Flow distribution effects in the loading and catalytic regeneration of

203

Flow distribution e ects in the loading and catalytic regeneration of wall- ow diesel particulate lters G A Stratakis and A M Stamatelos* Mechanical and Industrial Engineering Department, University of Thessaly, Volos, Greece

Abstract: The study of catalytic regeneration characteristics of porous ceramic diesel particulate lters (DPFs) is of growing interest to industry as diesel soot emissions are limited by legislation to levels below 0.01 g / km (for passenger cars). More speci cally, pressure drop computations and correlations are important factors employed in the design and control of diesel lter systems. However, in numerous cases, computations and models fail to match the experimentally observed evolution of soot combustion in the lter. In this paper, the role of ow maldistribution in this issue is investigated, by means of full-scale tests of the loading and regeneration behaviour of a particulate lter installed on a modern diesel engine run on catalyst-doped fuel. Loading tests were performed at three characteristic engine operation points with markedly di erent levels of engine exhaust gas mass owrate. In these tests, it becomes apparent that complex ow maldistribution phenomena exist during the loading phase, which are not directly re ected by the behaviour of the pressure drop versus time curve. However, these phenomena are shown to a ect the distribution of collected soot mass in the di erent channels of the lter and, consequently, the regeneration behaviour. The evolution of ow maldistribution was also studied in a number of regeneration experiments. It was con rmed that the variation of the volatile organic fraction in the lter and the associated partial catalytic regenerations at low temperatures interact with ow and soot maldistribution in a complex way. The conclusions from this study set the scene for future, more detailed investigations that are expected to improve understanding and modelling of diesel lter pressure drop and regeneration characteristics. Keywords: diesel exhaust emissions, diesel particulate lters, catalytic soot incineration, volatile organic fraction, ow maldistribution

NOTATION A E k m MFR t T TF u U

area (m2) substrate thickness (m) permeability (m2) mass (kg) exhaust gas mass owrate (kg /s) time (s) temperature ( K ) temperature of lter (°C ) velocity (m /s) mean ltration velocity (m /s)

¢p

pressure drop across the single channel lter (Pa)

The MS was received on 22 April 2003 and was accepted after revision for publication on 23 September 2003. * Corresponding author: Laboratory of Thermodynamics and Thermal Engines, Mechanical and Industrial Engineering Department, University of Thessaly, Pedion Areos, 383 34 Volos, Greece. D05903 © IMechE 2004

dynamic viscosity (Pa s) density (kg /m3) soot layer density times permeability product (kg /m)

m r (rk) p

Subscripts f p s T V

1

lter particulate ceramic throat vessel

INTRODUCTION

Diesel lters are already in use by automotive manufacturers in order to reduce particulate emissions to ultralow levels. A diesel lter system mechanically lters and collects the particulate matter from the exhaust gas, Proc. Instn Mech. Engrs Vol. 218 Part D: J. Automobile Engineering

204

G A STRATAKIS AND A M STAMATELOS

thus increasing lter backpressure. Filter backpressure increases fuel consumption and reduces available torque [1]. Thus, it is necessary to clean the trap periodically by burning o the collected particulate ( lter regeneration). Numerous regeneration techniques have been suggested over the last 20 years, the simplest and most e ective being based on catalysts (fuel doping [2, 3] or a catalytically coated lter [4, 5]). The lowering of the soot combustion temperature by doping the fuel with catalytic additives (usually in the form of organometallic compounds) is a workable catalytic regeneration technique [6 ]. However, the design of successful catalytic fuel additive-assisted trap systems depends on the solution of problems related to lter durability and additive ash accumulation [7]. Any active regeneration system [8] requires some kind of monitoring of accumulated soot mass on the lter. An intensively sought objective in this context is to derive a valid correlation of soot mass, which cannot be directly measured, with pressure drop and other directly measurable variables (exhaust temperature, pressure, etc.). This brings into the foreground the need to de ne the parameters a ecting pressure drop and the respective expressions allowing their calculation or estimation. As shown in Fig. 1, the determination of pressure drop involves a complex interaction between design and operating parameters of the engine and lter. The pressure drop signal is a ected by numerous parameters and

Fig. 1

carries a lot of information that is di cult to decipher to extract the necessary information with respect to the soot mass loading level. In a previous paper [9], measurements of pressure drop resistance of individual channels of a lter were carried out. The results indicated that signi cant variations exist in soot loading or quality [10] among di erent channels (the centre versus the periphery of the lter) during the loading phase. This fact hints at the existence of ow maldistribution at the lter inlet during the loading phase (the centre versus the periphery). Similar phenomena of ow maldistribution in catalytic converters in gasoline cars have been investigated by numerous researchers [11–13]. Flow maldistribution in catalytic converters is shown to be a ected by the geometry of the inlet devices (piping and di user) that create an initial non-uniform distribution of the gas ow before its entrance to the converter. Although this irregularity is reduced along the cell of the catalyst due to the pressure drop, a certain degree of non-uniformity is always transmitted downstream. Some researchers [14] report that cell geometry produces some internal maldistribution by itself, even in case of initial uniform distribution of the gas ow. Despite signi cant literature studies for catalytic converters, no studies are known to date regarding ow maldistribution in diesel lters. The physical mechanism a ecting ow distribution is di erent from that of a catalytic converter. In the case

