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In this work, numerical investigations of methane catalytic combustion in the opposed counter-flow microcombustor are conducted under various inlet velocities, equivalence ratios, and geometric parameters. The results indicate that the high temperature zone is mainly located at the front and middle parts of the reaction zone. With the increase of inlet velocity, both methane conversion and exhaust gas temperature decrease, while the methane concentration in the downstream area increases. Its maximum velocity limit is 2.9 m/s. Moreover, temperature step zones of opposed counter-flow are obviously located at the front and middle parts with different equivalence ratios. The combustion efficiency decreases slowly with the increase of equivalence ratios. More importantly, critical values about the geometric parameters are determined for keeping better thermal performance. It is concluded that inlet velocity limit and methane conversion rate can be significantly increased and the temperature distribution is more uniform via reducing inlet width L2 and inlet height H, increasing the length of the downstream parts L1 and the downstream entrance length L3. In general, the opposed counter-flow microcombustor with optimized structure has better combustion stability. This design offers another way for developing the opposed counter-flow microcombustor.

The excavation depth of foundation pits has been increasing along with the continuous development of underground space and high-rise buildings. As a result, traditional double-row vertical piles cannot meet the ground settlement and deflection requirements. This study proposed a double-row pile optimization method to extend the suitability of double-row retaining piles to greater excavation depth. The optimization model was established by adjusting the inclination angle of the front and rear piles. Physical scale model tests were performed to analyze the effect of the inclination angle on the pile head displacements and bending moments during excavations and step loadings using three schemes, namely, traditional double-row piles with vertical piles, double-row contiguous retaining piles with batter pile in the front row, and double-row contiguous retaining piles with batter pile in both rows. Numerical simulations were also conducted to verify the effectiveness of the inclination angle adjustment in optimizing the double-row piles. Results indicate that the increase in the displacement and bending moment of the double-row contiguous retaining batter piles is not evident during excavation and step loading when compared with those of the double-row vertical piles and the double-row contiguous retaining piles with batter pile in the front row. Thus, double-row contiguous retaining batter piles can be used in deep foundation pits. The tilt angle is also excessively small to reduce the lateral displacement of the foundation pit, and the optimal tilt angle is 8°-16°. Although the embedment depth can influence the deformation of the double-row contiguous retaining batter piles significantly, a critical embedment depth may be reached. The findings of this study can provide references for the optimization of double-row piles in foundation pits.