Research Paper MATHEMATICAL MODELING OF FLAME QUENCHING PHENOMENA IN A CATALYTIC COATED FOUR STROKE SI ENGINE

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1 Research Paper MATHEMATICAL MODELING OF FLAME QUENCHING PHENOMENA IN A CATALYTIC COATED FOUR STROKE SI ENGINE 1 Dr. P. Ponnusamy, 2 Dr. N. Nedunchezhian, 3 W.Edwin Santhkumar Address for Correspondence 1 Professor, Department of Mechanical Engineering, Surya Engineering College, Erode, Tamilnadu, India Associate Professor, Department of Automobile Engineering, Institute of Road and Transport Technology, Erode, Tamilnadu, India Assistant professor, Department of Mechanical EngineeringExcel Engineering College,Komarapalayam Tamilnadu, India ABSTRACT Some researchers report that, the HC emission increases due to catalytic coatings in SI engine. The argument proposed by them is the catalysts deplete the air-fuel availability inside the boundary layer by activating them and when the flame reaches the boundary layer, the flame extinguishes due to lack of air-fuel mixture. To investigate the mechanism of flame quenching in a SI engine coated with catalysts, a numerical model of flame quenching phenomena is developed and the results are reported here. Boundary layer thickness and flame quenching distances are calculated. Temperature variations, concentration of species inside the boundary sublayer are also predicted. The model results show that flame quenches due to thermal loss and not by fuel depletion. It has been found that each catalyst has a particular surface temperature above which the surface catalytic reaction increases. KEY WORDS: flame quenching, air-fuel mixture, catalyst, fuel concentration, Combustion. 1. INTRODUCTION Quenching in terms of combustion refers to extinguishing a flame. There are many ways in which a flame can be extinguished. A flame can be extinguished by thermal effects (heat loss), chemical suppression and aerodynamic effects. One of the main researched areas is quenching by a cold wall. Thermal quenching is the process where a flame is quenched because of temperature differences between two mediums. The quenching medium could be a cooler surface or wall or an inert gas at relatively lower temperature than the flame. Many works on catalytically activated combustion in SI engine, report the improvement in fuel economy and reduction in exhaust emissions as reported by Pfefferle (1979), Thring (1980) and Dhandapani et al (1992). However, there have been some contradictions regarding the effect of catalytic coating on HC emissions.while commenting on the work of Rychter et al (1981), Robben noted that, the interaction of gas phase combustion with catalytic surface quench the gas phase reaction, mainly by removing the fuel present in the boundary layer. However, many researchers report decrease in HC emissions when catalysts are employed by Thring (1980), Rychter (1981), Jones (1997) and Dhandapani et al (1992). Heywood (1989) reported that flame quench distance depends upon the air-fuel ratio, the wall temperature, diffusion rate and flame speed. In addition, flame quenching is considered as the main source of HC emissions and it becomes necessary to understand the phenomena. There have been many studies, experimental and theoretical, reported in the literature on the near wall phenomenon and flame quenching. One of the evidences of flame quenching at the walls of combustion chamber of spark ignited engine was given by Daniel (1957). Combustion photographs were obtained in an L-head engine using a portraittype camera equipped with an extension to increase the magnification. Quench distances were calculated from the intensity of light near the walls of combustion chamber. Comparison of the observed distances with the correlation proposed by Friedman and Johnston (1950) was done, which is of the form q d 1/( p 0.91 T 0.5 ) for rich and stoichiometric mixtures (1) q d 1/(p 0.76 T 0.85 ) for lean mixtures (2) Daniel also indicated that the flame quenching is an important source of hydrocarbon emissions and observed that the HC emission levels are proportional to the quench distance. Nedunchezhian and Dhandapani (2006) studied the catalytic activation of charge near the combustion chamber wall and of the flame quenching phenomenon to identify whether flame quenches due to catalytic activation or due to thermal quenching. The results of the flame quench model indicate that the flame quenches due to the heat loss to walls. The depletion of fuel due to the catalyst coated on the combustion chamber walls does not affect flame quenching. The catalysts coated on the combustion chamber surface do not contribute increased hydrocarbon emissions, but actually reduce them. From the above literatures, it can be observed that only few studies have been made on the flame quenching phenomena coupled with the thermal and chemical processes. There is no literature available for the effect of catalytic activation of charge on flame quenching applicable to IC engine combustion. As the present work is with catalytic coated engines, it becomes necessary to study the catalytic activation of charge along with flame quenching. Hence, a mathematical model has been developed to study the catalytic activation and flame quenching process. 2. MATHEMATICAL MODEL Most of the available models predict the quenching phenomena during the flame quenching period only. In the present work, chemical activation may take place even during the compression stroke. Hence, the development of boundary layers and the prediction of catalytic activation of charge are carried out throughout the compression process. The details of the mathematical model are explained briefly in the following sections. 2.1Conservation equations The near wall analysis of catalytic activation is based on one-dimensional, steady flow, single-step reaction mechanism. The analysis confines the space bounded by the combustion chamber wall and the boundary layers. The boundary layers are divided into number of sub-layers of thickness y. Temperature, velocity, and species concentrations in the sub-layer are assumed to be uniform The conservation of mass,