Principle of interaction among design and operation parameters related to the loading and regeneration of diesel lters (EGR, exhaust gas recirculation; ECU, electronic control unit)

Proc. Instn Mech. Engrs Vol. 218 Part D: J. Automobile Engineering

D05903 © IMechE 2004

FLOW DISTRIBUTION EFFECTS IN THE LOADING AND CATALYTIC REGENERATION OF DPFs

of diesel particulate lters, the soot deposition modes that result from the two-phase ow produce variations in the soot loading pro le along the channel. Apart from the factors that a ect the packing of soot on the channel wall, a series of secondary factors, (a) volatile organic fraction ( VOF ) desorption phenomena and (b) stochastic regeneration phenomena,

adsorption–

which are triggered by the temperature di erences between the central and peripheric channels of the lter, should also be taken into account. These phenomena lead to a continuous redistribution of the properties of the accumulated soot layer that a ect the ow distribution across the lter channels. This paper aims to present initial experimental results that con rm the importance of ow maldistribution in diesel lter loading and regeneration processes. Transient measurements of the ow velocity at the exit of typical central and peripheric cells of a full-sized lter, during loading and regeneration under steady state engine operating conditions, are employed in this investigation. Furthermore, a soot mass distribution estimation technique described in detail in reference [9] is employed in the experimental study of soot distribution across the lter channels and con rmation of the results of the ow velocity measurements. This technique involves the recording of the discharge process of a vessel containing air, connected through a specially designed nozzle to typical cells of the lter. Based on the processing of the experimental results, a better understanding of the volatile organic fraction adsorption– desorption phenomena and the stochastic regeneration behaviour can be obtained. 2

THE PROBLEM OF SOOT MALDISTRIBUTION IN THE DETERMINATION OF PRESSURE DROP

A starting point for the calculation of the total pressure drop across the particulate lter would be the generally accepted correlation of Darcy’s law [15], which takes into account ow resistance of the lter wall plus accumulated soot layer. According to this law, the total pressure drop due to ow through the ceramic wall and the soot layer can be approximated by the following simpli ed relation: mUE mUE s+ p (1) k k s p By using the concept of e ective particulate layer thickness, the latter formula can be rewritten as follows [16]: ¢p=

mUE mUm s+ p (2 ) k A ( rk) s f p where A denotes the total ltration area of the channel, f ¢p=

D05903 © IMechE 2004

205

which could be approximated by the sum of the inlet channel areas, after subtraction of the area blocked by the lter plugs. Both factors of the above product are widely variable, thus the product of (rk) varies depending on engine p type, injection pressure, lter type, operation point, possible use of fuel additives, etc. In a previous paper, an attempt was made to narrow the range of variation of this parameter by a speci c experimental test rig, which is presented in more detail in reference [9]. The results of this study show how the product ( rk) depends on the: p (a) prevailing engine operation point during loading ( VOF content–exhaust temperature) and (b) loading history of the lter– lter channel (soot mass, etc). The above formula would allow, in principle, the backward approximate calculation of collected soot mass as a function of measured lter backpressure at a certain engine and lter loading operation point, once an approximate value for the product (rk) , i.e. soot p layer density ×soot layer permeability, was known for the speci c engine– lter–operation point combination. However, the above formula is non-dimensional, considering the total ltration surface of the lter as a homogeneous membrane. Use of the above-mentioned expression would only be justi ed if the super cial velocity is assumed to be absolutely uniform. This assumption could only rarely represent the real situation. Soot maldistribution phenomena caused by the inlet geometry and soot loading history (residual soot mass from previous incomplete regeneration) severely a ect the pressure drop signal. Such a situation is presented in Fig. 2 where the periphery of an SiC lter is, apparently, incompletely regenerated (presumably due to the lower temperatures prevailing there). As explained above, in reality, the situation is threedimensional and the simpli ed relations are of very little use. Now, if a more accurate calculation of the pressure drop of a full-sized lter is attempted, the added complexity depends on how many dimensions are inserted in the calculation. As a preliminary step, an attempt could be made to add one more dimension to the calculation. Based on experimental evidence, the radial dimension (the di erence between the lter centre and the periphery) would be the most signi cant. A simpli ed, stepwise variation of soot layer thickness (with soot density–permeability uniform throughout ) is assumed, which distinguishes between two zones in the lter: a central zone (core) and a peripheric zone (outer), which are characterized by two distinct soot layer thickness values. The purpose of this simpli ed calculation would be to visualize the e ect of stepwise radial maldistribution on the pressure drop (attempting to study situations resembling those of Fig. 2) under typical owrates in a typical full-sized lter. Proc. Instn Mech. Engrs Vol. 218 Part D: J. Automobile Engineering

