Active and Passive Aftertreatment Systems: A Numerical Analysis of Energetic Performances

7/30/2019 Active and Passive Aftertreatment Systems: A Numerical Analysis of Energetic Performances

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The high temperature gas flow determines thegradual heating of the solid. The correspondingmaximum energy is:

gasinTcmE =max (12)

STORED ENERGY FRACTIONThe evaluation of the aftertreatment systemperformance was achieved through a newparameter, the stored thermal energy fraction:

minmax

min)(

EE

EtE

= (13)

whereE(t) is the thermal energy stored in the solidat time t:

L

dxxTcmtE

L

s

= 0)(

)( (14)

The fraction of the stored thermal energy is afunction of time, and it assumes values between 0and 1 for both operating conditions (cooling andheating process).The parameter decreases from 1 to zero duringthe cooling process, and thus represents an heatretention efficiency.Conversely, during the heating phase, raisesfrom zero to unit. In this case represents anefficiency index of the heating process.

PERCENTAGE RESIDENCE TIMEThe parameter is related to the energeticperformance of the solid material. A newparameter that measures the performance of thegas phase is also useful. To this purpose, thepercentage residence time (PRT) was defined asfollows:

100=RT

tPRT (15)

where t represents the length of time that the gas

stays in the converter within a fixed thermalwindow (T1TT2).

( )

)2/(

,

)(2/

0 0

cycle

t L

t

dtdxtxv

Ta

t

cycle

= (16)

where v(x,t) is the gas velocity at time t;while a(T) =1 ifT1TT2;

a(T) = 0 elsewhere.

The proper temperature range, in fact, is crucial toassure a reliable and efficient aftertreatmentprocess. Furthermore, the mean residence time(MRT) can be calculated from the equation 16,considering always a(T) = 1.

RESULTS

A one-dimensional single channel model wasused to predict the stored energy fraction andthe percentage residence time PRT of anaftertreatment system with and without reverseflow (active and passive flow control respectively).Furthermore, the effects of the cycle time and thehydrocarbons oxidation were also analyzed.Specifically, for the active flow control mode, asymmetrical thermal cycle was studied (theforward and reverse flow time are the same).

Figure 2 shows the evolution of temperature

profiles of solid phase along the regenerator as afunction of time. The figure refers to the passiveflow control mode, neglecting the oxidationcontribution.

Figure 2 - Temperature profiles of the solid phase along theaftertreatment system as a function of time. Passive controland cooling phase.

The initial temperature of the solid is set equalto 700 C while the inflow gas temperature is 200C. This operating condition simulates the exhaust

gas at low load conditions (Zheng at al., 2004).The analysis shows the great influence of time onthe temperature distribution within theregenerator. The leading part of the converter isalmost completely cooled after 25 seconds.Conversely, at the outlet, the temperature valuesare close to 700 C. Furthermore, the regeneratorappears thoroughly cooled after 100 seconds.

The evolution of solid temperature profiles withthe active control system in a range of 100 s isshown Figure 3. The cycle time is 20 seconds,and the energy contribution of oxidation is alsoneglected. Specifically, the bold lines correspond

to the time values reported in Figure 2.


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Figure 8 - Temperature profiles of the solid phase along the aftertreatment system as a function of time. Active control and heatingphase.

The analysis demonstrates that, if hightemperatures are required for the properfunctioning of the aftertreatment system, theactive control is useful at low load operatingcondition with the converter at high temperature.The reversal mode allows to reduce or to avoidthe supplemental fuel, which has a negativeimpact on the engine energy efficiency. Activecontrol could be an attractive solution also if lowtemperature operations are requested. The higherthermal inertia of the reversal mode permits, infact, the maintenance for a longer time of theinitial temperature level in the aftertreatmentsystem after sudden variations in engine load.Conversely, the passive control system isrecommended during the warm-up phase and/orto accelerate the cooling or the heating process.

Figure 9 - Stored thermal energy fraction with active andpassive flow control. Heating phase.


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EFFECT OF OXIDATION - The following analysisis restricted to the cooling phase (inlet gastemperature Tgas = 200 C, initial solid temperatureTs = 700 C).

Figure 10 shows the evolution of the solidtemperature profiles with active control, with asemi-cycle period of 10 seconds. The energetic

contribution of the unburned hydrocarbonsoxidation is taken into account. The oxidationeffect on temperature profiles is not negligible,and a growing influence over time operatingsystem is observed. In fact, it should be noted thatthe weight of the energy exothermic reactionincreases with the number of reversals as a resultof the exchange convective heat decrease.

