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air combustion in the micro-planar combustor with a cross-plate insert
Applied Thermal Engineering 136 (2018) 177–184
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Research Paper
Experimental and numerical study of premixed propane/air combustion in the micro-planar combustor with a cross-plate insert
T
Aikun Tanga, Jiang Denga, Yiming Xub, Jianfeng Pana, Tao Caia a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China Cleaning Combustion and Energy Utilization Research Center of Fujian Province, Jimei University, Xiamen 361021, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Micro-combustion Propane Cross-plate Radiation efficiency Flammable velocity Plate-length
Over the past decade, the micro-thermophotovoltaic (MTPV) system has aroused widely public attention. Microcombustor is an important part, which can determine the working performance of this micro-power generator. In this paper, experimental investigations as well as a three-dimensional CFD simulation have been carried out to study the performance of propane/air premixed combustion in a new kind of cross-plate micro-planar combustor. Benefited from the heat transfer enhancement by the setting up of cross-plate, the average wall temperature of the new combustor is increased by more than 90 K, which results in the growth of radiation efficiency. Besides, the blowout limit is apparently extended in the cross-plate combustor. Compared to the singlechannel combustor, the blowout limit of propane/air in the cross-plate combustor can be raised by 0.4 m/s at equivalence ratio 0.7. It is also found that the cross-plate length can significantly affect the flame shape in the micro-channel and temperature distribution of the external wall. In contrast, the dimensionless plate length of 5/ 9 is suggested as the optimal structure parameter for the micro-combustor, which is due to the highest radiation efficiency.
1. Introduction Recently, the micro-power generators based on micro-combustion of hydrocarbon fuels, which own a series of advantages such as high energy density, small volume and long operating time, have aroused the lively interest of researchers. These apparatus inspire high hopes in the solution to the power problem of micro-electro-mechanical systems (MEMS), and their potential applications have enlarged rapidly to the portable electronics, wireless communication equipment and vehicles [1,2]. The micro-thermophotovoltaic (MTPV) system is one of the most promising power generation devices, which is composed of a microcombustor, an optical filter and photovoltaic cells. Along with thermo electric device, MTPV system also has the advantages of no moving components and convenient fabrication [3]. Micro-combustor, as one of the most important parts of the power generator, determines the stability of micro-combustion process and total power density of this system. However, micro-scale combustion also has some disadvantage: the short residence time of mixture and high heat loss resulting from the small volume-to-surface ratio. These drawbacks will result in the flame instability in the micro-scale combustor [4]. Various methods have, as yet, been implemented to enhance the combustion stability in each micro-scale combustor. Of them the excess
E-mail address: [email protected] (J. Pan). https://doi.org/10.1016/j.applthermaleng.2018.03.001 Received 1 May 2017; Received in revised form 28 February 2018; Accepted 1 March 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
enthalpy combustion [5–8], porous media combustion [9–11], catalytic combustion [12–15] and blended fuel combustion [16–18] in microscale condition have been proposed and investigated by many scholars. In addition, some attempts on the optimum structural design of microcombustors have also been conducted over decades. Fan et al. [19] fabricated a bluff body into the micro-combustor, and found that 3–5 times extension of blow-off limit can be obtained through this structure. Hosseini and Wahid [20] pointed out that a triangular bluff-body will improve the flame stability both on premixed and micro-flameless combustion model. Zuo et al. [21] numerically investigated the thermal performance of an improved counterflow double-channel micro-combustor hoping to obtain a higher and more uniform wall temperature. The formation of a very high frequency spinning combustion mode in the multiple-stepped micro-tubes with almost zero emissions has been reported by Deshpande and Kumar [22]. Akhtar et al. [23] evaluated the effect of cross sections on the external wall temperature distributions. Their results showed that the trapezoidal and triangular crosssections have better performance. Zuo et al. [24] found that the counter flow double-channel micro-combustor has a better thermal performance than that of the micro-combustor with coflow double-channel, and the non-uniformity of the temperature distribution over the external wall is much lower. Shamim et al. [25] pointed out that the outer wall temperature and the overall energy conversion efficiency of a
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S8200) is used to give a clear demonstration of the experimental photos, especially the shape and location of high temperature domain. The novel micro-planar combustor with cross-plate is exhibited in Fig. 2. The apparent sizes of this combustor are 18 mm, 9 mm and 4 mm in x, y, z directions, respectively. Due to the 0.5 mm wall thickness, the inner width and height of the combustion channel are 8 mm (x-direction) and 3 mm. A cross-plate is arranged at the latter part of the original combustor, which divides the downstream of the combustion channel into four segments and forms four outlets. The thickness of cross-plate is fixed as 0.5 mm, while the plate length (L2) varies for comparison. The dimensionless plate length is expressed as l = L2/L1. In contrast, the design method of cross-plate in micro-combustor can make the process of manufacturing and assembling much easier. Besides, the material of combustor and cross-plate are all chosen as the 316 stainless-steel, which is found capable of withstanding high temperature of 1400 °C.
thermophotovoltaic (TPV) system can be increased and improved in curved micro-combustors as compared to the straight channel. Baigmohammadiet et al. [26] inserted a wire to the micro-tube combustor, and the simulation results showed this method is beneficial to modify the flame location. However, Yang et al. [27] inserted a block to the micro-combustor and investigated its combustion process using entropy generation method. Wan et al. [28] analyzed the effect of cavities in a micro-planar combustor, which can also bring about an apparent improvement of combustion stability. From references above, it can be known that the change in inner structure can improve the working performance of the micro-combustor. However, it should be pointed out that the method can inevitably increase the difficulty of manufacturing and assembling of micro-combustor. Therefore, various combustors with simple inner structure and more significant enhancement effect should be further developed. In our previous work [29], numerical study indicates that the working performance of micro-planar combustor can be improved with the arrangement of parallel separating plates. To this end, a new type of micro-planar combustor with cross-plate will be proposed in this paper, whose structure is much simpler and can be easily assembled. Both experimental and numerical investigations have been adopted to analyze the combustion process of premixed propane/air in this kind combustor. The effects on the improving stable combustion range in mixture velocity and the heat transfer between high temperature exhaust gas and walls are also revealed. Finally, the optimum plate length for combustion characteristic in this micro-combustor has been obtained.
