Single Stage Fan - GasTurb

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Single Stage Fan

Modern high bypass turbofan engines have single stage fans with a low hub-tip radius ratio. The variability of the bypass ratio can be taken into account with a split map. However, getting the data for a split map is a complex and expensive task.

An alternative single stage fan performance description employs the hypothesis that the overall fan performance is not affected by variations in bypass ratio. The overall fan performance map is completed by an additional table with core stream efficiency.

The enhanced single stage fan map is used as follows:

During cycle design calculations, when scaling the map, the bypass ratio as well as the pressure ratio and efficiencies for the core and bypass streams are known. Given or assumed values for fan tip speed, hub-tip radius ratio and fan inlet Mach number yield the core stream velocity triangle. The rotor blade exit flow angle from this triangle remains the same in all other operating conditions.

During off-design simulations, the core flow velocity triangle analysis with known rotor blade exit angle yields the work done on the core stream. The pressure ratio is calculated from this work and the efficiency read from the core stream efficiency table mentioned above. Finally, the bypass stream pressure ratio and efficiency are calculated from the overall map and the core stream data employing the actual bypass ratio.

Smooth C 8.3 makes the creation of a core efficiency table simple. After smoothing the overall fan map click the Go to Secondary Map button.You will get an estimate for the core efficiency map which you can modify easily. Due to the low Mach numbers the core stream efficiency values will be higher than the overall efficiency of the fan. Furthermore, the efficiency islands will be wider.

Compare the overall fan map with overall efficiency contour lines and with core efficiency contour lines to see the difference.

More details about this enhanced fan performance model can be found in:

An Enhanced Off-Design Performance Model for Single Stage Fans
J. Kurzke
ASME 2014-GT-26449

 
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