Chaoqun Nie

Micro Air Injection and Its Unsteady Response in a Low-Speed Axial Compressor

A new approach, steady micro air injection from the casing, is proposed to improve the stability of a three-stage low-speed axial compression system. Although the injection rate is designated to be only a few ten thousandth of the compressor flow rate, such an injection is able to trigger the unsteady response and thus lower the mass flow rate at stall for up to 5.83%. At the same time, it keeps the steady compressor characteristic with no injection unchanged. In order to verify that the compressor response is indeed unsteady, experiments at various injection configurations are performed, which include different injection angles, axial gaps between injector and blade leading edge, radial penetration of injector and the amount of injected air. Evidences of the unsteady response are further demonstrated through dynamic signal analysis using a wavelet-based method to show the behavior of early flow disturbances under the influence of injection. Numerical analyses performed at near stall condition show that the tip clearance vortices do response to the micro-injection, and thus delay the inception of stall.

Flow Structures in the Tip Region for a Transonic Compressor Rotor

Numerical simulations are carried out to investigate flow structures in the tip region for an axial transonic rotor, with careful comparisons with the experimental results. The calculated performance curve and two-dimensional (2D) flow structures observed at casing, such as the shock wave, the expansion wave around the leading edge, and the tip leakage flow at peak efficiency and near-stall points, are all captured by simulation results, which agree with the experimental data well. An in-depth analysis of three-dimensional flow structures reveals three features: (1) there exists an interface between the incoming main flow and the tip leakage flow, (2) in this rotor the tip leakage flows along the blade chord can be divided into at least two parts according to the blade loading distribution, and (3) each part plays a different role on the stall inception mechanism in the leakage flow dominated region. A model of three-dimensional flow structures of tip leakage flow is thus proposed accordingly. In the second half of this paper, the unsteady features of the tip leakage flows, which emerge at the operating points close to stall, are presented and validated with experiment observations. The numerical results in the rotor relative reference frame are first converted to the casing absolute reference frame before compared with the measurements in experiments. It is found that the main frequency components of simulation at absolute reference frame match well with those measured in the experiments. The mechanism of the unsteadiness and its significance to stability enhancement design are then discussed based on the details of the flow field obtained through numerical simulations.

Flow Structure of Short-Length-Scale Disturbance in an Axial-Flow Compressor

Short-length-scale disturbances, also called spikes, are often responsible in triggering rotating stall in axial-flow compressors. One hypothesis suggests that spikes can be the consequence of dynamic interaction among forward-spilled tip-leakage flow, the main throughflow, and the reversed flow. However, the transit process of such a dynamic interaction in the vicinity of the rotor tip clearance and, thus, the physical images of the flow structure of a spike are still unknown. In this paper, we present a numerical study with a novel scheme for a low-speed axial-How compressor by incorporating rotating inlet distortion. Because the inlet distortion will overload a portion of the blades while keeping the rest working normally, the short-length-scale disturbances can be observed without advocating the computational difficulty of simulating a fully stalled compressor. Two unsteady simulations using a commercial, three-dimensional, time-accurate, Reynolds-averaged Navier-Stokes solver are performed: one for a one-fourth rotor annulus with finer grids and the other for the entire rotor annulus with coarser grids. After the code is validated by comparing experimental results with the simulation for the entire rotor annulus, the one-fourth-annulus simulation is used to unveil the flow physics. As elucidated from the computational results, the complete birth-to-decay process of the short-length-scale disturbances is captured for the first time. The corresponding 3-D flow structure is also revealed. It is shown that the effects of dynamic How interaction at the tip extend beyond the tip region and deeply into the blade span. A horn-shaped vortex with one end at about 30% of the blade span and the other end at the casing is formed, which generates a low-pressure dip in the casing-pressure measurement. The spike, as identified from casing-pressure measurement, corresponds to a flow image in which the vortex rotates around the annulus.

The Dual Mechanisms and Implementations of Stability Enhancement With Discrete Tip Injection in Axial Flow Compressors