3. Computational model In this paper, a three dimensional model is also established to give a comprehensive analysis on the combustion and radiation characteristics of the new combustor. During the simulation, only half of the combustor is adopted to save the computational time. Meanwhile, centerline temperature of combustor without cross-plate at three mesh densities has been selected to check grid-independence, as shown in Fig. 3. It can be seen from the figure that the centerline temperature of medium and fine mesh cases varies little. Finally, the medium size mesh (0.1 mm in each direction) is selected in the following study. Fluent (Ansys 15.0) is selected as the software for the calculation and analysis tool. The propane/air mechanism M5 which has 28 species and 73 reactions is also used in the simulation process [30]. On account of the small flow rate, the laminar model is used to depict the flow process, and the finite-rate model are selected as reaction model [17,31]. The basic governing equations of mixture can be seen in reference [18], which discretized through a first-order upwind scheme. Moreover, the under relaxation method, implicit solver and the finite volume method are used in this model. In addition, the pressure–velocity coupling is calculated by SIMPLE algorithm. As for the physical properties of mixture, density is calculated by the ideal gas law, while the thermal conductivity and viscosity are defined as a mass fraction-weighted average [32]. When it comes to each component, the specific heat is calculated by the fourth-order
2. Experimental set up The experimental system is shown in Fig. 1. This system includes a propane gas tank, an air compressor, two reducing valves, two flow controllers, a mixing chamber, a micro-planar combustor, an infrared thermal imager and a computer for recording. During the experiment, the pressure of propane (purity: 99.0%) from the gas tank will be reduced to 0.08 MPa by reducing valve. The flow rates of propane and air are monitored by the mass flow controllers (DSN-2000B, accuracy: 0.5%), and then the propane and air will be fully mixed in the mixing chamber. At the same time, the infrared thermal imager (Type: ThermovisionTM A40; Maximum measure temperature: 2000 °C; Measuring accuracy: ± 2%) is used to measure the temperature distribution of the combustor external surface. Besides, a digital camera (Type: Nikon
Infrared thermographer Flow controller
Micro-combustor Flash-back arrestor
Connector
Micro flowmeter Mixing chamber Reducing valve
Flow controller Propane Air compressor Fig. 1. Schematic diagram of the experimental system.
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Inlet
Wall
9 mm
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z m 18 m
0.5 mm
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x Outlet Fig. 2. Design characteristic of micro-planar combustor with cross-plate.
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(a) Centerline temperature along the combustor external wall.
Fig. 3. Centerline temperature at three grid sizes (without cross-plate).
T/K 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400
piecewise-polynomial function of temperature [16], the viscosity and thermal conductivity are calculated through kinetic theory [17]. The boundary conditions are illustrated as follows. The inlet is set as the velocity-inlet boundary condition. Meanwhile, each component is specified in the form of mass fraction. According to the practical situation, pressure-outlet boundary condition is adopted for the exhaust. No slip and zero diffusive flux species boundary conditions are employed at the gas solid interfaces, while the mixed thermal conditions are selected to evaluate the heat loss rate of each external wall, which include both natural convection and surface radiation heat transfer process [33]. The convective heat transfer coefficient between the external wall and environment is calculated to be 15 W/(m2·K), while the wall emissivity is set as 0.65, which is tested by the Fourier Transform Infrared Spectrometer. To validate the computational model, the comparison of centerline temperature along the external walls and the temperature distribution patterns has been made with respect to the experimental result. In order to make a good distinction, the combustor without cross-plate will be defined as straight-channel combustor, and the other is cross-plate combustor. As seen in Fig. 4, both of the high and low temperature zone location matches well, and the maximum relatively error of external wall temperature is only 3.4%, which illustrates the accuracy of the calculation model.
(b) External wall temperature distribution pattern of the combustor without cross-plate. Fig. 4. Experimental validation of computational model (inlet velocity: 0.6 m/s; Φ = 1.0 ).
4. Results and discussions 4.1. Effect of cross-plate on the working performance A comparative study with the straight-channel combustor has been conducted so as to give a comprehensive investigation on the basic performance of the novel combustor. In this section, the dimensionless plate length of 5/9, and the equivalence ratio of fuel and oxidant is chosen as 1.0. For the micro-power generator like MTPV system, 179
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(a) 0.6 m/s
(b) 0.8m/s
1250 1225
1350 Straight-channel combustor Cross-plate combustor
1200 1175
Maximum wall temperature (K)
Average wall temperature (K)
Fig. 5. Experimental photos of the external wall at same inlet velocity (left: straight-channel combustor; right: cross-plate combustor).
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Inlet velocity (m/s)
Inlet velocity (m/s)
(a) Average wall temperature
(b) Maximum wall temperature
Fig. 6. Wall temperature at different inlet velocities.
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improving the external wall temperature is the most reliable method to increase energy utilization efficiency [34,35]. Firstly, the experimental photos of the external wall between the two micro-combustors are shown in Fig. 5, which are taken by a digital camera in dark environment. It can be seen that the external wall of each micro-combustor becomes brighter with the rising of mixture flow rate, which indicates the expanding of high temperature zone. Under the same condition, the appearance of cross-plate combustor is much brighter than that of the straight-channel case, which is in accordance with the phenomenon shown in Fig. 6. Fig. 6 compares the average and maximum wall temperature difference between the straight-channel and cross-plate micro-combustor, which are obtained from the infrared thermal imager. The wall temperatures of the two kinds of micro-combustor grow steadily along with the inlet velocity in each case. Besides, the temperature differences between the straight-channel and cross-plate combustors are also enlarged with increasing inlet velocity. At the 0.6 m/s case, the maximum and average wall temperature of cross-plate combustor are 1213 K and 1125 K respectively, which exceed the straight-channel cases by 58 K and 70 K. However, when it reaches 0.9 m/s, the corresponding increments of cross-plate cases will be more than 100 K. These results express that at high inlet velocity, the inserted cross-plate will significantly enhance the heat transfer intensity. To further demonstrate the effect of this new structure on the energy output and overall efficiency of MPTV system, the radiation energy from external wall of straight-channel and cross-baffle combustors are calculated as depicted in Fig. 7. It can be seen that the radiation energy of cross-plate combustor is almost twice as large as straight-channel combustor. Meanwhile, the radiation energy increment becomes large
Radiation energy W
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Straight-channel combustor Cross-plate combustor
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Inlet Velocity (m/s) Fig. 7. Comparison of radiation energy of external wall versus inlet velocity.
with an increase in inlet velocity, which is in accordance with the results shown in Fig. 6(a). With respect to MTPV system, the spectral radiation is much important. Therefore, the energy spectrum of combustor’s external wall at different wavelengths is calculated adopting numerical method as shown in Fig. 8. It can be seen that due to the higher temperature of external wall, the spectral radiance of cross-plate combustor under different inlet velocities are drastically higher than that of straight-channel combustor, which is beneficial to MTPV system. To further clarify the phenomenon, the heat transfer coefficients between the mixture and the micro-channel at different cases are calculated (as seen in Table 1), which are based on the simulation results.
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Straight-channel (0.6 m/s) Cross-plate (0.6 m/s) Straight-channel (0.7 m/s) Cross-plate (0.7 m/s) Straight-channel (0.8 m/s) Cross-plate (0.8 m/s)
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Straight-channel (Blowout)
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Fig. 8. Variation of combustor’s external wall energy spectrum at different wavelengths.
Fig. 10. Flame stabilization of the two micro-combustors at different equivalence ratios. Table 1 Heat transfer coefficients at different cases. Inlet velocity (m/s)
Heat transfer coefficient/ straight-channel combustor (W·m−2·K−1)
Heat transfer coefficient/ cross-plate combustor (W·m−2·K−1)
0.6 0.7 0.8
19.18 21.57 23.24
39.41 48.35 61.59
fuel. It is observed that the radiation efficiency of straight-channel combustor case reduces gradually along the inlet velocity. However, the values of new type combustor are evidently higher, and remain above 0.29 in the first four flow rate conditions. As the inlet velocity reaches 0.9 m/s, the efficiency begins to drop and reaches about 0.28. It should be noted that the increase in chemical energy input does not absolutely mean the rising of radiation energy. As a result, the method of fuel input increasing may be not the best way to promote the system efficiency of the micro-power generators. Accordingly, it is suggested that the inlet velocity of cross-plate case should be kept at a reasonable and optimum working range. Furthermore, the flame stabilizations of the two micro-combustors are investigated, as shown in Fig. 10. Based on our previous research [36], the flame stability is defined: under a certain range of flow rates, the fuel/air can keep stable combustion in the micro-combustor without the phenomenon of flame oscillations or repetitive extinction and ignition occurring. Besides, the flame is not apparently asymmetrical. At the same time, the distribution of external wall temperature is relatively steady. The maximum inlet velocity of stable combustion represents the blowout limit, while the minimum inlet velocity is extinction limit. It can be seen that the flammable range shows various degrees of increase by the setting up of cross-plate. Benefited from the arrangement, the maximum stable combustion range in mixture velocity at stoichiometry ratio case rises from 1.0 m/s to 1.2 m/s. However, the positive effect of cross-plate could be much apparent under fuel-lean condition. For instance, the blowout limit of propane/air in the cross-plate combustor reaches 0.9 m/s under the condition of equivalence ratio 0.7, which is nearly twice of the straight-channel case. Besides, the extinction limit at this condition is also reduced to 0.4 m/s. When it comes to the fuel-rich conditions (equivalence ratio 1.1 and 1.2), the blowout limit of inlet velocity merely increases by 0.2 m/s and 0.1 m/s. Thus, it is concluded from the experimental results that the lean fuel case is a more suitable working condition for the cross-plate combustor. It follows that the cross-plate also has a function like bluff body, which is conducive to realize a longer residence time and stable flame [15,19,20].
For the micro-planar combustor without cross-plate, the heat transfer coefficient keeps rising as the inlet velocity increases, but the increment amplitude is small. When it comes to cross-plate combustor, the corresponding value is much higher than that of the straightchannel cases, and the increasing tendency is completely different. At the 0.6 m/s inlet velocity condition, the heat transfer coefficient of cross-plate combustor is 39.41 W/(m2·K), which is twice of straightchannel case. When increasing to 0.8 m/s, the value of cross-plate combustor reaches 2.7 times than that of straight-channel. This gives a direct explanation for the gradual rising of temperature difference in the Fig. 6. Therefore, it is concluded that when the cross-plate is inserted into the micro-combustor, the disturbance degree of the mixture should be improved. In addition, the equivalent diameter of the downstream channel will decrease apparently, which also contributes to the growing of the heat transfer coefficient. It is certain that the increase in average wall temperature will absolutely bring a growth of the radiation energy. However, the total chemical energy input will also rise gradually with an increase of fuel amount. Fig. 9 shows the radiation efficiency to further illustrate the advantage of cross-plate, which can be defined as the ratio of radiation energy output from the external wall to total chemical energy input by 0.350
Radiation efficiency
0.325 0.300 0.275 0.250
4.2. Effects of cross-plate length
0.225
It is universally known that the cross-plate length is a critical parameter, which can directly influence the working performance of micro-combustor. In this section, four dimensionless plate lengths have been selected, which are 4/9, 5/9, 6/9 and 7/9, respectively. Temperature distributions on the cross section of micro-combustor with different cross-plate lengths are shown in Fig. 11. Considering the thickness of the cross-plate, this surface is 0.5 mm away from the center section in z-direction, which can depict the temperature distribution of mixture gas. It is found that the temperature field at each case can be
0.200
Straight-channel combustor Cross-plate combustor
0.175 0.150
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Inlet Velocity (m/s) Fig. 9. Radiation efficiency at different inlet velocities.
