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The combustion characteristics of the swirl micro combustor with twisted vanes (Swirl-MC-TV) and the conventional micro combustor (Conventional-MC) are investigated and compared under different inlet velocities (8–40 m/s), wall materials (quartz, steel, and SiC), and equivalence ratios (0.6–1.4). The results show that the larger area of recirculation zones and the stronger recirculation intensity are the key factors for Swirl-MC-TV to stable combustion. When the inlet velocity is 40 m/s, compared with the Conventional-MC, the wall heat loss of the Swirl-MC-TV is reduced by 15.9%, and the reaction heat and combustion efficiency of the Swirl-MC-TV are increased by 17.5% and 5.9%, respectively. When the wall materials of steel and SiC, combustors have a better preheating effect and higher combustion intensity. When the equivalence ratio is greater than 0.6, the wall heat loss of Swirl-MC-TV is larger but the combustion efficiency and the reaction intensity are still higher than Conventional-MC.

Aimed at improving the energy output performance of the Microthermal Photovoltaic (MTPV) system, it is necessary to optimize the structure of the micro combustor. In this paper, micro combustor with in-line pin fins arrays (MCIPF) and micro combustor with both end-line pin fins arrays (MCEPF) were presented to realize the efficient combustion and heat transfer enhancement, and the influence of inlet velocity, equivalent ratio, and materials on thermal performance was investigated. The results showed that pin fins embedding is beneficial to improving combustion, and the combustion efficiency of MCIPF and MCEPF reaches 98.5% and 98.7%, which is significantly higher than that of the conventional cylindrical combustor (MCC). However, with the increase of inlet velocity from 8 m/s to 14 m/s, MCIPF exhibits the highest external wall temperature with a range of (1302–1386 K), while MCEPF maintains the best temperature uniformity. As the inlet velocity increases to 10 m/s, the external wall temperature and temperature uniformity reach the optimum. Besides, under the conditions of different equivalence ratios, both external wall temperature and heat flux increases first and then decreases, meanwhile the temperature uniformity of MCEPF is significantly improved compared with that of MCIPF, they all exhibit the highest external wall temperature with an equivalence ratio of 1.1, and the thermal performance is greatly enhanced. By comparing the heat transfer performance of combustors with different materials based on MCEPF, it is interesting to find that the application of high thermal conductivity materials can not only increase the external wall temperature, but also improve the temperature uniformity. Therefore, materials with high thermal conductivity such as Aluminum, Red Copper and Silicon Carbide should be selected for application in micro combustors and their components. The current work provides a new design method for the enhanced heat transfer of the micro combustor.

Micro-scale combustion is facing the problems of ignition difficulty, combustion instability, and low combustion efficiency. Therefore, it is necessary to improve the combustion characteristics in micro-combustor to expand the application range of micro-combustor. Focusing on the problem of the weak preheating effect of the micro cavity-combustor, the guide vanes are constructed to enhance the combustion near the cavity, and further combine with the bluff body to enhance combustion. The combustion characteristics of three types of cavity combustors with guide vanes (CCGV), bluff body (CCBB), bluff body, and guide vanes (CCGB) respectively are compared and analyzed under different inlet velocities (8–32 m/s), equivalence ratios (0.6–1.4), and wall materials (quartz glass, steel, and SiC). Results show that the guide vanes can greatly improve the combustion intensity near the cavity and improve the combustion stability of the cavity combustor. The combustion efficiency of CCGV and CCGB are increased by 43.04% and 85.96% respectively than CCBB when the inlet velocity is 32 m/s. The reaction heat of CCGV is 244.5 W when inlet velocity is 32 m/s, which is 0.55 times and 1.57 times that of CCGV and CCBB, respectively. The temperature uniformity and mean temperature of the outer wall of CCGV and CCGB both are better than that of CCGB. The combustion efficiency of CCGB is the highest among three combustors under the same equivalence ratio, especially when the equivalence ratio is less than 1. The reaction intensity in the cavity of the CCGV and CCGB with steel wall material is highest than that of the combustor with the other two wall materials. Wall materials with high thermal conductivity have a better preheating effect. Compared with quartz as the wall material, the mean temperature of the external wall of CCGV and CCGB using steel and silicon carbide as the wall material both increase by more than 130 K, and the wall heat loss both increase by more than 50%.

