Ronald Mailach

Bio: Ronald Mailach is an academic researcher from Dresden University of Technology. The author has contributed to research in topics: Axial compressor & Gas compressor. The author has an hindex of 11, co-authored 71 publications receiving 683 citations. Previous affiliations of Ronald Mailach include Ruhr University Bochum.

Rotating Instabilities in an Axial Compressor Originating From the Fluctuating Blade Tip Vortex

Abstract: Rotating instabilities (RI) have been observed in axial flow fans, centrifugal compressors as well as in low-speed and high-speed axial compressors. They are responsible for the excitation of high amplitude rotor blade vibrations and noise generation. This flow phenomenon moves relative to the rotor blades and causes periodic vortex separations at the blade tips and an axial reversed flow through the tip clearance of the rotor blades.The paper describes experimental investigations of RI in the Dresden Low-Speed Research Compressor (LSRC). The objective is to show that the fluctuation of the blade tip vortex is responsible for the origination of this flow phenomenon.RI have been found at operating points near the stability limit of the compressor with relatively large tip clearance of the rotor blades. The application of time-resolving sensors in both fixed and rotating frame of reference enables a detailed description of the circumferential structure and the spatial development of this unsteady flow phenomenon, which is limited to the blade tip region.Laser-Doppler-Anemometry (LDA) within the rotor blade passages and within the tip clearance as well as unsteady pressure measurements on the rotor blades show the structure of the blade tip vortex.It will be shown that the periodical interaction of the blade tip vortex of one blade with the flow at the adjacent blade is responsible for the generation of a rotating structure with high mode orders, termed as rotating instability (RI).Copyright © 2000 by ASME

Periodical Unsteady Flow Within a Rotor Blade Row of an Axial Compressor—Part II: Wake-Tip Clearance Vortex Interaction

Abstract: In this two-part paper results of the periodical unsteady flow field within the third rotor blade row of the four-stage Dresden Low-Speed Research Compressor are presented. The main part of the experimental investigations was performed using Laser-Doppler-Anemometry. Results of the flow field at several spanwise positions between midspan and rotor blade tip will be discussed. In addition time-resolving pressure sensors at midspan of the rotor blades provide information about the unsteady profile pressure distribution. In part II of the paper the flow field in the rotor blade tip region will be discussed. The experimental results reveal a strong periodical interaction of the incoming stator wakes and the rotor blade tip clearance vortices. Consequently, in the rotor frame of reference the tip clearance vortices are periodical with the stator blade passing frequency. Due to the wakes the tip clearance vortices are separated into different segments. Along the mean vortex trajectory these parts can be characterised by alternating patches of higher and lower velocity and flow turning or subsequent counterrotating vortex pairs. These flow patterns move downstream along the tip clearance vortex path in time. As a result of the wake influence the orientation and extension of the tip clearance vortices as well as the flow blockage periodically vary in time.Copyright © 2007 by ASME

The Periodical Interaction of the Tip Clearance Flow in the Blade Rows of Axial Compressors

Abstract: Current models on the tip clearance flow in turbomachines only describe the time-averaged behaviour of the flow structures. However, the real tip clearance flow is periodically fluctuating in time. This fact has to be considered for the design of turbomachine bladings especially with regard to blade vibrations and tip clearance noise.Detailed experimental investigations on the time-resolved behaviour of the flow in the rotor blade tip region were carried out in a four-stage low-speed research compressor. A strong time-periodic interaction of the blade tip vortices of adjacent blades can be shown for relatively large tip clearance of the rotor blades for operating points near the stability limit of the compressor. The resulting flow pattern, which frequency is not related to the rotor frequency, moves along the blade row. It can be described as a multicell configuration with strongly fluctuating cell number and size. The structure and propagation of the flow instability can be summarized in a model of the periodic fluctuating tip clearance flow (Mailach et al., 2000).Additional experiments were carried out in a straight cascade to improve the understanding of this flow phenomenon. It can be shown by means of time-resolved measurements that the same disturbance exists for comparable inlet flow conditions in the blade tip region of the cascade. Flow visualizations show that the blade tip vortex is strongly fluctuating and moves sometimes ahead of the leading edge of the adjacent blade. The result of this is a short-lengthscale flow pattern, which is propagating along the blade row. These experiments confirm the model of the time-periodic tip clearance flow proposed for compressors. A Strouhal-number for the estimation of the frequency of the flow fluctuation will be presented, which includes both design and aerodynamic parameters.Copyright © 2001 by ASME

