Incipient surge detection in automotive turbocharger ...

of 14

2019-24-0186

Incipient surge detection in automotive turbocharger compressors

S. Marelli, P. Silvestri, V. Usai, M. Capobianco

Università degli Studi di Genova

Abstract

Nowadays, turbocharging is a technique widely used to improve fuel

consumption and exhaust emissions in automotive engines.

Centrifugal compressors are typically adopted, even if an efficient

engine integration is often restricted by surge phenomena.

The focus of the present work is to describe an experimental analysis

developed with the aim at characterizing and identifying compressor

behavior in incipient surge conditions. The acoustic and vibrational

operative response of two automotive centrifugal compressors has

been experimentally analyzed on the test facility operating at the

University of Genoa.

Each compressor is characterized by a classical architecture and one of

them is equipped with a “ported shroud”, which enlarges stable zone.

Compressors characteristic curves have been measured under steady

flow conditions for different levels of corrected rotational speed from

the choking region to the surge line. Moreover, transient operations

have been taken into account where, starting from a stable condition,

surge phenomenon was generated progressively closing a valve

positioned downstream the compressor or increasing rotational speed

keeping fixed the circuit characteristics.

Accelerometer and microphone signals have been acquired and

analyzed both in the sub-synchronous contents and in higher

frequencies range related to blade passage phenomenon.

In addition, instantaneous compressor outlet pressure, turbocharger

rotational speed and temperature (captured close to the rotor) have

been recorded.

Different signal processing techniques in time and frequency domain

have been considered to analyze components and to separate the whole

signal into individual source contributions to identify the system status.

The obtained results provide an interesting diagnostic and predictive

solution to detect compressor instabilities at low mass flow rate

operating conditions.

Introduction

Nowadays, downsizing and turbocharging techniques are widely used

to improve fuel economy in automotive engines.

Centrifugal compressors are key components in the modern

turbocharged Internal Combustion Engine (ICE) design and an

efficient implementation is often restricted by the surge phenomenon.

The prediction and the prevention of surge are relevant aspects to avoid

unacceptable operating conditions and damaging problems [5, 6].

By further reducing the mass flow rate, rotating stalled flow patterns

are triggered. In this case, one or more stalled flow cells can propagate

along the impeller annulus, and thus the resulting pressure oscillations

frequency is a fraction of the impeller rotational speed [1, 4]. Emmons

et al. [1] presented an experimental investigation on compressor surge

and stall propagation through a hot-wire anemometer system. It was

observed that the whole compressor flow system may be unstable in

the manner of a self-excited Helmholtz resonator. Fink et al. [2]

investigated the turbocharger surge in a radial impeller-vaneless

diffuser free-spool system. Measurements showed impeller stall at the

inducer tips to be a key phenomenon in initiating surge. Yano [3] et al.

studied the effect of pulsating flow on compressor performance

through theoretical calculations and experiments to validate the model.

The study of compressor instability is also extremely relevant for

transient performance and boost pressure control purposes, as

indicated by the numerous contributions available on the topics of

modeling and model-based control [4].

The main focus on surge investigation is the analysis of the inception

process and the origin of surge occurrence, in order to early predict the

approaching of this undesired operating condition.

Experimental investigation on compressor performance together with

the analysis of the vibro-acoustic operational system response seems

to give interesting diagnostic tools to correctly predict this

phenomenon [7-12]. Munari et al. [7] performed experimental

investigations in the stable and unstable operating conditions of an

aeronautic turboshaft gas turbine axial-centrifugal compressor. In the

paper the test field data are analysed showing the performance of the

turbomachinery and its transient behaviour toward and during surge

using pressure, temperature and mass flow sensors located in strategic

positions along the circuit. In [8] the concept of “quasi-dynamic”

pressure measurements is introduced in order to obtain compressor

spectral maps describing the process of surge onset in a centrifugal

blower. Kabral et al. [9] proposed a new method based on the full

acoustic 2-port model for investigations of centrifugal compressor stall

and surge inception. Marelli et al. [10] adopted a specific flexible

circuit installed downstream the compressor to investigate the effect of

circuit geometry on a turbocharger compressor performance with

special reference to surge phenomenon. Instantaneous static pressures

are measured in several locations upstream and downstream the

compressor. Besides, dynamic sensors to measure noise and vibrations

are also adopted. In [11] experimental results obtained from a T100

microturbine connected with different volume sizes are shown with

special reference to surge operations. Ferrari et al. [12] presented a

surge prevention technique for advanced gas turbine cycles carrying

out a vibration analysis.

The vibro-acoustic signal analysis showed a significant increase of

energy content in specific frequency bands, not only during surge

events, but also close to the unstable condition [13, 14]. This technique

seems to be promising due to the adoption of not intrusive standard

sensors, able to be easily implemented in the on board automotive

diagnostic system.

In previous works, high noise levels near the Helmholtz frequency

were observed at compressor low mass flow rate levels, moving

towards the undesired working region. Moreover, a significant signal

content in the sub-synchronous range seems to be present close to the

unstable condition and it may be related with rotating stall

phenomenon [13].

Recently, the research approach based on vibro-acoustic surge

precursors seems to be promising and complementary to other

techniques based on compressor performance studies [7-12].

A turbocharger compressor was tested by the researchers of National

Technical University of Athens [15], observing an increase in low

of 14

frequency energy content below rotational frequency (sub-

synchronous frequency contents) through microphones. Therefore, the

RMS value of the sub-synchronous part of the spectrum was proposed

as a parameter to distinguish compressor operating conditions. In [16,

17], the Authors developed significant studies to find stall or surge

precursors starting from turbocharger vibro-acoustic signals. A six

axial and one centrifugal compressor stages were tested highlighting a

sub-synchronous content in the spectrum map of the accelerometers

mounted in correspondence of the axial stages when the throttle valve,

used to control external circuit characteristic curve, was nearest to the

closed position.

In general, the significant low frequency noise and vibration level at

low mass flow rates are a well-recognized problem of the centrifugal

compressors and it well worth to deep these aspects for early

identification of surge condition.

In the work here presented, a method to investigate surge phenomenon

of turbocharger centrifugal compressors is proposed, mainly based on

vibro-acoustic response signal. The aim of the activity is to detect an

incipient surge condition in a reliable way, avoiding false positives.

The experimental campaign was developed on the test facility

operating at the University of Genoa (UNIGE), considering different

operating points ranging from the choking region to the surge line, of

two compressors characterized by different size. During the tests,

pressure signals and instantaneous turbocharger rotational speed are

measured using high frequency response transducers together with

vibro-acoustic measurements.

A preliminary analysis has been performed to better understand the

frequency contents of the acquired signals and to identify the more

suitable sensors for diagnostic purpose.

The frequency characteristics of the vibro-acoustic signals were

obtained by computing a “Fast Fourier Transform” (FFT) and

discussed at different operating conditions from stable to unstable

conditions.

In addition, the information of a fast response pressure probe mounted

downstream the compressor has been analyzed and compared with the

others instantaneous recorded signals.

Sub-synchronous spectrum has been evaluated highlighting a

significant operational content close to the Helmholtz frequency of

inlet and outlet compressor circuits previously identified with a

specific experimental activity. Moreover, the sub-synchronous

characteristics, probably associated to the rotating stall, has been

considered.

In addition, high frequency range has been also analyzed to detect the

intensification of unusual frequency contents in the response signal

while surge condition is approaching. This effect is maybe related to

other sources interacting with typical compressor operational

phenomena like blade passage. A method based on high frequency

signal demodulation is used to better identify sub-synchronous

phenomena as rotating stall, not always easily detectable through a

direct analysis of the sub-synchronous spectra.

Signal energy levels have been calculated and their trend on

compressor characteristic curves has been analyzed on a wide

operative range with particular attention to the low mass flow rate

region.

This information could be used to find precursors of surge occurrence

to be used as control data (i.e., an on/off signal for a bleed valve)

implemented in an on board engine control system.

