomaas et al. [20] used a C1-C3 mechanism and an ethylene mechanism to calculate the laminar burning velocity of C2 - C3 hydrocarbons. From what had been discussed above, many scholars have determined the laminar burning velocity of propane. However, due to differences in measurement

methods, the values of laminar burning velocity measured by different scholars are quite different. The elementary reactions that promote the laminar burning velocity under high pressure conditions also need to be further clarified. For expanding premixed flames, the flame inherent instabilities are also the focus of research. In the process of flame propagation, the initial perturbations caused by the flame inherent instabilities, which act on


3

Energy & Fuels 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

Page 4 of 23

the flame front, and cause the formation of cellular structure, will affect the flame propagation speed. Kitagawa et al. [21] quantitatively studied the scale of flame inherent instabilities using non-dimensional wave number, showing that the mean wave number of the instability increased with the flame propagation. Furthermore, Jiang et al. [22] and Li et al. [23] decomposed the perturbations in flame front by using wavelet transform and Fast Fourier Transform (FFT), respectively. Critical radius and critical Peclet number based on the evolution of flame cellular structure are also the two important quantization parameters to study the flame inherent instabilities [24, 25]. Jomaas et al. [26] studied the transition of outwardly propagating spherical flame to cellularity, indicating that the critical radius and critical Peclet number decreased with the increase in equivalence ratio. However, for the laminar premixed flame of propane/air mixtures is relatively stable, and the critical radius and critical Peclet number of the flame under lower pressure and equivalent ratio often cannot be measured in the range of quartz window. How to establish the effective evaluation parameters of flame structural characteristics is of great significant. For the expanding premixed flame, the cracks formed before the formation of cellular structure, and the extraction of cracks can effectively characterize the flame inherent instabilities. To further reveal the influence of chemical reaction kinetics on the laminar burning velocity of propane/air mixtures and flame intrinsic instabilities on the flame cellular structure, experimental and numerical investigations were conducted for the laminar premixed flame of propane/air mixtures under room temperature and elevated pressure at various equivalence ratios. 2. Experimental setup and procedures 2.1 Experimental setup All the experiments were conducted in a constant volume combustion chamber, having an inner diameter of 380mm and the net volume of 28.73L. Two pairs of quartz windows were installed in the horizontal direction. The Schlieren system combined with a FASTCAM SA-X2 high-speed digital camera was used to capture the flame images. The resolution of the camera was 1024*1000 and the sampling frequency was 13500 frames per second. The air used in the experiments was synthesized air with 21% oxygen and 79% nitrogen by volume fraction, while the purity of propane used in the experiments was 99.95%. The gas phase pressures of


4

Page 5 of 23 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

Energy & Fuels

all components were calculated using Dalton’s law. After the mixtures were maintained for 5 minutes, ignition was activated. Exhaust gas after combustion was pumped out by using a vacuum pump, and an air flow was used to wash the chamber trice to eliminate the impact of residual gas. In addition, the ignition electrodes and ignition energy were kept unchanged in the experiments. The error bars were calculated based upon the results obtained from repeated experiments. The schematic of the experiment setup is shown in Fig. 1.

Fig. 1. Schematic of the experiment setup 2.2 Definition of parameters Considering the great advantages of MATLAB in image processing, it was chosen to extract the contour of flame front. In order to extract the cracks in flame front accurately, Gauss smoothing was used to process flame images. In order to avoid the interference of the background on the crack extraction, each image was subtracted from the image when the flame had not yet developed. While processing the flame information, the flame profile, ignition electrodes and crack information were extracted. In order to calculate the cracks information of the flame front, ignition electrodes before ignition and flame profile at a certain moment needed


5

Energy & Fuels 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

Page 6 of 23

to be extracted. Then, the cracks need to be refined and the pixel value be converted to real length using the conversion factor. Fig. 2 shows the extracted cracks information, which was in good agreement with the original image. For cracks to be generated as shown in Region B, extraction is not possible due to its low pixel value; For cracks at the edge of the flame as shown in Region A, due to the difference between the real threedimensional flame and the two-dimensional flame, it is impossible to extract more accurately. Nevertheless, cracks length extracted from two-dimensional images can still qualitatively reflect the effect of flame inherent instabilities on flame structural characteristics.

