Numerical Investigation of Pin Fin Types on the Thermal Improvement of H2/Air Micro Burner

The micro-thermophotovoltaic system is a prospective power generation system. The radiation performance of exterior walls is a critical component of MTPV, as it effectively determines the overall energy output. At a mixture intake speed of 10 m/s, the MCPFs-Diamond exhibits the most favourable thermal performance, as evidenced by the thermal distribution and the highest average temperature of the exterior walls, which measures 1255.3 K. Its available radiation energy can reach up to 43.6 W. In the case of micro combustors that incorporate alternative shapes of pin fins, the highest values for both the average external wall temperature and the available radiational energy are achieved when the gas intake speed is set to 8 m/s.


Introduction
The increasing consumption of fossil fuel causes a serious global issue to the environmental pollution and energy crisis present [1].The micro energy generator devices have the potential to become a widely energy source due to its high energy output capabilities [2].Among various micro energy generator devices, the micro-thermophotovoltaic (MTPV) system has gained significant attention because of its outstanding performance.
The energy supply is primarily derived from the radiation energy emitted by the outer walls in MTPV system.Therefore, the temperature uniformity and value are two critical factors that significantly influence the energy output.E et al. [3] demonstrated the non-premixed H 2 /air with catalytic combustion in the backward-facing step combustor.Although the catalysts weakened the homogeneous reaction, they led to a better outer wall temperature.When the catalytic combustor was operating at 10 m/s, both the mean flow speed and pressure drop were reduced by up to 5.6% and 250 Pa, respectively.Zhao et al. [4] designed a micro-opposed flow porous burner for propane/air, which could maintain the stable flame in a 2 mm chamber.An energy output of 5.16 W with a total energy conversion efficiency of 4.27% was realized at 0.18 W/cm 3 .Xue et al. [1] investigated the impact of the Y-shaped fins on the energy conversion and exergy of a micro burner.The exergy efficiency could reach up to 44.19%, 44.80%, and 44.80% for fins with angles of 30°, 60°, and 90°, respectively.Besides, it was found the new materials with high thermal conductivity was superior to traditional materials.To achieve improved combustion stability, more uniform and elevated outer wall temperatures, various types of pin fins have been numerically analyzed when inserted into micro combustors.

Numerical Simulation
Fig. 1 briefly depicts the energy conversion network in MTPV system.In the MTPV system, the chemical energy (E C ) of hydrogen or hydrocarbon is firstly released by combustion reaction and it will be converted into thermal energy (E T ), which includes the energy in high temperature exhausted gas (E G ), and the radiation energy (E R ) and convective heat transfer energy (E H ) of external walls.Then the radiation energy (E R ) of external walls will be transformed into electric energy (E E ) in the photovoltaic cells through filters.