Experimental set-up

The experimental activity was developed on two different turbocharger

compressors: a turbocharger IHI RHF3 adopted for downsized spark

ignition engines, and a turbocharger Borg Warner EFR 7670-C for

heavy duty diesel engines. The tested compressors are characterized

by a vaneless diffuser, different size, and the Borg Warner EFR 7670-

C is equipped with a ported shroud to enlarge flow stability [18]. In

Table 1, the main geometric characteristics of the tested turbochargers

are reported.

Table 1 – Turbochargers geometric characteristics

Turbocharger type Borg Warner IHI

Compressor impeller inlet

diameter

78.6 mm 40 mm

Turbine impeller outlet diameter

69.5 mm 33 mm

N° of compressor blades 14 10

N° of turbine blades 8 11

The experimental activity was developed on the turbochargers test

bench of the University of Genoa (Figure 1), fully described in

previous papers [19-21]. Three screw compressors supply a total mass

flow rate of 0.6kg/s at a maximum pressure of 8bar. The turbine inlet

temperature can be controlled in order to develop test in cold and hot

condition (up to 750°C, depending on the size of tested turbocharger).

The compressor inlet pressure can be controlled by regulating systems

or can directly work under ambient pressure condition (AF, in Figure

1). An automatic data acquisition system allows measurements of

thermodynamic parameters to be performed at different measuring

sections.

Average and instantaneous static pressures were measured through

piezoresistive transducers (accuracy of ±0.15% of full scale). K type

thermocouples (accuracy ±1.5°C) and platinum resistance

thermometer (Pt 100 Ohm class A, accuracy of ± 0.15°C + 0.2% of

measured value) were used to measure air temperature. Besides, an

exposed junction K type thermocouple, placed close to the inducer of

IHI turbocharger, was used to better appreciate the temperature rise

and fluctuation close to unstable zone. An eddy current probe mounted

close to the compressor wheel was used for turbocharger rotational

speed measurement (accuracy of ±0.009% of full scale). Compressor

mass flow rate was measured using a thermal mass flow meter

(accuracy of ±0.9% of measured value and ±0.05% of the full scale).

Instantaneous pressure and turbocharger rotational speed were

measured with high-frequency response probes and two high speed

data acquisition cards.

of 14

Figure 1. The turbocharger test facility of the University of Genoa.

Vibration and acoustic measurements were conducted using a

dedicated data acquisition system, which allows to acquire 16 different

channels with frequencies up to 204.8kHz sampling rate per channel,

at the same time (Table 2). Structural measurements were performed

using mono accelerometers located at the compressor inlet (called a:1,

Table 2) and outlet (called m:2, Table 2). Sensor frequency response

allowed to investigate signals components up to 10kHz. Three micro

mono axial accelerometers (with a resonance frequency higher than

55kHz) were mounted on the compressor housing in radial (X),

tangential (Y) and axial (Z) direction with respect to the volute (called

c:1, Table 2 and Figure 2 on left side). These probes allow to extend

vibration investigation to the higher frequencies, in the range of blade

pass frequency (BPF) phenomena.

Acoustic measurements were carried out with pre-polarized

microphones with range between 2 and 50kHz of dynamic response.

When the filter (AF, in Figure 1) is adopted, radial oriented

microphones (with reference to the duct) equipped with ellipsoidal

windscreen was assembled at the compressor inlet in order to capture

acoustic contribution to dynamic phenomena related to the wave

propagation in the feeding line (called asp:1, Table 2). Radial and axial

oriented microphones were positioned near compressor housing in

order to capture vibro-acoustic radiated noise from the turbocharger

(called comp:1, b&k:1, Table 2 and Figure 2 on right side).

Besides, a high-frequency response pressure transducer was placed

downstream the compressor to capture inflow pressure time history

(called kul:1, Table 2).

Table 2. List of measured vibro-acoustic variables.

Location Variable Description

c:1

c:1:+X Radial acceleration on compressor

housing

c:1:+Y Tangential acceleration on compressor housing

c:1:+Z Axial acceleration on compressor housing

a:1 a:1:+X Acceleration on inlet compressor line

m:2 m:2:+X Acceleration on outlet compressor line

b&k:1 b&k:1:+S Acoustic pressure on compressor housing (radial oriented)

comp:1 comp:1:+S Acoustic pressure on compressor housing

(axial oriented)

asp:1 asp:1:+S Inlet filter acoustic pressure

kul:1 kul:1:+S Inlet flow pressure outlet compressor

temp:1 temp:1 Temperature fluctuations in the

compressor inlet

Figure 2. Detail of the three accelerometers on the compressor housing (the

arrows indicate the measurement directions) on left side; microphones close to

IHI compressor housing on right side.

The experimental tests were conducted by controlling the turbine

power to drive the compressor rotational speed to a desired value. The

compressors performance is evaluated over an extended range, using

the motorized valve shown in Figure 1 to control the position of the

operating point on the characteristic curve. The turbocharger shaft

speeds were varied up to 130krpm for Borg Warner and to 220krpm

for IHI. Acquisitions were developed under steady flow conditions to

define a set of different stable operating points on the compressors

characteristic curve for different corrected rotational speeds (ncr)

defined as

n𝑐𝑟 =𝑛∙√𝑇0

√𝑇𝑇1

where:

n is turbocharger rotational speed

T0 is reference condition (equal to 293.15K)

TT1 is compressor inlet total temperature

Then, the motorized valve was progressively closed to induce surge in

the compressor. To detect the transition from stable to unstable

operations, i.e. the position of the surge line, the compressor outlet

pressure was recorded as the reference signal, monitoring its frequency

spectrum, and detecting the harmonics associated with the surge

phenomenon [10].

Besides, a very slow transient operation in compressors speed was

analyzed, maintaining the circuit (i.e., the motorized valve) in the same

condition.

Analysis on sensor frequency contents and

harmonic response of the circuit

Before performing a purely diagnostic analysis, the dynamic behaviour

of both turbochargers were experimentally investigated in the stable

operating region to obtain the vibro-acoustic responses in safe

operating conditions.

Frequency analysis allowed to define signal spectrum contents and a

preliminary identification of the sources was also performed aiming to

classify the principal contributions in the operational system

responses. Moreover, the sensors ability to recognise a specific source

based on the typology (structural, acoustic, fluid-dynamic) and the

measurement position were analyzed.

The aim of this preliminary activity was to improve the monitoring

system in order to identify which kind of response is more useful and

suitable to identify an incipient surge condition.

The rotor-dynamic behavior of each compressor is very similar, due to

the same radial hydrodynamic floating ring bearings adopted, which

AF Air Filter LM Laminar Flow Meter

AH Air Heater PC Pressure Control

APH Air Pre-Heater PG Pulse Generator

AR Air Reservoir SC Screw Compressor

C Compressor T Turbine

LC Lubricating Circuit TM Thermal Mass Flow

Meter

of 14

are fundamental in defining the dynamic characteristic of a rotating

machine [22, 23].

IHI turbocharger stable behavior was already analyzed and reported in

[10] and its operational response is mainly related to residual

unbalance, whirl and whip instabilities in hydrodynamic bearings and

blade pass phenomena (both of compressor and turbine). Moreover,

broadband contents related to fluid-dynamic phenomena and system

resonances have been found in some operating conditions [10].

In the following figures, Borg Warner turbocharger stable operational

response is analyzed at a constant corrected rotational speed of 80krpm

close to the maximum efficiency operating point. Figure 3 reports two

acoustic autopower spectra recorded with microphones placed at the

filter section (red trace) and close to the compressor volute (green

trace). In Figure 4, the autopower spectra of a pressure signal measured

downstream compressor (red trace) and of a structural signal (green

trace) recorded through an accelerometer installed on the compressor

housing are shown. For each signal, the obtained function describes

the distribution of power into frequency components. The autopower

algorithm allow to identify both random and deterministic

components. The presence of synchronous 1X component related to

residual unbalance (black dashed line cursor at 1357Hz) is apparent.

The 2X components (black dashed line cursor at 2714Hz) can be

detected only for the spectra of accelerometer and pressure signals

downstream the compressor, maybe due to system nonlinearities and

to a resonant behavior able to amplify the energetic content in the

system responses.

Figure 3. Example of microphone signals spectra at the inlet filter (red trace, at the bottom) and at compressor housing (green trace at the top) for ncr = 80krpm,

in maximum efficiency condition.