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roughly divided into three stages. Firstly, when the mixture gas gradually moves forward, the combustion reaction begins appearing and the temperature of mixture rapidly increases to the flame center temperature. Secondly, the reaction has not happened near the inlet of the channel, and the mixture temperature is approximately closed to the initial temperature. Then, the fluid will be obstructed by the cross-plate, which leads to the generation of disturbances. Finally, the burned gas flows into the four downstream regions, and the released heat will be absorbed by the cross-plate and inner wall. As a result, the gas temperature gradually decreases along the exhaust channels, and the dimensionless plate length of 7/9 case owns the smallest high temperature region. With the increasing of cross-plate length, the contact area of fluid and solid zone will also be larger, which ensures more exhaust heat absorbed by the walls. However, it is found that the average temperature of external wall does not keep on growing with the increase of cross-plate length, which can be seen in Fig. 12. The changing tendency at different length cases are very similar, that is to say the average temperature of external wall increases as the inlet velocity rises. In contrast, the corresponding values of 5/9 cases are apparently higher than that of other cases at the same inlet velocity. Meanwhile, when the inlet velocity ranges from 0.5 m/s to 0.7 m/s, the mean temperature of external wall for the dimensionless plate length of 4/9 are slightly higher than 6/9 and 7/9 cases. At the last tested inlet velocity condition, the temperatures of the three cases are nearly the same, which are all about 40 K below the 5/9 case plate-length micro-combustor. To better understand the effect under different plate lengths, mean heat transfer coefficient in the micro-channel and maximum flame
Average wall temperature (K)
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l=4/9 l=5/9 l=6/9 l=7/9
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temperature are investigated, as seen in Fig. 13, which are obtained by the numerical simulation. The heat absorption intensity between the inner wall and the mixture can be demonstrated by the mean heat transfer coefficient, while the maximum flame temperature can illustrate the combustion performance of fuels. That is the reason why these two parameters are selected as analytical subjects. From Fig. 13, it is indicated that the growth of cross-plate length will also bring out a persistent increase of heat transfer coefficient. The coefficient reaches 140 W/(m2·K) when the dimensionless plate length is 7/9, which is nearly 3 times the value of 4/9 case. However, the flame temperature is also directly affected by the cross-plate length. Long cross-plate will apparently result in the flame shape changing (As shown in Fig. 11) and the reducing of reaction space. It can be acknowledged from Fig. 13 that the flame temperature gradually decreases, especially when the dimensionless plate length exceeds 6/9. The maximum flame temperature is only 1882 K when the dimensionless plate length is 7/9, which is about 100 K lower than 4/9 case. From this point of view, choosing a proper length of cross-plate is of great importance to obtain higher wall temperature. In order to further analyze the effect of cross-plate length to the combustor working performance, radiation efficiency is contrasted in Fig. 14. Due to the higher average wall temperature, the value of 5/9 case remains about 0.29 at each inlet velocity condition, which are also significantly higher than that of other cases. Besides, the radiation efficiencies of 4/9 case remain unchanged at low inlet velocity condition, which are slightly higher than 6/9 and 7/9 cases. When up to 0.8 m/s, although the mean wall temperature keeps on increasing, the growthrate of radiation energy output suddenly begins to decline, and the
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Fig. 13. Average heat transfer coefficient and maximum flame temperature at different cross-plate lengths (inlet velocity: 0.8 m/s).
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160 150
Maximum flame temperature (K)
Fig. 11. Temperature field on the cross section at different plate lengths.
0.8
Inlet velocity (m/s) Fig. 12. Average wall temperature at different inlet velocities and cross-plate lengths.
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0.32
l=4/9 l=5/9 l=6/9 l=7/9
Radiation efficiency
0.31 0.30
[3] [4]
0.29 [5]
0.28 [6]
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[7]
0.26 [8]
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0.5
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0.7
[9]
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Fig. 14. Radiation efficiency at different inlet velocities and cross-plate lengths.
radiation efficiency is apparently lower than 6/9 case. In fact, when the chemical energy input is a constant, the working efficiency of MTPV system is directly affected by radiation efficiency of the external wall. Therefore, it is suggested that the dimensionless plate length of 4/9 would be the most suitable choice.
[11]
[12]
[13]
5. Conclusions [14]
An original micro-planar combustor with cross-plate is designed and fabricated for the micro-thermophotovoltaic system, so as to improve the combustion process and working behavior. Experiment and numerical simulation are conducted to contrast the combustion characteristic of micro-combustors with and without cross-plate. Furthermore, the effects of cross-plate length are investigated. The main conclusions can be drawn as follows:
[15]
[16] [17]
[18]
(1) Heat transfer between mixture and combustor inner wall can be enhanced by the arrangement of cross-plate. When compared to the straight-channel combustor, the average external wall temperature of the cross-plate micro-combustor is increased by at least 90 K. Meanwhile, the ratio of radiation energy output to total chemical energy input is also much higher. (2) The cross-plate can be capable of improving the flame stability, which results in the increasing of flammable range at a certain degree. Especially as equivalence ratio is 0.7, the blowout limit of propane/air in the cross-plate combustor can be raised by 0.4 m/s. Hence, the lean fuel condition is more suitable for the working process of cross-plate combustor. (3) Cross-plate length is an important structural parameter of the new combustor, which has an influence on the flame shape, and leads to various promotion amplitude of the external wall temperature. By contrast, the dimensionless plate length of 5/9 is adopted as the optimum plate-length because of the highest average temperature of external wall and radiation efficiency.
