The Effect of Ultrapolish on a Transonic Axial Rotor

Back-to-back testing has been done using NASA fan rotor 67 in the Glenn Research Center W8 Axial Compressor Test Facility. The rotor was baseline tested with a normal industrial RMS surface finish of 0.5-0.6 μm (20-24 microinches) at 60, 80 and 100% of design speed. At design speed the tip relative Mach number was 1.38. The blades were then removed from the facility and ultrapolished to a surface finish of 0.125 μm (5 microinch) or less and retested. At 100% speed near the design point, the ultrapolished blades showed approximately 0.3 0.5% increase in adiabatic efficiency. The difference was greater near maximum flow. Due to increased relative measurement error at 60 and 80% speed, the performance difference between the normal and ultrapolished blades was indeterminate at these speeds. INTRODUCTION As fuel costs continue to increase, the industries that use gas turbine engines have a pressing need to improve in-service performance. This need is especially true in the airline industry, for which fuel burn and performance retention in high by-pass ratio turbofan engines is a major concern. Any approach to improve the efficiency of present equipment will be considered if cost effective. Previous research by Suder et.al. [1] has shown that surface finish of axial fan and compressor blading is very important. This earlier research indicates that there is a significant performance penalty if the surface finish degrades from an industry-standard RMS finish of 0.5-0.6 μm (20-24 microinches) to a finish of 2.5-3.0 μm (100-125 microinches). Figure 1, taken from Suder et.al. [1], shows the impact of surface roughness of 2.5-3.0 μm on the design speed performance of a transonic compressor rotor. The six configurations shown in this figure differ in the location of roughness on the blade surface. One key finding from this work was that surface roughness on the leading edge of the blade, which exists for configurations G, H, and I, causes the most severe performance penalty. Suder’s work leads to the question of whether or not a gain in efficiency could be achieved if axial fan and compressor blade surfaces are polished to finishes less than 0.5-0.6 μm. According to Koch and Smith [2], the blade surface is considered hydraulically smooth for equivalent sand roughness Reynolds number less than 90. In theory, a better (lower RMS) finish than hydraulically smooth offers no benefit to performance. The standard industry surface finish of 0.5-0.6 μm is typically near or less than a roughness Reynolds number of 90 at high altitude cruise operating conditions where most fuel burn occurs. At the test conditions used in this work, a surface finish of 0.125 μm (5.0 microinches) corresponds to a roughness Reynolds number of approximately 20. Therefore, there is some justified skepticism as to whether ultrapolish, defined as a surface finish of 0.125 μm or better, can improve efficiency. However, occasional airline trials with ultrapolished blading have indicated that ultrapolishing is beneficial in that lower fuel burn is observed during acceptance testing after engine refurbishment. To verify and quantify the impact of ultrapolished surface finish on blade performance, back-to-back testing was done in the NASA Glenn Research Center’s W8 Axial Compressor Test Facility using NASA fan rotor 67. Baseline testing at a nominal surface finish of 0.5 μm (20 microinches) was followed by testing with the blades This is a preprint or reprint of a paper presented at a conference. Since revisions may be made prior to formal publication, this version is made available with the understanding that it will not be cited or reproduced without the permission of the author. https://ntrs.nasa.gov/search.jsp?R=20050196726 2018-03-25T18:04:34+00:00Z