Figure 4. Example of pressure signal spectra measured downstream

compressor (red trace, at the bottom) and accelerometer signal spectra on

compressor housing (green trace, at the top) for ncr = 80krpm, in maximum efficiency condition.

The sub-synchronous content at 574Hz due to the floating ring radial

bearings is mainly present in the accelerometer signal (black dotted

cursor). This contribution is more prominent than the one related to

residual unbalance underlining how bearings are significant in the

overall turbocharger vibrational response. The floating ring radial

bearing content is well detectable in both microphone spectra, while it

is almost absent in pressure signal downstream the compressor

denoting that this phenomenon does not seem to involve fluid domain.

In high frequency range, compressor BPF is detectable for all acquired

signals (blue dashed line cursor), while turbine BPF (magenta dashed

line cursor) can only be seen in autopower spectra of microphone close

to the turbocharger and of accelerometer on the compressor housing.

In some cases, peaks are detectable at a frequency close to the BPF and

may be related to mechanic sources in low frequency (synchronous

component 1X e floating ring radial contribution) that interact with the

blade pass phenomenon [24].

Some marked frequency contents at low frequency are present in the

microphone spectrum at the filter inlet section (see blue ellipse in

Figure 3 and green dashed line cursor at 205Hz), differently from the

accelerometer signals. These contributions may be related to circuit

acoustic modes and to wave dynamics phenomena in the inlet duct.

The system structural properties at these frequency levels seems to

filter these contributions reducing their energy in structural responses.

In Figure 5 the spectrum of the accelerometer placed on the compressor

housing in radial direction is reported for IHI turbocharger at a constant

corrected rotational speed of 90krpm close to the maximum efficiency

operating point. The frequency components show a strict analogy with

those detected for Borg Warner turbocharger. Also in this case, the

frequency contents may be easily associated to the principal system

sources (dashed line black cursor at 1505Hz for 1X, blue and magenta

cursors respectively for compressor and turbine BPFs, dotted line

black cursor at 437Hz and its second order for whirl/whip in radial

bearings).

For IHI turbocharger, the contributions related to the bearings

instability are much more apparent and dominant maybe related to the

lower rotor inertias, which induce higher instability in the bearings [22,

23]. Also for IHI turbocharger high frequency peaks near BPFs related

to low frequency sources (1X, whirl/whip) interacting with high

frequency ones are evident [24].

10000 200005000 15000 25000

Hz

comp:2:S (CH7)

0.00

0.02

Pa

1357 9519 13561

572

205

AutoPower comp:2:S

10000 200005000 15000 25000

Hz

asp:1:S (CH6)

0.00

0.02

Pa

1357 9519 13561

572

205

AutoPower asp:1:S

0 10000 200005000 15000 25000

Hz

c:1:+X (CH1)

0.0

0.1

0.2

g

1357

574

9512 13561

AutoPower c:1:+X

0 10000 200005000 15000 25000

Hz

kul:2:S (CH11)

0

4

8

12

16

Pa

1357

574

9512 13561

AutoPower kul:2:S

of 14

Figure 5. Example of compressor housing accelerometer spectrum – IHI

turbocharger - for ncr = 90krpm, in maximum efficiency condition.

Synchronous and sub-synchronous components related to residual

unbalance and bearing phenomena are not evident in the accelerometer

signal at the compressor inlet section. This may be justified by the fact

that elastic junctions have been used for the connection of the

compressor with the test bench pipes so that, at these frequencies, they

are structural uncoupled (see Figure 2).

Therefore, no significant structural path is available allowing

propagation of structural components from turbocharger to the pipe

line. Moreover, it seems that these mechanical sources have only a

slight interaction with fluid and so fluid-dynamic path also is not

available.

Surge is an unstable operation state, characterized by components in

sub-synchronous domain (rotating stall and broad band phenomena

that excite pipe line acoustic modes) and in high frequency domain

(BPF) [4, 7, 11]. For an easy early detection, it seems useful to analyze

high sensitivity signal to fluid-dynamics phenomena instead to

mechanical ones. System transmissibility seem fundamental as it can

attenuate or amplify the responses to certain sources. As reported

hereafter, downstream compressor pressure sensor and microphone

located in correspondence of the filter seem to be more useful for this

purpose.

In the proposed paper, the acoustic Frequency Response Functions

(FRF) of inlet and outlet compressor sections were identified to

validate whether this information can help the interpretation of the

system behavior in the proximity of unstable operating condition.

The excitation was introduced using an electromechanical shaker

instead of the compressor. The exciter head was positioned

alternatively in the inlet and outlet pipe line in correspondence of the

connection section of the compressor. The shaker was driven in sine

mode with variable frequency and allowed to generate plane waves

inside the ducts.

FRFs were evaluated between microphone response signal in a specific

point of the analyzed pipe line, and a reference signal correlated with

the generated acoustic excitation in the duct. In this case, the electrical

signal driving the external sources was selected as the reference.

During the investigation on the surge onset, the compressor interacts

with the system differently from the shaker, introducing different

boundary conditions in the pipe line and leaving a coupling between

the intake and outlet circuit that is totally absent in the case of shaker

tests. Furthermore, the use of the shaker does not consider the

contribution of the compressor related to its operating condition, which

could significantly influence the dynamic response of the whole

system [9].

Despite these considerations, a comparison between the FRFs

obtained through the exciter and the system operational response

spectra is performed to improve the system response analysis in order

to assess possible correlations.

Figure 6 shows the acoustic FRFs obtained for the inlet (on the top)

and outlet (on the bottom) lines in the 3-200Hz range. In the case of

the outlet circuit, a significant sensitivity of the system dynamics

related to the fractional opening of the throttling valve was observed.

In particular, red and green lines are respectively referred to the fully

open and closed position of the throttle valve. The functions trends are

well defined, and different peaks at certain frequencies maybe

corresponding to possible acoustic modes of the system can be easily

identified.

The peaks seem to be related to the propagation phenomena of

perturbations in the ducts for both inlet and outlet sections. The

frequency values are fairly near to the Helmholtz frequency and higher

multiple frequencies, defined as

f𝐻 =𝑎∙√𝐴

2𝜋√𝑙∙𝑉

where:

a is the speed of sound

l and A are respectively the equivalent length and cross-sectional area

of the duct

V is the system air volume [4].

Figure 6. Identification of the inlet circuit (on the top), outlet pipe (on the

bottom) for two extreme values of throttle valve opening.

Surge identification - analysis of instantaneous

signals

0 10000 20000 300005000 15000 25000

Hz

c:1:+X (CH1)

0

g

1505 14981

16489437

AutoPower c:1:+X

10 1003 4 5 6 7 8 9 20 30 40 50 60 70 80

Hz

10e-3

100e-3

1

10

6e-3

20e-3

30e-3

50e-3

70e-3

200e-3

300e-3

500e-3

700e-3

2

3

5

7

Pa

/V

14.9 29 45 66 86

FRF - inlet circuit

10 1003 4 5 6 7 8 9 20 30 40 50 60 70 80

Hz

100e-3

1

10

70e-3

200e-3

300e-3

400e-3

600e-3

2

3

4

6

20

Pa

/V

12 29 41 55 70 84 101 117

130

147

25

80

fully open position - V=0%closed position - V=100%

FRF - outlet circuit

of 14

For incipient surge detection, the vibroacustic compressor response for

both turbochargers is initially analysed for different values of

compressor mass flow rate, considering a fixed compressor rotational

speed. This technique allows to analyse different operating points on

the compressor map and to observe transition from stable operation to

unstable conditions. FFT analysis has been used to detect how

frequency signal contents in the system responses change from a stable

to a near unstable condition. Hereafter, the results are subsequently

presented with a particular emphasis in the sub-synchronous frequency

range and at high frequency value in correspondence of the blade pass

frequency (BPF).

As reported in literature [10-12, 15-17], the sub-synchronous

frequency range are attractive for the surge detection because it does

not take into account contents due to vibrational sources such as

residual unbalance or other phenomena associated to the turbocharger

rotational speed and higher multiples.

There is a sub-synchronous mechanicals contribution due to the

floating ring radial bearings adopted in both turbochargers, which is

clearly identifiable as it depends mainly on the rotational speed and

can be easily separated and removed through an order-stop filter [10].

