Review of tip air injection to improve stall margin in axial compressors

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Abstract

The present paper provides a summary of investigations of the tip air injection, which has been studied for over 30 years to extend the stall margin in axial flow compressors. The review covers the design of injected parameters, understanding of stability-enhancing mechanism, application of active control, and self-recirculation based on tip air injection. First, the injected parameters that can influence the stability-enhancing capability are surveyed and classified. These include injected mass flow and momentum, structure, pitch and yaw angle, axial location, and circumferential distribution. Next, the effect of tip air injection on the internal flow field is introduced; the action on the main stream flow or tip leakage flow depends on the injected moment ratio. Finally, the applications of active control with tip air injection and self-recirculating injection from a single-rotor to multi-stage compressors are listed and discussed. The active control methods are based on the Moore–Greitzer model and online prediction of stall inception. This study aims to aid researchers who are working on stability enhancement with tip air injection in compressors to improve their work. Moreover, considering that the regulation of injected mass flows or momentums is flexible, this study would facilitate the immediate application of the tip air injection in actual aero-engine compressors.

Introduction

In the pursuit of developing modern aircrafts that can fly faster, higher, and farther, they have to be equipped with advanced aero-engines that satisfy the requirements of a wide and safe operating range, high thrust to weight ratio, and high efficiency. However, the safe operation of aero-engines is commonly constrained by the rotating stall, which mainly occurs in axial flow compressors. As a type of aerodynamic instability, the rotating stall is considered as the most unfortunate occurrence that should be avoided at all cost in aero-engine compressors (Day [1]). Thus, numerous efforts have been exerted over the last seven decades to understand how and why the compressor stalls [[1], [2], [3]]. At the same time, a fairly consistent effort has been devoted to the development of active and passive control techniques aimed at increasing compressor stability. Hathaway [2] has summarized different passive endwall treatments, including the traditional axial slots and circumferential grooves with different geometries [[4], [5], [6], [7], [8], [9], [10]]. However, the above passive control technologies frequently entail efficiency penalty that cannot satisfy the demands of high thrust to weight ratio and high efficiency required of modern aero-engines.

Another common form of a stability-enhancing technology is the tip air injection, which injects into the blade tip region by means of high-pressure air. The earliest reported concept about this form was a 1950-patent filed by Geoffrey Wilde of Rolls-Royce Limited. It proposes aft stage bleeds that feed flow upstream of the inlet guide vane (IGV) to be injected along the casing ahead of the IGV [11]. Under this guidance, a few researchers have separately extracted the upstream reinjection and developed tip air injection [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24],[31], [32], [33], [34], [35], [36], [37], [38]]. Since the tip air injection has been confirmed to effectively extend the stall margin in an axial flow compressor, the studies on this type of injection are divided into two routes as follows.

The first route is the mechanism of tip air injection, which mainly focuses on the influence of injected parameters on the stall margin and its corresponding stability-enhancing mechanism [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21]]. A varied selection of injected parameters generates not only a different stall margin improvement (SMI), but also lead to a different understanding of the stability-enhancing mechanism [12]. For example, a few researchers think that the tip air injection can influence the main stream flow (inlet incidence and radial loading) [13,14,20,21], whereas others assume it can only act on the tip clearance flow [[15], [16], [17]].

The second route is the application of tip air injection, which includes active control [[22], [23], [24],[31], [32], [33], [34], [35], [36], [37], [38]] and self-recirculating injection [[39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]]. Day [22] firstly realized the active control through the use of a pulse jet over the blade tip and detection of a spike-type stall inception. This method was further validated in the Viper engine by Freeman et al. [23] and in Lazac 04 turbofan engine by Leinhos et al. [24]. However, existing results showed that the time interval from the spike-type inception to the deep stall is less than 1s for some axial compressors [22,[25], [26], [27]]; thus, a timely detection of the spike-type stall inception is a serious problem in actual applications. Consequently, with the help of the Moore–Greitzer model [28], some researchers developed the active control prediction based on the development of harmonics with different order numbers in the model [[29], [30], [31], [32], [33], [34], [35], [36]]; however, it is limited to the detection of modal-wave inception. Accordingly, recent studies have shifted to the prediction and detection of pre-stall inception and active control; the active control uses the pre-stall inception as a feedback signal [37,38]. As another application of the tip air injection, the self-recirculating injection is also a recent topic of considerable interest because existing studies indicate that it can consider stability and efficiency in the compressor [[39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]].

This review article presents the fundamental technical problems related to the tip air injection. It focuses on the stability-enhancing mechanisms and applications of tip air injection. The stability-enhancing mechanism of the tip air injection is categorized into two parts—selection of injected parameters and response to steady/unsteady parameters. Thereafter, the realizations of active and self-recirculating injections, which were anticipated to be applied in actual aero compressor in previous decades, are presented. The prospects and problems pertaining to tip air injection are discussed in the conclusion.

Section snippets

Design of injector parameters

The idea of tip air injection for extending the stall margin of the compressor originated from the boundary layer control in delaying flow breakdown in cascade tunnels and inlets [[54], [55], [56]]. In 1960, researchers in the National Aeronautics and Space Administration (NASA) Lewis Research Center conducted experiments using casing blowing and bleeding to control the end wall boundary layer in the rotor tip region in a high-speed single-stage axial compressor [[57], [58], [59], [60], [61],

Stability-enhancing mechanism

With the aim of immediately enhancing stability in actual compressors, researchers have attempted to reveal the stability-enhancing mechanism of tip air injections. In the past, because of the different selection of injected parameters, the understanding of the stability-enhancing mechanism is divided into two explanations.

Active control

The concept of active control for extending the stall margin in compressors has been proposed in 1989 [77]. The main principles of active control include predicting the stall inception and feeding back signal to the control; thereafter, the control can drive the actuator to extend the stall margin. Two important factors have to be carefully considered. One of the two is feedback detection. The Moore-Greitzer model made a significant contribution in terms of predicting the modal-wave inception [

Self-recirculating control

The self-recirculating injection is derived from the 1950 patent filed by Geoffrey Wilde of Rolls-Royce [11] Limited. It proposes aft stage bleeds that feed flow upstream of the IGV to be injected along the casing ahead of the IGV. With this concept, some researchers have separately extracted the upstream reinjection and developed the tip air injection (e.g., Day [22], Weigl et al. [33], van Schalkwyk et al. [34], and Li et al. [38] for active tip air injection; Suder et al. [13], Kefalakis et

Discussion

The tip air injection, including the external and recirculating injections, has been confirmed to effectively extend the stall margin in axial compressors. In the past, most of the works have been performed in the laboratory, such as the typical studies by Suder et al. [13], Day [22], Weigl et al. [33], and Hathaway [39]. Inspired by the previous experience, the numerical and experimental works on the tip air injection were implemented; examples of these are the following: parameter studies of

Conclusions

A wide stall margin is one of the design targets for compressors; moreover, it is the key parameter for ensuring safe engine operation. As an effective stability-enhancing method, the tip air injection, such as steady injection, active injection, and recirculating injection, has been widely studied in the past. However, previous results showed that the stability-enhancing capability of tip air injection is uncertain. To aid in the design of an effective tip air injection system in actual

Acknowledgement

The authors are thankful for the support of the National Science Foundation of China with Project No.51676183 and No.51727810. And the Special Fund for the Member of Youth Innovation Promotion Association of CAS (2018173). The authors also acknowledge Prof. Jingyi Chen, Prof. Feng Lin, Dr. Zhiting Tong, Dr. Xi Nan and Dr. Shaojuan Geng for the insightful discussion.

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