Numerical Study of the Pin Fin Types on Combustion Characteristics of Micro Combustor

The influence of different shapes of pin fin arrays in micro combustors on combustion performance is investigated numerically. All the micro combustors show a high combustion efficiency and the one with triangle-B pin fin arrays exhibits superiority in combustion efficiency. Additionally, the combustor efficiency of all micro combustors is relatively low and reduces as the velocity increases. This works provides a great reference and guidance for the development of the micro burner in MTPV system.


Introduction
MEMS is an interdisciplinary technical field that integrates various sciences and technologies, offering vast potential applications in fields such as medical, biological, military, and aerospace.In particular, micro power systems that provide energy typically employ hydrocarbon fuels, which are preferred because of their greater energy density, compact size, and lightweight properties.The MTPV system represents a critical power system among the various micro power systems available, as it facilitates the conversion of thermal energy to electrical energy [1].The thermal characteristics of the micro burner acts a critical role in determining the overall performance of the Micro Thermophotovoltaic (MTPV) system.Rahbari et al. [2] conducted a study to examine the impact of micro burner size and geometry parameters on the thermal performance and flame modes of premixed H 2 /air flames.They observed that increasing the equivalence ratio caused upstream movement of the flame front, and that incorporating a bluff body into the micro combustor improved combustion efficiency.In another study, Mansouri [3] proposed a wavy micro-combustor design for MTPV systems, and the findings indicated that the combustor efficiency of the wavy design was 8.3% higher than that of the conventional design.
Compared to conventional burners, micro-burners have a smaller volume, resulting in a larger specific surface area, robust heat dissipation, shorter fuel retention time, and a substantial pressure drop caused by friction.Multiple factors directly contribute to low combustion efficiency, a narrow working range, and challenging stable combustion in micro-burners.To achieve stable combustion in microburners, current micro-scale stable combustion technologies mainly include catalytic combustion technology, porous media combustion technology, mixed combustion technology, efficient heat recovery, uniform load compression ignition technology, pulsating combustion technology.Li et al. [4] conducted a study to examine the flame stability limits and thermal performance of a porous medium burner.The findings indicated that the incorporation of a porous medium could improve thermal recirculation and extend the flame stability limits.Ma et al. [5] developed a Swiss-roll micro-burner with dual chambers for premixed CH 4 /air and evaluated the impact of SiC, stainless steel, and copper on the blown-off limit.The superior thermal recirculation performance and low heat loss ratio significantly influenced the blow-off limit.Specifically, stainless steel demonstrated the greatest blowoff limit and excellent flame anchoring capability.Chen and Pang [6] investigated the catalytic raction performance of CH 4 /air in a micro-burner with a heat recirculation system.They observed that the system's stable operation was limited to a relatively broad flow pattern, and velocity showed a crucial role in achieving flame stability.Chouraqui et al. [7] experimentally examined the impact of diluents on premixed CH 4 /air flame.The diluents had a crucial effect on flame stability and the adiabatic flame temperature.The reduction of inflow velocity could suppress the generation of the flames with repetitive extinction and ignition (FREI).
Thus, the present study aimed to study the influence of pin fin type on the combustion characteristics of the micro-combustor.

Numerical Simulation
Figure 1 illustrates the structure of micro-burners with different shapes of pin fin arrays (MCPFs).All micro-combustors possess a length (L) of 16 mm (x-direction, longitudinal), width (W) of 6 mm (ydirection, transverse), and height (H) of 2 mm (z-direction, vertical).They comprise 13 columns and six pin fins in each column.The distance between the center of the first column and inlet is 2 mm, and between the center of the last column and outlet is also 2 mm.The pin fins are arranged in a staggered manner, and their shapes are circular, square, diamond, triangle (A), and triangle (B), respectively.The pin fin structural feature size (D) of all micro combustors is 0.5 mm, with an external wall thickness (δ) of 0.5 mm.The longitudinal gap (S w ) between two pin fin central elements and the transverse gap (S H ) between two pin fin centers are set at 1 mm.Additionally, an offset gap of 0.5 mm is introduced in the transverse direction (S L ).All micro combustors are fabricated using quartz material.