Therefore, its energy (RMS value) is strictly related to fluid

characteristics and it is more sensitive approaching the incipient surge

condition. In Figure 7, the auto-power spectra acoustic response for 5

different valve openings from far (trace no. 1) to immediately above

surge event (trace no. 5) for the Borg Warner compressor (ncr =

80krpm) are shown. Microphone signal positioned along the filter has

been considered as it contains information regarding the circuit fluid

dynamic phenomena and the compressor operating condition.

The FFT analysis highlights that low-frequency contents (up to 300Hz)

result in a significant increase of their energy level, when approaching

surge (blue elliptical marker on trace no. 5). This effect seems to be

related to the surge occurrence broadband source due to not optimal

flow condition in the compressor exciting system acoustic resonance (

dashed line black cursors, Figure 7) causing significant peaks in the

response system signal.

Figure 7. Auto-power spectra of the microphone at the filter section for ncr =

80krpm. Sub-synchronous vibration contents highlighted in the blue ellipse (frequency axis in logarithmic scale) – Borg Warner turbocharger compressor.

For each trace it is noticeable the presence of the synchronous

contribution 1X (blue dotted cursor at 1360Hz), characterized by more

significant intensity in the last stable operating point probably due to

anomalous outflow condition in the rotor which generates an

additional unbalance in the system [4]. Besides, frequency components

close to the 1X (orange ellipse in Figure 7) related to new phenomena

that arise in the system approaching the incipient surge condition.

For traces no.4 and 5, a peak can be observed close to 400Hz frequency

(green dashed line cursor in Figure 7) probably related to the rotating

stall of the compressor. This aspect will be later on discussed by

correlating this contribution in the sub-synchronous field to the blade

passage within an analysis in high frequency signal contents.

In Figure 8 (on the top) the envelope trace of the previous spectra

calculated in stable conditions (shown in red) and the spectrum of the

acoustic response in deep surge (shown in green) are reported. The

envelope trace highlights the significant peaks of all operational

responses before reaching the unstable condition. In Figure 8 (on the

bottom) the same traces for the IHI turbocharger are reported for a

corrected rotational speed level of 118krpm.

Figure 8. Envelope auto-power spectrum trace evaluated referring to the

previous stable spectra (left side - primary axis) and auto-power spectrum in deep surge condition (right side - secondary axis) - (Borg Warner compressor,

ncr = 80krpm on the top; IHI compressor, ncr = 118krpm on the bottom).

For each tested compressor, common peaks close to low frequency

domain (up to 300Hz) can be observed for the envelope and the deep

surge spectra.

Slight variations in the frequency contents of the system operational

response when the compressor is operating in a stable or deep surge

condition can be highlighted. It seems that some dynamic properties

and some specific natural frequencies, that cause system operational

response, are not significantly influenced by a change of compressor

operating condition.

However, the energy contents at certain frequencies can instead be

very sensitive to changes in the operating condition of the machine.

For both compressors the signal amplitude is more significant in deep

surge condition (see different range in the two secondary axes).

Slight differences in operating frequencies can be observed when

comparing the spectra of the two turbochargers due to a low sensitivity

10 100 10005 6 7 8 9 20 30 40 50 60 7080 200 300 400 500 700

Hz

1

2

3

4

5

0.12

Pa

4341360

14.87.1 28 43 65 87

10 100 10003 4 5 6 7 8 20 30 40 50 60 80 200 300 400 600 2000

Hz

asp:1:S (CH6)

0.0

0.1

Pa

0

1

2

3

Pa

14.8 146228 45 73 118

1360

2057.0 286

AutoPower asp:1:S envelopeAutoPower asp:1:S deep_surge

10 100 10003 4 5 6 7 8 20 30 40 50 60 80 200 300 400 600 2000

Hz

asp:1:S (CH6)

0.0

0.1

Pa

0

1

Pa

13.8 208827 46 71 120

2176

7.0 194 291

AutoPower asp:1:S envelopeAutoPower asp:1:S deep_surge

of 14

of the vibro-acoustic characteristics of the entire system with respect

to the compressor.

In the case of a deep surge, low frequency component is highlighted in

the spectrum at 7.0Hz for IHI turbocharger. In this operating condition,

the periodic reversal flow could excite low frequency modes not

receiving enough energy in stable operation.

For the Borg Warner turbocharger, the operating envelope (on stable

operating points) and deep surge spectra are calculated for further

levels of corrected rotational speed (ncr = 50, 65, 95 and 118krpm). For

each iso-speed level, a low dependence of the frequency contents is

confirmed by changing the mass flow rate.

Besides, the operational response spectra for different levels of

rotational speed exhibit similar trends in the low frequency range (up

to 300Hz) with the corresponding peaks at similar frequency values.

This seems to highlight a low dependence on the nature of the

responses (in terms of frequency contents), hence on the dynamic

properties of the system for a change in the compressor rotational

speed (Figure 9).

Figure 9. Envelope auto-power spectrum trace evaluated on stable spectra (left

side - primary axis) for Borg Warner turbocharger (on the top) and (auto-power

spectrum in deep surge condition for 5 rotational speed levels (on the bottom).

An invariance in terms of the characteristic frequencies of the system

response below 300Hz is observed for each operating conditions and

for both considered turbochargers.

In the field of sub-synchronous frequencies, it is possible to identify

other frequency bands characterized by a modified energy content,

significantly when the compressor approaches a surge condition.

At higher frequencies (0.5-1X range) a high peak density can be

observed probably due to the pipe line acoustic modes and to the

compressor operating condition.

A global energy trend at higher frequency levels is not always suitable

to detect when the compressor approaches an unstable operating

condition.

If the previous operational spectra (Figures 7-9) are compared with the

FRF obtained through the harmonic response of the circuit (Figure 6),

a good agreement may be found on the frequencies peak for both

functions (i.e., acoustic modes of the line identified by shaker and

operational responses in different compressor operating conditions).

The low frequency operational content seems to be highly dependent

on the compression line characteristics, with particular reference to the

inlet circuit and not to the compressor type and operating conditions.

The frequency values are close to the calculated Helmholtz frequency

and higher multiple frequencies (blue cursors in fig. 6 at 14.9, 29, 45,

66, 86 Hz).

Therefore, the experimental investigation on the pipe line response

seems important in the absence of the compressor and in the condition

of zero mass flow rate, to obtain information on the signal

contributions which can provide useful diagnostic information to

identify the incipient surge. In Figure 10, the measured operational

spectra for the sub-synchronous frequency range (1X blue dashed

cursor at 1360Hz) of the accelerometer mounted on the Borg Warner

compressor inlet are shown for the five previously considered

operating points at ncr = 80krpm.

Figure 10. Accelerometer signal spectra evaluated for ncr = 80krpm (from 0 to

4 operating points, i.e. from the maximum flow condition towards the surge

occurrence) – Borg Warner turbocharger

The contributions in the vibrational response seem not relevant with a

slight sensitivity to the surge up to 40Hz. A more prominent sensitivity

can be observed at higher frequencies (> 40Hz) probably related to the

dynamic properties of the system which cause the measured structural

responses. A significant increase in structural signal energy was

observed in a frequency range close to the 0.5X-1X range, where the

spectra trends change with an increase in energy content passing from

operating point 1 to 5, i.e. from the maximum flow condition towards

the surge occurrence. Structural signals measured at the compressor

housing highlight a worst sensitivity in identifying the surge

occurrence in the field of sub-synchronous frequencies, maybe due to

the presence of marked mechanical contributions which dominate the

effects of fluid dynamic response [10]. If the mechanical contributions

are properly removed, a diagnostic information suitable in the high

frequency field can be achieved.

For each compressor, the signals contents were taken into account for

diagnostic purposes, even during a surge transient. To observe the

transition from stable to unstable condition, the operational responses

were analysed during a turbocharger run up, increasing the compressor

rotational speed for the same position of the throttle valve (i.e., same

external characteristic curve of the circuit).

The transient investigation was run in a sufficiently slow manner to

reproduce a sequence of steady operating condition of the system.

Therefore, acquired signals represent a sequence of working points

characterized by a fixed valve position for each rotational speed value.

In Figure 11, the results are reported for a run-up starting from a

rotational speed of 60krpm for Borg Warner compressor: the

beginning of deep surge occurs at a rotational speed of about 92krpm.

The time-frequency analysis on the signal spectrum was conducted to

evaluate the instant of the surge inception, confirming that sub-

synchronous content is more significant for the surge detection. Figure

11 shows time-frequency analysis through a zoomed color-map of

10 1003 4 5 6 7 8 9 20 30 40 50 60 7080 200 300 400 500

Hz

asp:1:S (CH6)

0.1

0.2

0.3

0.4

0.5

0.6

Pa

14.8 28 45 68 95 2057.0 286

AutoPower asp:1:S envelope 050krpmAutoPower asp:1:S envelope 065krpmAutoPower asp:1:S envelope 080krpmAutoPower asp:1:S envelope 095krpmAutoPower asp:1:S envelope 118krpm

10 1003 4 5 6 7 8 9 20 30 40 50 60 7080 200 300 400 500

Hz

asp:1:S (CH6)

2.0

4.0

6.0

8.0

Pa

14.8 28 45 68 95 2057.0 286

AutoPower asp:1:S deep_surge 050krpmAutoPower asp:1:S deep_surge 065krpmAutoPower asp:1:S deep_surge 080krpmAutoPower asp:1:S deep_surge 095krpmAutoPower asp:1:S deep_surge 118krpm

10 100 10003 4 5 6 7 8 20 30 40 50 60 80 200 300 400 600 2000

Hz

a:1:+Z (CH5)

0.00

0.01

0.02

0.03

g

1093.2

1360

86657

735

153 309 461

AutoPower a:1:+Z 1AutoPower a:1:+Z 2AutoPower a:1:+Z 3AutoPower a:1:+Z 4AutoPower a:1:+Z 5

of 14

acoustic pressure signal, measured at the filter section. The horizontal

(x) and vertical (y) axis represent the frequency content and the time,

respectively. The amplitude is related with the color intensity with

reference to the color-bar. Low frequency components (red dashed line

cursors in Figure 11) highlight a significant variation together with an

energy content rise during the transient operation approaching the

surge condition.

Figure 11. Time frequency analysis of microphone signal at the filter section

during surge transient operation (zoomed map range 3-1000Hz).

For better identify the most significant contents of the system response

during transient, the average function of all measured spectra was

assessed. In this function, the contents at a specific frequency common

for each spectrum are apparent, while the variable ones are not

detectable. Figure 12 shows the trend of the average spectra for each

compressor and in both cases the presence of very sharp peaks

indicates the existence of stable contributions that remain at constant

frequencies during the run up.

As under steady state investigation, it is possible to note significant

components in the acoustic responses, which remain at a fixed

frequency value during the transient for both compressors. This aspect

highlights the significant influence of the circuit on the operational

response.

The frequency contents evaluated in the transient investigations

present the same behavior of steady state, so linked to the Helmholtz

frequency and its multiples.

Figure 12. Average spectrum function of the acoustic response at the inlet

section during transient investigation (red line - Borg Warner compressor, green

line – IHI compressor).

A similar behavior can be observed for the microphone signal located

close to the compressor housing (Figure 13). In this case, the

contributions related to the rotational speed are apparent, such as the

sub-synchronous of the floating ring radial bearings (see orange arrow)

and synchronous of residual unbalance (see green arrow). In the low

frequency band, contributions related to the pipe line characteristic can

be observed in the compressor housing microphone signal. The

structural responses (Figure10) are not characterized by low frequency

contents. An acoustic path could be assumed between the inlet filter

and the microphone located close to the compressor housing.

Figure 13. Time frequency analysis at the compressor housing of the

microphone signal during transient operation (zoomed map range 3-1000 Hz).

Vibration data were recorded through a high sample frequency value

(100 kHz) to investigate vibrational and acoustic response in high

frequency domain close to the blade pass frequency (BPF).

Figure 14 shows high frequency components of the auto-power spectra

measured on the Borg Warner compressor housing in radial direction

from stable (operating point 1, higher spectrum) to incipient surge

(operating point 5, lower spectrum) conditions for ncr = 80krpm

(operating point 4, middle spectrum).

It is possible to note that the frequency peak associated with the BPF

(dotted blue line cursors at 9.4kHz and 18.8kHz for the BPFs without

or with splitter blades, Figure 14) becomes less marked when moving

from point 1 to point 5. Near unstable conditions, BPF component

seems to lose energy and, at the same time, new contributions rise

related to fluid-dynamic phenomena linked to the incipient surge.

Figure 14. High frequency detail of the radial acceleration auto-power spectra

evaluated on the compressor housing in three different operation conditions.

In the last stable condition before incipient surge, significant energy

content is observed in the 6X-7X frequency band (8079-9425Hz, see

left black dashed line and blue dotted line cursors). In this range it is

possible to identify a main contribution at 9019Hz (cyan dashed

cursor) which is a frequency that differs from the BPF by a value equal

to 9425-9019 = 406Hz, lower than the synchronous frequency

(406Hz<1X = 1346Hz).

In the spectrum, a side band near the main peak corresponding to the

BPF arises and it is clearly identifiable. This could lead to the

hypothesis of the rise of a new phenomenon that interacts with the

blade passage characterized by a sub-synchronous characteristic

frequency of 406Hz (<1X) [24].

10 100 10003 4 5 6 7 8 9 20 30 40 50 60 7080 200 300 400 500 700

Hz

asp:1:S (CH2)

10

20

30

40

50

60

70

5

15

25

35

45

55

65

s

0.50

0.00

Pa

7.2 15.9 27.6 46.1 68.6 94.7 225

10 1003 4 5 6 7 8 9 20 30 40 50 60 70 80 90 200 300

Hz

0

10

20

5

15

25

Pa

28.0 45.5 68.5 93.5 116.57.6 15.7

135.3

190.0 246.1

225.6

AutoPower asp:1:S - Borg WarnerAutoPower asp:1:S - IHI

10 100 10003 4 5 6 7 8 9 20 30 40 50 60 7080 200 300 400 500 700

Hz

comp:2:S (CH3)

10

20

30

40

50

60

70

5

15

25

35

45

55

65

s

0.20

0.00

Pa

7.2 15.9 27.6 46.1 68.6 94.7 225

10000 150007000 8000 9000 11000 12000 13000 14000 16000 17000 18000 19000

Hz

0.00

0.10

0.20

0.30

0.40

0.50

g

94259019 13465 18814108548079

AutoPower c:1:+X

10000 150007000 8000 9000 11000 12000 13000 14000 16000 17000 18000 19000

Hz

0.00

0.10

0.20

0.30

0.40

0.50

g

188459420 134598075 10854

AutoPower c:1:+X

10000 150007000 8000 9000 11000 12000 13000 14000 16000 17000 18000 19000

Hz

0.00

0.10

0.20

0.30

0.40

0.50

g

9404 18804108448064 13441

AutoPower c:1:+X

of 14

This could be justified by the establishment of rotating stall, a typically

sub-synchronous phenomenon, attributable to the approach of the

compressor to a condition of low-flow instability that interacts with the

phenomenon of blades passage [4].

In the following, a joint analysis of the signals in the sub-synchronous-

high frequency field is carried out to further verify this hypothesis.

Similar considerations can be extended to the 13X-14X band assuming

the blade passage as a main phenomenon that also considers the splitter

blades.

It is interesting to note that passing from point 1 to point 5 there are no

obvious side bands for the component to the BPF of the turbine to

indicate the absence of the occurrence of anomalous sub-synchronous

phenomena that interact with this component (green cursor at 13465Hz

= 10X, Figure 14).

However, a reduction in the amplitude of this content could be related

to a reduction in power and therefore to a lower aerodynamic load on

the impeller blades.

The same contents were found in the operational response of the

system in the transient near the surge condition. In Figure 15 a time

frequency analysis of the accelerometer signal on the compressor

housing, with a colormap representation that adopts the normalization

of the frequency axis with the value of the rotation speed (1X) is

shown. The contents corresponding to the BPF (and any other system

order) are at a fixed abscissa value during the run-up. Consistently with

the iso-speed analysis, a contribution correlated to the BPF is identified

in the 6X-7X interval which is evident in the last 15s of the transient

before the surge occurrence. In this final phase of the transient, an

energy loss of the 7X component in conjunction with the onset of new

contributions in the 6X-7X band appears.