206


known and kept constant: A MFR outer U (7) core A outer r A core gas core The system of algebraic equations (5) and (7) may be employed, for example, to calculate the evolution of pressure drop if a constant, low soot layer thickness is kept in the core zone of the lter and a variation of soot layer thickness is assumed in the periphery, starting from low and reaching extremely high values. The results of the respective calculations are presented in Table 1 and the characteristic evolution of the lter pressure drop as a function of total soot loading is presented in Fig. 3. This gure explains why a lter that is heavily loaded in the periphery continues to demonstrate reasonable backpressure values if its core zone happens to be kept relatively clean by frequent partial regenerations. Obviously, the rate of lter pressure drop increase as a function of the total soot loading in the speci c, simpli ed loading scenario is directly a ected by the relative dimensions of the two lter zones, which can be expressed by the ratio between the core and the total ltration area. In the example calculation of Fig. 3, this ratio is assumed to be of the order of 0.6 (assuming a situation similar to that of Fig. 2, where the central part up to the marked circle remains lightly loaded and the rest of the lter is gradually loaded up to high values). U

Fig. 2

Front view of a partially regenerated lter. The appearance of a zone that becomes gradually darker moving from the central to the outer cells of the lter indicates that the lter is incompletely regenerated at the periphery. This type of behaviour could be the cause of exhaust ow maldistribution between the centre and periphery of the lter

Starting with the expression for an equal pressure di erence through the two zones of the lter, ¢p

= ¢p core outer The above non-dimensional correlations employed for each of the two distinct zones: mU

(3) can

be

E mU E mU E mU E core s + core p,core = outer s + outer p,outer k k k k s p s p (4)

This results in a correlation between the super cial velocities of the two zones: U

outer

=

a+bE p,core U core a+bE p,outer

(5)

where mE m s b= and k k s p On the other hand, the continuity equation can be pro tably employed, if the realistic assumption is made that the exhaust gas mass owrate ( MFR) is more or less dictated by the engine characteristics at the speci c operation point, which are only slightly a ected by backpressure at low backpressure levels [1]: a=

U (A U +A ) (6) gas core core outer outer Using this additional equation, one more correlation between the super cial velocities of the two zones may be deduced, provided that the exhaust mass owrate is MFR=r


=

3

EXPERIMENTAL

The measurements were performed on an SiC lter tted to the exhaust system of a 2.0 litre displacement highpressure direct injection ( HDI ) turbocharged passenger car engine, running on one of the test benches of the Laboratory of Thermodynamics and Thermal Engines, on carefully selected steady state operation points. The engine and diesel lter speci cations are presented in Table 2. Figure 4 presents the experimental layout of the lter loading and regeneration experiments. Temperatures were measured simultaneously at the exhaust pipe 50 mm after the turbine exit, at the inlet of the lter and inside the lter, along a lter diameter 1.5 cm deep from the end (Fig. 4). The existence of ow maldistribution is investigated in this paper by means of ow velocity measurements at a downstream lter (no catalyst in front ) during: (a) the loading phase and (b) the regeneration phase. The experimental determination of ow velocity is done by means of Pitot tubes, as a simple and e ective method to meet the high temperature and corrosive conditions inside the exhaust system during engine operation. The measurements were taken at the lter exit for the following reasons: D05903 © IMechE 2004


Table 1

Summarized results of calculated lter backpressure and wall ow velocity at the central and peripheral channels of a non-uniform loaded lter assuming constant exhaust gas mass owrate and variable soot layer thickness only for the cells of the outer area. The ratio of the central to outer areas is tted to the value of 1.5 according to the situation described in Fig. 2 for the case of a lter loaded with an exhaust gas mass owrate of the order of 70 g /s

Soot layer thickness at core (m) 1×10Õ 1×10Õ 1×10Õ 1×10Õ 1×10Õ 1×10Õ 1×10Õ 1×10Õ 1×10Õ 1×10Õ

4 4 4 4 4 4 4 4 4 4

Fig. 3

Table 2

Soot layer thickness at periphery (m)

Core velocity (m /s)