Results and discussion
Hydrogen is a well-known fuel that exhibits a high combustion efficiency and hydrogen conversion rate due to its easy-burning nature.The impact of intake mixture speed on the combustion efficiency is illustrated in Fig. 2. The combustion efficiency reduces with increasing inlet velocity, while remaining high at a level of at least of 93%.The combustion efficiency decreases by 4.02%, 4.67%, 4.36%, 3.58%, and 3.43% for the combustor with circle, square, diamond, triangle(A) and triangle(B) pin fins, respectively, when the speed increases from 4 to 12 m/s.The MCPFs-Triangle(B) shows the best combustion efficiency, followed by MCPFs-Triangle(A), with some exceptions such as the MCPFs-Square and MCPFs-Diamond.Overall, the combustion efficiency decreases gradually and then sharply with increasing inlet velocity.When the velocity reaches a certain level, the mixture is blown out from the outlet, which significantly shortens the residence time.Figure 3 demonstrates the impact of mixture speed on the average temperature of the exhaust gas.It is noted that the average temperature of the exhaust gas increases with the inflow speed increasing.For the combustor with circle, square, diamond, triangle(A) and triangle(B) pin fins, when the inflow inlet velocity changes from 4 m/s to 12 m/s, the temperature of the exhaust gas increases from 1381.9 K, 1351.1 K, 1389.7 K, 1369.8K, and 1375.4K to 1778.7 K, 1759.4K, 1774.8K, 1732.8K, and 1749.1 K, respectively.It should be noted that the MCPFs-Diamond shows the greatest average temperature of the exhaust gas, followed by the MCPFs-Circle, while the MCPFs-Square exhibits the lowest average temperature of outflow when the mixture inflow speed is less than 8 m/s.The MCPFs-Triangle(A) and MCPFs-Triangle(B) fall in the middle sequence.When the speed is greater than 8 m/s, the average temperature of the exhaust gas in MCPFs-Circle reaches its peak.Seitzman et al. [8] suggested that the distribution of hydroxyl radical (OH) could represent both the flame vortical structures and the flame region.Fig. 4 displays the OH contour at speed of 6 m/s.It is found that the shape of pin fins styles shows a crucial impact on the flame region and the hydroxyl radical distribution.There is a high OH distribution region in the MCPFs-Circle, MCPFs-Square and MCPFs-Triangle(A).However, there is no obvious high OH distribution region in the MCPFs-Triangle-B.It means the combustion intensity of the MCPFs-Circle, MCPFs-Square and MCPFs-Triangle(A) are higher than other micro combustors at 6 m/s.This also corresponds with the results in Fig. 3, the sequence of mean temperature of exhausted gas is MCPFs-Diamond, MCPFs-Circle, MCPFs-Triangle(A), MCPFs-Triangle(B) and MCPFs-Square, respectively.It was found that the pin fins styles in micro combustors can not only prevent the free radicals blowing out, but also improve the blown-off limit and combustion efficiency.velocity of 6 m/s.The impact of pin fin styles on temperature distribution in micro combustors at 6 m/s is presented in Fig. 5.An evident high-temperature zone is observed in all cases, but the size of the high-temperature region differs among the micro combustors with various pin fin array shapes.The MCPFs-Circle exhibits the largest high-temperature region, followed by the burner with diamond, triangle(A), and square pin fins, whereas MCPFs-Triangle(B) displays the smallest high-temperature region.The temperature distribution in MCPFs corresponds with the hydroxyl radical distribution contour, which can be attributed to the combustion intensity determining the temperature distribution.6 illustrates the impact of pin fin shapes on velocity contour and vector in micro burners at 6 m/s.The shape of pin fins plays a crucial role in determining the overall performance and output energy of micro combustors in MTPV systems.The presence of pin fin arrays results in high velocity regions between two pin fins in the transverse direction and velocity recirculation regions behind the pin fins in the longitudinal direction due to the Bernoulli effect.Pin fins also entrain unburned high temperature mixture and low temperature premixed gas, which can prolong the residence time.Notably, MCPFstriangle(A) exhibits the largest velocity recirculation regions, followed by the burners with diamond, triangle(B), square, and circle pin fins.

Conclusions
All of the micro combustors with pin fins demonstrate high combustion efficiency.However, the MCPFs-Triangle(B) exhibits the highest combustion efficiency under the same operating conditions.It should be noted that the combustion efficiency decreases with increasing intake velocity.In addition, the available radiation energy in micro combustors initially increases and then decreases.Notably, the MCPFs-Diamond exhibits the best comprehensive performance with an average outer wall temperature of 1255.3K and an available radiation energy of 43.6 W at 10 m/s.Furthermore, its temperature distribution is the most uniform compared to all other micro combustors.

Figure 1 .
Figure 1.Schematic diagram of micro combustor and various shapes of pin fin.

Figure 2 .
Figure 2. Impact of shapes of pin fins styles on combustion efficiency.

Figure 3 .
Figure 3. Influence of various shapes of pin fins in micro combustor on exhausted gas average temperature.

Figure 4 .
Figure 4. Influence of different shapes of pin fin arrays on the OH contour at the center plane at inletvelocity of 6 m/s.The impact of pin fin styles on temperature distribution in micro combustors at 6 m/s is presented in Fig.5.An evident high-temperature zone is observed in all cases, but the size of the high-temperature region differs among the micro combustors with various pin fin array shapes.The MCPFs-Circle exhibits the largest high-temperature region, followed by the burner with diamond, triangle(A), and square pin fins, whereas MCPFs-Triangle(B) displays the smallest high-temperature region.The temperature distribution in MCPFs corresponds with the hydroxyl radical distribution contour, which can be attributed to the combustion intensity determining the temperature distribution.

Figure 5 .
Figure 5. Influence of pin fins styles on the temperature contour at inlet velocity of 6 m/s.

Figure
Figure6illustrates the impact of pin fin shapes on velocity contour and vector in micro burners at 6 m/s.The shape of pin fins plays a crucial role in determining the overall performance and output energy of micro combustors in MTPV systems.The presence of pin fin arrays results in high velocity regions between two pin fins in the transverse direction and velocity recirculation regions behind the pin fins in the longitudinal direction due to the Bernoulli effect.Pin fins also entrain unburned high temperature mixture and low temperature premixed gas, which can prolong the residence time.Notably, MCPFstriangle(A) exhibits the largest velocity recirculation regions, followed by the burners with diamond, triangle(B), square, and circle pin fins.

Figure 6 .
Figure 6.Influence of pin fins styles on the velocity distribution and vector at the center plane at 6 m/s.