Figure 15. Velocity transient (fixed position of the throttle valve), time frequency analysis accelerometer signal on the compressor housing.

Rotating stall identification has been also considered through an

analysis of the operational responses considering the information in the

sub-synchronous field where the characteristic frequency of the

phenomenon falls. For this purpose, the downstream compressor

pressure spectrum is considered suitable for capturing this

phenomenon as the sensor performs a direct measurement on the fluid,

differently from a structural and acoustic sensor where an additional

path is interposed between the source and the probe position.

Figure 16 compares Borg Warner compressor downstream pressure

spectra for the previous considered stable (point 1) and last stable point

before reaching the surge (point 5) conditions at 80krpm. Close to

surge condition, a 400Hz component (see dashed line blue cursor) can

be observed, maybe associated with a rotating stall phenomenon, as

confirmed by the presence of sidebands for the 7X and 14X orders with

an offset close to 400Hz (see double cursors at 9134-9505Hz and

18611-19045Hz) to indicate a correlated effect of the rotating stall on

the phenomenon of blade passage [24].

Figure 16. Compressor downstream pressure spectra for ncr = 80krpm, red trace

(at the bottom) -> stable, green trace (at the top) -> last stable condition before

surge occurrence.

In the microphones acoustic spectrum close to the filter and to the

compressor housing (Figure 17) the rotating stall sub-synchronous

content is present, even if its identification is not completely clear due

to the presence of the components related to the Helmholtz frequency

and its multiplier in the same frequency range. The modulating effect

of the stall on the BPF is clearly visible as sidebands of both the 7X

and the 14X.

Figure 17. Spectrum obtained from the microphone close to the compressor

housing for ncr = 80krpm in the last stable operating point.

The accelerometer at the compressor housing, differently from the

pressure sensor placed within the fluid, identifies the stall frequency

only as a high frequency contribution related to the BPF, while it is not

clearly present as a specific component of the spectrum in the field of

sub-synchronous frequencies. It seems that the energy of the

phenomenon and the transmissibility system properties at these

frequencies do not significantly contribute to obtain structural response

well detectable.

The cross spectra between inflow pressure and acoustic pressure

housing signals were calculated to avoid noise and uncorrelated

contributions of the fluid in two signals (turbulence noise) and make

more evident the common content related to the rotating stall [24].

In Figure 18 the analysis of the phenomenon during a rotational speed

transient is reported. In the last stable instants of the transient

condition, a component associated with the rotating-stall phenomenon

(see dashed red cursor at 395Hz and the red ellipse) can be identified.

This component identification seems to be coherent with the high

frequency analysis where the interaction of the blade pass phenomenon

with the sub-synchronous one is underlined by the presence of a side

5.0 6.0 7.0 8.05.5 6.5 7.5 8.5

order

Derived Order (t:1)

10

20

30

40

50

60

70

5

15

25

35

45

55

65

s

0.40

0.00

g

7.0 8.06.0

0 100005000 15000

Hz

kul:2:S (CH11)

0

10

20

30

40

Pa

9134

9505

18611

19045

399 1370

AutoPower kul:2:S

0 100005000 15000

Hz

kul:2:S (CH11)

0

10

20

30

40

Pa

9134

9505

18611

19045

399 1370

F AutoPower kul:2:S

0 100005000 15000

Hz

comp:2:S (CH7)

0

10e-3

20e-3

Pa

9134

9505

18611

19045

399 1370

AutoPower comp:2:S

of 14

band of the BPF. In the spectra, low frequency contributions can be

also detected as identified in the previous analysis related to the

dynamic response of the circuit.

Figure 18. Cross power spectrum between pressure sensor signal in the fluid

vein and microphone close to the compressor housing: common contributions between the signals (not-related ones are excluded).

Energy level analysis of filtered signal

The analysis on the frequency contents of the operational response

signals made it possible to identify which components mostly change

their value near an incipient surge condition.

This information was used to filter the acquired signals in order to keep

mainly sensitive contents and then calculate their Root Mean Square

(RMS) values. This technique allows to obtain a filtered signal

characterized by an energetic level affected by the surge occurrence,

thus more suitable to identify compressor instabilities.

At first, 3-300Hz frequency range, where frequency contents in system

response become more significant near surge, were considered and

shown in Figure 19 with reference to the microphone signal measured

at the filter.

In Figure 19, a colour contour of RMS filtered value is shown, in the

steady flow compressor pressure ratio and total-to-total efficiency

map. By reducing the mass flow rate for a constant rotational speed,

the filtered signal energy content increases approaching surge

condition, with a maximum level reached in deep surge.

Figure 19. Colormap of RMS value in the frequency range of 3-300 Hz for the

acoustic pressure measured at the filter section (Borg Warner compressor).

In Figures 20 and 21, the colour contour of RMS value in sub-

synchronous range respectively referred to the accelerometer signal

located at the inlet feeding line and to the outlet pressure signal in the

specific high frequency band 6X-7X is represented. The sensitivity of

these quantities is adequate to identify the system approach to a

condition of incipient surge.

In the case of the accelerometer, it seems interesting to note that the

lower RMS values occur near the zone of maximum efficiency, where

the flow incidence angle for the impeller seems to be optimum to

improve compressor vibro-acoustic response.

Figure 20. Colormap of RMS value in the whole sub-synchronous frequency

range for the accelerometer signal at the inlet section (Borg Warner).

Figure 21. Colormap of RMS value in frequency band 6X-7X of the outlet pressure signal (Borg Warner).

In general, at low mass flow rate level when the slope of the

compressor characteristic curves becomes near to zero, RMS values

previously considered highlight an increase, approaching the unstable

condition, resulting a possible surge precursor for diagnostic purpose.

Figure 22 shows the RMS value trends for the microphone (at the filter

and compressor housing) and inflow pressure signals during the

transient test for a specific sub-synchronous band between 3Hz and the

300Hz. In all cases, an increasing tendency in the trend of the energy

of the spectrum during the transient and a high variation of slope in

correspondence with the achievement of surge can be observed. This

aspect further highlights the significant change of low-frequency sub-

synchronous contents from a stable operating condition to a condition

close to the surge with low mass flow rate levels.

Figure 22. Borg Warner compressor - Spectral energy in the range 3-300Hz – inlet filter section and compressor housing microphones, in flow pressure signal

100 1000 1000050 60 70 80 200 300 400 500600 800 2000 3000 4000 6000

Hz

10

20

30

40

50

60

70

5

15

25

35

45

55

65

s

3.00

0.01

Pa

2/H

z

395

50000 60000 70000 80000 9000055000 65000 75000 85000

rpm

t:1 (T1)

80

90

100

110

dB

Pa

91511

Frequency band 3-300Hz asp:1:SFrequency band 3-300Hz comp:2:SFrequency band 3-300Hz kul:2:S

of 14

Near incipient surge condition, significant frequency contents close to

the BPF (7X) may be noted as previously observed in the high

frequency signal analysis and this may be related to an interaction

between two sources (blade pass and stall) of the system generating a

modulation phenomenon [24].

To better analyse this behaviour, signal has been filtered in a band

between 6X and 8X orders (where these contents appear most

significant) and then a frequency analysis has been computed on the

filtered signal Hilbert envelope. In this way, it is possible to perform a

demodulation of the blade pass content adopting a methodology

analogous to the one used for detecting gear defects (in the considered

case, the gear mesh frequency corresponds to BPF) [25, 26].

Figure 23 represents a plot of the signal Hilbert envelope frequency

analysis for some operational response signals of the system in a stable

point (maximum efficiency condition - dotted line) and in the last

stable point for ncr = 80krpm. This technique seems to identify, for all

the signals considered in the second point of operation, a modulating

effect on the BPF, which could be due to a rotating stall phenomenon

(peak at 402Hz).

