Surface Ageing of Soot Particles Reactions on the surface of soot are the principal route by which...

Surface Ageing of Soot Particles Reactions on the surface of soot are the principal route by which mass is transformed from the gas to solid phase in laminar premixed flames. However, the reactivity of soot particles decreases significantly over time so an accurate model is necessary to predict soot mass. R I A Patterson, J Singh & M Kraft Department of Chemical Engineering, University of Cambridge References: References: 6 Summary 6 Summary •The decay constant can be used to fit the final soot volume fraction for three different flames. •Surface decay never seems to reach its asymptotic value in some flames which are therefore unsuitable for testing this part of the model. •A detailed chemical model for the surface reactions is needed. [1] J Appel at al. Kinetic Modelling of soot formation with detailed chemistry and physics: Laminar premixed flames of C2 hydrocarbons. Combust. Flame 121 : 122-136 (2000) [2] M Frenklach and H Wang in: H Bockhorn (Ed), Soot Formation in Combustion - Mechanisms and Models. Springer Verlag, Berlin 1994, 165-192 [3] R I A Patterson et al. The Linear Process Deferment Algorithm: A new technique for solving population balance equations. Technical Report 26, c4e-Preprint Series, Cambridge, 2004 1 Soot Model 1 Soot Model The model we use consists of three main processes • Particle inception – when two pyrene molecules ‘coagulate’ • Coagulation – two particles form a new sphere with the same total mass • Surface events – chemical reactions on the particle surface (4 types) o Pyrene condensation o C 2 H 2 addition o OH oxidation o O 2 oxidation 2 Stochastic Algorithm (LPDA) 2 Stochastic Algorithm (LPDA) Wait an exponentially-distributed time step and update current time, t Initialize system Probabilistically choose an event to perform Coagulation Particle Inception yes no Perform deferred processes for all particles yes no Perform jump Update particle ensemble Pyrene condensation Select particle(s) Perform deferred processes on selected particle(s) Place updated particle(s) back into ensemble • Fictitious events are a technical convenience in which time is advanced and the event selection procedure followed upto and including the simulation of the deferred processes but the updated particle(s) are then replaced without performing the selected event. • The algorithm is described in [3] Is t > t stop ? Fictitious event ? Stop 3 Active Sites Model 3 Active Sites Model This exponential decay is calculated along with the deferred processes. When two particles coagulate the age of the new particle is calculated from the surface areas a 1 , a 2 and ages τ 1 , τ 2 of the old particles as An important part of the model for the rate of C 2 H 2 addition and O 2 oxidation is the number of active sites on the surface of the soot particle [1,2]. This is known to decrease over time but little is known about the decay process except that it never reaches 0 so we model the density of active sites, α,on the surface of a particle as an offset exponential decay process in the particle age τ : C e 1 . 2 1 2 2 1 1 a a a a We present results for three acetylene flames for which some experimental data is available. One can see that the simulated soot volume fraction has a significant dependence on the decay constant C but that by choosing the value carefully good agreement with the experimental data can be achieved. The data were taken from [1]. Lines: simulation results Points: experimental data Pres s / bar C/O ratio Initial gas velocity /cm s -1 JW 1.69 1 0.69 5.9 JW 10.673 10 0.673 3.0 JW 10.68 10 0.68 6.0 4 Decay constant 4 Decay constant University of Cambridge Department of Chemical Engineering 7 th WCCE, Glasgow CH 4 4 CH 2 2 O 2 O 2 OH H 2 PA H s PA H s O 2 O 2 O 2 OH CH 2 2 H 2 H 2 burner reaction zone residence time

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Transcript of Surface Ageing of Soot Particles Reactions on the surface of soot are the principal route by which...

Surface Ageing of Soot Particles

Reactions on the surface of soot are the principal route by which mass is transformed from the gas to solid phase in laminar premixed flames. However, the reactivity of soot particles decreases significantly over time so an accurate model is necessary to predict soot mass.

R I A Patterson, J Singh & M Kraft

Department of Chemical Engineering, University of Cambridge

References:References:

6 Summary6 Summary•The decay constant can be used to fit the final soot volume fraction for three different flames.

•Surface decay never seems to reach its asymptotic value in some flames which are therefore unsuitable for testing this part of the model.

•A detailed chemical model for the surface reactions is needed.

[1] J Appel at al. Kinetic Modelling of soot formation with detailed chemistry and physics: Laminar premixed flames of C2 hydrocarbons. Combust. Flame 121 : 122-136 (2000)

[2] M Frenklach and H Wang in: H Bockhorn (Ed), Soot Formation in Combustion - Mechanisms and Models. Springer Verlag, Berlin 1994, 165-192

[3] R I A Patterson et al. The Linear Process Deferment Algorithm: A new technique for solving population balance equations. Technical Report 26, c4e-Preprint Series, Cambridge, 2004

1 Soot Model1 Soot Model

The model we use consists of three main processes• Particle inception – when two pyrene molecules ‘coagulate’• Coagulation – two particles form a new sphere with the same total

mass• Surface events – chemical reactions on the particle surface (4 types)

o Pyrene condensationo C2H2 additiono OH oxidationo O2 oxidation

2 Stochastic Algorithm (LPDA)2 Stochastic Algorithm (LPDA)

Wait an exponentially-distributed time step and update current time, t

Initialize system

Probabilistically choose an event to perform

CoagulationParticle Inception

yesno

Perform deferred processes for all particles

yesno

Perform jump

Update particle ensemble

Pyrene condensation

Select particle(s)

Perform deferred processes on selected particle(s)

Place updated particle(s) back into ensemble

• Fictitious events are a technical convenience in which time is advanced and the event selection procedure followed upto and including the simulation of the deferred processes but the updated particle(s) are then replaced without performing the selected event.

• The algorithm is described in [3]

Is t > tstop ?

Fictitious event ?

Stop

3 Active Sites Model3 Active Sites Model

This exponential decay is calculated along with the deferred processes. When two particles coagulate the age of the new particle is calculated from the surface areas a1, a2 and ages τ1, τ2 of the old particles as

An important part of the model for the rate of C2H2 addition and O2 oxidation is the number of active sites on the surface of the soot particle [1,2]. This is known to decrease over time but little is known about the decay process except that it never reaches 0 so we model the density of active sites, α,on the surface of a particle as an offset exponential decay process in the particle age τ :

Ce1

.21

2211

We present results for three acetylene flames for which some experimental data is available. One can see that the simulated soot volume fraction has a significant dependence on the decay constant C but that by choosing the value carefully good agreement with the experimental data can be achieved. The data were taken from [1].

Lines: simulation results

Points: experimental data

Press / bar

C/O ratio Initial gas velocity /cm s-1

JW 1.69 1 0.69 5.9

JW 10.673 10 0.673 3.0

JW 10.68 10 0.68 6.0

4 Decay constant4 Decay constant

University of CambridgeDepartment of Chemical Engineering

7th WCCE, Glasgow

C H4 4

C H2 2