Fan Facility Research Articles

Rotating Stall Control in an Axial Fan with Pulsed-Direct-Current Plasma Actuation

The use of dielectric barrier discharge plasma actuators to suppress traveling stall cells and thereby improve stall margin is examined. This involved the design and construction of a nonconductive high-solidity fan facility. The experiments were meant to support the application of similar systems in transonic axial compressors. Eight 63.5-mm-long plasma actuator segments were installed in the casing over the blade row at three axial locations that were indicated in the literature to be optimum for stall margin improvement. Independent control of fan revolutions per minute (RPM) and mass flow was used to set conditions that produced traveling stall cells. These were documented using an azimuthal array of microphones. The microphone pressure time series were spatially correlated to reveal the coherent wave packets. Different azimuthal combinations of the plasma actuators were investigated from a single actuator to all eight. A single actuator reduced the local spatial correlation metric (SCM), a measure of stall cell activity, by 40% and the azimuthally averaged SCM by 30%. A “memory effect” on the stall cell suppression was revealed, and it persisted for approximately 180° of the casing azimuth. The effect of the plasma actuator axial location was investigated at the design RPM and at two off-design RPMs. At the design RPM, the two most optimum locations were slightly upstream of the blade row and at the midchord of the blade row. This changed for the off-design conditions. For the range of the fan RPMs and actuator locations investigated, the stall margin improvements ranged from 1.63 to 5.45%. Finally, a plasma actuator stall extension model is presented for relating the required plasma actuator body force to the achieved stall margin improvement.

Application of Large Gurney Flaps on Low Reynolds Number Fan Blades

On the basis of a semi-empirical model, large Gurney flaps of 10%, 20%, and 30% of the fan blade chord length were tested in a specially designed ventilation fan facility. At the highest volumetric flowrates tested, the flapped blades all produced higher pressures than the baseline nonflapped case. When proper accounting was made of fan rotational speed, all flapped blades produced consistently higher dimensionless pressures, with the 30% flap producing the highest pressures at large volumetric flowrates. Based on the assumption that sound power varies with the sixth power of fan rotation speed, it was shown that the sound pressure level could be reduced by nearly 4 dB. All flapped configurations produced higher mechanical efficiency than the baseline case but the mass of the flap relative to that of the blade emerged as an important parameter. A 10% flap, whose mass was negligible relative to the blade, produced the largest increase of 18% in static efficiency. Further research will focus on testing the flaps over the entire operational range, as well as on redesigning stiffer and lighter Gurney flaps. The introduction of three-dimensionality such as spanwise spaced holes, slits, or serrations that have previously been used to reduce airfoil drag will also be considered.

INLET GUIDE VANE WAKES INCLUDING ROTOR EFFECTS

Fundamental experiments are described directed at the investigation of forcing functions generated by an inlet guide vane (IGV) row, including interactions with the downstream rotor, for application to turbomachine forced response design systems. The experiments are performed in a high-speed research fan facility comprised of an IGV row upstream of a rotor. IGV-rotor axial spacing is variable, with the IGV row able to be indexed circumferentially, thereby allowing measurements to be made across several IGV wakes. With an IGV relative Mach number of 0·29, measurements include the IGV wake pressure and velocity fields for three IGV-rotor axial spacings. The decay characteristics of the IGV wakes are compared to the Majjigi and Gliebe empirical correlations. After Fourier decomposition, a vortical–potential gust splitting analysis is implemented to determine the vortical and potential harmonic wake gust forcing functions both upstream and downstream of the rotor. Higher harmonics of the vortical gust component of the IGV wakes are found to decay at a uniform rate due to viscous diffusion.

Active cancellation of rotor/stator interaction tones in turbofan engines

Rotor/stator-generated aerodynamic noise in turbofan engines, HVAC axial fans, and other turbomachinery is often dominated by blade passage harmonics produced by dipole sound sources. Radiation toward the inlet and exhaust depends upon the effective orientation of the dipole sources relative to the propagation angles of the excited modes. Absorption of this tonal noise using passive treatment is often compromised because of a lack of knowledge of the relative phases and amplitudes of the propagating modes. A modal-based active cancellation system has been developed for global suppression of rotor/stator interaction tones. Axially spaced annuli of controlled wall-mounted actuators are located immediately upstream and downstream of the hubs and tips of stator vanes. Wall-mounted error microphone arrays are located in the inlet nacelle and exhaust duct. Actuator drive vectors are determined in an adaptive-quadrature control algorithm that seeks to minimize total radiated noise. The concept was initially developed and tested on a 24-in. axial fan facility and then extended and demonstrated on the 48-in. Active Noise Control Fan facility at NASA Lewis Research Center. Simultaneous suppression of modes (4,0) and (4,1) of 8–17 dB toward the inlet and 3–11 dB toward the exhaust was achieved at the second blade-passage harmonic.

High speed rotor wake total temperature measurements using a hot-film anemometer

To develop a quantitative understanding of unsteady and interacting turbomachine flow fields, it is necessary to quantify the instantaneous efficiency of high speed turbomachines. This requires the measurement of both the unsteady velocity and total temperature variation in the exit flow of a high speed rotor. In this paper, techniques to utilize a single slant-film anemometer to measure unsteady total temperature are developed and evaluated. Then a series of preliminary experiments are performed in a high speed axial fan facility to quantify the instantaneous rotor efficiency. This is accomplished by utilizing these single slant-film methods to measure the total temperature in the rotor wakes. Results show that measurements at multiple overheats and several probe orientations are required. The simplest method proves to be useful for determining parameters used in other methods. An analysis based on King’s law gives good results even when measurements are outside the calibration range. Within the calibration range, a polynomial representation of the wire response to mass flux and total temperature yields good total temperature fluctuation results. A model analysis technique is also assessed.

Measured Rotor Wake and Potential Forcing Functions, Including Blade Row Interactions

A fundamental experiment is directed at the acquisition and analysis of data dee ning compressible forcing functions generated by a rotor, including interactions with the upstream-generated inlet guide vane (IGV) wakes, for application to wake forcing function models in turbomachine forced-response design systems. The research fan facility consists of an IGV row and a downstream rotor. IGV‐ rotor axial spacing is variable, with the IGV row able to be indexed circumferentially, thereby enabling the rotor wakes to be measured both in the IGV freestream and wakes. At a rotor-relative Mach number of 0.6 unsteady measurements are made of the rotor wake pressure and velocity e elds for two IGV‐ rotor axial spacings and with the rotor wake measured in the IGV freestream and wake regions. The decay characteristics of the rotor blade wakes are compared to empirical correlations. After Fourier decomposition, a vortical‐ potential gust splitting analysis is implemented and applied to the e rst-harmonic data to determine the vortical and potential harmonic wake gust forcing functions both upstream and downstream of the rotor. The potential gust component of the rotor wakes upstream of the rotor is found to be dominated by the e rst-harmonic component with small contributions from the second and third harmonics. Higher harmonics of the vortical gust component of the rotor wakes measured both in and out of the IGV wakes are found to be signie cantly reduced in the IGV wake regions.