ARTICLE

J. Cent. South Univ. (2019) 26: 2259-2271

DOI: https://doi.org/10.1007/s11771-019-4171-y

Numerical study of effect of front cavity on hydrogen/air premixed combustion in a micro-combustion chamber

CHEN Hai(陈海), LIU Wei-qiang(刘伟强)

College of Aerospace Science and Engineering, National University of Defense Technology,Changsha 410073, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: The micro-combustion chamber is the key component for micro-TPV systems. To improve the combustor wall temperature level and its uniformity and efficiency, an improved flat micro-combustor with a front cavity is built, and the combustion performance of the original and improved combustors of premixed H₂/air flames under various inlet velocities and equivalence ratios is numerically investigated. The effects of the front cavity height and length on the outer wall temperature and efficiency are also discussed. The front cavity significantly improves the average outer wall temperature, outer wall temperature uniformity, and combustion efficiency of the micro-combustor, increases the area of the high temperature zone, and enhances the heat transfer between the burned blends and inner walls. The micro-combustor with the front cavity length of 2.0 mm and height of 0.5 mm is suitable for micro-TPV system application due to the relatively high outer wall temperature, combustion efficiency, and the most uniform outer wall temperature.

Key words: micro-combustor; hydrogen; front cavity; numerical study; energy conversion efficiency

Cite this article as: CHEN Hai, LIU Wei-qiang. Numerical study of effect of front cavity on hydrogen/air premixed combustion in a micro-combustion chamber [J]. Journal of Central South University, 2019, 26(8): 2259-2271. DOI: https://doi.org/10.1007/s11771-019-4171-y.

1 Introduction

The micro-electro-mechanical system technology has received much attention in recent years [1-3]. Micro-power generation devices based on combustion offer several advantages, including their light weight, high-power density, long charging duration, and short recharging time [4, 5]. In addition, the hydrogen and methane are less pollutional to the environment and hardly produce nitrogen oxides. Moreover, as renewable energy sources, hydrogen and methane can effectively alleviate energy shortages. Therefore, these devices are deemed suitable alternatives to conventional batteries and can be used in micro fuel cells, micro gas turbines, and micro thermo-electric and thermo- photovoltaic (TPV) systems.

A micro-TPV system comprises PV cells, an emitter, filter, and heat source. A micro-combustor is a core part of a micro-TPV system, in which the chemical energy of fuel is converted into other types of energy via combustion [6-8]. Therefore, micro-combustors with a large operation range have received much research attention. However, the short residence time of mixtures and high heat loss resulting from the high surface area-to-volume ratio of micro-combustors lead to the flame instability and low blow-off limit of the system [9, 10]. To address these problems, the flame stability must be enhanced and the operating range of the system must be extended. The flame stability and combustion performance of micro-combustors can be improved by using a structure design.

ZHANG et al [11] found that the blow-off limit of the combustor with blunt body is 2.5-fold higher than that of the combustor without blunt body when the equivalence ratio is 1. PENG et al [12] numerically examined the thermal performance of a micro-combustion chamber with and without a front cavity and found that the front cavity enhances the stability of the micro-combustor. E et al [13] established a methane/air premixed combustion model in a micro-combustor and found that the combustion performance is improved at a 45° backward-facing step angle. PAN et al [14] found that the micro pin-fin in combustor increases the area of the reaction zone and the blow-out limit of the combustion chamber. AKHTAR et al [15] numerically examined a novel planar micro- combustion chamber with a combined bluff and baffle structure for micro-TPV systems. WAN et al [16-19] numerically investigated the behavior of an H₂/air blend in a micro-combustion chamber with cavities and found that these cavities can also expand the working range of the inlet velocity. ARAVIND et al [20] proposed a micro-combustion chamber with a recirculation hole. They found that the porous media preheat the mixture through a thermal cycle and that the flame stability is significantly improved. BAGHERI et al [21] numerically studied the combustion characteristics of premixed H₂/air in micro-combustors with different bluff body structures and found that the combustion chamber with a wall fin as a bluff body shows the highest flame temperature and emission efficiency. YILMAZ et al [22, 23] established a combined structure of cavity, post-stage, and micro-channel in a combustor. They found that the wall temperature and combustion efficiency of the new combustor have been significantly improved. NI et al [24] numerically studied the thermal performance of combustion chamber with rectangular, U-shaped, and N-shaped ribs and revealed that the average temperature of the U-shaped rib combustion chamber increases by 25.4 K compared with other combustors. E et al [25] found that the inlet pipe structure of the combustor improves the mixing performance of H₂/O₂ and enhances flame stabilization and heat recirculation by using a backward-facing step and cavity. WAN et al [26] designed a micro-combustor with a flat flame stabilizer and two preheating channels and found that the blow-out limit of the combustor initially increases and then decreases along with an increasing plate length. ZUO et al [27] proposed a micro-cylindrical combustion chamber with a rib and found that the outer wall temperature of the improved combustion chamber is higher and more uniform than that of the micro-combustion chamber without a rib. TANG et al [28] found that the cross-plate enhances the heat transfer and increases the average wall temperature by more than 90 K. FAN et al [29] proposed a double-wall micro-combustor and found that the combustion efficiency of the double-layer micro-combustor is higher than that of single-layer SiC and quartz micro-combustor at a large inlet velocity. LI et al [30] numerically studied the effects of channel height and inlet velocity on combustion characteristics in a 2D planar micro- combustion chamber with a separating plate.

Adding hydrogen to hydrocarbon is another effective way for improving combustion performance. However, hydrogen cannot be easily applied in small portable installations because of issues regarding its price, production, and storage [31]. Methane is a promising alternative fuel that produces minimal pollution emission. However, methane has its own disadvantages, including its low burning velocity and high ignition temperature, which may result in low combustion efficiency especially on a micro scale. Adding hydrogen to methane can improve the combustion and extend operability ranges of the latter due to its advantages, such as its high flame propagation speed, high diffusivity, and low ignition energy.

TANG et al [32] found that the proportion of total radiation energy and effective radiation energy to the total input chemical energy significantly increases along with the H₂ addition ratio. WU et al [33] found that the soot-free length and flame temperature increase along with the addition of hydrogen. The addition of hydrogen can also reduce the CO emission. WANG et al [34] found that the flame propagation speed significantly increases with the increase in hydrogenation amount and inlet temperature. AMANI et al [35] found that the combustion chamber with a 70% hydrogen volume shows the best performance. TANG et al [36] experimentally studied the combustion performance of some hydrocarbon fuels in a planar combustion chamber and found that the combustion of CH₄/air cannot be sustained when the height of channels is less than 2.5 mm and that H₂/air can be ignited when the micro-combustor is 1 mm high.

A high and uniform combustor wall temperature and a high combustion efficiency are both desired for thermoelectric and TPV devices yet are difficult to achieve because the heat loss sharply increases along with the surface area-to-volume ratio of the micro-combustor. In addition, the residence time of the mixture decreases when the combustion chamber is scaled down. To address these problems, cavities have been included in a planar micro-combustion chamber [16-19] and the effects of cavity geometry on combustion performance have been evaluated. Based on Refs. [16-19], an improved micro-combustor by applying a front cavity was built. The combustion performance of micro-combustors with and without a front cavity under various inlet velocities and equivalence ratios is then compared, and the effects of front cavity height and length on the outer wall temperature and efficiency are numerically studied.

2 Numerical methods

2.1 Geometric model of a micro-combustor

The schematic diagram of a micro-combustion chamber with and without a front cavity is shown in Figure 1. The thickness of the combustion chamber walls (W₃) and gap distance (W₁) are 2 mm and 1 mm, respectively. The total length (L₀) and cavity length (L₂) of the micro-combustion chamber are 18.0 mm and 3.0 mm, respectively. The width (W₀) of the combustion chamber is 10 mm, whereas the length (L₁) and depth (W₂) of the cavities are 3 mm and 1 mm, separately. The micro-combustor with a front cavity has a varying front cavity height (H) and length (L) as shown in Figure 1(b).