As previously highlighted, a direct FFT analysis on the signal (and not

on the envelope of the high pass filtered signal) with the exception of

the fluid vein pressure signal, generally does not allow to correctly

identify the sub-synchronous content of the rotating stall. This is due

to the presence of other contributions that make the identification more

complex or due to dynamic aspects related to the nature of the sensor

or the transmissibility of the system.

Figure 23. FFT analysis of the envelope function in the case of microphone

signals (asp:1, comp:2), structural signals (c: 1: + X) and fluid vein pressure (kul:1) - (Dotted line -> stable condition; continuous line -> last stable operating

point.

This method for rotary stall identification applied to the fluid vein

pressure sensor signal has been extended to both compressors. In Table

3 the obtained results and the evaluated rotating stall frequency (frs) are

reported. In some operating conditions (especially for low rotational

speed levels), the identification of the phenomenon frequency was not

possible, as for IHI turbocharger when the inlet section was

characterized by controlled pressure level and not by the ambient. This

layout configuration may be considered a more complex, and hence

the application method is less reliable.

Table 3. Rotating stall frequency (obtained from fluid vein pressure sensor

signal – last stable operational point) – (1) not identifiable from the experimental data; (2) controlled compressor inlet pressure; (3) from demodulation method

Turbocharger ncr

[krpm]

frs

[Hz]

1X

[Hz]

frs

[1X]

frs [Hz]

(3)

Borg Warner 50 (1) 845 (1) (1)

Borg Warner 65 199 1097 0.19 186

Borg Warner 80 399 1370 0.31 402

Borg Warner 95 450 1591 0.26 465

Borg Warner 110 432 1881 0.22 367

IHI (2) 59 (1) 993 (1) (1)

IHI (2) 89 (1) 1497 (1) (1)

IHI (2) 118 386 1986 0.19 389

IHI (2) 162 506 2743 0.19 464

IHI 89 383 1502 0.25 293

IHI 118 389 1992 0.19 408

Compressor inlet temperature trend analysis

In the case of the IHI turbocharger, for a rotational speed transient

leading to surge condition, the trace of the output voltages of the

thermocouple at compressor inlet is reported, indicating the qualitative

trend of the temperature at the compressor intake section; in addition,

its first order derivative is also reported (at the top of Figure 24,).

The signal and the derivative trends are compared with the filtered

RMS value in the frequency band 3-300Hz of the microphone signal

at the compressor intake line (at the bottom of Figure 24). The similar

trend seems to highlight the potential of this measurement in providing

further diagnostic information. The two derivatives are characterized

by a peak when surge low frequency component in the downstream

compressor pressure signal appears [10, 20]. Figure 25 shows time

frequency analysis of the pressure signals where it is possible to detect

the previous time instant when the low frequency contents appear in

the related spectra (t=140s).

Further investigations will be conducted in future work by correlating

this signal to other measured vibro-acoustic signals.

Figure 24. Trend analysis during the rotational speed transient of the time history and relative derivatives of the temperature and acoustic pressure signals

to the compressor inlet section

1000500250 750 1250

Hz

0

10e-3

Pa

0.00

10.00

Pa

0.00

0.10

g

420.2

AutoPower asp:1:SAutoPower comp:2:SAutoPower asp:1:SAutoPower comp:2:SAutoPower kul:2:SAutoPower kul:2:SAutoPower c:1:+XAutoPower c:1:+X

0 50 100 150

s

Time

10

Pa

-1.0

0.0

1.0

2.0

Pa

/s

140.1

time historyfirst order derivative

microphone

0 50 100 150

s

Time

1e-3

2e-3

3e-3

V

-0.5e-3

0.0

0.5e-3

1.0e-3

1.5e-3

(VH

z)

140.6

time historyfirst order derivative

thermocouple

of 14

Figure 25. Time frequency analysis during the transient of the downstream

compressor pressure – low frequency content detail (0-100Hz)

Rotational speed signal analysis

In this section, the rotational speed signal was analyzed to assess if

variations in frequency content are detectable during transition from a

stable to the unstable operating condition. The measurement system

returns a square wave whose period corresponds to a whole rotation or

part linked to the distribution of the blades on the impeller according

to the adopted settings. During the activity, the measurement device

provides two cycles in the output signal for a complete impeller

rotation.

The FFT analysis of the rotational speed signal shows a frequency

content consisting in a fundamental frequency (two pulses per

revolution) and only higher multiples in the case of stable compressor

condition (Figure 26, red trace). When the unstable condition is

approached, sidebands are detectable at the fundamental frequency of

the signal correlated to signal frequency modulation phenomena [24].

This could be due to an irregularity of the impeller motion correlated

to an incipient surge condition (see green and blue traces in the spectra,

Figure 26). In the presence of deep surge, the modulating phenomena

in the rotational speed signal are predominant, and their frequencies

are correlated to those identifiable in other system responses (low

frequency contents related to the dynamic response of the circuit).

The modulating phenomena take place at relatively low frequencies

(<10Hz) to exclude the presence of significant phenomena related to

torsional vibration due to the shaft line turbocharger structural

characteristics.

Figure 26. Frequency analysis of the rotational speed signal (at the top) and

rotational velocity time history (at the bottom) during surge transient - Borg

Warner turbocharger

Conclusions

In the paper, the main results of an experimental investigation on two

different turbocharger centrifugal compressors in stable and unstable

operating conditions are shown, with special reference to the transition

from a stable condition to the low flow instability region.

The aim of this study is to evaluate whether it is possible to predict the

incipient surge condition using suitable quantifier calculated from

instantaneous signals. Vibrational, acoustic and inflow pressure

system responses have been considered and their aptitude in detecting

the presence of specific operational sources has been preliminary

analyzed.

The spectrum energy level seems to be a useful quantifier for surge

prediction due to sub-synchronous and higher frequencies signal

contents.

For both compressors, incipient surge conditions are characterized by

a low frequency content close to the Helmholtz frequency of the

system and its multipliers. The system behavior seems strictly related

to the dynamic response of the circuit and slightly to the compressor

features and its operating conditions.

For the investigated components at frequencies up to 300 Hz, this

contribution seems to be well captured by microphone positioned at

the filter, differently from structural signals. This may be justified by

system transmissibility characteristics suitable to reduce the energy of

this specific low frequency content.

The deep surge condition can be easily recognized, since it is always

accompanied by high vibration levels and discernible noise. Both the

incipient surge and the deep surge seem to be characterized by similar

spectra contents.

In the downstream compressor pressure signal, a sub-synchronous

narrow band content is often present near unstable operating condition

may be related to a rotating stall. The detection of this phenomenon is

useful for diagnostic purpose as rotating stall often occur just before

the surge onset.

This specific content is difficult to be identified in acoustic and

structural system responses with sub-synchronous frequency band

0 10 20 30 40 50 60 70 80 905 15 25 35 45 55 65 75 85 95

Hz

0

100

50

s

6171.5

0.7

Pa

3.0 ; 1421.7

7.0 ; 4279.0

6.9 27.0 46.9 72.9

140.0

141.0

2600 2700 2800 2900 3000 3100 32002650 2750 2850 2950 3050 3150 3250

Hz

t:1 (CH10)

0.0

1.0

0.5

V

2693 3063

3047 3078

3143

3129

3156

3115

3169

2887

Spectrum t:1 80804 rpmSpectrum t:1 86704 rpmSpectrum t:1 91936 rpmSpectrum t:1 94279 rpm

60 7052 54 56 58 62 64 66 68 72 74 76

s

80000

90000

85000

95000

rpm

80804

94279

86704

91936

of 14

analysis due to the presence of other contributions related to

resonances.

The rotating stall identification seems to be possible by detecting the

rise of its interaction with the high frequency blade passage

phenomenon. To this aim, a demodulation technique has been used by

means of a procedure based on the envelope analysis applied on band-

pass filtered acquired signal. In the envelope spectrum, the rotating

stall onset is well detectable since a specific sub-synchronous

frequency contents appears near low mass flow rate operating

conditions.

Through this method, the rotating stall diagnostic information may be

obtained not only starting from pressure signal, but also considering

the sub-synchronous frequency content of micro mono axial

accelerometers on compressor housing and microphones positioned

near the turbocharger.

