Tsuguji Nakano

Impact of Inter-Stage Dynamics on Stalling Stage Identification

A key objective of compressor rig tests is the identification of compressor stall boundary. A complementary goal is the identification of the stalling stage based on test data. This serves two purposes: 1) Validate the pre-test prediction of the stage loading distribution, and 2) identify the weak stages, should improvements in operating range be desired in subsequent design iterations. Typically the pertinent test data is in the form of static pressure measurements. Many engineers believe that a stalling stage is accompanied by a transient upstream pressure rise coupled with a downstream pressure loss. However, inter-stage dynamics may cloud the identification of the stalling stage. To this end, an analysis of inter-stage dynamics, immediately preceding the stall event, could provide an alternate assessment of the stalling stage. This work reviews existing stall models for studying compressor dynamics. The main focus of this work is to develop ability to capture inter-stage dynamics. A 3-state equation lumped Moore-Greitzer (MG3) model is widely used to study the dynamic compressor response during surge and rotating stall transients. However the evolution of MG3 model may not provide a suitable framework for the investigation of inter-stage dynamics. On the other hand, an unsteady time marching 1-D fluid dynamic model (e.g. similar to the DynTECC formulation which includes body forces), while unable to capture the rotating stall dynamics, is sufficient for this purpose. A numerical simulation has been developed to investigate the impact of stage characteristics, as well as load distribution on the compression and expansion waves that develop prior to a surge event. Through a controlled weakening of selected stages, the time evolution of these waves is related back to the stalling stage. It is found that the weakened stage is not necessarily the stalling stage as identified via the pressure rise and downstream pressure drop pattern.Copyright

Modeling and Analysis of Ice Shed in Multistage Compressor of Jet Engines

Simulation of ice shed into a multistage axial compressor involves a coupled two phase flow of a continuous phase comprising of air and water vapor and a discrete phase with ice crystals and water droplets. A first principles based discrete phase model is formulated to capture the heat and mass transfer processes of ice flow in air. A quasi one-dimensional model is used to represent the continuous phase. An exchange of information at every time step between the two models leads to a coupled response that alters characteristics like temperature and pressure distributions across the compressor. However, an understanding of the impact of various assumptions used for modeling of the icing physics is imperative in order to establish the fidelity of the developed icing model, before its use in gas turbine engine ice ingestion studies. This paper describes the assumptions and semi-numerical models used in the coupled discrete-continuous phase flow numerical models. The input characteristics of the discrete phase related to the size and distribution of ice crystals, the assumed percentage of ice particles escaping through compressor bleed ports, simplifications associated with ice and droplet breakup on impact with compressor blades, moisture content affecting the dry air properties, are some of the factors that are variables in the icing study. The impact of these factors on the compressor flow dynamics is estimated through a parametric analysis.

A Method for Evaluating the Aerodynamic Stability of Multistage Axial-Flow Compressors

A new simple engineering parameter to evaluate the stability of multistage axial compressors has been derived. It is based on the stability analysis for a small circumferential disturbance imposed on the steady-state flow field. The analytical model assumes that the flow field is two dimensional and incompressible in the ducts between blade rows although the steady-state density is permitted to change across the blade rows. The resulting stall parameter contains terms that relate to the slope of the pressure rise characteristic of the blade rows and the inertia effects of the fluid in the blade rows and ducts. The parameter leads to the classical stability criteria based on the slope of the overall total to static pressure rise coefficient in the limit where constant density and constant blade rotational speed are assumed across the compressor. The proposed stall parameter has been calculated for three different multistage axial flow compressors, and the results indicate that the parameter has a strong correlation with the measured stability of the compressors. The good correlation with the test data demonstrates that the newly derived stall parameter captures much of the fundamental physics of instability inception in multistage compressors, and that it can be a good guideline for designers and engineers needing to evaluate the stability boundary of multistage machines.