Aerodynamic Blade Row Interactions in an Axial Compressor: Part I — Unsteady Boundary Layer Development

Abstract: This two-part paper presents experimental investigations of unsteady aerodynamic blade row interactions in the first stage of the four-stage Low-Speed Research Compressor of Dresden. Both the unsteady boundary layer development and the unsteady pressure distribution of the stator blades are investigated for several operating points. The measurements were carried out on pressure side and suction side at midspan. In part I of the paper the investigations of the unsteady boundary layer behaviour are presented. The experiments were carried out using surface-mounted hot-film sensors. Additional information on the time-resolved flow between the blade rows were obtained with a hot-wire probe. The unsteady boundary layer development is strongly influenced by the incoming wakes. Within the predominantly laminar boundary layer in the front part of the blade a clear response of the boundary layer to the velocity and turbulence structure of the incoming wakes can be observed. The time-resolved structure of the boundary layer for several operating points of the compressor is analyzed in detail. The topic “calmed regions”, which can be coupled to the wake passing, is discussed. As a result an improved description of the complex boundary layer structure is given.Copyright © 2003 by ASME

Rotor-Stator Interactions in a Four-Stage Low-Speed Axial Compressor—Part I: Unsteady Profile Pressures and the Effect of Clocking

Abstract: This two-part paper presents detailed experimental investigations of unsteady aerodynamic blade row interactions in the four-stage Low-Speed Research Compressor of Dresden. In part I of the paper the unsteady profile pressure distributions for the nominal setup of the compressor are discussed. Furthermore the effect of blade row clocking on the unsteady profile pressures is investigated. Part II deals with the unsteady aerodynamic blade forces, which are calculated from the measured profile pressure distributions. The unsteady pressure distributions were analysed in the first, a middle and the last compressor stage both on the rotor and stator blades. The measurements were carried out on pressure side and suction side at midspan. Several operating points were investigated. A complex behaviour of the unsteady profile pressures can be observed, resulting from the superimposed influences of the wakes and the potential effects of several up- and downstream blade rows of the four-stage compressor. The profile pressure changes nearly simultaneously along the blade chord if a disturbance arrives at the leading edge or the trailing edge of the blade. Thus the unsteady profile pressure distribution is nearly independent of the convective wake propagation within the blade passage. A phase shift of the reaction of the blade to the disturbance on the pressure and suction side is observed. In addition clocking investigations were carried out to distinguish between the different periodic influences from the surrounding blade rows. For this reason the unsteady profile pressure distribution on rotor 3 was measured, while stator 1–4 were separately traversed stepwise in the circumferential direction. Thus the wake and potential effects of the up- and downstream blade rows on the unsteady profile pressure could clearly be distinguished and quantified.© 2004 ASME

Criteria for Spike Initiated Rotating Stall

Abstract: A computational study to define the phenomena that lead to the onset of short length-scale (spike) rotating stall disturbances has been carried out. Based on unsteady simulations, we hypothesize there are two conditions necessary for the formation of spike disturbances, both of which are linked to the tip clearance flow. One is that the interface between the tip clearance and oncoming flows becomes parallel to the leading-edge plane. The second is the initiation of backflow, stemming from the fluid in adjacent passages, at the trailing-edge plane. The two criteria also imply a circumferential length scale for spike disturbances. The hypothesis and scenario developed are consistent with numerical simulations and experimental observations of axial compressor stall inception. A comparison of calculations for multiple blades with those for single passages also allows statements to be made about the utility of single passage computations as a descriptor of compressor stall.

