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gsphelp  Fan

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fan

 

The fan component is used to represent the fan or low pressure compressor in turbofan engines including the splitter dividing the fan exit flow in a duct- (or "bypass") and core flow. The compression process is modelled entirely equal to the compressor component, except that separate design data (pressure ratio and efficiency) and compressor maps for the duct- and core flows are used.

 

The fan has similar bleed flow functionality as the compressor component. The fan's bleed flow functionality applies to the core flow (no bypass bleed functionality).

 

The bypass ratio determines the ratio of the duct and core mass flows and is user specified in the design point (Design bypass ratio). The off-design bypass ratio usually deviates from the design value depending on the thermodynamic state of the gas turbine system (and is calculated as a state variable). In the design point, the duct flow is compressed using the duct design data and duct map; the core flow using the core design data and map. With deviating off-design bypass ratios, the "dividing streamline" between the two flows will tend to move, depending on effects such as the position of the splitter behind the fan. Since the core and duct maps represent geometrical parts of the fan, the distribution of the entry flow to the core and duct maps then may well have to change for best simulation accuracy. This change is controlled by the user specified Cf factor in the General tab sheet. The Cf factor ranges between 0 and 1:

Cf = 0 implies the dividing streamline is not affected by the bypass-ratio at all and flow division between core and duct maps remains unaffected by deviating off-design bypass ratios. This case could well be thought of as a flow "splitter" at infinite distance from the fan exit.
Cf = 1 represents the other extreme with the flow division between the two maps being entirely proportional to the bypass-ratio. This case seems hard to realise in practice, but a flow splitter right behind the fan would require a Cf at least larger than 0.

In practice, best results are obtained with Cf values close to 0.

 

For both duct and core flow paths options are available off-design performance prediction. Using the radiogroup box one of the following options can be chosen:

Map

The standard option uses a map similar to the compressor map of type component map. The map file consists of tables with corrected mass flow, efficiency and pressure ratio as a function of corrected normalised rotational speed and beta. The map operating point corresponding to the design operating point is specified using the map design rotational speed and beta values. Click the Graph button in the Map tab sheet to view the map graphically and note the yellow rectangle which can be used to move the map design point. In the map graph also the beta-lines can be show after activating the appropriate item in the Options menu.

Operating Line

 This advanced option allows the engine modeller to use the compressor operating line. In case the maps of the compressors cannot be obtained from the manufacturer, the operating line of an existing engine can inserted (or a predicted operating line for a new engine). When using this option note that the mass flow error equation for the turbine needs to be deactivated to obtain a solvable equation system. In case the operating line is the actual running line of the engine, the error will remain very small (can be visualised through the option Werror on the turbine output tab sheet). Using a predicted operating line, the error increases the further the operating point is located from the real operating point.

No map (DP only)

This advanced option allows the engine modeller to calculate design point performance without having to specify an off-design component map.

 

The option Reset map scale factors sets the map scaling factors to 1 during off-design calculations. This enables the engine modeller to match existing turbo components using their actual/original maps.

 

The DownStream Calculation option is to specify which downstream gas path (duct or core) is calculated first. This is for cases where bleeds must be calculated in one downstream gas path that is used in the other. For example: is fan duct bleed is inserted in core flow (e.g. behind LPT), the duct flow must first be calculated (DownStream Calculation = Duct first).

In the case of both flows from duct to core and vice versa, the problem can not be made 'determinate' anymore and splitter and mixer components must be used (instead of bleeds/sec. airflows) to model the secondary flows, like in the ABFAN_HEM model.

 


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