1×10Õ 4 1.5×10Õ 2×10Õ 4 2.5×10Õ 3×10Õ 4 3.5×10Õ 4×10Õ 4 4.5×10Õ 5×10Õ 4 5.5×10Õ

5.96×10Õ 6.81×10Õ 7.36×10Õ 7.74×10Õ 8.02×10Õ 8.24×10Õ 8.41×10Õ 8.55×10Õ 8.67×10Õ 8.76×10Õ

4 4 4 4 4

Outer velocity (m /s) 2 2 2 2 2 2 2 2 2 2

5.96×10Õ 4.68×10Õ 3.86×10Õ 3.28×10Õ 2.85×10Õ 2.52×10Õ 2.26×10Õ 2.05×10Õ 1.87×10Õ 1.72×10Õ

2 2 2 2 2 2 2 2 2 2

Total soot mass loading (g)

¢p for maldistributed loading (kPa)

¢p for homogenous loading (kPa)

0.0146 0.0175 0.0204 0.0233 0.0262 0.0291 0.0320 0.0349 0.0378 0.0408

5.42×10 6.19×10 6.68×10 7.03×10 7.29×10 7.48×10 7.64×10 7.77×10 7.87×10 7.96×10

5.42×10 6.40×10 7.37×10 8.35×10 9.33×10 1.03×102 1.13×102 1.23×102 1.32×102 1.42×102

Calculated e ect of soot maldistribution on lter backpressure and wall ow velocities at the central core and the outer area of the lter, assuming a constant exhaust gas mass owrate and a constant soot layer thickness at the core. The soot layer thickness at the outer cells varies from 0.1 to 0.55 mm

Engine technical data and diesel lter speci cations

Engine type Cylinders Displacement Rated power per r/min Rated torque per r/min Filter type Filter Diameter×length Cell pitch Filter wall thickness

HDI turbocharged engine 4, in-line 1997 cm3 80 kW/4000 r/min 250 N m/2000 r/min SiC 14/200 cpsi (cells per square inch) 143.8 mm×150 mm 1.89 mm 0.4 mm

1. Pitot tubes could distort the velocity eld upstream of the lter. 2. Pitot tubes would be quickly clogged with soot emitted by the engine (subsequently collected by the lter). 3. A ow maldistribution downstream of the lter also accounts for the e ects of the in-channel ow variD05903 © IMechE 2004

207

ation that could not be measured using simple equipment. Figure 5 presents the experimental layout of the exhaust gas velocity measurements. Two Pitot tubes were placed, the rst one at the centre and the second at the periphery of the lter. The distance between the measurement point and the channel exit was about 20 cell diameters as the theory demands for a complete development of the ow pro le at the exit of the lter. The monitoring of the Pitot tubes is carried out by recording the signal of a di erential pressure sensor generated by the di erence between the total or Pitot pressure and the static pressure. Assuming that the exhaust gas stream behaves as a steady state one-dimensional ow of an incompressible frictionless uid, the general relationship between the velocity and the pressure caused by the gas moving over Proc. Instn Mech. Engrs Vol. 218 Part D: J. Automobile Engineering

208


Fig. 4

Experimental layout. The engine and digitally controlled dynamometer installation is shown along with exhaust gas analysers, main lter measurement lines and the data acquisition system

the Pitot tube (total pressure minus static pressure) [18] may be used:

S

2¢p Pitot (7) r g where C is a Pitot tube constant and r is the exhaust g gas density determined by measuring the temperature at the Pitot tube inlet. V =C g

3.1 Steady state loading experiments The steady state lter soot-loading experiments were performed with the engine running with 25 ppm Ce DPX9-doped fuel. Engine speed points were selected to cover a range between 2000 and 4000 r/min as more representative of the full range of city and extra-urban driving conditions, and engine load was varied to result in a range of lter wall temperatures from 300 to 400 °C. As presented in reference [10], this range of lter wall temperatures results in varying VOF content of accumulated soot between 2 and 9 per cent and allows the study of the e ects of VOF adsorption–desorption and stochastic regeneration. After completion of each loading scenario the lter was regenerated and cleaned with a reverse ow of water in order to exclude any secondary e ects from residual fuel additive ash. The loading test protocol, carefully designed based on the engine maps, is presented in Table 3. Proc. Instn Mech. Engrs Vol. 218 Part D: J. Automobile Engineering

3.2 Regeneration experiments Two typical regeneration modes that appear under actual vehicle operating conditions were studied: 1. The rst one involves high speed and load engine operation associated with a high exhaust gas mass owrate and high exhaust gas temperature. 2. The second involves medium speed and load engine operation resulting in a low owrate and low exhaust gas temperature. Each regeneration process was initiated after loading at the respective engine operation point, until the same level of lter loading was achieved by a step increase in engine load. The engine was left to run for 10 min at 2000 r/min and 40 N m before the regeneration strategy was applied, with the duration considered long enough for thermal and chemical equilibrium to be reached. The regeneration engine operation points are presented in Table 3. 3.3 Soot loading distribution measurements In order to con rm the measurements with the Pitot tubes and indirectly evaluate the soot loading distribution from the lter centre to the periphery, the expansion device of Fig. 6, described in detail in reference [9], was employed in a series of measurements at selected inlet channels of the engine lter after the end of the loading process. This simple device consists of a vessel containing air at 1 bar initial gauge pressure. This vessel D05903 © IMechE 2004