As an example, the differences between themaximum temperature values with and withoutthe effect of the oxidation are about 37, 72, 92and 153 C for the second, third, fourth and fifthcycle respectively.

The effect of the retention effectiveness foractive and passive control is visible in Figure 11.For a long time of operation, when the convectivethermal exchange becomes negligible, does nottend to zero due to the energy oxidation effect.

EFFECT OF CYCLE TIME - In order to explore inmore detail the influence of the control mode onthe converter heat retention, a parametric study ofthe effect of the cycle time was carried out (Figure12). Specifically, the cooling phase is considered.The curve, related to the active controlaftertreatment systems, is catenary. Moreover, the

figure shows the presence of peaks associatedwith the switching time.

Furthermore, Figure 12 shows that theeffectiveness of the aftertreatment system withactive flow control tends to overlap with the curvecorresponding to the passive control as the cycletime increases. As expected, a perfectcorrespondence between the effectiveness of twocontrol systems during the interval of the firstsemi-cycle (before the first reversal) is evident.

CONCLUSIONS

A single channel one-dimensional model wasproposed in order to analyse the energy efficiencyof an aftertreatment system, and to evaluate theinfluence of the key operating parameters on theprocess characteristics. Specifically, the codeenabled the simulation of the heat exchangebetween the converter and the gas, and thedetermination of the gas and solid phasetemperature profiles. The analysis was carried outconsidering both passive and active flow control.The latter is based on periodic reversal of flowthrough the aftertreatment system. The

temperature profiles of gas and solid phase were

used for the characterization of the regeneratorenergy efficiency.

The comparison between active and passiveflow control showed the greatest thermal inertia ofreversal operation. As a consequence, thereversed flow converters appear more suitable tomaintain the monolith initial temperature level for

a longer time after sudden variations in engineload. Conversely, if rapid cooling or heating ofthe solid phase are requested, the unidirectionalflow is preferable.

Furthermore, the analysis showed that theeffect of the hydrocarbons oxidation on thetemperature profiles is not negligible. Specifically,its influence increases with the cycle time and thedifferences between the maximum temperaturevalues raise from 37 to 153 C going from thesecond to the fifth operating cycle.

Finally, the study demonstrated the significantinfluence of the cycle time on the system during

the cooling phase. Specifically, the energyperformances of the active control systemasymptotically approaches those of the passivecontrol system as the cycle time increases.

REFERENCES

Amelio M., Florio G., Morrone P., 2005,Simulazione dello Scambio Termico e dellePerdite di Carico all'Interno di Rigeneratori adImpaccamento Casuale in Ossidatori Termici

Rigenerativi, Atti del convegno "60 CongressoNazionale ATI", Roma.

Amelio M., Morrone P., 2007, Numericalevaluation of the Energetic Performances ofStructured and Random Packed Beds inregenerative thermal oxidizers, AppliedThermal Engineering, Vol. 27, 762-770.

Baik J. H., Yim S. D., Nam I.,. Mok Y. S., Lee J.-H., Cho B. K., Oh S. H., 2004, Control of NOxemissions from diesel engine by selectivecatalytic reduction (SCR) with urea, Topics inCatalysis, vols. 30/31, 37-41.

Burch R., Breen J. P., Meunier F. C., 2002, A

review of the selective reduction of NOx withhydrocarbons under lean-burn conditions withnon-zeolitic oxide and platinum group metalcatalyst, Applied Catalysis B: Environmental,vol. 39, 283-303.

Cauda E., Fino D., Saracco G., Specchia V.,2007, Preparation and regeneration of acatalytic diesel particulate filter, ChemicalEngineering Science, vol. 62, 5182-5185.

Guglielmini G., Pisoni C., 1996, Elementi ditrasmissione del calore, Maon EditorialeVecchi.

Gthenkea A., Chatterjeea D., Weibela M.,

Waldbera N., Kob P., Marekb M.,


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Figura 10 - Temperature profiles of the solid phase along the aftertreatment system as a function of time, with oxidation. Active control

and cooling phase.

Figure 11 - Influence of the oxidation on the stored thermalenergy fraction. Heating phase.

Figure 12 - Stored thermal energy fraction with active andpassive flow control.


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Active and Passive Aftertreatment Systems: A Numerical Analysis of Energetic Performances

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