[19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
Acknowledgements [27]
This work is supported by National Natural Science Foundation of China (No. 51676088, No. 51376082), Practice Innovation Program for College Postgraduates of Jiangsu Province (No. SJLX16_0428), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experimental and numerical study of premixed propane/air combustion in the micro-planar combustor with a cross-plate insert
T
Aikun Tanga, Jiang Denga, Yiming Xub, Jianfeng Pana, Tao Caia a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China Cleaning Combustion and Energy Utilization Research Center of Fujian Province, Jimei University, Xiamen 361021, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Micro-combustion Propane Cross-plate Radiation efficiency Flammable velocity Plate-length
Over the past decade, the micro-thermophotovoltaic (MTPV) system has aroused widely public attention. Microcombustor is an important part, which can determine the working performance of this micro-power generator. In this paper, experimental investigations as well as a three-dimensional CFD simulation have been carried out to study the performance of propane/air premixed combustion in a new kind of cross-plate micro-planar combustor. Benefited from the heat transfer enhancement by the setting up of cross-plate, the average wall temperature of the new combustor is increased by more than 90 K, which results in the growth of radiation efficiency. Besides, the blowout limit is apparently extended in the cross-plate combustor. Compared to the singlechannel combustor, the blowout limit of propane/air in the cross-plate combustor can be raised by 0.4 m/s at equivalence ratio 0.7. It is also found that the cross-plate length can significantly affect the flame shape in the micro-channel and temperature distribution of the external wall. In contrast, the dimensionless plate length of 5/ 9 is suggested as the optimal structure parameter for the micro-combustor, which is due to the highest radiation efficiency.
1. Introduction Recently, the micro-power generators based on micro-combustion of hydrocarbon fuels, which own a series of advantages such as high energy density, small volume and long operating time, have aroused the lively interest of researchers. These apparatus inspire high hopes in the solution to the power problem of micro-electro-mechanical systems (MEMS), and their potential applications have enlarged rapidly to the portable electronics, wireless communication equipment and vehicles [1,2]. The micro-thermophotovoltaic (MTPV) system is one of the most promising power generation devices, which is composed of a microcombustor, an optical filter and photovoltaic cells. Along with thermo electric device, MTPV system also has the advantages of no moving components and convenient fabrication [3]. Micro-combustor, as one of the most important parts of the power generator, determines the stability of micro-combustion process and total power density of this system. However, micro-scale combustion also has some disadvantage: the short residence time of mixture and high heat loss resulting from the small volume-to-surface ratio. These drawbacks will result in the flame instability in the micro-scale combustor [4]. Various methods have, as yet, been implemented to enhance the combustion stability in each micro-scale combustor. Of them the excess
E-mail address: [email protected] (J. Pan). https://doi.org/10.1016/j.applthermaleng.2018.03.001 Received 1 May 2017; Received in revised form 28 February 2018; Accepted 1 March 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
enthalpy combustion [5–8], porous media combustion [9–11], catalytic combustion [12–15] and blended fuel combustion [16–18] in microscale condition have been proposed and investigated by many scholars. In addition, some attempts on the optimum structural design of microcombustors have also been conducted over decades. Fan et al. [19] fabricated a bluff body into the micro-combustor, and found that 3–5 times extension of blow-off limit can be obtained through this structure. Hosseini and Wahid [20] pointed out that a triangular bluff-body will improve the flame stability both on premixed and micro-flameless combustion model. Zuo et al. [21] numerically investigated the thermal performance of an improved counterflow double-channel micro-combustor hoping to obtain a higher and more uniform wall temperature. The formation of a very high frequency spinning combustion mode in the multiple-stepped micro-tubes with almost zero emissions has been reported by Deshpande and Kumar [22]. Akhtar et al. [23] evaluated the effect of cross sections on the external wall temperature distributions. Their results showed that the trapezoidal and triangular crosssections have better performance. Zuo et al. [24] found that the counter flow double-channel micro-combustor has a better thermal performance than that of the micro-combustor with coflow double-channel, and the non-uniformity of the temperature distribution over the external wall is much lower. Shamim et al. [25] pointed out that the outer wall temperature and the overall energy conversion efficiency of a
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S8200) is used to give a clear demonstration of the experimental photos, especially the shape and location of high temperature domain. The novel micro-planar combustor with cross-plate is exhibited in Fig. 2. The apparent sizes of this combustor are 18 mm, 9 mm and 4 mm in x, y, z directions, respectively. Due to the 0.5 mm wall thickness, the inner width and height of the combustion channel are 8 mm (x-direction) and 3 mm. A cross-plate is arranged at the latter part of the original combustor, which divides the downstream of the combustion channel into four segments and forms four outlets. The thickness of cross-plate is fixed as 0.5 mm, while the plate length (L2) varies for comparison. The dimensionless plate length is expressed as l = L2/L1. In contrast, the design method of cross-plate in micro-combustor can make the process of manufacturing and assembling much easier. Besides, the material of combustor and cross-plate are all chosen as the 316 stainless-steel, which is found capable of withstanding high temperature of 1400 °C.
thermophotovoltaic (TPV) system can be increased and improved in curved micro-combustors as compared to the straight channel. Baigmohammadiet et al. [26] inserted a wire to the micro-tube combustor, and the simulation results showed this method is beneficial to modify the flame location. However, Yang et al. [27] inserted a block to the micro-combustor and investigated its combustion process using entropy generation method. Wan et al. [28] analyzed the effect of cavities in a micro-planar combustor, which can also bring about an apparent improvement of combustion stability. From references above, it can be known that the change in inner structure can improve the working performance of the micro-combustor. However, it should be pointed out that the method can inevitably increase the difficulty of manufacturing and assembling of micro-combustor. Therefore, various combustors with simple inner structure and more significant enhancement effect should be further developed. In our previous work [29], numerical study indicates that the working performance of micro-planar combustor can be improved with the arrangement of parallel separating plates. To this end, a new type of micro-planar combustor with cross-plate will be proposed in this paper, whose structure is much simpler and can be easily assembled. Both experimental and numerical investigations have been adopted to analyze the combustion process of premixed propane/air in this kind combustor. The effects on the improving stable combustion range in mixture velocity and the heat transfer between high temperature exhaust gas and walls are also revealed. Finally, the optimum plate length for combustion characteristic in this micro-combustor has been obtained.