The energy output and energy conversion efficiency of MTPV system are relatively low due to the energy loss. In order to improve the energy output of micro-thermophotovoltaic (MTPV) system, the internal straight and spiral fins are introduced into the micro combustor. The impact of hydrogen mass flow rate, equivalence ratio, and materials on the thermal performance are investigated. The increase of hydrogen mass flow rate brings higher average outer wall temperature, but the temperature difference also increases and the temperature uniformity becomes worse. The equivalence ratio of 1 is suggested to obtain higher average outer wall temperature and better temperature uniformity. The materials with higher thermal conductivity can obtain better thermal performance. Meanwhile, the higher thermal conductivity can also reduce the impact of introduction of internal fins.

Targeting at decreasing the pressure drop in the micro heat sink, a gradient distribution pin fin arrays and narrow slots (GPN) is proposed. In addition, in order to further reduce the pressure drop of the micro heat sink, three improved are proposed, including GPN48, GPN2244, GPN264 by changed the number and position distribution of the pin fin arrays in the narrow slots. The results show that the pressure drop of the three improved gradient distribution pin fin arrays and narrow slots decreases by 28.7% ∼ 33.4% at a certain volumetric flow rate (Qv=120 ml/min, Re =563) when comparing with the conventional gradient distribution. Furthermore, when Re ranged from 188 to 1125, the maximum wall temperature of the three improved gradient distribution pin fin arrays and narrow slots decreases by 4.4 K ∼ 26.6 K when comparing with the gradient distribution (GPN). When the Re =563, the heat flux limit of GPN is only 82 W/cm2, while GPN48, GPN2244 and GPN264 reached 109 W/cm2, 114 W/cm2 and 120 W/cm2 respectively. Especially, comparing with GPN, the average heat transfer coefficient of GPN48, GPN2244 and GPN264 increases by 11.9% ∼ 23.0% (Re =118), which greatly improves the application reliability in the micro heat sink.

Micro-scale combustion is facing the problems of ignition difficulty, combustion instability, and low combustion efficiency. Therefore, it is necessary to improve the combustion characteristics in micro-combustor to expand the application range of micro-combustor. Based on the traditional cavity combustor (TCC), a cavity combustor with guide vanes (CCGV) was constructed to further strengthen the effect of the cavity. The combustion characteristics of TCC and CCGV at different inlet velocities and equivalence ratios were compared and analyzed. And the combustion characteristics of CCGV were researched under different guide vane geometries. The temperature and the area of high-temperature zones in the cavity of CCGV are significantly higher and bigger than the TCC respectively. The cavity of CCGV has a better preheating and ignition effect than TCC. When the inlet velocity is 8 m/s, the maximum temperature in cavities of CCGV is 326 K higher than TCC. When the equivalence ratio increases from 0.6 to 1.2, the blow-off limits of TCC and CCGV increase from 3 m/s and 33 m/s to 14 m/s and 121.5 m/s, respectively. The CCGV with S/L3 = 0.12/0.4 and α = 45° has a better practical application value. When the inlet velocity is 14 m/s, compared with S/L3 = 0.12/0.4, the combustion efficiency of S/L3 = 0.15/0.5, S/L3 = 0.18/0.6 and S/L3 = 0.21/0.7 increases by 1.21 %, 2.25 % and 2.61 %, respectively, while the pressure loss increases by 3.79 %, 5.70 % and 13.68 %, respectively. When the inlet velocity increases are 20 m/s, the combustion efficiency and is 74.46 %, 87.97 %, and 87.51 %, respectively. The pressure loss of α = 60° is 4.35 % higher than that of α = 45°.

To improve temperature distribution is uneven in Y-shaped microchannel (Y) heat sink, connected Y-shaped microchannel (CY) is proposed and heat dissipation performance is compared. Results revealed that CY model has a lower peak temperature and makes temperature distribution more uniform due to two symmetrical vortexes are formed at both sides of branch structure, while there are still high temperature zones in the second stage channels under high heat flux. Therefore, the micro pin fins are embedded in CY heat sink (FCY) to examine heat transfer performance. Results illustrated that there is more pressure loss in FCY heat sinks, while temperature uniformity of FCY model is better than CY heat sink, which strengthens heat dissipation of the second stage channels. Heat transfer coefficients of FCY-1, FCY-2 and FCY-3 heat sinks are 1.12, 1.28 and 1.11 times than CY model, respectively. The composite performance factor, both FOM2/1 and FOM2/3 are greater than 1. The FCY-2 model demonstrates a good thermal and hydraulic performance can achieve the best cooling capacity.