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Measurement of the local gas velocity at the outlet of a wall flow

particle filter for constant filter loading

Bernd Benker, Annett Wollmann, Michael ClaussenCUTEC-Institut GmbH, Clausthal Zellerfeld, Germany

ABSTRACT

Recent measurements and models show that the soot distribution and the permeability of the

soot layer depend on the selected engine operating points both while loading and during

regeneration. The influence of an upstream oxidation catalyst on the soot distribution has been

shown. Most interestingly, a radial velocity profile has been observed by an optical measuring

technique.

However, in actual practice only the overall pressure drop due to the thousands of parallel

channels of a wall flow filter is measured. In order to find out experimentally whether or notaging and thermal instability of diesel particle filter correlate with inhomogeneous flow

conditions a simple set-up was developed to measure the outlet velocity profile.

A Prandtl-tube with an inner diameter of 1 mm was scanned in x- and y-direction across the

outlet channels of a catalytically coated wall flow filter, which was attached to a blower. The

settings of its electro motor were kept constant in all the experiments. The resolution of themicro Prandtl-tube is sufficient to resolve the flow profile of individual channels. Different

loading strategies and a regeneration were examined in a time-sequence.

INTRODUCTION

The modelling of soot deposition and regeneration in Diesel particulate filters (DPF) hasreached a high standard. However, there is a discrepancy between the high number of

parameters going into these models and the low number of data actually measured on the test

bed at the tail pipe of the exhaust gas treatment system. Data such as pressure drop, particlesize distribution, and gas composition are available as time series, but each data point

represents the overall performance of many DPF channels, which in addition may vary alongits length.

Only few non-invasive methods such as computer tomography can measure the thickness of

ash layers with 3-dim resolution [1], but its sensitivity at present is rather low. However, it

could be shown that ashes are accumulating in the dead-end sections of the inlet channels. It

is expected [2] that the remaining surface area of each channel shows a self-stabilizing

behaviour: Areas where the particle deposition rate is initially high develop a higher pressuredrop, so that throughput and particle deposition rate will decrease subsequently. So, although

a 3-dim resolution would be most interesting, a measurement of the overall performance of each channel is already very helpful for modelling and for the practical improvement of DPF

design with respect to long-term stability.Subsequently, Laser Doppler Velocimeter (LDV) has been applied to examine the flow field

at a DPF outlet [3]. The optical instrument allows in principle for excellent resolution, but

practical limitations arise from the fact that tracer particles introduced upstream of the DPF

would not pass through the filter. So the tracers have to be injected downstream of the DPF

and the space required for doing so cannot be examined with the optical instrument. Due to

this loss of resolution the flow field of individual channels was not observed, but still it could

clearly be seen that the outlet velocity across the DPF outlet was far from constant. These

variations were used as a sensitive measure to characterize different filter systems andregeneration strategies.



This experimental work was continued by replacing the LDV with a classical instrument

providing under these specific circumstances higher resolution at lower costs. The dynamic

pressure was measured with a Prandtl-tube that was scanned across the whole outlet area of

the DPF using 2 translation stages. Positioning of the probe and collection of the pressure data

were controlled by computer, so that pressure profiles containing about 0.25 Mega bites could

be handled. Thus the grey scale “image” of the flow profile has a resolution similar to theoptical image of the DPF face obtained with a digital camera (see Fig. 1). For the DPF

examined it can be seen that there are strong variations both on a large scale, i.e. betweencentre and periphery of the DPF, and on a small scale, i.e. near the wall of adjoining filter

elements.The DPF examined here had been used in many earlier experiments. New DPF systems

should have a better design and performance – an assumption, which can be tested in detail

with the experimental set-up.

Fig.1: Photo of DPF face Fig.2: Grey scale presentation of the pressure

values using 520*550 pressure “pixels”

and image pre-processing

EXPERIMENTAL

TEST ENGINE

The test engine used is a modern 1.9 litre 4 stroke diesel engine with pump-unit-injection,

VGT and EGR is examined on a test bed equipped with a 220 kW Schenk DYNAS

dynamometer. The emission standard of the engine is EURO III.ENGINE OPEARTION POINTS

The engine operation points for loading and regeneration strategies are summarized in Tab. 1.

