Flow effects of microperforated-panel casing treatments in a contra-rotating fan

Abstract

The flow effects of microperforated-panel (MPP) casing treatments installed over the front rotor of a small contra-rotating (CR) fan are numerically studied, with emphasis put on the reshaped front-rotor tip leakage flow, the blade response to rotor-rotor interaction, and the unsteadiness of static pressure on blade surfaces. It is found that the secondary flow through the sub-millimeter holes of the MPP hinders the formation of the front-rotor tip leakage flow and reduces its circumferential stretching, resulting in an improved inflow to the rear rotor. Consequently, the unsteady characteristics of static pressure and lift force on the rear rotor are generally attenuated. An enlarged back cavity can enhance such attenuation, in which case a 14.5% decrease in pressure unsteadiness is observed. The first, second, and third harmonics of the unsteady lift force experience reductions of 15.6%, 37.4%, and 60.9%, respectively. However, the unsteady characteristics on the front rotor dominated by potential interaction are not significantly influenced by the MPP. Furthermore, analysis of blade-to-blade phase differences shows that, whether the CR fan is treated with the MPP or not, the phase differences are not precisely repeated but vary randomly and that the higher the order of the force harmonic, the more likely the phase relationship is disturbed by the treatments. The varying nature of the phase relationship would render the acoustic energy of an induced interaction tone leaked into shaft tones and broadband noise. Radiation efficiency analysis of force harmonics demonstrates that the front rotor is not an efficient contributor to interaction noise compared with the rear rotor, which radiates more interaction tones with much higher efficiency. The current work is expected to be useful not only for devising effective MPP casing treatments to control small CR fan noise but also for understanding their underlying flow effects on rotor-rotor interaction.