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The stall zone for the single axial fan and axial fans operated in parallel are shown in the figure. [4] The Figure shows the Stall Prone Areas differently for One fan and Two fans in parallel. [4] The following can be inferred from the graph : For the Fans operated in parallel, the performance is less when compared to the individual fans.
It includes separate axial fan factors and is aimed primarily at the designer of the air moving system. AMCA Publication 202 - Troubleshooting. Part 3 of the Fan Application Manual, helps to identify and correct problems with the performance and operation of the air moving system after fan installation.
Axi-symmetric stall, more commonly known as compressor surge; or pressure surge, is a complete breakdown in compression resulting in a reversal of flow and the violent expulsion of previously compressed air out through the engine intake, due to the compressor's inability to continue working against the already-compressed air behind it.
This arrangement introduces an additional gas leakage path compared to a front-fan configuration and was a problem with this engine with higher-pressure turbine gas leaking into the fan airflow. [47] An aft-fan configuration was later used for the General Electric GE36 UDF (propfan) demonstrator of the early 1980s.
Where noise is an issue, larger, slower-turning fans are quieter than smaller, faster fans that can move the same airflow. Fan noise has been found to be roughly proportional to the fifth power of fan speed; halving the speed reduces the noise by about 15 dB. [22] Axial fans may rotate at speeds of up to around 38,000 rpm for smaller sizes. [23]
A fan may have two maps, one for the bypass (i.e. outer) section and one for the inner section which typically has longer, flatter, speed lines. Military turbofans tend to have a much higher design fan pressure ratio than civil engines. Consequently, the final (mixed) nozzle is choked at all flight speeds, over most of the throttle range.
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There are two solutions to this problem: 1) Open up the cold nozzle area at low flight speeds, which moves the fan working line away from the surge. This has little effect on the position or slope of the surge line. OR 2) Make the effective surge line of the fan shallower by progressively reducing the pitch of the fan as the engine is throttled.