Rotor blade pressure measurement in a high speed axial compressor using pressure and temperature sensitive paints

Pressure and temperature sensitive paints have been utilized for the measurement of blade surface pressure and temperature distributions in a high speed axial compressor environment. Four blades were painted, two with temperature sensitive paints and two with pressure sensitive paints. This combination allows temperature distributions to be accounted for when determining the blade suction surface pressure distribution. Measurements were taken and pressure maps on the suction surface of a blade were obtained over a range of rotational speeds. The formation of a suction surface shock was detected at the higher speeds. Introduction Pressure and temperature sensitive paints (PSP and TSP) offer a unique and inexpensive means of determining surface pressure and temperature distributions. Continuous surface pressure and temperature distributions, impossible to obtain using conventional measurement techniques, are critical for understanding complex flow mechanism and allow direct comparisons with results from computational fluid dynamic calculations (CFD). Cost for a PSP/TSP measurement system can be justified when one considers the cost of a single permanently installed pressure f Postdoctoral Research Assistant, Member AIAA * Graduate Research Assistant, Student Member AIAA t Professor, Member AIAA Copyright © 1997 by the American Institute of Aeronautics and Astronautics, Inc. All Right Reserved. transducer. Not only is the cost of installing these conventional probes an issue, but the aerodynamics and structural dynamics of the model can be seriously altered by modifications to accommodate the transducers and static pressure taps. Data rates for PSP and TSP are several orders of magnitude faster than data rates for conventional techniques. The theory and applications of PSP and TSP techniques are detailed in the later sections. Aerodynamic loads on turbomachine blade rows result in fatigue failure of rotating components. Increased fatigue life of all internal components would lead to reduced costs incurred by the owner. Hence, advancement in the fundamental understanding of blade surface pressure distribution will lead to improved designs and reduced costs in both development and maintenance of a gas turbine engine. Pressure measurements obtained with pressure sensitive paints will provide not only the variations in pressure on the whole blade surface, but will provide a large data base of pressure data which will be useful in development of advanced forced response models for use in turbomachine design. Recently, PSP measurements on rotating machinery were conducted by Burns and Sullivan [1.] with a laser-PMT based system. They obtained pressure distributions on a small wooden propeller and a TRW Hartzell propeller. Their PSP-derived pressure distributions across the blades show reasonable trends. In this study, the PSP and TSP techniques are further used to measure pressure and temperature distributions on rotor blades in a high speed axial compressor. Axial Compressor Facility The Purdue Research Axial Fan Facility features a 30.48 cm (12 in.) diameter, 2/3 hubtip ratio compressor rotor which is integral with the shaft. The drive system consists of a 400 horsepower AC motor driving a magnetic clutch with a variable speed output that drives a gearbox, as shown in Figure 1. Eighteen inlet guide vanes are twisted to produce a free vortex whirl into a 19 blade axial flow rotor. Both the inlet guide vanes and the rotor blades are designed with NACA 65 series airfoil sections on circular arc meanlines. The aluminum rotor blades have 2 in. chord and 2 in. span from the hub to tip. Located downstream of the rotor are eight aerodynamic struts that support the rotor bearing housing. Axial spacing between the trailing edge of the rotor and the leading edge of one of the struts nondimensionalized by the rotor chord CR is L/CR = 3.47. For these experiments, axial IGV-to-rotor spacing to chord ratio is set at Z/CR = 0.6 (10° IGV stagger angle). Note that the selection of rotor speeds is dictated by th&vibrational level of the machine. The velocity measurements have been made using hot-film sensors between the IGV and rotors. Typical spanwise distrtbjjjfions of relative Mach numbers and absolute flow angles at 14,750 rpm are shown in Fig, 2 (a) and (b), where %U/CR is the relative upstream distance from the rotors. A detailed discussion about hot-film measurements in the compressor is given by Johnston and Fleeter [2], PSP and TSP Theory and Measurement Techniques The physical processes involved in behavior of PSP and TSP have been thoroughly presented in literature [3-11], but will be covered briefly in this section for completeness. A probe molecule embedded in a polymer^ binder is elevated to an excited state by absorbing light of a particular wavelength. The molecule will return to the ground state by releasing the excitation energy. Energy can be dissipated via emission of light (fluorescence and phosphorescence) or through radiation less deactivation processes such as oxygen quenching and thermal quenching. For pressure sensitive paints (PSPs), the excess energy can be absorbed by oxygen molecules through a process in which energy is transferred to oxygen molecules in a collisional manner. This process, known as oxygen quenching, depends upon the concentration of oxygen molecules. Since the concentration of oxygen molecules is proportional to the partial pressure of oxygen, luminescence is sensitive to pressure due to the oxygen quenching process and can be modeled with the Stern-Volmer relation: