С-4. Simulation of Smoke Particles Coagulation in the Exhaust System of Piston Engine

Transcription

1 Kaliningrad 2012

2 Simulation of Smoke Particles Coagulation in the Exhaust System of Piston Engine С-4 Sergey M. Frolov1, Konstantin A. Avdeev1, Vladislav S. Ivanov1, Branislav Basara2, Peter Priesching2, Maik Suffa2 1Semenov Institute of Chemical Physics,4 Kosigin Str., Moscow, Russia?AVL LIST GmbH, 1 Hans-List-Platz, A-8020 Graz, Austria Starting from 2012, the new Euro-V I regulation lim its both the content o f sm oke and soot particles (no m ore than 5 m g/km, 66 % reduction as com pared to Euro-V ) and their num ber (no m ore than /km) in the exhaust gases o f transportation piston engines [1]. Such string ent regulations require solution o f tw o principal problem s, nam ely (1) im proving the engine operation process, and (2) im proving the aftertreatm en t o f exhaust gases using advanced sm oke filters. To succeed in solving these challenging tasks, there is a need in better understanding o f the evolution o f particles size distribution function (SD F) during th eir m otion in the engine exhaust system. As a m atter o f fact, since all em issions are measured at the outlet o f the exhaust system the know ledge o f the SDF at the exhaust system outlet could help in solving the inverse problem that is to obtain the true SDF in the engine itself. The latter is then can be used for im proving the engine operation process and decrease the am ount and the num ber o f particles form ed in engine. O ne o f the reasons o f SD F variation in the particle-laden turbulent flow is particle coagulation [2, 3]. In high-speed turbulent flow s w ith subm icron- and nanosize particles at relatively low tem peratures the coagulation o f particles is governed m ainly by three m echanism s [2 5]: Brow nian, turbulent diffusion and turbulent kinetic. It is expected that the Brow nian m echanism dom inates for particles o f size com parable w ith the size o f m olecules o f the carrier gas [2, 3]. The turbulent diffusion driven coagulation is caused by turbulent velocity fluctuations in the carrier gas w hich prom ote p article collisions. T he turb u len t kinetic coagulation 51

3 m echanism is based on the fact that there is a velocity slip betw een gas and particles and therefore particles o f different size are involved in the turbulent m otion differently. The com plex turbulent flow field in m odern engine exhaust system s m akes it difficult to foresee the dom inating coagulation m echanism. It is likely that the various coagulation m echanism s can dom inate in different parts o f the exhaust system thus replacing each other or contributing com parably to the SD F evolution. In this com m unication, the m odel o f sm oke particle coagulation in the confined turbulent flow o f engine exhaust gases has been developed. The m odel incorporates all three coagulation m echanism s m entioned above. M athem atically, the model is based on several sim plifying assumptions: (1) the gas flow in the exhaust system is one-dim ensional, quasi-stationaiy and is know n a priory, (2) particles are so small that their velocity does not differ from the local mean gas velocity; (3) particle size varies only due to coagulation; (4) the coagulation probability o f colliding particles o f different size is equal to 1; (5) the coagulation probability o f colliding particles o f identical size is equal to 0; (6) the initial SDF is log-normal. These assum ptions result in the following differential equation governing the evolution o f particle num ber density n ( d,x ) : w here x is the coordinate, Vg is the gas velocity, d is the equivalent particle diam eter, param eters nin, din and a jn are the total num ber density, m ean diam eter and variance o f the initial log- norm al SD F; and the differential term in the right-hand side o f Eq. (1) represents the coagulation rate [2]: = I n(r2 )l \P ( rh r2)«(п)dr\ \dr2 (2) coag 0 [O H ere, r = d l 2 is the equivalent particle radius, n(r)dr is the num ber density o f particles possessing radius from r to r+ {^^2 ) = Pb{t\, r2 )+ Ptd{r\, r2 )+ Ptk{rh r2 ) is the coagulation core for 52

