LOW ASPECT RATIO TRANSONIC ROTORS .2. INFLUENCE OF LOCATION OF MAXIMUM THICKNESS ON TRANSONIC COMPRESSOR PERFORMANCE

Transonic compressor rotor performance is sensitive to variations in s everal known design parameters. One such parameter is the chordwise lo cation of maximum thickness. This article reports on the design and ex perimental evaluation of two versions of a low aspect ratio transonic rotor that had the location of the tip blade section maximum thickness moved forward in two increments from the nominal 70 percent to 55 and 40 percent chord length, respectively. The original hub characteristi cs were preserved and the maximum thickness location was adjusted prop ortionately along the span. Although designed to satisfy identical des ign speed requirements, the experimental results reveal significant va riation in the performance of the rotors. At design speed, the rotor w ith its maximum thickness located at 55 percent chord length attains t he highest peak efficiency among the three rotors but has lowest flow rollback relative to the other two versions. To focus on current rugge dization issues for transonic blading (e.g., bird and ice ingestion), detailed comparison of test data and analysis to characterize the aero dynamic flow details responsible for the measured performance differen ces were confined to the two rotors with the most forward location of maximum thickness. A three-dimensional viscous flow analysis was used to identify the performance-enhancing features of the higher efficienc y rotor and to provide guidance in the interpretation of the experimen tal measurements. The computational results of the viscous analysis sh ow that the difference in performance between the two rotors can be at tributed to the higher shock losses that result from the increased lea ding edge ''wedge angle '' as the maximum thickness is moved closer to the leading edge. The test data and the three-dimensional viscous ana lysis also reveal that the higher efficiency rotor achieves the same s tatic pressure rise potential and loading at a higher flow level than its less efficient counterpart and this is responsible for its resulti ng lower flow rollback and apparent loss in stall margin. Comparison o f the peak efficiencies attained by the two rotors described in this a rticle with the baseline ruggedized rotor performance presented in par t 1 of this paper suggests the existence of an optimum maximum thickne ss location at 55 to 60 percent chord length for such low aspect ratio transonic rotors.