Fig. 6

209

Measuring device for the soot loading distribution measurement

of a routine calculation procedure that is also described and demonstrated in reference [9]. The exhaust pipe of the vessel of Fig. 6 was carefully connected to the inlet of typical, representative trap channels and the pressure drop characteristics were recorded. Fig. 5

Schematic diagram of the Pitot tubes assembly on the engine exhaust system at the exit of the lter

4 may be connected to the inlet of selected lter channels with an airtight seal. The vessel discharge pipe is controlled by a solenoid valve and opens on demand by the data acquisition software, triggering the expansion of the compressed air from the vessel through the walls of the speci c channel to the atmosphere. Monitoring of the expansion process is carried out by means of recording the signal of a pressure sensor that is also mounted on the vessel. The results of this recording procedure are curves of the type shown in Figs 8, 10 and 12. These curves are employed in the comparative estimation of the soot density ×permeability product, (rk) , by means p Table 3 Process

N (r/min)

Torque (N m)

RESULTS AND DISCUSSION

4.1 Loading experiments As already mentioned, the steady state loading experiments were performed to investigate the ow and soot loading characteristics along the lter channels under various mass owrates and lter wall temperatures. Initially a low mass owrate (35 g /s) loading experiment was performed with the engine running at a speed of 1800 r/min and a load of 80 N m, resulting in a lter wall temperature of the order of 370 °C as measured at the centre of the lter by thermocouple T/C9. The results are presented in Fig. 7.

Experimental protocol

MFR (g /s)

T exhaust (°C )

TF inlet (°C)

TF wall centre (°C )

TF wall side (°C )

Loading

1800 3000 3600

80 40 50

35 68 91

410 316 402

385 307 391

380 304 388

365 290 364

Regeneration

2000 3000

180 160

58 91

533 600

516 584

510 580

490 550

D05903 © IMechE 2004


210


Fig. 7

Low mass owrate (35 g /s) loading scenario. The exhaust gas velocity is measured simultaneously at the exit of the central and peripheral channels of the lter during loading at 1800 r/min, 80 N m with 25 ppm additive. Evolution of lter backpressure is also monitored. The e ects of the lter wall temperature on the loading process are measured by thermocouple T/C9 (closer to the centre) and T/C8 (closer to the periphery)

Three characteristic parts were observed in the recording of the lter loading process: 1. The rst incorporates the phase of the beginning of loading until the accumulation of soot mass becomes uniform. During this initial short phase, the backpressure and velocity increase rapidly due to the lling of lter surface pores. After this initial period, the particulates start to accumulate on the wall surface and the pressure drop curve takes a more linear form. It can be observed that the exhaust gas velocity for all this period is higher in the central region of the lter, indicating that the ow passes mainly through the central channels of the lter. This behaviour could be attributed to the e ects of the inlet pipe and inlet di user geometry, which directs the ow through the central region. As the accumulated mass increases, the core velocity decreases and the ow is diverted towards the outer area of the lter, resulting in a velocity increase through the periphery channels. 2. In the second part, the loading process is continued without any changes to the soot loading pro le. The velocities at the centre and the periphery are equal and tend to decrease as a result of the increase of lter backpressure. This increase of lter backpressure due to the increase in the accumulated soot mass within the lter channels reduces the velocity di erences between the centre and periphery and makes the ow distribution more uniform. 3. In the third part of the loading process, the e ects of Proc. Instn Mech. Engrs Vol. 218 Part D: J. Automobile Engineering

stochastic phenomena start to show up. A sudden decrease of the lter backpressure is associated with an increase in the lter inlet temperature, exceeding 380 °C. This is shown to disturb the previously described uniform ow distribution. It is thought that a VOF desorption process could be taking place in the central channels of the lter where the temperatures are higher. Comparison of wall temperatures at the lter centre and periphery shows that the periphery is systematically colder than the centre by about 25 °C. Presumably, VOF desorption would cause a permeability increase of the soot layer of the central channels of the lter, resulting in a reduction of the ow resistance through this region. Thus, a marked increase of exhaust gas velocity at the central region of the lter is observed, indicating a non-uniform redistribution of the ow eld over the lter channels. The percentage of VOF that could eventually remain adsorbed on the dry soot accumulated in the lter mainly depends on the lter temperature [10]. If the lter temperature exceeds the range of 380–420 °C, most VOF is vaporized and only dry soot remains accumulated on the lter wall. On the other hand, prolonged operation of a lter heavily loaded with dry soot under low load and speed conditions can lead to the readsorption of heavy hydrocarbons emitted in the particulate layer, thus increasing its VOF content. In order to con rm the above observations, a set of measurements with the expansion device have been perD05903 © IMechE 2004