3. Computational model In this paper, a three dimensional model is also established to give a comprehensive analysis on the combustion and radiation characteristics of the new combustor. During the simulation, only half of the combustor is adopted to save the computational time. Meanwhile, centerline temperature of combustor without cross-plate at three mesh densities has been selected to check grid-independence, as shown in Fig. 3. It can be seen from the figure that the centerline temperature of medium and fine mesh cases varies little. Finally, the medium size mesh (0.1 mm in each direction) is selected in the following study. Fluent (Ansys 15.0) is selected as the software for the calculation and analysis tool. The propane/air mechanism M5 which has 28 species and 73 reactions is also used in the simulation process [30]. On account of the small flow rate, the laminar model is used to depict the flow process, and the finite-rate model are selected as reaction model [17,31]. The basic governing equations of mixture can be seen in reference [18], which discretized through a first-order upwind scheme. Moreover, the under relaxation method, implicit solver and the finite volume method are used in this model. In addition, the pressure–velocity coupling is calculated by SIMPLE algorithm. As for the physical properties of mixture, density is calculated by the ideal gas law, while the thermal conductivity and viscosity are defined as a mass fraction-weighted average [32]. When it comes to each component, the specific heat is calculated by the fourth-order
2. Experimental set up The experimental system is shown in Fig. 1. This system includes a propane gas tank, an air compressor, two reducing valves, two flow controllers, a mixing chamber, a micro-planar combustor, an infrared thermal imager and a computer for recording. During the experiment, the pressure of propane (purity: 99.0%) from the gas tank will be reduced to 0.08 MPa by reducing valve. The flow rates of propane and air are monitored by the mass flow controllers (DSN-2000B, accuracy: 0.5%), and then the propane and air will be fully mixed in the mixing chamber. At the same time, the infrared thermal imager (Type: ThermovisionTM A40; Maximum measure temperature: 2000 °C; Measuring accuracy: ± 2%) is used to measure the temperature distribution of the combustor external surface. Besides, a digital camera (Type: Nikon
Infrared thermographer Flow controller
Micro-combustor Flash-back arrestor
Connector
Micro flowmeter Mixing chamber Reducing valve
Flow controller Propane Air compressor Fig. 1. Schematic diagram of the experimental system.
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piecewise-polynomial function of temperature [16], the viscosity and thermal conductivity are calculated through kinetic theory [17]. The boundary conditions are illustrated as follows. The inlet is set as the velocity-inlet boundary condition. Meanwhile, each component is specified in the form of mass fraction. According to the practical situation, pressure-outlet boundary condition is adopted for the exhaust. No slip and zero diffusive flux species boundary conditions are employed at the gas solid interfaces, while the mixed thermal conditions are selected to evaluate the heat loss rate of each external wall, which include both natural convection and surface radiation heat transfer process [33]. The convective heat transfer coefficient between the external wall and environment is calculated to be 15 W/(m2·K), while the wall emissivity is set as 0.65, which is tested by the Fourier Transform Infrared Spectrometer. To validate the computational model, the comparison of centerline temperature along the external walls and the temperature distribution patterns has been made with respect to the experimental result. In order to make a good distinction, the combustor without cross-plate will be defined as straight-channel combustor, and the other is cross-plate combustor. As seen in Fig. 4, both of the high and low temperature zone location matches well, and the maximum relatively error of external wall temperature is only 3.4%, which illustrates the accuracy of the calculation model.
(b) External wall temperature distribution pattern of the combustor without cross-plate. Fig. 4. Experimental validation of computational model (inlet velocity: 0.6 m/s; Φ = 1.0 ).
4. Results and discussions 4.1. Effect of cross-plate on the working performance A comparative study with the straight-channel combustor has been conducted so as to give a comprehensive investigation on the basic performance of the novel combustor. In this section, the dimensionless plate length of 5/9, and the equivalence ratio of fuel and oxidant is chosen as 1.0. For the micro-power generator like MTPV system, 179
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(a) 0.6 m/s
(b) 0.8m/s
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Fig. 5. Experimental photos of the external wall at same inlet velocity (left: straight-channel combustor; right: cross-plate combustor).
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Fig. 6. Wall temperature at different inlet velocities.
