SIMULATION OF POROUS MEDIUM COMBUSTION IN ENGINES

Transcription

1 SIMULATION OF POROUS MEDIUM COMBUSTION IN ENGINES Jan Macek, Miloš Polášek Czech Technical University in Prague, Josef Božek Research Center Introuction Improvement of emissions from reciprocating internal combustion engines (ICE) working in unsteay moe is possible using homogeneous combustion with equalize temperature fiel One possibility is to realize it in (highly) porous meium () well-trie alreay in burners at TU Erlangen-Nürnberg - [1] an initiate there for ICE use as well Thermoynamic consierations are necessary to evaluate potential of this concept concerning efficiency an emissions at ecrease combustion temperatures From the other sie, insert creates a source of heat uring compression accoring to the amount of fresh mixture transferre to it Finite volume (FV) approach is useful then to simulate the reality of thermoynamic changes Spatial rag an heat transfer istribution in a influences the whole process Current paper aims to evaluate these phenomena an optimize the performance of an ICE equippe with a porous meia burner in the part (approx 50%) of compression volume before serious experiments are starte During them the moel will be fitte to the reality an continuously improve using knowlege from steay operation boiler burners, as well Thermoynamics of Engine Heat Regeneration Capabilities An appropriate tool for the more precise but yet prompt ugement of new cycles (eg, with combustion in a porous meium) has been foun in the stanar iealize cycle moel but aapte to changing heat capacities [], [3], to the influence of low pressure part of cycle cause by turbocharging an limite in maximum pressure an temperature (isothermal burning) The moel has been base on general polytropic reversible changes The equalize an ecrease temperature in a working space, where no more flat (planar) flame front occurs, improves significantly emissions both from high an low temperature zones However, the low temperature limits in general the efficiency of a cycle If porous meia of sufficient thermal capacity are present in a combustion chamber, which is a stanar case of current ceramic porous material, heat regeneration (accumulation an release) takes place It is so especially uring thermoynamic changes when most of cyliner charge is concentrate in the Inicate Efficiency Incluing Gas Exchange eta i [1 5100% 5000% 4900% 4800% 4700% 4600% 4500% 4400% 4300% Inicate Efficiency of the Stanar (v-p-t ) Combustion Cycle an That Combine with Concurrent or Countercurrent Regenerator at v=const mip 1800 MPa; T b=var; Cooling Loss 15%; eta s exh 15%; eta s TC 50% Cooling by Regenerator Maximum Air Firing Temperature Excess Pressure Countercurrent Concurrent None Concurrent None Exceee Firing Pressure Limit Concurrent 400% None 0 Compression Ratio eps [1] combustion chamber, ie, at the en of compression an start of expansion strokes It is a process typical for uncontrolle regenerator that works usually as a concurrent recuperator The first approximation has been calculate concerning the highest temperature of regeneration as that of the en of expansion The results accoring to the presente figure are promising, showing especially high

2 efficiency with low epenence on air excess an compression ratio for regenerate cycles unlike the case of stanar cycle Nevertheless, especially for the case of uncontrolle regenerator the more precise moel is neee Application of Finite Volume Moel to Combustion The application of general simulation methos base on AMEM moel presente alreay before [4] for engine requires introuction of some new source terms in conservation equations concerning spatially istribute heat transfer an flow resistance (momentum sink) cause by soli matrix presence For the first approximation, the following assumptions are use when estimating require parameters Geometry of a insert is characterize using a filament moel of its matrix If the mean iameter of a filament is an porosity of a matrix (free space share) is P then the length of a filament in the volume of a zone V fille with insert is ( 1 ) L 4 = (1 P V ) π The mass an heat capacity of the filament of a ensity r an specific heat capacity c ( ) m = ( 1 P) V ρ ; C = c m Using the assumption of stochastic but in space isotropic vector that characterizes irection of filament (like in naive approach to kinetic theory of gas), 1/3 falls into every orthogonal coorinate axis Then, effective heat conuctivity l eff of the whole length an cross-section of a zone (approximately V 1/3 an V /3 respectively) calculate from the number of parallel heat conucting channels of a filament cross-section is π L (1 ( 3 ) 4 P λ eff = λ = λ ) V V 3 Heat transfer surface for heat convection is obviously 4(1 P) ( 4 ) S = π L = V Flow resistance an heat transfer coefficient might be estimate following the filament approach in the form of results vali for tube bunles in heat exchangers Require geometrical values are the number of cyliner rows in flow irection n an istance between the rows s ( 5 ) n = 3 (1 P) 3 π = 3 V ; s 3 = = 3 n (1 P π L 3 V V ) ; reference minimum flow cross-section A ref π 1 P ( 6 ) A 3 ref = A n = V 4 3 Reynols number ( 7 ) w ref ρ Re = µ ( T)

