Experimental investigation of serpentine nozzles for turbofan

Abstract

Serpentine nozzles are widely used in stealth bombers and unmanned aerial vehicles to significantly suppress the infrared radiation signatures and to acquire excellent stealth performance with the goal to improve the survivability on the battlefield. The flow characteristics of serpentine nozzles are complex and noticeably different from those of axisymmetric nozzles due to the geometric configuration. This paper aims to obtain the flow characteristics of both serpentine nozzles and axisymmetric nozzles and to compare them under various working conditions. We obtained the flow characteristics of serpentine nozzles and axisymmetric nozzles with the employment of a schlieren system, PSI electronic pressure scanning valves, a six-component force balance system and flowmeters. The results show that the static pressure distributions on the upper and down walls of serpentine nozzles are completely different from those of axisymmetric nozzles, which are mainly affected by the flow tube near the walls. The flow velocity increases and the static pressure drops when the flow tube contracts. The outcome is opposite once the flow tube expands. The nozzle configuration and the bypass flow pressure ratio have little influence on the schlieren photographs of the flow fields downstream of the nozzle exit, which are composed of expansion waves and shock waves alternately. The values of the flow coefficient and the thrust coefficient both decrease when the difference between the core flow pressure ratio and the bypass flow pressure ratio gets large. The configuration of the serpentine nozzle, especially the wall curvature, affects the values of its flow coefficient and thrust coefficient. The value of the flow coefficient for serpentine nozzle 2 is about 1.5% lower than that for serpentine nozzle 1, and the value of the thrust coefficient for serpentine nozzle 2 is about 0.5% lower than that for serpentine nozzle 1.