2.2 Modeling equations

Given that the characteristic scale is notably larger than the molecular mean-free path of the mixture in the original micro-combustion chamber, the blends are regarded as continuous [16-19].

The Reynolds number exceeds 500 when the inlet velocity in the original combustor is equal to or above 8 m/s. KUO et al [37] reported that the realizable k-epsilon model can obtain much better predictions than the laminar model when the Reynolds number is equal to or above 500; therefore, the realizable k-epsilon model is applied in the numerical simulation. Also a 2D steady-state model is employed due to the large aspect ratio (W₀/W₁=10:1) of the micro-combustor.

Figure 1 Schematic diagram of micro-combustor:

The continuity equation,

                  (1)

The momentum equation,

         (2)

The energy equation,

          (3)

The species equation,

                     (4)

where C_p,i, R_i, Y_i denote specific heat capacity, generation or consumption rate, and the mass fraction of species i, separately; while l_ft and l_f are turbulent thermal conductivity and thermal conductivity of the blends, separately.

Hydrogen is the only fuel in the micro- combustor. Thus, the combustion efficiency of the micro-combustor can be represented as follows:

                        (5)

where and are the hydrogen mass flow rates of the inlet and outlet, respectively.

2.3 Boundary conditions

Fluent is used to solve the mass, momentum, energy, and species conservation equations. The density of the H₂/air blends is calculated by following the ideal gas assumption, while the specific heat is computed by using the mixing law. The mass-weighted mixing law is calculated based on the thermal conductivity and viscosity of all species, while kinetic theory is used to determine the blend mass diffusivity. Temperature patches of 2000 K and 1350 K on the fluid and solid zones, respectively, are used to ignite the H₂/air blends.

The chemical kinetic mechanism of the H₂ reaction as reported in Ref. [38] was adopted. This mechanism comprises 13 species and 19 reversible elementary reactions. To verify the chemical mechanism, WAN et al [16-19] performed an H₂/air simulation in a combustor with cavities and then compared the simulation results with the experimental data reported in Ref. [39]. They found that the numerical results coincide with the experimental data.

The boundary conditions are set as follows. The velocity and pressure outlet conditions were employed at the inlet and outlet, respectively. The absolute pressure and temperature of the inlet were fixed at 101325 Pa and 300 K, respectively. The specific heat capacity, density, and thermal conductivity of the quartz solid material were 750 J/(kg·K), 2650 kg/m³, and 1.05 W/(m·K), respectively.

The heat loss in the solid wall greatly influences the combustion. Therefore, the heat transfer in the channel walls is considered in the numerical simulation. The heat loss in the exterior surfaces includes radiation, and the natural convection is computed as

                (6)

where h₀ is the natural convection heat transfer coefficient, 20 W/(m²·K); T_w is the exterior wall temperature; ε is the emissivity of the solid surface (0.92); T_∞ is the ambient temperature (300 K); and σ is the Stephan-Boltzmann constant, 5.67×10⁸ W/(m²·K⁴).

The outer wall temperature difference △T_w and the mean outer wall temperature`T_w of the combustor are computed as follows, separately:

                      (7)

                           (8)

where T_w,min and T_w,max are the minimum and maximum temperatures on the wall, respectively; T_w,i and A_w,i are the temperature and the surface area of grid cell i on the outer wall, respectively.

2.4 Grid independence

A grid independent test was conducted by using the 2D grid of the micro-combustion chamber without a front cavity as shown in Table 1. The mesh intervals employed in the test were 1×10^-5, 2×10^-5, 3.3×10^-5 and 4×10^-5m. The inlet velocity and equivalence ratio of the test case were 10 m/s and 0.4, respectively; the radiation emissivity of the outer wall was 0.92, and the heat transfer coefficient was 20 W/(m²·K) [16-19]. Table 1 shows that these four mesh intervals only have slight differences in temperature and hydrogen at the outlet. To ensure accuracy and save computation time, the grid spacing of 2×10^-5 m was employed in the numerical simulation. The numerical model of the micro-combustor was meshed by using approximately 227600 structured cells as shown in Figure 2.

Table 1 Calculation results of different grid spacings in 2D structure of a micro-combustor

Figure 2 Grid system near cavity

2.5 Model verification

To validate the accuracy of our numerical solution, the predicted and measured exhaust blends temperature in the original micro-combustor as reported in the literature were compared [16-19], as shown in Figure 3. The maximum temperature difference was 98 K, thereby indicating the reasonable accuracy of the proposed numerical solution.

Figure 3 Comparison between experimental data and numerical results for exhaust gas temperature in original micro-combustor

3 Results and discussion

3.1 Effects of inlet velocity on combustion characteristics of micro-combustors with and without front cavity

Figure 4 shows the outer wall temperature profiles for the original and improved micro- combustors at inlet velocities of 10 and 24 m/s. The equivalence ratios are maintained at 0.4, and the length and height of the front cavity in the improved micro-combustor are 2 and 0.5 mm, respectively. These figures show that the outer wall temperature profile increases from the inlet and reaches a peak around the cavity center. Thereafter, the temperature profile descends with the further increase in the axis length because of the heat losses from the outer wall. Under various inlet velocities, the outer wall temperature of the combustion chamber with the front cavity at the front part is significantly higher than that of the combustion chamber without the front cavity; the outer wall temperature of the combustion chamber with the front cavity at the back part is slightly lower than that of the combustion chamber without the front cavity. This phenomenon is attributed to the recirculation and low-velocity zone formed in the front cavity and the enhanced heat transfer effect between the blends and the inner wall.

Table 2 quantitatively compares the average wall temperatures, outer wall temperature nonuniformity, and efficiency of the micro combustion chambers without and with the front cavity (H=0.5 mm, L=2.0 mm) under different inlet velocities; here, the equivalence ratio, f, is maintained at 0.4. The average wall temperature of the combustion chamber with the front cavity is higher than that of the combustion chamber without the front cavity by 46.9, 56.6, 69, 79.9 and 101.8 K. The outer wall temperature difference is less than that of the combustion chamber without the front cavity by 277.9, 206.7, 190.6, 176.4 and 177.1 K. The combustion efficiency of the combustion chamber with the front cavity is higher than that of the combustor without the front cavity by 0.01%, 0.2%, 2.8%, 5% and 5.6%. Thus, the micro- combustion chamber with the front cavity has high outer wall temperature uniformity, level, and combustion efficiency compared with those of the original combustor. The positive effect of the front cavity on the average wall temperatures, outer wall temperature uniformity, and efficiency strengthens with the increase in inlet velocity because the front cavity increases the flame temperature and enhances the convective heat.

Figure 4 Outer wall temperature profiles of micro-combustors with and without front cavity heights under Φ=0.4

Table 2 Mean outer wall temperature, outer wall temperature difference and efficiency for combustors with and without front cavity under various inlet velocities, where Φ=0.4

Figure 5 compares the temperature distributions for the original and improved micro combustion chambers at inlet velocities of 10 and 24 m/s. The equivalence ratio is maintained at 0.4. A high-temperature zone is located in the front cavity, which suggests a recirculation and low- velocity zone is formed; and the flame is stabilized by the front cavity. On the contrary, the original micro-combustion chamber only forms a high- temperature zone in the cavities and boundary layers. The front cavity increases the area of high-temperature zone and makes the flame front move forward the inlet, which enhances the heat transfer between the blends and upstream inner walls. The residence time of the mixture and preheating of blends is also prolonged. Consequently, the newly proposed combustion chamber has a higher and more uniform wall temperature than that of the original one.

Figure 6 presents the radical OH distribution of the original and improved micro combustion chambers. The inlet velocities are maintained at 10 and 24 m/s, and the equivalence ratio is kept at 0.4. The variation in the reaction regions is consistent with that in the high-temperature zones, as shown in Figure 5. The location of the highest OH mass fraction zone in the improved micro-combustor is closer to the inner wall compared with that in the original micro-combustor. The reaction zone in the micro-combustion chamber with the front cavity is narrower and longer than that in the original micro-combustor. Hence, more released heat is transferred to the wall, and the wall temperature of the micro-combustion chamber with the front cavity is higher than that of the original one.