Results and discussion
The overall performance of a MTPV system is contingent upon the energy output of the micro burner.Figure 2(a) illustrates the impact of intake flow velocity on the combustion efficiency of micro burners with different configurations of pin fin arrays.The combustor efficiency reduces with the intake flow inlet velocity increasing.Specifically, in the MCPFs-Circle, MCPFs-Square, MCPFs-Diamond, MCPFs-Triangle(A), and MCPFs-Triangle(B), the combustor efficiency decreases from 26.1%, 26.5%, 26.1%, 25.3%, and 26.5% to 9.0%, 8.6%, 9.1%, 9.1%, and 8.5%, respectively, as the intake flow speed increases from 4 to 12 m/s.Comparing various shapes of pin fin arrays in micro burners, when the intake flow speed is not higher than 8 m/s, the MCPFs-Triangle(B) exhibits the highest combustor efficiency, while the MCPFs-Triangle(A) has the lowest combustor efficiency.The MCPFs-Diamond and MCPFs-Circle are always in the middle sequence.The MCPFs-Diamond exhibits the highest combustor efficiency when the intake flow velocity exceeds 8 m/s.However, despite the high combustion efficiency observed in all micro combustors, the overall combustor efficiency is generally not very high.This is mainly attributed to a significant amount of energy being lost through convective heat loss from the surface and in the outlet exhaust gas, resulting in a relatively small portion of available radiation energy.Figure 3(a) illustrates the impact of different pin fin styles on the average outer wall temperature.With a growth of the flow velocity, the average outer wall temperature initially increases and then decreases.At 8 m/s, the MCPFs-Circle, MCPFs-Square, MCPFs-Triangle(A), and MCPFs-Triangle(B) reach their maximum average outer wall temperature values of 1230.5 K, 1247.9K, 1230.8K, and 1249.1 K.However, the MCPFs-Diamond exhibits a maximum outer wall temperature value of 1255.3K at an inflow speed of 10 m/s.This phenomenon is attributed to the interaction between the actual H 2 combustion flow rate and thermal performance between the gas mixture and inner walls.As reported in our previous study [5], the flame center shifts towards the outlet with an increase of inflow speed, which is detrimental to heat transfer at the front of the micro combustor.This phenomenon has been validated in other literature sources as well.
Fig. 3(b) shows the influence of inflow speed on the temperature range of the radiation walls in micro burners with various shapes of pin fin arrays.It is observed that the temperature range in all micro burners increases with velocity increasing.The temperature range in MCPFs-Circle, MCPFs-Square, MCPFs-Diamond, MCPFs-Triangle(A), and MCPFs-Triangle(B) increases from 287.5 K, 291.0 K, 284.6 K, 276.3 K, and 287.6 K to 682.3 K, 732.0 K, 723.1 K, 724.2 K, and 748.7 K, respectively, with the inlet speed increasing from 4 to 12 m/s.With the increase in inlet velocity, the maximum temperature of outer walls increases, while the minimum temperature of outer walls decreases for all micro combustors.Moreover, the high-temperature region and flame region in all micro combustors move towards the outlet, which weakens the heat transfer performance, resulting in a temperature reduction at the front of the combustor.Therefore, the minimum temperature of outer walls in all micro combustors decreases.Additionally, the growth in inflow speed leads to a higher actual hydrogen combustion mass flow rate, resulting in the release and conversion of more thermal energy from chemical energy and an increase in the maximum temperature of outer walls in all micro combustors.These results indicate that the uniformity of temperature of outer walls worsens as the inlet velocity increases.Moreover, the MCPFs-Diamond exhibits the lowest temperature range among all micro combustors when the inlet velocity is below 10 m/s.
The non-dimensional temperature uniformity coefficient (T NDT ) takes into account both the temperature range and mean square deviation of surface temperature to provide a comprehensive measure of temperature uniformity.As the inlet velocity increases, the non-dimensional temperature uniformity coefficient increases in all micro combustors with different pin fin arrays.At low inlet velocities, there is no significant difference in the non-dimensional temperature uniformity coefficient among the micro combustors.This corresponds with the temperature distribution of each micro combustor, which shows no significant difference at low inlet velocities.However, when the inlet velocity is below 12 m/s, the MCPFs-Diamond exhibits the lowest non-dimensional temperature uniformity coefficient, indicating that it has the best temperature uniformity distribution at these velocities.

Figure 2 (
b) depicts the available radiation energy at various intake velocities within micro combustors equipped with distinct styles of pin fins.It is observed that as the inflow velocity increases, the available radiation energy initially increases and then decreases in all micro combustors.At 8 m/s, the MCPFs-Circle, MCPFs-Square, MCPFs-Triangle(A), and MCPFs-Triangle(B) achieve their maximum available radiation energy of 40.3 W, 42.6 W, 40.3 W, and 42.8 W, respectively.The MCPFs-Diamond exhibits its highest available radiation energy of 43.6 W at 10 m/s.When the intake flow velocity does not exceed 8 m/s, the MCPFs-Triangle(B) attains the highest available radiation energy, followed by the MCPFs-Square, MCPFs-Diamond, MCPFs-Circle, and MCPFs-Triangle(A).Conversely, when the intake flow velocity surpasses 8 m/s, the MCPFs-Diamond demonstrates the highest available radiation energy.

Figure 2 .
Figure 2. Effect of pin fins styles on energy output in micro combustors.(a) Combustor efficiency and (b) radiation energy.

Figure 3 .
Figure 3. Thermal characteristics of micro combustors with different shapes of pin fin arrays at different inlet velocities of premixed gas.(a) Average temperature of external walls; (b) Temperature range; (c) Non-dimensional temperature uniformity coefficient.4.ConclusionsThe MCPF-Circle has the largest high temperature region, followed by the MCPFs-Diamond, MCPFs-Triangle(A) and MCPFs-Square.While the size of the high temperature region in MCPFs-Triangle(B) is smallest.The MCPFs-Diamond exhibits the best comprehensive performance at the inflow velocity of 10 m/s.Its average outer wall temperature and available radiation energy can reach up to 1255.3K and 43.6 W, respectively.Meanwhile, its temperature distribution is the most uniform among all micro combustor.