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Discrepancies between predicted and measured results have been reported by users of ASHRAE Standard 8Z 1-1983, "Method of Testing Dynamic Characteristics of Propeller Fans-Aerodynamically Excited Fan Vibration and Critical Speeds/’ It was believed that these discrepancies were associated with the fact that the standard did not take into account the effects of centrifugal stiffening in determining propeller fan critical speeds° To determine whether this was true, ASHRAE initiated ASHRAE Re= search Project RP-477, "Development of a Method to Predict Vibration Response of Propeller Fans under Actual Operating Conditions." The objectives of this project were:

  1. Identify and modify, if necessary, a commercially available finite element program that runs on personal microcomputers and that takes into account centrifugal stiffening; and
  2. Use the program identified in (1) to help explain discrepancies that reportedly exist between propeller fan critical speeds predicted in accordance with ASHRAE Standard 87.1 and the corresponding critical speeds measured on the same fans in simulated operating conditions

This paper presents the technical bases for procedures that use measured spatial coordinates of specified key points on one flat fan blade and associated spider sector of a propeller fan to perform three geometric transformations: 1. place a camber, a screw, or both in a fan blade; 2. twist the spider; and & generate a specified number of identical blades. The procedures were included in a computer program that generates input source files that can be used by a commercially available, microcomputer-based, finite element program that can be used to perform vibration analyses of propeller fans.

 

Citation: ASHRAE Transactions, vol. 95, pt. 2, Vancouver, BC 1989