Figure 5 Temperature distribution in micro-combustors with various inlet velocities:

Figure 6 Mass fraction contours of OH in different combustors with various inlet velocities:

3.2 Effects of front cavity length and height on combustion characteristics

Figure 7 shows the outer wall temperature profiles for the micro-combustors with different front cavity height and lengths, in which the equivalence ratio is maintained at 0.4. The outer wall temperature of the combustor with the height (H) of 0.25 mm is the highest, followed by the combustors with H=0.5, 0.75 mm under different inlet velocities. The outer wall temperature in the front part increases with the increase in front cavity length when the inlet velocity is constant and the reaction region moves upstream with the increase in front cavity length, which releases considerable heat to the upstream inner wall. Figure 7 also shows that the discrepancy in the outer wall temperature of the different combustors increases when the front cavity length increases and the inlet velocity is fixed constant.

Figures 8 and 9 present the average wall temperatures and wall temperature difference of the different micro combustion chambers with various front cavity heights and lengths when the inlet velocity equals 10 and 24 m/s. The equivalence ratio is maintained at 0.4. The average wall temperature increases as the front cavity length increases, with the front cavity height kept at constant under various inlet velocities. While the average wall temperature changes inversely with front cavity height. As the front cavity length increases, the wall temperature difference decreases. Furthermore, it can be found from Figure 9 that the wall temperature nonuniformity first decreases with the increase in the front cavity height, and then it increases with the increase in the front cavity height under various inlet velocity and front cavity length.

Figure 10 presents the combustion efficiency of the micro combustion chambers with various front cavity heights and lengths under inlet velocities of 10 and 24 m/s, in which the equivalence ratios are maintained at 0.4. The combustion efficiency of different micro combustion chambers is higher than 99% when the inlet velocity equals 10 m/s. As the inlet velocity increases to 24 m/s, the combustion efficiency of the combustors drops due to the short residence time of blend. With increases in front cavity height and length, the areas of low-velocity and recirculation zones and the combustion efficiency increase.

Figure 7 Outer wall temperature distribution under various front cavity lengths and heights:

Figure 8 Average wall temperature for combustor under various front heights and lengths where Φ=0.4

Figure 9 Outer wall temperature difference for combustor under various front heights and lengths where Φ=0.4

Figure 10 Combustion efficiency for combustors under various front heights and lengths where Φ=0.4

Figure 11 shows the contours of the temperature distributions for the micro-combustors with different front cavity heights and lengths. The flame is stabilized in the cavity, front cavity, and boundary layer. As the inlet velocity increases, the wall temperature downstream of the cavities also increases. This result is due to the decrease in the residence time of the mixture in the combustor as the inlet velocity increases, which leads the reaction regions to move downstream. The temperature near the cavities increases as the front cavity length increases, whereas the front cavity height is constant. This result is due to the increased area of recirculation and the low-velocity zone in the front cavity, which means that considerable heat is released on the upstream wall. The outer wall temperature near the cavities and the boundary layer in downstream decreases when the front cavity length is set at a constant.

Figure 12 presents the radical OH distribution of the temperature distributions for the micro- combustors with different front cavity heights and lengths when inlet velocities are kept at 10 and 24 m/s. The equivalence ratio is maintained at 0.4. As the front cavity height and length increase, the highest OH mass fraction location moves forward to the inlet, which means that the flame is stabilized by the front cavity. As the front cavity height increases, the reaction zone in the front cavity is farther to the inner wall of the front cavity, which implies that minimal heat is transferred to the wall of the front cavity. The width of the internal flow cross section will increase correspondingly, which will decrease the convective heat transfer effect between the inner wall and burning blends.