Fig. 2 Comparison between the flame and the extraction of cracks For the expanding premixed flame, the flame propagation speed (Sn) can be derived using the flame radius evolution (Eq.1).

Sn =

dRu dt

(1)

where Ru is the equivalent radius of the flame area. The flame stretch rate has a significant influence on the evolution of the flame, which is given by Eq.2.

α=

d (ln A) 1 dA = dt A dt

(2)

where A is the flame area.


6

Page 7 of 23 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

Energy & Fuels

There is a linear relationship between the flame propagation speed and the flame stretch rate [27].

Sl − Sn = Lbα

(3)

where Lb is the Markstein length of the burnt gas and Sl is the unstretched flame propagation speed, which was derived from the extrapolating of Sn to flame stretch rate of zero. The laminar burning velocity can be calculated according to the mass conservation across the flame front using Eq. 4.

ul =

ρb S ρu l

(4)

where ρu and ρb are the density of the unburnt and burnt mixtures calculated by using the chemical equilibrium program GASEQ, respectively. Additionally, the thermal expansion ratio was defined as the ratio of the unburnt gas density to that of the burnt one. Flame thickness, which can be used to characterize the hydrodynamic instability, was calculated using Eq.5.

δ=

Tad − Tu ( dT dx )max

(5)

where Tad and Tu are the adiabatic flame temperature and unburnt mixture temperature, respectively. Furthermore, (dT/dx)max is the maximum value of temperature gradient. Numerical calculations were performed with the Premixed Laminar Flame-Speed module in CHEMKIN software. Mixture-average transport was conducted and the Soret diffusion was considered [28, 29]. The mechanism was used to simulate the laminar burning velocity of propane/air mixtures, including 71 species and 469 reactions [30]. The adiabatic flame temperature was calculated using the chemical and phase equilibrium calculations module in CHEMKIN software. 3. Results and discussion 3.1 Laminar burning velocity


7

Energy & Fuels 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

Page 8 of 23

Fig. 3 shows the unstretched flame propagation speed with equivalence ratios under different initial pressures. With the increase in equivalence ratio, the unstretched flame propagation speed increased firstly and then decreased, and reached the maximum value when the equivalence ratio was 1.1. For equivalence ratios ranging between 0.9 - 1.1 and 1.1 - 1.3, there was a linear relationship between the unstretched flame propagation speed and equivalence ratio. With the increase in initial pressure, the unstretched flame propagation speed decreased.

Fig. 3 Effect of equivalence ratio on the unstretched flame propagation speed of propane/air mixtures under various initial pressures Fig. 4 shows the effect of equivalence ratio on the laminar burning velocity of propane /air mixtures. Similar to the unstretched flame propagation speed, the laminar burning velocity increased firstly and then decreased with the increase in equivalence ratio. With the increase in initial pressure, the laminar burning velocity decreased.


8

Page 9 of 23 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

Energy & Fuels

Fig. 4 Effect of equivalence ratio on the laminar burning velocity of propane/air mixtures under various pressures Fig. 5 shows the comparison of experimental and numerical laminar burning velocities at 2 bar. The present experimental values were consistent with the ones reported by Hasaan et al. [32] and Jomaas et al. [20]. The laminar burning velocities of Huzayyin et al. [31] and Desoky et al. [12] were slightly higher than the experimental ones within the range of equivalence ratio of 1.0 - 1.2. The differences between the present experiment and simulation values were relatively small, which laid the foundation for follow-up research.

Fig. 5 Comparison of experimental and numerical laminar burning velocity of propane/air mixtures at 2bar In order to identify the core elementary reactions and radicals dominating the laminar burning velocity, the sensitivity analysis of mass flow was conducted under different initial conditions. Fig. 6 shows the


9

Energy & Fuels 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

Page 10 of 23

sensitivity coefficient to laminar burning velocity for different equivalence ratios. The laminar burning velocity showed the highest sensitivity to the chain branching reaction of H+O2=O+OH, followed by the reactions of CO+OH=CO2+H, HCO+M=CO+H+M and CH3+OH=CH2*+H2O, which generated the active radicals to influence the process of combustion. The reactions of CH3+H (+M) =CH4 (+M), H+O2+H2OHO2+H2O and C3H8+H=H2+iC3H7 were the main chain branching reactions, which resulted from the reduction of active radicals during the combustion process. With the increase in equivalence ratio, the active radicals generated by the elementary reaction of H+O2=O+OH increased, and the consumption of active radical of H by the elementary reaction of CH3+H (+M) =CH4 (+M) and H+O2+H2OHO2+H2O increased and decreased, respectively. The shift in the effect of core elemental reaction on the laminar burning velocity, caused by the increased of the equivalence ratio, resulted in an increase first and then decrease in the laminar burning velocity.