The mechanisms and implementation scheme of discrete tip air injection are studied in this paper. A map that summarized the routes to stall is then proposed. It is argued that there exists a critical tip clearance ratio that separates two different routes to stall, which infers that the stability enhancement can also be based on two different mechanisms. A summation of tip injection test data in the literatures demonstrates that this is actually the case. For each compressor, there are two trends in the curve of stall margin improvement (SMI) versus injected momentum ratio, which is separated by a demarcation ratio of injected momentum. A series of tests are done in a low-speed compressor to show that the micro injection, wherein the injected momentum ratio is less than the demarcation ratio, can only act on the tip leakage flow (TLF) and thus provide small SMI by weakening the self-induced unsteadiness of the tip leakage flow (UTLF), while in contrast the macro injection can provide much larger SMI by acting on the main flow, decreasing the inlet angle-of-attack and thus unloading the blade tip. Based on these findings, a novel detecting-actuating scheme is designed and implemented onto a low-speed axial compressor. A cross-correlation coefficient is used to detect the UTLF in the prestall process way before stall inception and then to guide the opening of proportional electromagnetic valves. The injected flow rate can be smoothly varied to cover both micro-and macroinjection, which saves energy when the compressor is stable, and provides protection when it is needed. The same principle is applied to a high-speed compressor with a recirculation injection and the preliminary test results are very encouraging.

The Self-Induced Unsteadiness of Tip Leakage Vortex and Its Effect on Compressor Stall Inception

The self-induced unsteadiness of tip leakage vortex (TLV), which appears in a compressor rotor working in a range of operating points on its characteristics, from wide-open throttle all the way to the stall limit, is investigated experimentally. The research aims are twofold, to clarify the three modes in TLV development process through experimental evidences and to explore the effect of this in-blade TLV unsteadiness on stall inception. In the first half of the paper, in order to detect the unsteadiness and ensure its existence in the experimental environment (not just in computational results), phase-locked Mean and Root-Mean-Square (RMS) contours are used to track the time-averaged trajectories of the TLV, while a power spectral density (PSD) analysis provides a means to identify the magnitude and the frequency of the oscillation. With all of the above, the three modes of the TLV development, which are steady, in-blade unsteady and cross-blade unsteady TLV, can be clearly demonstrated. In the second half of this paper, various tip jet injections are applied to test the effects of the unsteady TLV on stall inception. It is found that a spike stall precursor is originated from circumferential locations where the strongest unsteady TLV are. At those locations, tip jet injections that are designated to directly alter the characteristics of TLV improve the stall margin effectively. Further, the injections are arranged over the rotor tip in difference axial locations and switched on at different points of compressor characteristic, demonstrating that if the injection misses the tip vortices or interferes with TLV too late, little or even no improvement in stall margin can be gained. These results show that the unsteady TLV are closely related to spike stall inception in this single rotor, which implies that the initiation of compressor stall could be manipulated by properly altering the characteristics of TLV unsteadiness.Copyright

Further Investigation on Transonic Compressor Stall Margin Enhancement With Stall Precursor-Suppressed Casing Treatment

A kind of casing treatment, named as stall precursor-suppressed (SPS), has been developed recently, which was proved to be able to effectively improve stall margin (SM) without significant efficiency loss in low-speed axial flow compressors and a transonic compressor rotor. In this paper, the effectiveness of the SPS casing treatment is investigated in a single-stage transonic compressor. Based on an extended stall inception model, the quantitative evaluation of the SM enhancement by the SPS casing treatment is presented for the transonic compressor stage. The model predicts that a 2.5–6.8% of stall margin improvement (SMI), which is defined in terms of mass flow rate at stall inception, can be achieved at the design rotational speed. The experimental results show that the SPS casing treatment can achieve 3.5–9.3% of the SMI at 95% design rotational speed. Due to the fact that the distributions of the total pressure ratio along the spanwise direction are kept the same as those of the solid wall casing at the same mass flow rate, the SPS casing treatments with a small open area ratio and large backchamber enhance the SM without a recognizable efficiency loss and a migration of the pressure-rise characteristics. Furthermore, the mechanism of SMI with the SPS casing treatment is investigated in the experiments. In comparison with the solid wall casing, the emergence and the evolution of the stall inception waves are suppressed and the nonlinear development of the stall process is delayed with the SPS casing treatment.

Flashback Limit and Mechanism of Methane and Syngas Fuel

Flashback is one of the major problems in lean premixed combustion of gas turbine combustor. Due to the effulgent future of co-product system and IGCC, lean premixed combustion, one of the approaches to ultra low NOx for rich hydrogen syngas fuel need farther research on anti-flashback and low pressure drop combustor. Mechanism and characteristics of methane and syngas flashback for 2 types of flame holders, i.e. ring shape and rod shape have been researched through experiment as well as numerical simulation. The partial premix model has been selected to simulate premixed combustion flashback process since it combined the advantage of PDF model and TFC model. Experiments demonstrate that, the flashback velocity of different fuel compositions or flame holder size generally can be correlated to the same dimensionless function by using Peclet number model if the structures of flame holders are the same. Peclet function curves were used to compare the anti-flashback performance of the 2 types of flame holders mentioned above with swirl holder. Boundary coaxial jet can change flashback through the wall into flashback in the core flow and significantly improve the anti-flashback performance of the ring-type flame holder on condition that the velocity of the boundary coaxial jet is in an optimal range. As the result, ring shape holder shows the best while swirl holder the worst on anti-flashback performance.© 2006 ASME

