The inter stage reflux of a multi-stage centrifugal blower refers to the non ideal flow phenomenon in which gas fails to enter the inlet of the next stage impeller smoothly after the outlet of the previous stage impeller, and some of the airflow undergoes reverse vortex, separation or recirculation in the inter stage reflux device (guide) or flow channel. This phenomenon is essentially the result of the combined effect of aerodynamic mismatch and flow channel design defects, directly affecting the overall efficiency and operational stability of the machine.

Physical mechanism and genesis

Pressure gradient mismatch: The airflow at the outlet of the previous stage impeller has high kinetic energy and static pressure. If the suction at the inlet of the next stage impeller is insufficient or the inlet flow field is uneven, it will cause a local area to have a reverse pressure gradient, which will promote airflow backflow.

Boundary layer separation: If there is a sudden expansion, sharp angle turning or rough surface in the reflux channel, it is easy to cause boundary layer separation, forming a vortex zone and causing the mainstream airflow to deviate from the design path.

Excessive clearance: If the axial or radial clearance between the impeller and the reflux device exceeds the design threshold (usually recommended to be controlled at 3% -5% of the impeller outlet width), leakage flow will occur, forming a local reflux channel.

Geometric mismatch in the flow channel: The angle of the guide vanes of the reflux device is not coordinated with the absolute velocity direction of the impeller outlet (such as an inlet placement angle deviation of>30 °), resulting in impact losses and secondary flow, exacerbating reflux.

Negative impact on performance

|Dimension of Influence | Specific Performance|

|Efficiency decline | Reflux airflow does not participate in effective compression, kinetic energy is converted into ineffective vortex kinetic energy, and the isentropic efficiency of the whole machine decreases by 5% -15%|

|Increased noise | Collision between backflow vortex and high-speed impeller generates wideband aerodynamic noise, with peak values exceeding 85 dB (A)|

|Deterioration of stability | Local backflow induces blade channel shedding, which may trigger the precursor of surge, resulting in significant fluctuations in flow rate, pressure, and current|

|Increased vibration | The asymmetric flow field caused by backflow results in an unbalanced force on the rotor, an increase in bearing load, and vibration values that can exceed the standard by 2-3 times|

Engineering Suppression and Optimization Design

Optimization of reflux structure: The combination design of wing shaped guide vanes and three-dimensional flow meridians is adopted to ensure smooth transition of flow channel curvature and reduce separation points.

Precise control of clearance: The axial clearance between the impeller and the reflux device is controlled at 3% -5% of the impeller outlet width to avoid leakage and reflux.

Guide vane angle matching: The placement angle of the guide vane inlet should be consistent with the absolute velocity direction of the impeller outlet, with a recommended range of 15 ° -30 °, and the outlet diffusion angle should be ≤ 8 °.

Smooth treatment of flow channel: Adopting a parabolic gradient cross-section design to avoid sudden expansion and contraction and reduce eddy current losses.

Materials and Manufacturing Process: Use electroslag casting special steel (such as 0Cr13Ni4Mo) to enhance cavitation resistance and ensure that the surface roughness Ra of the flow channel is ≤ 1.6 μ m.

Current research and technological trends

In international research, this phenomenon is referred to as "inter stage recirculation" or "flow separation in inter stage passage" and has been widely modeled and analyzed in high-pressure compressors.

Modern design generally combines CFD simulation (such as CFX, ANSYS Fluent) to numerically predict the inter stage flow field and achieve iterative optimization of the structure.

New technologies such as adjustable guide vanes and active flow control (such as wall blowing and suction) have been experimentally validated to reduce reflux losses by over 20%.

The current mainstream industrial grade multi-stage centrifugal blowers (such as in the fields of sewage treatment and chemical gas production) have widely adopted the above optimized design to control the inter stage reflux within an acceptable range, ensuring continuous operation for more than 1 year and noise levels below 85 dB (A).

Shandong Ruidian Environmental Protection Technology Co., Ltd. is located in the high-end equipment manufacturing base of Luozhuang District, Linyi City. It is a modern enterprise that focuses on the research and development, design, production, and manufacturing of multi-stage centrifugal blowers, centrifugal vacuum pumps, Roots blowers, pneumatic conveying systems, and soil remediation integrated special equipment. Its telephone number is 13287120066.