formed. The results are presented in Fig. 8. Two di erent zones are observed as the measuring device is moved from the central (hotter during operation) to the outer (colder during operation) region. The pressurized air is shown to discharge faster through the central channels than through those at the periphery, indicating that the ow resistance due to VOF desorption is lower. It must be mentioned that this type of measurement is performed after the engine stops, at lter wall temperatures that do not exceed 150 °C. Thus, a part of VOF that was present in the thick particulate layer could possibly condense on the colder outer channel walls after engine stop, resulting in a variation of permeability characteristics between the lter core and periphery channels. As a next step, the loading process was repeated with the engine running at 3000 r/min at 40 N m, resulting in a medium exhaust gas mass owrate of the order of 70 g /s and a lter wall temperature of the order of 350 °C. The results are presented in Fig. 9. Again, a higher exhaust gas velocity is initially observed at the central region of the lter. In comparison with the previous low mass owrate loading point, the di erence between the central and outer velocities is lower, indicating that the increase of backpressure diverts a part of the ow through the outer channels. With increasing lter soot loading, this e ect is counterbalanced, due to selective loading of the central part. After a period where the ow seems to be stabilized to a relatively homogeneous distribution, a reversal of the ow distribution is observed. This could be attributed to stochastic regen-

Fig. 8

D05903 © IMechE 2004

211

eration phenomena at the soot of the lter periphery, which is expected to be VOF rich due to the lower wall temperatures. The VOF content at this operation point varies from about 6 per cent at the central channels to 9 per cent at the circumferential channels. Owing to its very close contact with the catalyst oxides it may be oxidized at temperatures below 250 °C. Thus it may be assumed that a slow regeneration process takes place at speci c points on the lter periphery where the soot loading, composition and temperatures are favourable. This slow regeneration reduces the ow resistance through the periphery, allowing an increase in velocity through this region. Another phenomenon that could be involved in producing this behaviour is the appearance of signi cant inertia e ects at high owrates, due to the upstream piping and inlet di user causing a swirl ow component. The measurements with the expansion device con rm the above results. As shown in Fig. 10, the vessel discharge curves indicate two distinct zones as the nozzle is moved from the central to the peripheral channels. The pressurized air discharges faster through the circumferential channels than through the central channels. This means that the stochastic regenerations happening in the VOF-rich periphery channels produce a lower soot loading zone. This could be responsible for the redirection of exhaust gas ow from the central area towards the outer lter area. The study of the e ects of exhaust gas mass owrate on ow and soot maldistribution during loading is completed with a high owrate–high exhaust temperature

Estimation of soot mass distribution after the low mass owrate loading scenario of Fig. 7. Pressure drop measurements were performed with the expansion device described in Fig. 6 at speci c channels along a diameter of the lter Proc. Instn Mech. Engrs Vol. 218 Part D: J. Automobile Engineering

212


Fig. 9

Medium mass owrate (68 g /s) loading scenario. The exhaust gas velocity is measured simultaneously at the exit of the central and peripheral channels of the lter during loading at 3000 r/min, 40 N m with 25 ppm additive. Evolution of lter backpressure is also monitored. The e ects of the lter wall temperature on the loading process are measured by thermocouple T/C9 (closer to the centre) and T/C8 (closer to the periphery)

Fig. 10

Estimation of soot mass distribution after the medium mass owrate loading scenario of Fig. 9. Pressure drop measurements were performed with the expansion device described in Fig. 6 at speci c channels along a diameter of the lter


D05903 © IMechE 2004


loading test obtained with the engine running at 4000 r/min at 30 N m. This point results in a mass owrate of the order of 90 g /s and a lter wall temperature of the order of 390 °C. The results are presented in Fig. 11. As in the previous cases, the ow initially passes through the central core of the lter. In this case the velocity di erence between the centre and periphery takes its lowest value and is compensated very early by the increase of backpressure as a result of soot accumulation within the channels. After this period, a reversal of the ow is observed without any stabilization to a uniform ow distribution as in the previous cases. This could be attributed to the selective loading of the central part of the lter. Thus the previously described inertia phenomena may enforce the concentration of particulates at the central channels. After this phase a new reversal of the ow is observed, which is associated with a continuous increase in the measured velocity at the lter centre. This phase is characterized by a rapid increase in the lter wall temperature measured at the centre-line, indicating that a regeneration process takes place at the centre of the lter where the temperatures are higher. This mechanism results in a partial regeneration of the lter and is responsible for the signi cant variation of the exhaust ow distribution and the subsequent loading maldistribution, with the exhaust owing mainly through the central region of the lter.

Fig. 11

D05903 © IMechE 2004

213

The test with the expansion device con rms the above observation. The results shown in Fig. 12 indicate that the pressurized air discharges faster through the central channels than through the circumferential channels. This fact hints to a possible partial regeneration that may have taken place at the central part of the lter.