Stall, Surge, and 75 Years of Research

Abstract: Work on rotating stall and its related disturbances has been in progress since the Second World War. During this period, certain ‘hot topics’ have come to the fore — mostly in response to pressing problems associated with new engine designs. This paper will take a semi-historical look at some of these fields of study (stall, surge, active control, rotating instabilities etc.) and will examine the ideas which underpin each topic. Good progress can be reported, but the paper will not be an unrestricted celebration of our successes because, after 75 years of research, we are still unable to predict the stalling behaviour of a new compressor or to contribute much to the design a more stall resistant machine. Looking forward from where we are today, it is clear that future developments will come from CFD in the form of better performance predictions, better flow modelling and improved interpretation of experimental results. It is also clear that future experimental work will be most effective when focussed on real compressors with real problems — such as stage matching, large tip clearances, eccentricity and service life degradation. Today’s topics of interest are mostly associated with compressible effects and so further research will require more high speed testing.Copyright © 2015 by ASME

An Experimental and Numerical Investigation Into the Mechanisms of Rotating Instability

Abstract: This paper reports on an experimental and numerical investigation aimed at understanding the mechanisms of rotating instabilities in a low speed axial flow compressor. The phenomena of rotating instabilities in the current compressor were first identified with an experimental study. Then, an unsteady numerical method was applied to confirm the phenomena and to interrogate the physical mechanisms behind them. The experimental study was conducted with high-resolution pressure measurements at different clearances, employing a double phase-averaging technique. The numerical investigation was performed with an unsteady 3-D Navier-Stokes method that solves for the entire blade row. The current study reveals that a vortex structure forms near the leading edge plane. This vortex is the result of interactions among the classical tip-clearance flow, axially reversed endwall flow, and the incoming flow. The vortex travels from the suction side to the pressure side of the passage at roughly half of the rotor speed. The formation and movement of this vortex seem to be the main causes of unsteadiness when rotating instability develops. Due to the nature of this vortex, the classical tip-clearance flow does not spill over into the following blade passage. This behavior of the tip-clearance flow is why the compressor operates in a stable mode even with the rotating instability, unlike traditional rotating stall phenomena.Copyright © 2001 by ASME

Large-eddy simulation analysis of mechanisms for viscous losses in a turbomachinery tip-clearance flow

Abstract: The tip-leakage flow in a turbomachinery cascade is studied using large-eddy simulation with particular emphasis on understanding the underlying mechanisms for viscous losses in the vicinity of the tip gap. Systematic and detailed analysis of the mean flow field and turbulence statistics has been made in a linear cascade with a moving endwall. Gross features of the tip-leakage vortex, tip-separation vortices, and blade wake have been revealed by investigating their revolutionary trajectories and mean velocity fields. The tip-leakage vortex is identified by regions of significant streamwise velocity deficit and high streamwise and pitchwise vorticity magnitudes. The tip-leakage vortex and the tip-leakage jet which is generated by the pressure difference between the pressure and suction sides of the blade tip are found to produce significant mean velocity gradients along the spanwise direction, leading to the production of vorticity and turbulent kinetic energy. The velocity gradients are the major causes for viscous losses in the cascade endwall region. The present analysis suggests that the endwall viscous losses can be alleviated by changing the direction of the tip-leakage flow such that the associated spanwise derivatives of the mean streamwise and pitchwise velocity components are reduced.

Origins and structure of spike-type rotating stall

Abstract: In this paper we describe the structures that produce a spike-type route to rotating stall and explain the physical mechanism for their formation. The descriptions and explanations are based on numerical simulations, complemented and corroborated by experiments. It is found that spikes are caused by a loss of pressure rise capability in the rotor tip region, due to flow separation resulting from high incidence. The separation gives rise to shedding of vorticity from the leading edge and the consequent formation of vortices that span between the suction surface and the casing. As seen in the rotor frame of reference, near the casing the vortex convects toward the pressure surface of the adjacent blade. The approach of the vortex to the adjacent blade triggers a separation on that blade so the structure propagates. The above sequence of events constitutes a spike. The simulations show shed vortices over a range of tip clearances including zero. The implication is that they are not part of the tip clearance vortex, in accord with recent experimental findings. Evidence is presented for the existence of these vortex structures immediately prior to spike-type stall and, more strongly, for their causal connection with spike-type stall inception. Data from several compressors are shown to reproduce the pressure and velocity signature of the spike-type stall inception seen in the simulations.Copyright © 2012 by ASME