Transient Behavior in Axial Compressors in Event of Ice Shed

Incidents of partial or total thrust loss due to engine icing at cruise have been recorded over past several years. These events increase the demand for better understanding of compressor dynamics under such conditions. In the present study, physics based compressor blade row model (BRM) is used to evaluate the effect of booster ice-shed on axial high pressure compressor (HPC) at flight and approach idling conditions (65%–82% Nc). A representative aviation high-bypass turbofan engine HPC is used in this study. Transient behavior of compressor with varying ice ingestion conditions is compared and inter-stage dynamics is analyzed. Stage re-matching occurs due to heat exchange between air and ice which dictates the stall inception stage in the compressor. It is found that although T3 drop is closely related to compressor stall inception, the transient mechanism of ice-shed also plays an important role. Comparisons are made with steady energy balance equation to determine total water content (TWC) at HPC inlet to emphasize the importance of compressor transients. The ice amount, its ingestion duration and rate affect the onset of stall. HPC might sustain through a slower ice-shed while a faster ice-shed can lead to compressor stall with little or no chances of recovery. Understanding this transient behavior and inter-stage dynamics due to ice-shed will help in designing and implementing passive or active stall control mechanisms.© 2015 ASME

Analysis of Stall Onset in a Multistage Axial Flow Compressor in Response to Engine Icing

Propulsion system instabilities such as compressor surge and stall can arise due to ice ingested by an aircraft engine flying through high ice water content regions. Performance and operability are affected by ice ingestion into a gas turbine engine compression system. Since the 1980’s ingestion of ice particles into engines have caused over one hundred engine power loss events. This paper presents an analysis of a multistage compressor system response to ice ingestion. Towards this, an aero-thermodynamic model of the discrete particles that captures mass and heat balances with air, is constructed. The computational methodology integrates it with a quasi-one-dimensional unsteady flow model with additional source terms from the discrete phase. From numerical simulations of the coupled continuous-discrete phase flow model, it is observed that a re-matching of the stages across the compressor occurs with increasing ice flow rates to accommodate loss of energy to the ice flow. The axial flow and pressure oscillations with increasing ice flow rates results in an eventual irretrievable unsteady compressor operating point excursion to stall side. The flow solver simulates the onset of a surge-stall event and identifies the stalling stage of the compressor. The numerical simulations correlate the magnitude of ice flow rates to pressure disturbances ultimately causing compressor instability.

A Method for Evaluating the Effect of Circumferential Inlet Distortion on the Aerodynamic Stability of Multi-Stage Axial-Flow Compressors

A simple engineering parameter to evaluate the stability of high-speed multi-stage compressors with distorted inlet flow has been derived based on a simplified semi-compressible linear stability model. The parameter consists of steady-state flow quantities and geometric parameters of the compressor and it indicates that the circumferential integral of the slope of the steady-state individual blade row static pressure rise characteristics is important in the determination of the compressor stability limit in the presence of distortion. The parameter reduces to the author’s rotating stall inception parameter in the limit of non-distorted inlet flow. Since the model includes a downstream plenum and throttle, a condition for pure surge inception with undistorted inlet flow has been deduced. The pure surge conditions can be reduced to the classical dynamic and static instability conditions in the limit of a constant annulus area incompressible compressor. The results indicate that rotating stall always precedes surge instability, as many engineers and researchers would expect from experience. The parameter for instability with inlet distortion was calculated using test data measured in a high-speed 5-stage compressor with two different types of circumferential inlet distortion, and the results show that the parameter has a strong correlation with the data and is an improvement over the classical incompressible stability parameter. The results demonstrate that the parameter captures much of the physics important during the instability inception in a high-speed multi-stage compressor subjected to circumferential inlet distortion. The parameter clearly shows how each compressor component’s characteristics contribute to the overall stability in a high speed axial multi-stage compressor, therefore, it will aid engineers and designers in their understanding and prediction of the aerodynamic instability inception phenomena.Copyright

A Method for Evaluating the Aerodynamic Stability of Multi-Stage Axial-Flow Compressors

A new simple engineering parameter to evaluate the stability of multi-stage axial compressors has been derived. It is based on the stability analysis for a small circumferential disturbance imposed on the steady state flow field. The analytical model assumes that the flow field is two dimensional and incompressible in the ducts between blade rows although the steady state density is permitted to change across the blade rows. The resulting stall parameter contains terms that relate to the slope of the pressure rise characteristic of the blade rows and the inertia effects of the fluid in the blade rows and ducts. The parameter leads to the classical stability criteria based on the slope of the overall total to static pressure rise coefficient in the limit where constant density and constant blade rotational speed are assumed across the compressor. The proposed stall parameter has been calculated for three different multi-stage axial flow compressors and the results indicate that the parameter has a strong correlation with the measured stability of the compressors. The good correlation with the test data demonstrates that the newly derived stall parameter captures much of the fundamental physics of instability inception in multi-stage compressors, and that it can be a good guideline for designers and engineers needing to evaluate the stability boundary of multi-stage machines.Copyright