4.2 Regeneration experiments The processes taking place during catalytic regeneration are quite complex and not yet well understood. The instability of the appearance and evolution of regeneration at the temperature range from 250 °C (appearance of erratic regeneration) to 600 °C (thermal regeneration) is one of the most signi cant problems since it is related to the lter durability. The monitoring of ow velocity at the core and periphery of the lter during regeneration can provide important information about the role of ow distribution. As mentioned above, two regeneration scenarios were investigated here: a low mass owrate– medium exhaust gas temperature scenario and a high mass owrate–high exhaust gas temperature scenario. In the rst regeneration scenario, a lter previously loaded at 1800 r/min at 80 N m was subsequently regenerated at 2000 r/min with a step increase from 40 to 180 N m (mass owrate of 58 g /s), resulting in a

High mass owrate (91 g /s) loading scenario. The exhaust gas velocity is measured simultaneously at the exit of the central and peripheral channels of the lter during loading at 4000 r/min, 30 N m with 25 ppm additive. Evolution of lter backpressure is also monitored. The e ects of the lter wall temperature on the loading process are measured by thermocouple T/C9 (closer to the centre) and T/C8 (closer to the periphery) Proc. Instn Mech. Engrs Vol. 218 Part D: J. Automobile Engineering

214


Fig. 12

Estimation of soot mass distribution after the high mass owrate loading scenario of Fig. 11. Pressure drop measurements were performed with the expansion device described in Fig. 6 at speci c channels along a diameter of the lter

Fig. 13

Low mass owrate regeneration scenario of a previously loaded lter at 1800 r/min and 80 N m. Exhaust gas velocity is measured simultaneously at the exit of the central and peripheral channels of the lter during a step increase in the lter inlet temperature, produced by changing the engine operation point from 2000 r/min and 40 N m to 2000 r/min and 180 N m. Evolution of lter backpressure is also monitored. The e ects of the lter wall temperature on the regeneration process are measured by thermocouple T/C9 (closer to the centre) and T/C8 (closer to the periphery)


D05903 © IMechE 2004


temperature increase of the order of 250 °C. The recordings of ow velocities along with lter backpressure and lter wall temperatures are presented in Fig. 13. It can be seen that, for speci c levels of wall temperature and lter backpressure, the regeneration is initiated and the shape of ow velocity curves directly correlates with the temperature. Initially, due to the previous loading history, the ow is higher through the central region of the lter, indicating that a non-uniform soot loading distribution exists. After the step load increase, a slow increase in the ow velocity at the central region is observed, indicating the initiation of regeneration due to the higher temperatures that accelerate the soot oxidation. The velocity through the periphery reduces during the regeneration of the central part as a result of the ow deviation. However, as the regeneration process continues and the temperature at the periphery increases, the soot oxidation is transferred to the outer channels, thus increasing the ow velocity until completion of regeneration, where steady state conditions are obtained again. In the case of a high mass owrate–high temperature regeneration mode a lter previously loaded at 3000 r/min at 40 N m was regenerated with a step increase from 2000 r/min and 40 N m to 3000 r/min and 160 N m (mass owrate of 91 g /s), resulting in a tem-

Fig. 14

D05903 © IMechE 2004

215

perature increase of the order of 320 °C. The results of Fig. 14 show that initially the ow velocity at the periphery is somewhat higher due to the previous loading history. Higher wall temperatures at the central part resulted in the initiation of regeneration there. The resulting soot depletion in this part forced the ow to pass mainly from the centre, allowing the appearance of higher oxidation rates of soot by exhaust gas oxygen. Thus a rapid increase in the ow velocity at the centre associated with a higher heat release rate due to the higher exothermy is observed. At the same time, the ow velocity measured at the periphery is minimized, indicating the existence of signi cant ow maldistribution. Afterwards, regeneration sets up also in the periphery and nally the ow is homogenized again, with a somewhat higher ow through the centre. The above-described regeneration scenarios led to the conclusion that the time lag in the regeneration of the outer lter channels observed in the experimental results could be attributed to ow maldistribution e ects. 5

CONCLUDING REMARKS

1. A systematic e ort was made to record and explain ow maldistribution e ects across the channels of

High mass owrate regeneration scenario of a previously loaded lter at 3000 r/min and 40 N m. The exhaust gas velocity is measured simultaneously at the exit of the central and peripheral channels of the lter during a step increase in the lter inlet temperature, produced by changing the engine operation point from 2000 r/min and 40 N m to 3000 r/min and 160 N m. Evolution of lter backpressure is also monitored. The e ects of the lter wall temperature on the regeneration process are measured by thermocouple T/C9 (closer to the centre) and T/C8 (closer to the periphery) Proc. Instn Mech. Engrs Vol. 218 Part D: J. Automobile Engineering

216

2.

3.

4.