Tab.1: engine operation points

speed

[rpm]

torque

[Nm]

T

[°C]

Soot mass flow

[g/h]

NOx/soot

[ - ]

loading 2000 31 280 2.2 6.8

regeneration 2000 186 440 1.2 250



FILTER AND OXIDATION CATALYST

A 200 cpsi catalytic coated DPF already used in earlier experiments [4] was examined with

the Prandtl probe.

In addition a diesel oxidation catalyst (DOC) could be installed directly to the DPF, i.e.

without forcing the flow from the oxidation catalyst through a connecting pipe.The chemical and geometrical data of the DPF and oxidation catalyst are summarized in

Tab. 2 and Tab. 3.

Tab. 2: DPF specification

Filter Material

Coating

SiC

CeO2+75 g/ft3 Pt

Geometry 5.66” x 6”

Cell density [cpsi]Wall thickness [mil],

(mm)

20014, (0.356)

Diameter [mm]

Plug ring [mm]

143.5

1.5

Length [mm]

Plug length [mm]

150.5

4

Channel width [mm]

Porosity [%]

1.5

42

Tab.3: DOC specification

DOC Material

Coating

Ceramic

75 g/ft3 Pt

Geometry 5.66” x 3.54”

Cell density [cpsi]

Wall thickness [mil],

(mm)

400

6, (0.153)

Diameter [mm]

Plug ring [mm]

143.5

1.5

Length [mm] 88.8



MEASURING SYSTEM

When the DPF had reached the desired soot load, it was removed from engine test bed and

transferred to the Prandtl-tube apparatus. Due to its heat capacity oxidation reactions may

have continued until the DPF reached room temperature. Fig. 3 shows a scheme of the

experimental plant.

Fig. 3: scheme of the experimental plant

The Prandtl probe consists of a double wall tube coupling the stagnation pressure (pstag) andthe static pressure (pstat) to an external differential pressure transducer. Its outer diameter is

3 mm, its inner diameter 1 mm. From the pressure difference the gas velocity (v) can easily be

calculated according to Eq. (1). The pressure signal is rather insensitive to misalignment: An

angular deviation of less than 15 degrees between the flow and the probe is allowed according

to the specification of the manufacturer LAMBRECHT.

v =2 " #p

$Eq. (1)

"p = pstag #pstat

The pressure transducer FCO 510 by FURNESS CONTROLS has a resolution of 0.1% of its

full-scale value, which can be set to 200 Pa or 2000 Pa. The corresponding minimum

detectable velocity is 0.6 m/s and 1.9 m/s, respectively. The flow rate through the DPF

provided by a blower is equivalent to an average velocity of 3.6 m/s. The overall flow profile

across the DPF and the profile across each channel both show a significant peak at their

centres and therefore the larger range of the pressure transducer was used in most of theexperiments.

The maximum pressure signal was close to 500 Pa corresponding to a local outflow velocityof nearly 30 m/s.The instrument outputs a stabilized signal after about 0.4 sec. Much faster

response would be achieved by using a different version of the instrument allowing for direct

access to the raw signal of the pressure gauge.

transducer+-

PC

air

from blower

filterprobe

motor

controller



ACCURACY AND RESOLUTION

The positioning system of the Prandtl probe consists of two translation stages, one mounted

orthogonally on top of the other. Both of them move the tip of the probe in a vertical plane

parallel to the outlet face of the DPF at a distance of 1 mm. From a given reference point the

accuracy of the position is within 2 µm in horizontal direction and within 5 µm in verticaldirection. During each transversal scan the probe measures the pressure at about 5 positions

per channel. The scanning speed was chosen according to the desired spatial resolution and

the response time of the pressure transducer.The results obtained from ten horizontal scans at constant vertical position are shown in Fig.

4. It shows the average pressure signals and the corresponding standard deviation. Closedchannels and outflow channels can easily be distinguished and the reproducibility of the peak

values is high. However, the scatter of the data in the wings of the velocity profiles is much

higher.

Fig.4: averaged pressure signal from a horizontal scan (line)

with standard deviation (bars)

Both observations can be explained by error propagation. The measured flow profile may be

approximated by a parabola (see Fig. 5). For a given horizontal misplacement the resulting

error of the pressure signal in the wings is much stronger than in the vertex. Thus it was

estimated that the actually accuracy of the horizontal position is about 30 µm. It was

concluded that the biggest error in this direction is not due to the scanner itself but due to theinefficient synchronisation between positioning and data acquisition.