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improving the external wall temperature is the most reliable method to increase energy utilization efficiency [34,35]. Firstly, the experimental photos of the external wall between the two micro-combustors are shown in Fig. 5, which are taken by a digital camera in dark environment. It can be seen that the external wall of each micro-combustor becomes brighter with the rising of mixture flow rate, which indicates the expanding of high temperature zone. Under the same condition, the appearance of cross-plate combustor is much brighter than that of the straight-channel case, which is in accordance with the phenomenon shown in Fig. 6. Fig. 6 compares the average and maximum wall temperature difference between the straight-channel and cross-plate micro-combustor, which are obtained from the infrared thermal imager. The wall temperatures of the two kinds of micro-combustor grow steadily along with the inlet velocity in each case. Besides, the temperature differences between the straight-channel and cross-plate combustors are also enlarged with increasing inlet velocity. At the 0.6 m/s case, the maximum and average wall temperature of cross-plate combustor are 1213 K and 1125 K respectively, which exceed the straight-channel cases by 58 K and 70 K. However, when it reaches 0.9 m/s, the corresponding increments of cross-plate cases will be more than 100 K. These results express that at high inlet velocity, the inserted cross-plate will significantly enhance the heat transfer intensity. To further demonstrate the effect of this new structure on the energy output and overall efficiency of MPTV system, the radiation energy from external wall of straight-channel and cross-baffle combustors are calculated as depicted in Fig. 7. It can be seen that the radiation energy of cross-plate combustor is almost twice as large as straight-channel combustor. Meanwhile, the radiation energy increment becomes large
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with an increase in inlet velocity, which is in accordance with the results shown in Fig. 6(a). With respect to MTPV system, the spectral radiation is much important. Therefore, the energy spectrum of combustor’s external wall at different wavelengths is calculated adopting numerical method as shown in Fig. 8. It can be seen that due to the higher temperature of external wall, the spectral radiance of cross-plate combustor under different inlet velocities are drastically higher than that of straight-channel combustor, which is beneficial to MTPV system. To further clarify the phenomenon, the heat transfer coefficients between the mixture and the micro-channel at different cases are calculated (as seen in Table 1), which are based on the simulation results.
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Fig. 8. Variation of combustor’s external wall energy spectrum at different wavelengths.
Fig. 10. Flame stabilization of the two micro-combustors at different equivalence ratios. Table 1 Heat transfer coefficients at different cases. Inlet velocity (m/s)
Heat transfer coefficient/ straight-channel combustor (W·m−2·K−1)
Heat transfer coefficient/ cross-plate combustor (W·m−2·K−1)
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fuel. It is observed that the radiation efficiency of straight-channel combustor case reduces gradually along the inlet velocity. However, the values of new type combustor are evidently higher, and remain above 0.29 in the first four flow rate conditions. As the inlet velocity reaches 0.9 m/s, the efficiency begins to drop and reaches about 0.28. It should be noted that the increase in chemical energy input does not absolutely mean the rising of radiation energy. As a result, the method of fuel input increasing may be not the best way to promote the system efficiency of the micro-power generators. Accordingly, it is suggested that the inlet velocity of cross-plate case should be kept at a reasonable and optimum working range. Furthermore, the flame stabilizations of the two micro-combustors are investigated, as shown in Fig. 10. Based on our previous research [36], the flame stability is defined: under a certain range of flow rates, the fuel/air can keep stable combustion in the micro-combustor without the phenomenon of flame oscillations or repetitive extinction and ignition occurring. Besides, the flame is not apparently asymmetrical. At the same time, the distribution of external wall temperature is relatively steady. The maximum inlet velocity of stable combustion represents the blowout limit, while the minimum inlet velocity is extinction limit. It can be seen that the flammable range shows various degrees of increase by the setting up of cross-plate. Benefited from the arrangement, the maximum stable combustion range in mixture velocity at stoichiometry ratio case rises from 1.0 m/s to 1.2 m/s. However, the positive effect of cross-plate could be much apparent under fuel-lean condition. For instance, the blowout limit of propane/air in the cross-plate combustor reaches 0.9 m/s under the condition of equivalence ratio 0.7, which is nearly twice of the straight-channel case. Besides, the extinction limit at this condition is also reduced to 0.4 m/s. When it comes to the fuel-rich conditions (equivalence ratio 1.1 and 1.2), the blowout limit of inlet velocity merely increases by 0.2 m/s and 0.1 m/s. Thus, it is concluded from the experimental results that the lean fuel case is a more suitable working condition for the cross-plate combustor. It follows that the cross-plate also has a function like bluff body, which is conducive to realize a longer residence time and stable flame [15,19,20].
For the micro-planar combustor without cross-plate, the heat transfer coefficient keeps rising as the inlet velocity increases, but the increment amplitude is small. When it comes to cross-plate combustor, the corresponding value is much higher than that of the straightchannel cases, and the increasing tendency is completely different. At the 0.6 m/s inlet velocity condition, the heat transfer coefficient of cross-plate combustor is 39.41 W/(m2·K), which is twice of straightchannel case. When increasing to 0.8 m/s, the value of cross-plate combustor reaches 2.7 times than that of straight-channel. This gives a direct explanation for the gradual rising of temperature difference in the Fig. 6. Therefore, it is concluded that when the cross-plate is inserted into the micro-combustor, the disturbance degree of the mixture should be improved. In addition, the equivalent diameter of the downstream channel will decrease apparently, which also contributes to the growing of the heat transfer coefficient. It is certain that the increase in average wall temperature will absolutely bring a growth of the radiation energy. However, the total chemical energy input will also rise gradually with an increase of fuel amount. Fig. 9 shows the radiation efficiency to further illustrate the advantage of cross-plate, which can be defined as the ratio of radiation energy output from the external wall to total chemical energy input by 0.350
Radiation efficiency
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4.2. Effects of cross-plate length