3.3 Effect of equivalence ratio on combustion performances of micro-combustors with and without front cavity

Figure 13 presents the difference in the outer wall temperature of the combustors with and without the front cavity under various equivalence ratios. With different equivalence ratios, the outer wall temperature of the combustion chamber with the front cavity at the front part is significantly higher than that of the combustion chamber without the front cavity; the outer wall temperature of the combustion chamber with the front cavity at the back part is slightly lower than that of the combustion chamber without the front cavity.

Figure 11 Temperature distributions in combustors with different front cavity heights and lengths under equivalence ratio of 0.4:

Figure 12 Mass fraction contours of OH in different combustors with different front cavity heights and lengths under Φ=0.4:

Figure 13 Outer wall temperature distribution of micro- combustors with and without front cavity under various equivalence ratios where inlet velocity is kept at 10 m/s

Table 3 quantitatively compares the mean outer wall temperature and the outer wall temperature nonuniformity of the combustion chambers with and without the front cavity under different equivalence ratios. The mean outer wall temperature of the combustion chamber with the front cavity is increased by 48.4 K and 1.5 K and the outer wall temperature nonuniformity is decreased by 249 K and 42 K compared with that on the combustion chamber without the front cavity. Therefore, the combustion chamber with the front cavity has higher outer wall temperature level and temperature uniformity under different equivalence ratios compared with that without the front cavity.

Table 3 Mean outer wall temperature and outer wall temperature difference of micro-combustors with and without front cavity under various equivalence ratios where the inlet velocity is kept at 10 m/s

4 Conclusions

A micro-combustion chamber with the front cavity was designed in this paper. The effects of front cavity height and length on premixed H₂/air combustion were numerically studied. The performance of the original design was compared with that of the newly proposed designs under various inlet velocity and equivalence ratios. The conclusions are summarized as follows:

1) The combustor with front cavity has higher outer wall temperature, temperature uniformity, and combustion efficiency of the micro-TPV system under various inlet velocities and equivalence ratios than the original combustor.

2) The front cavity increases the area of the high temperature zone and enhances the heat transfer between the burned blends and inner walls.

3) The thermal performance is improved and reaches the peak when the front cavity length ranges from 1.0 mm to 2.0 mm and the front cavity height is kept constant under various inlet velocities. With the increase in front cavity height, the combustion efficiency increases, and the average wall temperature changes inversely.

4) The micro-combustor with the front cavity length of 2.0 mm and height of 0.5 mm has a relatively high outer wall temperature, combustion efficiency, and the most uniform outer wall temperature, which is suitable for the application of micro-TPV system.

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(Edited by YANG Hua)

中文导读

前置凹腔微燃烧室氢气/空气预混燃烧的数值模拟研究

摘要：为了提高微燃烧室的壁面温度、壁面温度均匀性以及燃烧效率，提出了具有前置凹腔的微型燃烧室。对改进结构燃烧室与原结构燃烧室在不同入口速度以及当量比的热性能进行了对比，并分析了前置凹腔高度和长度对改进结构燃烧室预混氢气/空气壁面温度与燃烧效率的影响。计算结果表明前置凹腔内形成了回流区，促进了高温气体与燃烧室壁面的传热，使微燃烧室在不同入口速度和当量比下的外壁温度、温度均匀性和燃烧效率得到显著提高。同时，当前置凹腔高度保持一定时，微燃烧室热性能随着前腔长度的增加而得到提高；当长度保持一定时，随着燃烧室前置凹腔高度的增加，燃烧室的燃烧效率随之增加，然而壁面平均温度随之下降。因此，当燃烧室前置凹腔高度与长度分别为0.5 mm和2.0 mm时，燃烧室有相对较高的壁面平均温度与燃烧效率，同时壁面温度均匀度最高。

关键词：微燃烧室；氢气；前置凹腔；数值模拟；能量转换效率

Foundation item: Project(11802336) supported by the National Natural Science Foundation of China

Received date: 2019-04-20; Accepted date: 2019-07-07

Corresponding author: CHEN Hai, PhD; E-mail: chfair@163.com; ORCID: 0000-0001-8661-8490