Fig. 5: Flow profiles from a square channel.symbols and fitted line: measured profile,

dotted line: calculated profile from a probe with a diameter of 2 mm;

line: theoretical profile [6]

0

50

100

150

200

250

300

350

400

0 100 200 300 400

x-position [pixel]

! p [ P a ]

0

0.2

0.4

0.6

0.8

1

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

position [mm]

n o r m a l i z e d v e l o c i t y [ - ]

ideal

calculated

measured



For a single square channel the theoretical velocity profile along its line of symmetry is

shown in Fig. 5. In the same figure the symbols of the measured curve represent the average

data from 6 channels.

The diameter of the Prandtl-tube used in these experiments is about as wide as a single

channel: The geometrical channel width is 1.5 mm, the measured cannel width defined as the

distance between the points where the velocity is down to 10% of the peak value is 3.0 mm.Therefore when traversing from a free channel to a closed one there is a considerable gradient

across its opening. A similar situation is known from Prandtl (or Pitot) measurements in boundary layers. As can be seen in Fig. 6 from [5] the centre line of the tube does not

coincide with the average velocity of the parabolic profile thus there is systematic shift of the profile. The corrections published in that paper cannot be used for our purposes since they

refer to different Reynolds numbers. We evaluated the pressure signal by understanding each

data point as the average stagnation pressure exerted by the flow profile onto the opening of

the tube. As can be seen in Fig.5 this pressure averaging procedure has successfully been

applied to convert the theoretical ideal flow profile from a rectangular channel into the

measured profile from a Prandtl-tube and evaluated with Eq. (1).

Fig.6: Illustration of centreline displacement effect [5]

The observed velocity can be estimated from the theoretical pressure profile averaged acrossthe effective probe size. As can be seen, an effective size of 2 mm, which is in between the

inner diameter of the probe of 1 mm and the outer diameter of 3 mm, results in a good

agreement with the measured profile.

The horizontal grid spacing was set to 0.27 mm, i.e. about then times the (overall) accuracy of

horizontal position. The vertical grid spacing was chosen equal to the horizontal one, thus

providing about 25 pressure data per channel. The actual number is not constant since the

channels do not possess a regular spacing. The offset and inclination the filter elements with

respect to each other can clearly be seen, both, in the photo in Fig. 1 and in the corresponding

“pressure image” in Fig. 2. Scanning the velocity profile of each channel (no matter if open or closed) at about 25 positions provides a stabile value for its average output velocity

independent of the relative position of grid and channel.

The resolution is sufficient to detect individual channels (i.e. the zero-signals between them),

calculate a well-defined position for the centre of the channel and characterize the output

velocity (i.e. the average pressure signal). For this purpose methods of image processing wereapplied.



IMAGE PROCESSING

The broadening of the flow profile can also be seen from the histogram of the original data of

Fig. 2. It shows 34 % of zero signals in contrast 67% expected from DPF geometry.

Thus the “pressure image” of the DPF outlet is “blurred” and it can be improved by setting a

suitable threshold and by applying routines such as “erosion” and “dilation”. The thresholdsets all values less than a chosen limit to zero, erosion removes single “pressure pixels”,

dilation fills single “holes”. Doing so leaves all the channels well separated from their

neighbours so that automatic objects counting routines can be applied for statisticalevaluation. The parallel alignment of the DPF walls, the scanning direction, and shape of the

matrices support the square appearance of the channels in the figures.The critical step is the choice of the threshold. It was used to match the calculated volume

flow rate to the actual throughput from the blower (see Tab. 3). A global threshold

corresponding to Δ p = 86 Pa was used (i.e. a constant value for all regions of the image) and it

was kept constant in all the experiments. In Tab. 3 “open” refers to opening, i.e. one erosion

following by one dilation.

Tab. 3: preset flow rates from the blower compared to calculated values from

the “pressure image” treated with different image routines

from the

blower

original

image

open threshold threshold

+ open

Flow

[m3/h]

200 393 326 274 216

Derivation

[%]- 49 39 27 7

The 0.25 Mega bites of original data from each picture are finally reduced to about 2000

tabulated data sets containing the size, average pressure signal and position of each outflowchannel. They are available for further statistical evaluation.



