Among the design parameters of axial flow fans, the hub ratio is a key indicator of fan performance. It refers to the ratio of the hub diameter to the impeller diameter. This seemingly simple numerical relationship profoundly impacts the fan's air volume, air pressure, efficiency, and operational stability. Understanding the relationship between hub ratio and performance is crucial for fan selection, operating condition adaptation, and energy-efficient operation.

The hub ratio directly shapes the axial flow fan's performance. Fans with a small hub ratio (typically less than 0.3) have a larger impeller blade area, resulting in a wider airflow cross-section. This structure excels at handling high flow rates, pushing more air axially at the same speed. For example, axial flow fans with small hub ratios are commonly used in cooling tower ventilation systems. Their wide blades efficiently move large volumes of air through the heat dissipation packing, meeting the cooling tower's high air exchange requirements. However, the shortcomings of the small hub ratio are also very obvious: due to the large length of the blades, the centrifugal force generated during rotation requires extremely high blade strength, and in scenarios with high wind pressure requirements, the blades have limited work capacity and are prone to efficiency loss due to airflow separation.

Axial fans with a medium hub-to-hub ratio (0.3-0.5) exhibit more balanced performance characteristics. This ratio achieves a good balance between blade length and hub support strength, enabling them to deliver substantial air volume while maintaining a reasonable level of air pressure. Fans with a medium hub-to-hub ratio are widely used in applications such as subway tunnel ventilation and large factory ventilation, where both air flow and pressure are crucial. Their design advantage lies in more optimal force distribution at the blade root, which allows them to maintain high efficiency over a wide range of operating conditions through optimized blade airfoil curves. Test data from a subway ventilation system showed that an axial fan with a 0.42 hub-to-hub ratio achieved an efficiency of 82% at the designed air volume, with an efficiency drop of no more than 5% when the air volume fluctuated by +15%, demonstrating excellent adaptability to operating conditions.

Axial fans with a large hub-to-hub ratio (greater than 0.5) tend to offer high pressure at low flow rates. Because the hub occupies a larger proportion of the impeller, the blades are shorter, allowing them to withstand higher pressure loads during rotation. Large hub ratio fans excel in applications such as mine main ventilation and boiler induced draft, where high-resistance pipe networks must be overcome. Their blades are typically designed with a large mounting angle, which exerts greater compression on the airflow and generates higher static pressure. However, this structure narrows the airflow channel, accelerates flow velocity, and easily forms a turbulent boundary layer on the blade surface, which in turn reduces efficiency under high-flow conditions. Experiments have shown that increasing the hub ratio from 0.4 to 0.6 increases fan pressure by 30%, but reduces airflow at the efficiency point by approximately 25%.

The hub ratio also indirectly affects fan noise characteristics by affecting the ratio of blade tip speed to hub speed. Fans with small hub ratios have higher blade tip speeds, resulting in more severe airflow disturbances in the gap with the casing, which can easily generate high-frequency aerodynamic noise. Fans with large hub ratios, however, have shorter blades and lower tip speeds, resulting in predominantly low-frequency noise. However, airflow separation at the blade root and hub junction can cause low-frequency vibration noise. In sound-sensitive places such as hospital air-conditioning systems, medium hub ratio fans are usually selected and combined with acoustic cover designs to balance air volume, air pressure and noise control requirements.

It is worth noting that the choice of hub ratio needs to be coordinated with the motor power and speed parameters. If a fan with a small hub ratio is paired with a high-speed motor, the structure may fail due to excessive centrifugal force on the blades; if the speed of a fan with a large hub ratio is too low, it will not be able to play its high-pressure advantage. In engineering practice, the parameter combination under specific working conditions is often determined through the ternary optimization model of "hub ratio-speed-power". For example, in the ventilation system of an offshore platform, a compact fan needs to be used due to space limitations. During the design, a larger hub ratio (0.55-0.6) will be selected in combination with a high-speed motor to achieve the required wind pressure within a limited size. At the same time, structural safety is ensured through material upgrades (such as the use of aluminum alloy blades).