Fig. 6 Effect of equivalence ratio on sensitivity coefficients for the laminar burning velocity of propane/air mixtures at 2bar The laminar burning velocity is significantly affected by the concentration of active radicals. According to the sensitivity analysis for the laminar burning velocity, the concentrations of active radicals H, O, OH, HO2 and CH3 were selected. The four active radicals of H, O, OH and CH3 had higher influence on the combustion process. The maximum concentration of H and OH increased firstly and then decreased with the increase in equivalence ratio, which reached the maximum value at the equivalence ratios 1.2 and 1.0, respectively. The


10

Page 11 of 23 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

Energy & Fuels

maximum concentration of CH3 increased with the increase in equivalence ratio and the growth rate gradually decreased after the equivalence ratio reached to the value of 1.2. With the increase in equivalence ratio, the maximum concentration of O decreased. When the equivalence ratio ranged between 0.9 - 1.1, the maximum concentration of HO2 changed slightly. With the further increase of equivalence ratio, the maximum concentration of HO2 gradually decreased (Fig. 7).

(a) H

(b) OH

(c) O

(d) CH3


11

Energy & Fuels 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

Page 12 of 23

(e) HO2 Fig. 7 Effect of equivalence ratio on the flame structure of propane/ air mixtures at 2bar When the equivalence ratios were within the range of 0.9 - 1.1, the enhancements in the chain branching reaction and laminar burning velocity were identical, and the influence of initial pressure on the elemental reactions was similar. The elemental reactions of H+O2=O+OH, HCO+M=CO+H+M, C2H2+H (+M) = C2H3 (+M) and CH3+OH=CH2*+H2O became important elementary reactions, which promoted the laminar burning velocity when the equivalence ratio was 1.3. With the increase in equivalence ratio, the chain branching reaction which had the greatest influence on the laminar burning velocity, was changed from H+O2+H2OHO2+H2O to CH3+H (+M) =CH4 (+M).


12

Page 13 of 23 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

Energy & Fuels

(a) φ =0.9

(b) φ =1.1

(a) φ=1.3 Fig. 8 Effect of initial pressure on the sensitivity coefficients for laminar burning velocity for various equivalence ratios Fig. 9 shows the effect of initial pressure on the concentration of core active radicals. In contrast to the active radicals of H, O, OH and CH3, the concentration of HO2 was relatively small, and the change was not obvious with the increase in initial pressure. The maximum concentrations of H, O, OH and CH3 decreased significantly with the increase in initial pressure, leading to a significant impact on the laminar burning velocity.


13

Energy & Fuels 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

Page 14 of 23

(a) H

(b) OH

(c) O

(d) CH3

(e) HO2 Fig. 9 Effect of initial pressure on the flame structure of propane/ air mixtures for equivalence ratio of 1.3 3.2 Flame structural characteristics Flame structural characteristic have a significant influence on the flame propagation characteristics. When the flame cellular structure developed to a certain degree, the wrinkle degree of the flame front increased, resulting in an increase of the flame propagation speed. For the laminar premixed flame, flame inherent instabilities are the core factors which induce the flame becoming unstable and forming cellular structure [34, 35]. For this reason, it is of great significance to study the flame inherent instabilities.


14

Page 15 of 23 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

Energy & Fuels

With the increase in equivalence ratio, the adiabatic flame temperature and thermal expansion ratio presented a trend of increase first and then decrease. The adiabatic flame temperature and thermal expansion ratio reached the maximum values at equivalence ratio of 1.0 and 1.1, respectively. With the increase in initial pressure, the adiabatic flame temperature and thermal expansion ratio increased. When the equivalence ratio was 1.0, the differences in adiabatic flame temperature and thermal expansion ratio at different initial pressure were the most significant. When equivalence ratio was greater than 1.0, the difference decreased with the increase in equivalence ratio (Fig. 10).

