Numerical Optimization of the Nozzle Air Inlet Structure in H 2 -CH 4 Micromix Combustor

As the world seeks to move away from fossil fuels, gas turbines are being asked to do something they were never designed to do: burn hydrogen alongside natural gas. Hydrogen is a clean fuel that produces no carbon dioxide when it burns, but it is also incredibly reactive. When mixed with air, it burns so quickly and fiercely that it can cause the flame to flash back into the fuel supply or create intense, localized hot spots that generate harmful nitrogen oxides. To solve this, engineers have developed a concept called micromix combustion. Instead of one large flame, this approach uses hundreds of tiny channels to create a vast field of microscopic flames. By breaking the fuel and air into small, separate jets, the mixture happens almost instantly, preventing the formation of hot spots and keeping emissions low. However, for this system to work, the fuel and air must be mixed with extreme precision before they ever ignite. If the mixture is uneven, the benefits of the tiny flames disappear.

A team of researchers from China's National Energy Group and research institutes set out to perfect the design of the nozzle that delivers this mixture. Their goal was to find the best way to arrange the holes where air enters the nozzle to ensure it blends perfectly with the hydrogen and methane fuel. They did not build a physical prototype to test in a lab; instead, they used powerful computer simulations to model the flow of gases. These simulations allowed them to watch how air and fuel interact inside the nozzle, tracking how the gases swirl, collide, and mix under different conditions. The researchers focused on a specific type of nozzle where air enters from the side walls and collides with fuel coming from the bottom, creating a turbulent environment that forces the two gases together.

The team began by testing how the number of air holes affected the mixing process. They compared a simple design with a single layer of four holes against more complex arrangements with eight and twelve holes. They found that adding more layers of holes created a chain reaction of swirling air currents. When air shoots out from the side, it blocks the upward path of the fuel, forcing the fuel to swirl around and creating a vortex, or a spinning pocket of air, near the bottom. With a single layer of holes, this swirling motion dies out quickly. But with two or three layers, the air from the upper holes catches the swirling motion from the lower ones, creating a second, third, and even more vortices as the mixture moves upward. These multiple layers of swirling air stretch and fold the fuel and air together, much like kneading dough, ensuring they are thoroughly blended before they reach the end of the nozzle. The researchers discovered that while twelve holes offered a slight improvement over eight, the extra complexity and manufacturing difficulty were not worth the tiny gain in performance. They settled on a double-layer design with eight holes as the most efficient balance.

Next, the researchers looked at the size of those holes. In their initial design, all the air holes were the same size, but they noticed that the air pressure inside the nozzle was not uniform. The air entering from the bottom holes faced more resistance and moved with less force than the air entering from the top. To fix this, they tried a "graded" design where the holes at the bottom were made larger and the holes at the top were made smaller. This simple change redistributed the momentum of the air. By making the bottom holes larger, more air was pushed through them with greater force, strengthening the swirling vortex at the bottom where the fuel first enters. This stronger swirl pulled the fuel into the air more effectively, eliminating pockets of unburned fuel that had lingered near the bottom in the uniform designs. The simulations showed that this graded approach improved the mixing quality significantly, pushing the uniformity of the mixture to nearly perfect levels.

The team then tested whether this optimized nozzle could handle different amounts of hydrogen. They simulated conditions where the fuel was pure natural gas, a mix with 60% hydrogen, and even pure hydrogen. The results showed that the new design worked exceptionally well for blends containing up to 60% hydrogen. In these scenarios, the mixing remained uniform and stable. However, when they pushed the hydrogen content to 100%, the system began to struggle. Pure hydrogen requires a much higher flow rate to deliver the same amount of energy, which means the fuel shoots out of the nozzle at a much higher speed. This high-speed fuel jet became too powerful, punching straight through the air currents instead of being caught and swirled by them. The delicate vortex system that held the mixture together collapsed, and the mixing quality dropped. This finding suggests that while the nozzle is highly effective for the transition period of adding hydrogen to gas, it may need further adjustments to handle pure hydrogen.

Finally, the researchers realized that the nozzle did not need to be as long as they had originally built it in their models. The simulations showed that the mixing happened very quickly; by the time the gases traveled 60 millimeters up the nozzle, they were already 97% mixed. The remaining 40 millimeters of length added very little to the final quality of the mixture. This insight means that engineers could build a much shorter, more compact nozzle that saves space and reduces manufacturing costs without sacrificing performance. The study concludes that by carefully arranging the number and size of the air holes, it is possible to create a nozzle that mixes hydrogen and natural gas efficiently and safely, providing a practical step toward cleaner energy for gas turbines.