## 1 Introduction  
According to the International Energy Agency's report \"Global Methane Tracker\", methane emissions from the energy sector reached nearly 130 million tons in 2023, accounting for over one-third of anthropogenic emissions, second only to the agriculture sector [1]. These emissions primarily arise from methane escaping during coal mining and unintentional emissions in the oil and gas sectors [2]. The gas escape from coal mines accounts for approximately 40% of the country's total methane emissions in China [3]. Therefore, the implementation of low-concentration gas utilization has become an important measure to reduce greenhouse gas emissions, and achieving a uniform gas mixture is essential for the effectiveness of various gas utilization technologies. In modern industrial applications, mixing is a critical process, particularly in chemical production and environmental engineering. However, traditional mixing methods struggle to achieve ideal results for high-flow-rate, high-turbulence flows and low-viscosity gases. When handling shear-insensitive gaseous media such as hydrogen or methane, mixing operations are typically conducted under turbulent flow conditions. Nevertheless, inadequate mixing in high Reynolds number turbulent states may lead to unstable flow patterns [4; 5], localized concentration anomalies (excessive or insufficient), increased system energy loss, reduced reaction rates, or potential safety hazards. To address these challenges, enhancing turbulent mixing efficiency through rational structural arrangements of mixing elements while strengthening momentum and mass transfer is critically important [6]. Compared with dynamic mixers [7], static mixers have the advantages of a simple structure and no moving parts, and are suitable for continuous mixing processes. With advancements in static mixer research, particularly improvements to mixing elements, their applications have expanded from traditional solid-liquid and liquid-liquid mixing to enhanced heat transfer, gas-liquid mixing, and gas-gas mixing [8]. Compared to conventional gas mixing methods such as pipeline blending [9; 10], nozzle mixing [11; 12], and venturi tube mixing [13], static mixers demonstrate superior overall performance in balancing energy loss and mixing uniformity [14]. The gas mixing performance of static mixers primarily depends on the geometric shape of elements, geometric parameters, swirl positioning, rotational orientation, and structural layouts. [PERSON] et al. [15] demonstrated that dynamic mixers require additional energy input while delivering mediocre mixing efficiency and lower safety performance. Therefore, static mixers are generally preferred for blending hazardous gases like methane and natural gas. [PERSON] et al. [7] showed that during gas mixing, SK-type and SX-type static mixers exhibit higher turbulent kinetic energy and turbulence intensity. SK-type static mixers demonstrate maximum shear strength, while HEV-type static mixers achieve the highest shear efficiency. However, HEV-type static mixers underperform in mixing effectiveness compared to the other three types, particularly under short mixing distances (<5D). Torque-modified mixing elements prove more suitable for gas mixing processes. Existing low-concentration methane utilization systems employ static helical mixing structures [16; 17], yet blending uniformity remains suboptimal. [PERSON] et al. [18] utilized numerical simulations to study hydrogen-methane blending processes in follow-flow hydrogen mixing equipment with different static mixer types and operating conditions. The results indicate that turbulence-enhancing elements in mixers decisively influence blending uniformity; the proper arrangement of these elements significantly improves mixing performance [19]. Current helical mixing element configurations in static mixers lack systematic design principles [20], with existing helical mixers demonstrating unsatisfactory blending uniformity.