3 Unfortunately, publishe correlations - see, eg, [5] - for pressure loss an heat transfer coefficient are vali for rather high Reynols numbers Further experiments are neee to provie more precise ata for this case Laminar bounary layer correlations for flow in circumference of a single cyliner have been use provisionally at Re accoring to ( 7 ) The rag of a cyliner with the length /3L perpenicular to the co-orinate irection a (gri of filaments in irections b, g) may be estimate in the form (using mass of gas in the zone), see [4] ( 8 ) F 1 4 c w w X ref ref = cx L ρ wref wref = (1 P) m 3 3 π Drag coefficient is for a single cyliner accoring to [6] may be for low Re<400 represente by a regression correlation ( 9 ) c X = Re Re Re For higher Re>400 c X» 11 If the velocity is generally oriente in space the formula ( 8 ) is not precise in cases where non-linear epenence of rag force on velocity exists, which is the case except for the linear omain of Re<1 The escribe simulation is thus a very rough approximation an experiments focuse to the real properties of s are neee The realistic Re uring piston movement an combustion for»1 mm is in the range of Heat transfer coefficient accoring to [7] may be use for an estimation of regeneration capability of a insert For a wie range of 35<Re< ( 10 ) = λ ( + (04Re + 006Re )Pr ) Convective heat flux calculation nees besie a an S ( 4 ) the temperature of insert surface T In the case of FV moel it is solve using Fourier equation with effective heat conuctivity of a matrix an its thermal capacity For a simple 0-D moel surface temperature can be estimate from ifferential equation of energy conservation T ( 11 ) C = S ( T T ) Q& cooling t Integrating the heat flux uring time of a cycle perio an fining the mean value for aiabatic case, the energy conservation yiels the simplest approximation ( 1 ) T = t per T t per All unknowns in a source term are now etermine for both zone 0-D moel an FV one except for raiation heat transfer insie Its influence on combustion of gases with narrow spectral bans of absorption must be evaluate in the future The moel evelope concerns the thermal conuctivity of the insert as well The cooling effect of walls will be taken into account optimizing thermal insert isolation in contact with engine metal part (eg, cyliner hea) It will give also bounary conitions for all involve engine parts concerning their esign an thermal stress calculation t t

4 The first results emonstrate interesting temperature an flow fiels cause by convective piston flows an those influence by heate gas expansion Examples are shown in the figures concerning TDC an beginning of expansion stroke Conclusion Moelling of Engine emonstrates the nees an possibilities of contemporary thermal engine evelopment The first thermoynamic analysis giving promising results has to be confirme by etaile FVM moel concerning the real impact of heat transfer uring compression an combustion Premature heat supply to compression is isavantageous as well as too intensive heat transfer uring combustion if insert is too col The mean temperature an cooling heat loss are sie-effects of this simulation The FVM moel itself has to be moreover verifie by experiments that are currently being prepare The question of -relevant source terms in turbulence moel is opene for future research LES-similar methos seem to be useful for it In the meantime, integral rag an heat transfer semi-empirical characteristics will be use in a form similar to propose ones but continuously improve on the base of engine an steay-flow test rig experiments Fig 1 Temperature fiel, TDC Fig Temperature fiel, 0 eg CA (40 eg CA after TDC) Acknowlegment This research has been subsiize by a Research Center proect LN00B073 of Ministry of Eucation, Czech Republic References [1]Durst, F et al: DER PORENBRENNER IN DER ÕLHEIZUNG Wärmetechnik 43, 1998 [] Macek,J: PROGRAM FOR IDEALIZED CYCLE SIMULATION USING GENERAL POLYTROPIC CHANGES Coe OBE_REGXLS, v 0001, Coe Library ÈVUT U01, Praha 000, 500 kb [3] Macek, J: COMPRESSION OR HEAT REGENERATION NEW RESULTS OF OLD METHODS In: XXXI Conference of ICE Depts, University of Žilina 000, pp [4] Macek, J Polášek, M: FINITE VOLUME METHOD MODIFIED FOR MOVEABLE BOUNDARIES - A VERSATILE TOOL FOR ENGINE DESIGN In: 8 th International Symposium on Computational Flui Dynamics Proceeings on CD-ROM Bremen: ZARM 1999 Pp 1-0 See also Euler an Navier Stokes Equation, UT AV ÈR 1999 [5] Kreith, F - Black, WZ: BASIC HEAT TRANSFER New York 1980, Chapter 54

5 [6] Schlichting, H: GRENZSCHICHT-THEORIE G Braun, Karlsruhe 1964 [7] Whitaker, S: FORCED CONVECTION HEAT-TRANSFER CORRELATIONS FOR FLOW IN PIPES, AIChE, 18 (197), p361