5.


a diesel lter during loading and regeneration operation. Full-scale tests of the loading and regeneration behaviour of a particulate lter installed on a modern diesel engine run on catalyst-doped fuel were employed in this investigation. Loading tests were performed at three engine operation points with markedly di erent levels of engine exhaust gas mass owrate. In these tests, it became apparent that complex ow maldistribution phenomena exist that vary during the loading phase, which are not directly re ected by the behaviour of the pressure drop versus time curve. These phenomena are shown to a ect the distribution of collected soot mass in the di erent channels of the lter and, consequently, the regeneration behaviour. The evolution of ow maldistribution phenomena was also studied in a number of regeneration experiments. It was observed that variation of the soot volatile organic fraction across the lter and the associated partial catalytic regenerations at low temperatures interact with ow and soot maldistibution in a complex way. A specially designed test rig for cold ow pressure drop and velocity distribution experiments will be employed in future, more detailed, investigations of the ow maldistribution behaviour of diesel lters.

ACKNOWLEDGEMENT The authors wish to thank Professor Herricos Stapountzis (Laboratory of Fluid Mechanics and Turbomachinery, Mechanical and Industrial Engineering Department, University of Thessaly) for his helpful discussions and valuable advice in the design and interpretation of ow measurement experiments. REFERENCES 1 Stamatelos, A. M. A review of the e ect of particulate traps on the e ciency of vehicle diesel engines. Energy Conversion and Managmt, 1997, 38(1), 83–99. 2 Salvat, O., Marez, P. and Belot, G. Passenger car serial application of a particulate lter system on a common rail direct injection diesel engine. SAE paper 2000-01-0473, 2000. 3 Versaevel, P., Colas, H., Rigaudeau, C., Noirot, R., Koltsakis, G. C. and Stamatelos, A. M. Some empirical


4

5

6

7

8

9

10

11

12

13

14

15

16

17

observations on diesel particulate lter modeling and comparison between simulations and experiments. SAE paper 2000-01-477, 2000. Giesho , J., Pfeifer, M., Shaefer-Sindlinger, A., Hackbarth, U., Teysset, O., Colignon, C., Rigaudeau, C., Salvat, O., Krieg, H. and Wenclawiak, B. W. Regeneration of catalytic diesel particulate lters. SAE paper 2001-01-0907, 2001. Maricq, M., Guo, G., Xu, N., Laing, P. and Hammerle, B. Performance of a catalyzed diesel particulate lter system during soot accumulation and regeneration. SAE paper 2003-01-0047, 2003. Vincent, M., Richards, P. and Catterson, D. A novel fuel borne catalyst dosing system for use with a diesel particulate lter. SAE paper 2003-01-0382, 2000. Ohno, K., Shimato, K., Taoka, N., Sungtae, H., Ninomiya, T., Komori, T. and Salvat, O. Characterization of SiC–DPF for passenger car. SAE paper 2000-01-0185, 2000. Herrmann, H. O., Lang, O., Miculic, I. and Scholz, V. Particel ltersysteme fur Diesel-Pkw. Motortechnische Z., 2001, 62(1), 652–660. Stratakis, G. A., Psarianos, D. L. and Stamatelos, A. M. Experimental investigation of the pressure drop in porous ceramic diesel particulate lters. Proc. Instn Mech. Engrs, Part D: J. Automobile Engineering, 2002, 216(D9), 773–784. Stratakis, G. A., Konstantas, G. S. and Stamatelos, A. M. Experimental investigation of the role of soot volatile organic fraction in the regeneration of diesel lters. Proc. Instn Mech. Engrs, Part D: J. Automobile Engineering, 2003, 217(D4), 307–317. Bella, G., Rocco, V. and Maggiore, M. A study of inlet ow distortion e ects on automotive catalytic converters. J. Engng Gas Turbines Power, 1991, 13, 419–426. Martin, A., Will, N., Bordet, A., Cornet, P., Gondoin, C. and Mouton, X. E ect of ow distribution on emissions performance of catalytic converters. SAE paper 980936, 1998. Cho, Y. S., Kim, D. S., Han, M., Joo, Y., Lee, J. H and Min, K. D. Flow distribution in a close-coupled catalytic converter. SAE paper 982552, 1998. Durakhiev, R. and Dodev, C. Gas ow distribution in packed columns. Chem. Engng and Processing, 2002, 41, 385–393. Sorenson, S. C., Hoj, J. W. and Stobbe, P. Flow characteristics of SiC diesel particulate lter materials. SAE paper 940236, 1994. Ebener, S. and Florchinger, P. Drukverlustmodel fur keramische Dieslpartikel lter. Motortechnische Z., 2000, 61(6), 414–422. Klopfenstein, R. J. Air velocity and ow measurement using a Pitot tube. ISA Trans., 1998, 37, 257–263.

D05903 © IMechE 2004