Fig. 10 Effect of equivalence ratio on the adiabatic flame temperature and thermal expansion ratio for various initial pressures Fig. 11 shows the effect of equivalence ratio on the flame thickness at various initial pressures. With the increase in equivalence ratio, the flame thickness decreased firstly and then increased. The flame thickness reached the minimum value at equivalence ratio of 1.1. As the initial pressure increased, the flame thickness and its growth rate decreased, and the flame hydrodynamic instability increased, leading to the flame more unstable at higher initial pressures.


15

Energy & Fuels 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

Page 16 of 23

Fig. 11 Effect of equivalence ratio on the flame thickness under various initial pressures Unlike hydrogen/air mixtures, the Lewis number of propane/air mixture gradually decreased with the increase in equivalence ratio [33]. As the Jomaas’s [26] results showed, the Lewis number was still greater than unity when equivalence ratio was 1.3. For the lean propane/air mixtures, the effect of the thermaldiffusive instability on the transition of flame to being unstable was relative weak. In the early stages of flame propagation, cracks formed due to the influence of electrodes. However, the cracks length was relatively short. The growth rate of cracks length was low and propagated along the electrodes. New cracks emerged after the flame radius reached the value of 40mm. When the equivalence ratio ranged from 0.9 to 1.1, both the hydrodynamic instability and thermal-diffusive instability increased with the increase in equivalence ratio. The length of cracks at the same radius increased, and cells gradually formed. When the equivalence ratio ranged from 1.1 to 1.3, with the increase in equivalence ratio, the hydrodynamic instability gradually decreased, and the thermal-diffusive instability gradually increased. With the development of flame, the growth rate of cracks increased rapidly. As the equivalence ratio increased, the cellular structure in flame front was significantly enhanced (Fig. 12). This may be due to the reason that the thermal-diffusive instability dominates the process of flame instability. φ

10mm

20mm

30mm

40mm


16

Page 17 of 23 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

Energy & Fuels

φ=0.9

φ=1.0

φ=1.1

φ=1.2

φ=1.3

Fig. 12 Effect of equivalence ratio on the evolution of propane/air premixed flame at 2 bar Flame stretch rate has a significant influence on the initial perturbations in flame front caused by the flame inherent instabilities, and finally reflected in the flame structural characteristics [36]. Fig. 13 shows the effect of equivalence ratio on the flame stretch rate. In the early stage of flame propagation, the flame stretch rate was relatively high. With the development of flame, the flame stretch rate decreased rapidly. When the flame developed to a certain degree, with the further development of flame, the growth rate of the flame tended to be constant. With the increase in equivalence ratio, the flame stretch rate at the same radius presents a trend of increase first and then decrease.


17

Energy & Fuels 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

Page 18 of 23

Fig. 13 Effect of equivalence ratio on the flame stretch of propane/air premixed flame at 2 bar In order to further investigate the effect of equivalence ratio on flame structural characteristics, cracks length in flame front was quantified. Fig. 14 showed that the total length of cracks in flame front increased linearly with time when the effect of flame inherent instabilities on the flame front was weak. With the development of flame, the limitation of flame stretch rate to the perturbations in flame front gradually decreased, while the perturbations got fully developed, leading to the emergence of new cracks and the nonlinear growth of cracks length [37]. With the increase in equivalence ratio, the hydrodynamic instability increased firstly and then decreased, while the thermal-diffusive instability significantly increased. The flame was easier to lose stability as a whole and cracks length at the same radius increased significantly. The generation of new cracks and the splitting of existing cracks increased with the enhancement of flame inherent instabilities, which led to an irregular fluctuation of the growth rate of cracks length.


18

Page 19 of 23 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

Energy & Fuels

Fig. 14 Effect of equivalence ratio on cracks length of propane/air premixed flame at 2 bar In order to investigate the influence of initial pressure on the flame structural characteristic, Fig. 15 shows the evolution of cracks length in flame front under various initial pressures. When the initial pressure was 1.0 bar, the flame front was relatively smooth and the cracks in flame front had not developed in the early stage of flame propagation. As the equivalence ratio increased, the cracks in flame front were enhanced. When the initial pressure was 2.0 bar, the cells had formed at the radius of 10mm. With the development of flame, the cellular structure in flame front was enhanced. Relatively uniform cellular structure was formed in flame front when the flame developed to the radius of 40mm. p0