RESULTS

LOADING WITHOUT DOC

The “pressure images” in Fig. 7 show in a step-by-step fashion the loading of a DPF (without

DOC). The cumulative loading time is indicated in the headline of each image. The initial profile with its central peak (yellow “pressure pixel” in the centre of “free” DPF) to an almost

flat profile after 8 hours loading.

DPF without DOC

Loading 2000 rpm 31 Nm

free 0.5 h 1 h

2 h4 h 8 h

Fig.7: pressure value distribution for different DPF loading states. The colour scale runs fromblue (low values) over red to yellow.

For a quantitative comparison the pressure data were sorted into 10 classes with respect totheir radial distance from the centre of DPF. For each of these ring-shape areas the averaged

flow rate and its standard deviation were calculated (see Fig. 8 and Fig. 9). Again, the central peak of the profile can be seen to become less pronounced with increasing soot load.



Fig.8: Radial dependence of the flow rate for different soot loadings. The pressure signal was

averaged over ring-shaped zone with outer radii as indicated on the abscissa.

Fig.9: Standard deviation related to the average flow rate shown in Fig.8

Each average curve in Fig. 9 has a relative minimum at small radii, which is due to the DPFgeometry: The outflow channels in the corner of a filter element are fed from only 2 rather

than 4 neighbouring inlet channels, and, in addition, in the centre a comparatively highfraction of the face area consists of element walls.

Outlet channels next to an element wall are connected to 3 inlet channels and therefore also

have a systematically lower throughput. This can be seen in the line scan in Fig. 5 but for

obvious reasons not in the averaged radial profile in Fig. 8. However, the systematically lower pressure signals contribute to the standard deviation shown in Fig. 9.

The most irregular pressure pattern, i.e. the highest standard deviation is found on the

periphery. It reflects the problem of matching a square ceramic structure into a round casing.

LOADING WITH DOC

After a forced complete regeneration the loading of the DPF is repeated with the DOCinstalled upstream. The corresponding set of images is shown in Fig. 10. The patterns

observed are similar to the ones obtained without DOC (see Fig. 7), they correspond to a

similar filter backpressure (see Fig. 11), but they are obtained at a later time.

DPF with DOC

2.E-06

3.E-06

4.E-06

5.E-06

6.E-06

7.E-06

8.E-06

9.E-06

1.E-05

0 10 20 30 40 50 60 70

radius [mm]

s t a n d a r d d e v i a t i o n [ m 3 / h ]

free

loaded 0.5 h

loaded 1 h

loaded 2 h

loaded 4 h

loaded 8 h

0.E+00

1.E-05

2.E-05

3.E-05

4.E-05

5.E-05

6.E-05

0 10 20 30 40 50 60 70

radius [mm]

f l o w r a t e [ m 3 / h ]

freeloaded 0.5 hloaded 1 hloaded 2 hloaded 4 hloaded 8 h



Loading 2000 rpm 31 Nm

0.5 h 1 h2h

4 h 8 h 12 h

Fig. 10: pressure value distribution on different DPF loading states.

However, especially for high loadings the same filter backpressure may refer to a different

mass of deposited soot and to a different velocity profile. It may be assumed that the small

fraction of high velocity channels observed in an otherwise flat flow profile is responsible for

this seemingly contradictory behaviour (see Fig. 10 at 12 hours).

Fig.11: deposited soot mass (full symbols and line fit) andfilter backpressure (empty symbols) as a function of timefor loading with DOC and without DOC

In the case of cyclic filter loading the throughput is higher than observed under constant

loading conditions [7].

5.84

5.96

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

0 2 4 6 8 10 12 14

time [h]

f i l t e r b a c k p r e s s u r e [ k P a ]

0

1

2

3

4

5

6

7

8

d e p o s i t e d s o o t m a s s [ g / L ]

backpressure w/o DOCbackpressure w DOCmass w/o DOCmass w DOC

full symbolsempty symbols



REGENERATION

As expected, using the DOC increases the reaction rate (compared Fig. 12 and Fig. 13). The

transient character of the regeneration is most clearly captured in Fig. 12 at 15 min without

DOC.