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It is universally known that the cross-plate length is a critical parameter, which can directly influence the working performance of micro-combustor. In this section, four dimensionless plate lengths have been selected, which are 4/9, 5/9, 6/9 and 7/9, respectively. Temperature distributions on the cross section of micro-combustor with different cross-plate lengths are shown in Fig. 11. Considering the thickness of the cross-plate, this surface is 0.5 mm away from the center section in z-direction, which can depict the temperature distribution of mixture gas. It is found that the temperature field at each case can be
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roughly divided into three stages. Firstly, when the mixture gas gradually moves forward, the combustion reaction begins appearing and the temperature of mixture rapidly increases to the flame center temperature. Secondly, the reaction has not happened near the inlet of the channel, and the mixture temperature is approximately closed to the initial temperature. Then, the fluid will be obstructed by the cross-plate, which leads to the generation of disturbances. Finally, the burned gas flows into the four downstream regions, and the released heat will be absorbed by the cross-plate and inner wall. As a result, the gas temperature gradually decreases along the exhaust channels, and the dimensionless plate length of 7/9 case owns the smallest high temperature region. With the increasing of cross-plate length, the contact area of fluid and solid zone will also be larger, which ensures more exhaust heat absorbed by the walls. However, it is found that the average temperature of external wall does not keep on growing with the increase of cross-plate length, which can be seen in Fig. 12. The changing tendency at different length cases are very similar, that is to say the average temperature of external wall increases as the inlet velocity rises. In contrast, the corresponding values of 5/9 cases are apparently higher than that of other cases at the same inlet velocity. Meanwhile, when the inlet velocity ranges from 0.5 m/s to 0.7 m/s, the mean temperature of external wall for the dimensionless plate length of 4/9 are slightly higher than 6/9 and 7/9 cases. At the last tested inlet velocity condition, the temperatures of the three cases are nearly the same, which are all about 40 K below the 5/9 case plate-length micro-combustor. To better understand the effect under different plate lengths, mean heat transfer coefficient in the micro-channel and maximum flame
Average wall temperature (K)
1200 1175
l=4/9 l=5/9 l=6/9 l=7/9
1125 1100 1075 1050 1025 0.5
0.6
0.7
140 130
1975
120 110
1950
100 90 80
1925 1900
70 60 50 40
1875
l=4/9
l=5/9
l=6/9
l=7/9
1850
temperature are investigated, as seen in Fig. 13, which are obtained by the numerical simulation. The heat absorption intensity between the inner wall and the mixture can be demonstrated by the mean heat transfer coefficient, while the maximum flame temperature can illustrate the combustion performance of fuels. That is the reason why these two parameters are selected as analytical subjects. From Fig. 13, it is indicated that the growth of cross-plate length will also bring out a persistent increase of heat transfer coefficient. The coefficient reaches 140 W/(m2·K) when the dimensionless plate length is 7/9, which is nearly 3 times the value of 4/9 case. However, the flame temperature is also directly affected by the cross-plate length. Long cross-plate will apparently result in the flame shape changing (As shown in Fig. 11) and the reducing of reaction space. It can be acknowledged from Fig. 13 that the flame temperature gradually decreases, especially when the dimensionless plate length exceeds 6/9. The maximum flame temperature is only 1882 K when the dimensionless plate length is 7/9, which is about 100 K lower than 4/9 case. From this point of view, choosing a proper length of cross-plate is of great importance to obtain higher wall temperature. In order to further analyze the effect of cross-plate length to the combustor working performance, radiation efficiency is contrasted in Fig. 14. Due to the higher average wall temperature, the value of 5/9 case remains about 0.29 at each inlet velocity condition, which are also significantly higher than that of other cases. Besides, the radiation efficiencies of 4/9 case remain unchanged at low inlet velocity condition, which are slightly higher than 6/9 and 7/9 cases. When up to 0.8 m/s, although the mean wall temperature keeps on increasing, the growthrate of radiation energy output suddenly begins to decline, and the
1150
1000
2000
Fig. 13. Average heat transfer coefficient and maximum flame temperature at different cross-plate lengths (inlet velocity: 0.8 m/s).
1250 1225
160 150
Maximum flame temperature (K)
Fig. 11. Temperature field on the cross section at different plate lengths.
0.8
Inlet velocity (m/s) Fig. 12. Average wall temperature at different inlet velocities and cross-plate lengths.
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0.32
l=4/9 l=5/9 l=6/9 l=7/9
Radiation efficiency
0.31 0.30
[3] [4]
0.29 [5]
0.28 [6]
0.27
[7]
0.26 [8]
0.25 0.24
0.5
0.6
0.7
[9]
0.8
Inlet velocity (m/s) [10]
Fig. 14. Radiation efficiency at different inlet velocities and cross-plate lengths.
radiation efficiency is apparently lower than 6/9 case. In fact, when the chemical energy input is a constant, the working efficiency of MTPV system is directly affected by radiation efficiency of the external wall. Therefore, it is suggested that the dimensionless plate length of 4/9 would be the most suitable choice.
[11]
[12]
[13]
5. Conclusions [14]
An original micro-planar combustor with cross-plate is designed and fabricated for the micro-thermophotovoltaic system, so as to improve the combustion process and working behavior. Experiment and numerical simulation are conducted to contrast the combustion characteristic of micro-combustors with and without cross-plate. Furthermore, the effects of cross-plate length are investigated. The main conclusions can be drawn as follows:
[15]
[16] [17]
[18]
(1) Heat transfer between mixture and combustor inner wall can be enhanced by the arrangement of cross-plate. When compared to the straight-channel combustor, the average external wall temperature of the cross-plate micro-combustor is increased by at least 90 K. Meanwhile, the ratio of radiation energy output to total chemical energy input is also much higher. (2) The cross-plate can be capable of improving the flame stability, which results in the increasing of flammable range at a certain degree. Especially as equivalence ratio is 0.7, the blowout limit of propane/air in the cross-plate combustor can be raised by 0.4 m/s. Hence, the lean fuel condition is more suitable for the working process of cross-plate combustor. (3) Cross-plate length is an important structural parameter of the new combustor, which has an influence on the flame shape, and leads to various promotion amplitude of the external wall temperature. By contrast, the dimensionless plate length of 5/9 is adopted as the optimum plate-length because of the highest average temperature of external wall and radiation efficiency.
[19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
Acknowledgements [27]
This work is supported by National Natural Science Foundation of China (No. 51676088, No. 51376082), Practice Innovation Program for College Postgraduates of Jiangsu Province (No. SJLX16_0428), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
[28]
[29]
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