10mm

20mm

30mm

40mm

p0=1.0bar

p0=1.5bar

p0=2.0bar

Fig. 15 Effect of initial pressure on the evolution of propane/air premixed flame at the equivalence ratio of 1.3 Fig. 16 shows the effect of initial pressure on cracks length of propane/air premixed flame. For the lean mixture, the effect of flame inherent instabilities on the flame front was relatively weak, the flame front was smooth and no cracks generated and developed in the quartz windows area. With the increase in initial pressure, the hydrodynamic instability enhanced, and cracks in flame front got significantly developed. When the flame developed to a certain degree, cracks emerged and developed with the flame. The cracks length


19

Energy & Fuels 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

Page 20 of 23

presented a nonlinear relationship with time when the flame developed to a certain degree. The radius, at which the cracks emerged, decreased with the increase in initial pressure. For the rich mixtures, the flame inherent instabilities exhibited a relatively strong effect on the flame front, while the cracks could be observed in the quartz windows range and the radius at which the cracks emerged decreased with the increase in equivalence ratio

(c) φ =0.9

(b) φ =1.1

(c) φ =1.3 Fig. 16 Effect of initial pressure on cracks length of propane/air premixed flame under various equivalence ratios


20

Page 21 of 23 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

Energy & Fuels

4. Conclusions In order to study the flame propagation characteristics and flame structural characteristics, experimental and numerical methods were used to study the propane /air mixtures to confirm the core elemental reactions and the active radicals, which showed a significant effect on the laminar burning velocity. In addition, the influence of flame inherent instabilities on the flame structural characteristics was quantitatively studied by extracting the cracks information in flame front. The main conclusions from this work can be summarized as follows. 1.

With the increase in equivalence ratio, the unstretched flame propagation speed and the laminar burning velocity increased firstly and then decreased, while both the parameters reached the maximum values at the equivalence ratio of 1.1. With the increase in initial pressure, the unstretched flame propagation speed and the laminar burning velocity decreased.

2.

The laminar burning velocity showed the highest sensitivity to chain branching reaction of H+O2=O+OH. The sensitivity coefficient increased with the increase in equivalence ratio. As the equivalence ratio increased, the main chain inhibiting reactions changed from H+O2+H2OHO2+H2O to CH3+H (+M) =CH4 (+M).

3.

The four active radicals of H, O, OH and CH3 exhibited an important influence on the combustion process. With the increase in equivalence ratio, the maximum concentration of H, OH and CH3 increased firstly and then decreased. The maximum concentrations of the four active radicals decreased significantly with the increase in initial pressure, eventually leading to a significant impact of laminar burning velocity.

4.

With the increase in equivalence ratio, the hydrodynamic instability increased firstly and then decreased, while the thermal-diffusive instability significantly increased, and the flame was easier to be stable as a whole. Additionally, cracks length at the same radius increased significantly. With the increase in initial pressure, the hydrodynamic instability enhanced, and the cracks in flame front got significantly developed.


21

Energy & Fuels 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

Page 22 of 23

Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51706014), the Fundamental Research Funds for the Central Universities (No. 2017YJS161, 2017JBZ102, 2017JBM049). References [1] Chen, L.; Raza, M.; Xiao, J. Energ. Fuel. 2017, 31(9):9429-9437. [2] Song, J.; Yao, C.; Liu, S.; Xu, H. Energ. Fuel. 2008, 22(6), 3806-3809. [3] Agudelo, J. R.; Lapuerta. M.; Moyer, O.; Boehman, A. L. Energ. Fuel. 2017, 31(3):2985-2995. [4] Askari, O.; Elia, M.; Ferrari. M.; Metghalchi, H. Appl. Energ. 2017, 189, 568-577. [5] Askari, M. H.; Ashjaee, M.; Karaminejad, S. Energ. Fuel. 2017, 31(12):14169-14179. [6] Beccari, S.; Pipitone, E.; Genchi, G. J. Energ. Inst. 2016, 89(1):101-114. [7] Krishnan, S. R.; Srinivasan, K. K.; Raihan, M. S. Fuel 2016, 184:490-502. [8] Lee, J.; Chu, S.; Cha, J.; Choi, H.; Min, K. Energy 2015, 93(4):1041-1052. [9] Akram, M.; Saxena, P.; Kumar, S. L. Energ. Fuel. 2012, 27(6):3460-3466.