4 particles w ith radii r\ and r2, w hereas the term s flb(r\,r2), Pld{rh r2) and p tk(rh r2) are the coagulation cores in the B row nian, turbulent diffusion and turbulent kinetic m echanism s [2 5]: Pb (n vr2 ) = k fin (n. r2 )K b {rh>r2 ) ( 3 ) Ptd (n >r2 ) = S (л Vr2 ( w'2 ) (4) P tk{rb rl ) = S (r\ S l ) A rb r2 ) (5) w here kfm is the coefficient o f free-m olecular coagulation; К the coagulation coefficient; S (i\,r2) is the effective collision cross- section; w 0, r2) is the relative velocity o f interacting particles in the turbulent flow ; and ( h 2^ is the m ean value o f the squared relative velocity o f interacting particles. The set o f equations (1 5), supplem ented w ith the data base o f therm ophysical data o f substances w as integrated num erically on the steady-state turbulent flow field in the exhaust system o f diesel engine o f total length 1.3 m (Fig. 1). Show n in Fig. 1 are the th ree-dim ensional d istrib utio ns o f m ean gas velocity Vg and dissipation o f the turbulent kinetic energy obtained using A V L FIR E r code. By averaging over tube cross-section, these distributions w ere reduced to one-dim ensional distributions used in the m odel. Figure 2 show s the typical result o f calculations for sm oke par- 1 X 3 tid e coagulation at din= lo n m, a in = 0.6 and njn= 2 10 m. The SD F is seen to undergo significant transform ation: the mean particle d iam eter changes from din - 10 nm at the inlet o f the exhaust pipe to doul a 120 nm at its outlet. In this exam ple, the B row nian coagulation m echanism w as dom inating in all pipe sections. V ariation o f initial SDF param eters at the pipe inlet results in the significant variation o f SD F at the outlet. For exam ple, Fig. 3 show s the effect o f crllt on dout. In [6], the results o f calculations w ere com pared with experim ental data. is 53

5 Fig. I. Three-dimensional distributions of mean gas velocity (left) and dissipation of the turbulent kinetic energy (right) in the diesel exhaust system used for studying the evolution o f smoke particle SDF. d, nm Fig. 2. Predicted variation of smoke particle SDF along the length of the exhaust pipe at initial log-normal SDF parameters nm = m '3, dln = 10 nm and <тш =0.6: 1.* = 0 (inlet); m; m; m; m (outlet). Fig. 3, Predicted dependence o f mean smoke particle diameter doul at the pipe outlet on the initial log-normal SDF variance a m at the pipe inlet at n n r \ din = 10 nm. Thus, w e have developed the m odel o f sm oke particle coagulation in the exhaust system o f piston engine, including three co ag u lation m echanism s: Brow nian, turbulent diffusion and turbulent kinetic. T he calculations revealed that th e coagulation process ex erts a significant influence on the particle SDF at the outlet o f the ex h au st pipe, and the dom in ating m echanism is the B row nian coagulation. This w ork w as partly supported by the RFBR. 54

6 1. Commission Regulation (EC) No 692/2008 of 18 July en. htm, p Payne J. F. B., Skyrme G. Int. J. Multiphase Flow, 1993, Vol. 19? No/ 3, pp Friedlander S. K. Smoke, Dust and Haze: Fundamentals o f Aerosols Dynamics, NY Oxford, Oxford University Press Inc., Levich V. G. Physico-Chemical Flydrodynamics. Moscow, Phyz.- Math. Lit. Publ., Piskunov V. N. Theoretical models of aerosol formation kinetics. Sarov, Rus. NRC VNIIEF Publ., Avdeev K. A., Ivanov V. S., Frolov S. М., Basara B., Priesching P., Suffa M. In: Combustion and Explosion, Moscow, Torus Press, 2012, Issue 5, pp Real Gas Equation of State for Methane C-5 Viktoria V. Kozynda, Alexei V. Dubrovskii, Sergey M. Frolov Semenov Institute of Chemical Physics, 4 Kosigin Str., Moscow, Russia M ethane is one o f the m ost w ide spread fuels and raw m aterials for chem ical industry used for production o f synthetic gas, acetylene, black carbon, etc. D espite its therm ochem ical and physical properties are in general w ell investigated, its num erous novel applications, in particular in transportation engines, require the know ledge o f accurate therm al and calorific equations o f state (EO S). The aim o f this com m unication is to develop the therm al EOS for m ethane in a supercritical param etric dom ain at 250 < 7 < 1000 К and 0.1 < P < M Pa. Figure 1 is the phase equilibrium diagram for m ethane obtained using the data o f [1]. The shaded area in Fig. 1 is the supercritical param etric dom ain o f our interest here. 55