Additional diagnostic information has been found from inlet section

thermocouple time-averaged signal trend analysis and by detecting the

presence of rotational speed irregularities near unstable low mass flow

rate conditions.

In future works, additional signal processing techniques (i.e., high

order spectral analysis [27], independent component analysis [28] and

blind sources separation [29]) will be considered to extract diagnostic

contents from the system response signals in order to define other

precursors and obtain a more reliable detection of incipient surge

condition. This diagnostic analysis will be considered [30, 31] to

develop an advanced control system able to prevent surge conditions

based on such precursors.

References

[1] Emmons, H.W., Pearson, C.E., and Grant, H.P., Compressor Surge

and Stall Propagation, Transactions of the ASME 77:455-469, 1955

[2] Fink, D.A., Cumpsty, N.A., and Greitzer, E.M., Surge Dynamics

in a Free-Spool Centrifugal Compressor System, J. Turbomachinery

114:321-332, 1992

[3] Yano, T. and Nagata, B., A Study on Surging Phenomena in Diesel

Engine Air-Charging System, The Japan Society of Mechanical

Engineers 14:364-376, 1971

[4] Greitzer, E.M., Surge and Rotating Stall in Axial Flow

Compressors. Part I: Theoretical Compression System Model. Journal

of Engineering for Power, 98 (1976) 190-198

[5] Biliotti, D., Bianchini, A., Vannini, G., Belardini, E., Giachi, M.,

Tapinassi, L., Ferrari, L., Ferrara, G., Analysis of the rotor dynamic

response of a centrifugal compressor Subject to aerodynamic loads due

to Rotating Stall, Journal of Turbomachinery ASME 2015, Vol. 137

[6] Bently, D. E., Goldman, P., Vibrational Diagnostics of Rotating

Stall in Centrifugal Compressors, ORBIT First Quarter 2000

[7] Munari, E., Morini, M., Pinelli, M., Spina, P.R., Suman, A.,

Experimental Investigation of Stall and Surge in a Multistage

Compressor, ASME Paper GT2016-57168, ASME Turbo Expo 2016,

Seoul, South Korea

[8] Liskiewicz G., Horodko, L., Stickland, M., Kryłłowicz, W.,

Identification of phenomena preceding blower surge by means of

pressure spectral maps. Experimental Thermal and Fluid Science, 54

(April 2014) Pages 267-278

[9] Kabral, R., Åbom, M. Investigation of turbocharger compressor

surge inception by means of an acoustic two-port model. Journal of

Sound and Vibration 412 (2018) 270e286

[10] Marelli, S., Misley, A., Taylor, A., Silviestri, P., Capobianco, M.,

Canova, M., Experimental Investigation on Surge Phenomena in an

Automotive Turbocharger Compressor. SAE Technical Papers, doi:

10.4271/2018-01-0976

[11] Ferrari, M.L., Silvestri, P., Pascenti, M., Reggio, F., Massardo,

A.F. Experimental Dynamic Analysis on a T100 Microturbine

Connected With Different Volume Sizes, ASME Transactions -

Journal of Engineering for Gas Turbines and Power, 140, 021701-1-

12, 2018

[12] Ferrari, M.L., Silvestri, P., Reggio, F., Massardo, A.F., Surge

prevention for gas turbines connected with large volume size:

Experimental demonstration with a microturbine, (2018) Applied

Energy, 230, pp. 1057-1064

[13] Dehner, R., Figurella, N., Selamet, A., Keller, P., Becker, M.,

Tallio, K., Miazgowicz, K., Wade, R., Instabilities at the low-flow

range of a turbocharger compressor, SAE Int. J. Engines 6 (2) (2013)

1356e1367, https://doi.org/10.4271/2013-01-1886

[14] Zhenzhong, S., Wangzhi, Z., Xinqian, Z., Instability detection of

centrifugal compressors by means of acoustic measurements,

Aerospace Scienceand Technology 82–83 (2018) 628–635

[15] Aretakis, N., Mathioudakis, K., Kefalakis, M., Papailiou, K.,

“Turbocharger unstable operation diagnosis using vibroacoustic

measurements, ASME Journal of Engineering for Gas Turbines and

Power,126 (Nov. 2004) pages 840-847

[16] Morini, M., Pinelli, M., Venturini, M., Acoustic and Vibrational

Analyses on a Multi-Stage Compressor for Unstable Behavior

Precursor Identification, ASME Paper GT2007-27040, ASME Turbo

Expo 2007, Montreal, Canada

[17] Munari, E., D’Elia, G., Morini, M., Mucchi, E., Pinelli, M., Spina,

P.R., “Experimental Investigation of Vibrational and Acoustic

Phenomena for Detecting the Stall and Surge of a Multistage

Compressor, ASME Journal of Engineering for Gas Turbines and

Power. 2018; 140(9):092605-092605-9. GTP-17-1451 doi:

10.1115/1.4038765

[18] Guillou, E., Gancedo, M., Gutmark, E., Experimental

Investigation of Flow Instability in a Turbocharger Ported Shroud

Compressor, Journal of Turbomachinery 2016, Vol. 138/061002-1

[19] Tanda, G., Marelli, S., Marmorato, G., Capobianco, M., An

experimental investigation of internal heat transfer in an automotive

turbocharger compressor, Applied Energy, 193, 531-539,

http://dx.doi.org/10.1016/j.apenergy.2017.02.053, 2017

[20] Marelli, S., Capobianco, M., Experimental investigation under

unsteady flow conditions on turbocharger compressors for automotive

gasoline engines, 10th International Conference on Turbochargers and

Turbocharging, Institution of Mechanical Engineers, pp. 219–229,

2012

[21] Marelli, S., Capobianco, M., Measurement of Instantaneous Fluid

Dynamic Parameters in Automotive Turbocharging Circuit, SAE

Technical paper 2009-24-0124, Proceedings of the 9th International

Conference on Engines for Automobile, doi: 10.4271/2009-24-0124,

2009

[22] Friswell, M. I., Penny, J. E. T., Garvey, S. D., Lees, A. W.,

Dynamics of Rotating Machines

[23] Muszyńska, A., Rotordynamics, CRC Press, Taylor & Francis

Group, ISBN 978-0-8247-2399-6, 2005

[24] Oppenheim, A.V., Schafer, R.W., Discrete-time signal

processing, Prentice-Hall, Inc. Upper Saddle River, NJ, USA, 1999.

[25] Randall, R.B. A new method of modelling gear faults, Journal of

Mechanical Design 104, 259-267, 1982

[26] Mcfadden, P.D. Detecting fatigue cracks in gear by amplitude and

phase demodulation of the meshing vibration, ASME J.of Vibration,

Acoustics, Stress, and Reliability in Design 108, 165-170

[27] Lotfi, S., Jaouher, B. A., Farhat, F., Application of higher order

spectral features and support vector machines forbearing faults

classification, ISA Transactions 54 (2015) 193–206

[28] Hyvärinen, A., Karhunen, J., Oja, E., Independent component

analysis, John Wiley & Sons, Inc

[29] Comon, P., Jutten, C., Handbook of Blind Source Separation

Independent Component Analysis and Applications, Elsevier Ltd

[30] Lucifredi, A., Silvestri, P. An overview of fundamental

requirements for a condition monitoring and fault diagnosis system for

machinery and power plants (2003) Proceedings of the Tenth

International Congress on Sound and Vibration, pp. 4691-4698

of 14

[31] Aonzo, E., Lucifredi, A., Silvestri, P. Diagnostic modules based

on chaos theory for condition monitoring of rotating machinery, (2000)

Proceedings of the 25th International Conference on Noise and

Vibration Engineering, ISMA, pp. 937-944

Contact Information

Silvia Marelli (Associate Professor)

DIME, University of Genoa, Genoa, Italy

E-mail: [email protected]

Paolo Silvestri (Assistant Professor)

DIME, University of Genoa, Genoa, Italy

E-mail: [email protected]

Acknowledgments

The work presented in this paper was in part conducted with support

from The H2020 UPGRADE (High efficient Particulate free Gasoline

Engines) Project (Grant Agreement Number: 724036).

Incipient surge detection in automotive turbocharger ...

Documents

Transcript of Incipient surge detection in automotive turbocharger ...