[10] Law, C. K.; Sung, C. J. Prog. Energ. Combust. 2000, 26(4):459-505. [11] Metghalchi, M.; Keck, J. C. Combust. Flame 1980, 38(2):143-154. [12] Desoky, A. A.; Abdel-Ghafar, Y. A.; El-Badrawy, R. M. Int. J. Hydrogen Energ. 1990, 15(12):895-905. [13] Razus, D.; Oancea. D.; Brinzea, V.; Mitu, M.; Moileanu, C. Energ. Convers. Manage. 2010, 51(12):29792984. [14] Liu, Q.; Zhang, Y.; Fang, N.; Li, Lei. Fuel 2015, 140:677-684. [15] Tang, C.; Zheng, J.; Huang, Z.; Wang, J. Energ. Convers. Manage. 2010, 51(2):288-295. [16] Tang, C.; Huang, Z.; He, J.; Jin, C.; Wang, X.; Miao, H. Energ. Fuel. 2009, 23(1):151-156. [17] Tang, C.; Huang, Z.; Jin, C.; He, J.; Wang, J.; Wang, X.; Miao, H. Int. J. Hydrogen Energ. 2008, 33(18):4906-4914. [18] Tang, C.; He, J.; Huang, Z.; Jin, C.; Wang, J.; Wang, X.; Miao, H. Int. J. Hydrogen Energ. 2008, 33(23): 7274-7285. [19] Li, Z.; Cheng, X.; Wei, W.; Qiu, Liang.; Wu, H. Int. J. Hydrogen Energ. 2017, 42: 24055-24066.


22

Page 23 of 23 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

Energy & Fuels

[20] Jomaas, G.; Zheng, X.; Zhu, D.; Law, C. P Combust Inst 2005, 30(1):193-200. [21] Kitagawa, T. Jsme International Journal 2005, 48(1):2-8. [22] Jiang, Y.; Li, G.; Li, H.; Li, L.; Li, F. Energ. Fuel.2017, 31(12):14237-14247. [23] Li, F.; Li, G.; Jiang, Y.; Li, H.; Sun, Z. Energies 2017(10):678-693. [24] Li, H.; Li, G.; Sun, Z.; Yu, Y.; Zhai, Y.; Zhou, Z. Fuel 2014, 135(11):279-291. [25] Li, H.; Li, G.; Sun, Z.; Zhai, Y.; Zhou, Z. Energies 2014, 7(7):4710-4726. [26] Jomaas, G.; Law, C.; Bechtold, J. J. Fluid Mech 2007, 583(583):1-26. [27] Xie, Y.; Wang, J.; Cai, X.; Huang, Z. Int. J. Hydrogen Energ. 2016, 41(40):18250-18258. [28] Meng, X.; Hu, E.; Li, X.; Huang, Z. Fuel, 2016, 185:916-924. [29] Wang, G.; Li, Y.; Yuan, W.; Zhou, Z.; Wang, Y.; Wang, Z. Combust. Flame 2017, 184:312-323. [30] Davis, S.; Law, C.; Wang, H. Combust. Flame 1999, 119(4):375-399. [31] Huzayyin, A.; Moneib, H.; Shehatta, M.; Attia, A. Fuel 2008, 87(1):39-57. [32] Hassan, M.; Aung, K.; Kwon, O.; Faeth, G. J. Propul. Power 2012, 14(4):479-488. [33] Clarke, A. Process Saf. Environ. 2002, 80(3):135-140. [34] Vu, T.; Park, J.; Kwon, O.; Bae, D.; Yun, J.; Keel, S. Int. J. Hydrogen Energ. 2010, 35(8):3868-3880. [35] Liu, F.; Bao, X.; Gu, J.; Chen, R. Int. J. Hydrogen Energ. 2012, 37(15):11458-11465. [36] Jiang, Y.; Li, G.; Li, H.; Li, L.; Li, F. Energ. Fuel. 2017.31:14237-14247. [37] Jiang, Y.; Li, G.; Li, H.; Li, L.; Zhang, G. Int. J. Hydrogen Energ. 2018,43(18):9022-9035.


23

Experimental and Numerical Study on the Combustion Characteristics

Recommend Documents