Effects of Pre-Chamber Internal Shape on CH ₄ -H ₂ Combustion Characteristics Using Rapid-Compression Expansion Machine Experiments and 3D-CFD Analysis 2023-24-0043

Pre-chamber (PC) natural gas and hydrogen (CH₄-H₂) combustion can improve thermal efficiency and greenhouse gas emissions from decarbonized stationary engines. However, the engine efficiency is worsened by prolonged combustion duration due to PC jet velocity extinction. This work investigates the impact of cylindrical PC internal shapes to increase its jet velocity and shorten combustion duration. A rapid compression and expansion machine (RCEM) is used to investigate the combustion characteristics of premixed CH₄ gas. The combustion images are recorded using a high-speed camera of 10,000 fps. The experiments are conducted using two types of long PC shapes with diameters φ=4 mm (hereafter, longφ4) and 5 mm (hereafter, long φ5), and their combustions are compared against a short PC shape (φ=12 mm). For all designs of the PC shapes, the PC holes are 6 with 2 mm in diameter. Initial recorded results using only CH₄ show that jet extinction does not occur using the short and long 5mm types. The combustion duration of φ=4 mm PC is the shortest compared to the short-type and 5 mm PC. With CH₄-H₂ blending (0%H₂, 10%H₂, 20%H₂) and 4 mm shape, the combustion durations of 10%H₂ and 20%H₂ can be shortened compared to the CH₄-only case (0%H₂). However, the jet extinction probability is not zero.

3D-CFD combustion simulations are performed using CONVERGE software, and a 1/6 sector-mesh, GRI-Mech 3.0, G-equation combustion, and law-of-wall models are utilized in conjunction with RNG k-epsilon turbulence model. Simulated in-cylinder pressure and burning rate are validated against the recorded data. PC jet velocity, turbulent kinetic energy (TKE) of the main chamber, PC jet temperature, total heat loss in PC, and PC heat loss rate of 12 mm, 4 mm, and 5 mm are compared. Experimental combustion images and 3D-CFD temperature distribution with and without H₂ blending are also reported. The results show that long PC types can accelerate the jet velocity and shorten combustion duration. The jet extinction can be prevented by designing a small PC diameter area larger than the nozzle’s cross-sectional area. With H₂ blending, laminar burning velocity increases, and the jet ejection timing can be advanced. The result shows that the combustion duration can be shortened by 15 degrees using CH₄-20%H₂ against CH₄ only.