Shrouded Axial Fan Stall Margin via Casing Wedges
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Solution Overview
Problem
Shrouded axial fans face premature stall due to leakage flow recirculation, which limits their stable operating range and application versatility, as existing casing treatments are often unsuitable or ineffective for shrouded fans.
Innovation Solution
A shrouded fan design featuring a fan shroud with an S-shaped cross-section and radially inwardly extending casing wedges that create controlled clearance gaps to manage recirculating flow, reducing swirl and stabilizing the airflow by shaping the casing wedges to conform to the shroud and controlling the reinjection angle and mass flow of recirculation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If leakage flow is reduced in shrouded axial fans, then stall margin is improved, but existing casing treatments are unsuitable or ineffective for shrouded fans
Solution Approach 1:
The patent applies different treatment configurations at different locations around the casing. Specifically, it uses a combination of radial clearance variations and axial wedge gaps positioned at specific angular locations to address the leakage flow problem locally where it occurs most severely, rather than applying a uniform treatment around the entire casing.
Solution Approach 2:
The casing treatment is divided into discrete segments or wedges rather than being a continuous structure. These segmented wedge elements can be independently positioned and sized to control the recirculating flow at different angular positions, making the treatment adaptable to the specific flow patterns in shrouded fans.
2Stability of the object's composition
If recirculating flow is controlled to reduce swirl, then flow stability is improved, but device complexity increases due to S-shaped shroud and wedge configurations
Solution Approach 1:
The shroud is given an S-shaped cross-sectional contour with curved surfaces that guide the recirculating flow smoothly. This curvature design naturally directs the leakage flow to rejoin the main flow at a more favorable angle, reducing swirl and improving stability without requiring additional active control mechanisms.
Solution Approach 2:
The patent modifies geometric parameters of the shroud and casing, specifically the S-shaped cross-sectional dimensions and the wedge gap angles, to optimize flow control. By carefully selecting these dimensional parameters, the design achieves flow stabilization through passive geometric means rather than complex active systems.
3Reliability
If radial clearance and axial wedge gaps are optimized, then leakage flow management is improved, but manufacturing precision requirements increase
Solution Approach 1:
The casing wedges are pre-positioned during assembly to establish the correct radial and axial gap dimensions before the fan begins operation. This preliminary positioning ensures that the critical clearance gaps are set to the optimal values, reducing the need for tight tolerances on the rotating components themselves.
Solution Approach 2:
Rather than requiring uniform high precision across all surfaces, the patent focuses precision requirements on specific critical surfaces where the radial and axial wedge gaps are formed. Other non-critical surfaces can be manufactured with more relaxed tolerances, reducing overall manufacturing complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enhances the stall margin and operational range of shrouded axial fans by stabilizing the airflow and reducing premature stall, allowing for broader application in HVAC systems and other environments.
Implementation Method 1
leakage flow from the high pressure side to low pressure side of the fan blades... This leakage flow is reingested into the fan... As the leakage flow reenters the fan, it gives rise to rotating swirl flow and instabilities
Implementation Method 2
As the leakage flow reenters the fan, it gives rise to rotating swirl flow and instabilities at the blade tip
Data Source
AI summary
A fan assembly includes a shrouded fan rotor having fan blades extending from a hub and rotatable about a central axis. A fan shroud extends circumferentially around the fan rotor and secured to the fan blades. The shroud includes a first axially extending annular portion secured to the fan blades, a second axially extending annular portion radially outwardly spaced from the first axially extending annular portion, and a third portion connecting the first and second axially extending annular portions. A casing is located circumferentially around the fan shroud defining a radial clearance between the casing and the fan shroud, and includes casing wedges extending from a radially inboard surface of the casing toward the shroud and defining a radial wedge gap between a first wedge surface and the shroud and an axial wedge gap between a second wedge surface and an upstream end of the fan shroud.