DPF without DOC

Regeneration 2000 rpm 186 Nm

8 min 15 min30 min

Fig. 12: Regeneration without DOC

It can be seen that the reaction is controlled by the element wall. Therefore, the square

symmetry of the elements is dominant over the rotational symmetry of the overall design.

DPF with DOC

Regeneration 2000 rpm 186 Nm

8 min 15 min30 min

Fig. 13: Regeneration with DOC

CONCLUSION

The effect of a DOC on the loading and regenerating behaviour of a DPF was examined. For

this purpose, the local gas velocity at its outlet face was measured with a Prandtl-tube and the

following results were observed:

The diffusor upstream of the DPF does not provide a flat velocity profile at its outlet.

Therefore the overall velocity profile from a fully regenerated DPF has a strong

maximum at its centre.



After 8 hours of loading the overall velocity profile from a DPF (without DOC) is

nearly flat. When using a DOC+DPF under the same conditions a similar state is

reached after 12 hours.

At the onset of regeneration the square structure of DPF elements becomes important

and overrules the rotational symmetry of the overall design (see especially Fig. 12 for t = 15 min).

Elements in the centre regenerate before those on the periphery.

Channels near element walls show comparatively high throughput at extremely high

soot loading. They seem to be the first to start regenerating within an element,

although a complete regeneration seems to be difficult to achieve (with and without

DOC).

The observed characteristics refer to the one DPF examined so far. It will be most interesting

to see whether or not the special behaviour of the channels close to the element wall is unique

to this type of DPF.The measuring method developed is capable of observing the flow profile of, both, individualchannels and the complete DPF face. Several improvements can be made in the future, such

as using smaller probes, a faster pressure sensor, or a better synchronisation with the scanner.

In its present state the probe is able to detect, e.g. local effects of the DPF, which may

consciously be used or avoided in future design work.

REFERENCES

1. Piesche, M.; Bergende, M.; Deuschle, T.; Hitzler, G.; Janoske, U.; Weitk, W.;

„Langzeitstabilität von Partikelfiltern in Dieselmotoren“, Informationstagung Motoren,Frankfurt 2003, Heft R521, 2003

2. Konstandopoulos, A.G.; Kostoglou, M.; Skaperdas, E.; Papaioannou, E.; Zarvalis, D.;Kladopoulou, E.; “Fundamental studies of diesel particulate filters: Transient loading,regeneration and aging”, SAE Technical Paper 2000-01-1016, 2000

3. Maly, M.; Claussen, M.; Carlowitz, O.; Kroner, P.; Ranalli, M.; Schmidt, S.; “Influence of the nitrogen dioxide based regeneration on soot distribution”, SAE Technical Paper 2004-01-0823, 2004

4. Konstandopoulos, A.G., et al; “The diesel exhaust aftertreatment (DEXA) cluster: Asystematic approach to diesel particulate emission control in Europe”, SAE technicalPaper 2004-01-0694, 2004

5. Grosser, W.I.; ”Factors Influencing Pitot Probe, Centerline Displacement in an Turbulent

Supersonic Boundary Layer”, NASA Technical Memorandum107341, 1997 (with areview of earlier studies from 1936 onwards)6. Shah, R.K.; London, A.L.; „Laminar Flow Forced Convection in Ducts“, Supplement 1-

Advances of heat transfer, Academic press, London, 19787. Benker, B.; Wollmann, A.; Claussen, M.; “Measurement of the local gas velocity at the

outlet of a wall flow particle filter, SAE technical Paper 2005-24-001, 2005

CONTACTDr. -Ing. Bernd Benker

*

++49 53 23 / 9 33 - 2 45 phone

[email protected]

Dipl.-Ing. Annett Wollmann*

++49 53 23 / 9 33 - 2 33 phone

[email protected]

Univ.-Prof. Dr.-Ing. Michael Claußen*

++49 53 23 / 9 33 - 2 05 phone

[email protected]

*CUTEC-Institut GmbH

Chemical Process TechnologyLeibnizstr. 23

38678 Clausthal-Zellerfeld, Germany ; ++49 53 23 / 9 33 - 1 00 fax

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Cutec Oil Fuel2006