Extensive Experimental Study of Circumferential Single Groove in an Axial Flow Compressor

Circumferential single-groove casing treatment becomes an interesting topic in recent few years, because it is a good tool to explore the interaction between the groove and the flow in blade tip region. The stall margin improvement (SMI) as a function of the axial groove location has been found for some compressors, such a trend cannot be predicted by steady high-fidelity CFD simulations. Recent efforts show that to catch such a trend, multi-passage, unsteady flow simulations are needed as the stalling mechanism itself involves cross-passage flows and unsteady dynamics. This indicates a need to validate unsteady numerical simulation results. In this paper, an extensive experimental study of a total of fifteen single casing grooves in a low-speed axial compressor rotor is presented, the groove location varies from 0.4% to 98.3% of axial tip chord are tested. The unsteady pressure data both at casing and at the blade wake with different groove locations are measured and processed, including the movement of trajectory of tip leakage flow, the evolution of unsteadiness of tip leakage flow (UTLF), the unsteady spectrum signature during the stall process, and the outlet unsteady flow characteristic along the span. These data provide a case study for validation of the unsteady CFD results, and may be helpful for further interpretation on the stalling mechanism affected by circumferential casing grooves.© 2014 ASME

The Dynamics of Prestall Process in an Axial Low-Speed Compressor With Single Circumferential Casing Groove

The fact that the location of single circumferential casing groove can have a large impact on the stall margin of axial compressors has been actively investigated in recent years. However, it remains a tough challenge to numerically predict the groove performance and clarify its underlying mechanism on the difference of stall margin improvement (SMI) for different groove locations. In this paper, a single rotor, which had been proven to be a tip sensitive rotor with spike type stall inception, is tested and numerically simulated with an unsteady Reynolds averaged Navier-Stokes (URANS) solver. The test results show that the rear grooves perform better than the front grooves in this rotor. A multi-passage numerical scheme is used to capture the prestall process involving the unsteady cross-passage flow interaction. Although the calculation did not fully capture the measured trend of stall margin improvement, the numerical result did show that the front groove, which is the closest to the leading edge, generates the worst stall margin extension, and the rear groove, which is located right behind the front groove, gives the best stall margin improvement.The prestall dynamics for smooth casing and the two typical grooves are chosen for a comparative study to clarify the underlying mechanism. Three different prestall processes are found. For smooth casing, a rotating disturbance evolves into spike after the interface between tip leakage flow (TLF) and incoming main flow (MF) spills in front of the leading edge. For the front groove, the interface is prevented by the groove to move forward during the throttling process. A modal wave is captured before stall. When the rear groove is applied, the interface location as a function of flow coefficient behaves much similar to the case of smooth casing. However, there is no any rotating disturbance, neither the modes nor the spike, with this groove. The flow is symmetric until all the passages break down at the same time.Copyright

A Summary of Stall Warning and Suppression Research With Micro Tip Injection

Micro tip injection is a stall control technique in which engineers inject small-flowrate yet high-velocity jets into the tip region of axial compressor rotor blades to extend the stable operation range, while keeping the entire compressor characteristic line nearly unchanged. This paper summarizes the related research performed in Chinese Academy of Sciences in the past several years, ranging from understanding the fundamental mechanisms to demonstrating the technology in a laboratory compressor. A brief review in tip injection research indicates that for each compressor, there exists a critical injection momentum ratio, below which the injection flows may work differently from those over the critical value. Detailed casing unsteady static pressure measurements and the related sophisticated data processing proved the existence of self-induced unsteady tip leakage flow, which was also confirmed by unsteady CFD simulations. It is showed that this kind of unsteadiness in tip leakage flow might be responsible to make the auto-correlation coefficient drop significantly as the compressor operated at lower flowrate close to stall limit. This warning signal, which was first successfully used by other research groups with little care taken on its flow mechanism, appears much earlier than the classical modal-wave or spike stall precursors. Micro tip injection was effective if it interacts with the unsteady tip leakage flow long before the stall precursor emerges. As a technology demonstration, a DSP board with a build-in auto-correlation early-stall-warning algorithm and an on-off controller of injection valve worked well to drive a micro tip injection system and successfully delayed the stall in a low-speed compressor with much less jet flows than the continuous micro tip injection.Copyright