2 species and energy are indicated in Figure 1. The momentum equation is satisfied by the assumption of uniform pressure. Among the conservation equations, the solution of species conservation equation becomes complicated because of the non-availability of activation energies of the heterogeneous catalytic activation. Figure 1. Mass, energy and momentum conservation at the control volume 2.2 Thermal boundary layer thickness The layer of air-fuel mixture adjacent to the combustion chamber wall is heated by the heat flowing from the wall during initial period of compression. When the cylinder temperature increases above the wall temperature the heat flows from the combustion chamber contents to the wall. Hence, the gas layer is heated by this heat flowing from the cylinder gases. During the heat transfer from the wall and to the wall, the gas layer adjacent to the wall behaves as a buffer layer and the temperature distribution in the layer varies uniformly. This layer is called thermal boundary layer. Figure 2 shows the development of thermal boundary layer and the associated symbols used to describe it. The oxidation of air-fuel mixture adjacent to the chamber wall is influenced by the wall temperature. Figure 2 Development of thermal boundary layer and laminar boundary layer The thickness of thermal boundary layer is calculated from the correlation proposed by Leyford-Pike and Heywood (1984). T = 0.6 Re 0.2 ( t) ½ (3) where,re = vx 0 /, = k/ C p, v = S p /(x 0 /x) The temperature change within the calculated boundary layer is assumed to vary uniformly. The profile of temperature distribution is given by / = (T-Tw)/(T -Tw) = 3/2(y/ T ) ½ (y/ T) 3 (4) The profile of temperature distribution is assumed to be of cubic polynomial with the boundary conditions of, T=Tw and d 2 T/dy 2 = y = 0 T=T and dt/dy = y = T 2.3 Laminar boundary layer thickness Gas motion within the engine is a major factor that controls the combustion process in spark-ignition engines. It also has a significant impact on the heat transfer and catalytic activation of charge. The initial in-cylinder flow pattern is set up by the intake process and subsequently amplified during compression process. The flow pattern inside the cylinder changes during compression and expansion strokes. The fluid flow inside the cylinder is turbulent and there exists a boundary layer in which the flow is laminar. The thickness of the laminar boundary layer varies with Reynolds number. Figure 2 shows the development of laminar boundary layer and the associated symbols to represent it. In the present work the thickness of the laminar boundary layer is calculated by, L = 4.65 (y/re) = 4.65 ( y /U ) (5) Velocity change within the laminar boundary layer is assumed to vary uniformly and follows the cubic polynomial profile. The velocity profile is given by, U/U = 3/2y/ L 1/2(y/ L ) 3 (6) with the following boundary conditions, U = y = 0 and U = y = L 3 RESULTS AND DISCUSSIONS 3.1 Boundary layer thickness The thermal boundary layer thickness (TBLT) and laminar boundary layer thickness (LBLT) are calculated on the piston crown and cylinder head for each crank interval using the flame quench model. The temperature and velocity distribution over the boundary layer are also predicted using the model. Figure 3 shows the variation of TBLT and LBLT with crank. It can be observed from the figure that the LBLT suddenly decreases and reaches a minimum value of m at an of 357 o before TDC after this starts growing and reaches a maximum value of m at 540 o. TBLT starts to increase and reaches a maximum value at m at 435 o then decreases. It is interesting to note that near TDC, the value of LBLT is lower than the corresponding values of TBLT. This shows that the LBLT dominates the control volume near TDC. In other words, the effects of cylinder flows are felt even inside the TBLT during this crank period. Hence, it is logical to assume the controlling boundary layer thickness as the minimum of TBLT and LBLT. This thickness will be referred simply as boundary layer thickness (BLT) as shown in the Figure 4. Figure 3 Variation of TBLT and LBLT with crank Figure 4 Variation BLT with crank Figure 5 shows the variation of boundary sublayer with crank. Each sublayer is assumed to have uniform properties. To compare the variation of temperature, velocity and fuel concentration inside the BLT, a non-dimensional BLT is calculated from

3 Normalized BLT = Boundary sublayer distance from the wall / BLT The above definition normalizes the BLT between zero and one. the lower limit of temperature variation in a BLT nearer to the wall. This is due to the effect of free stream temperature. 3.3 Velocity variation in sublayers Figure 5 Variation of sublayer thickness with crank 3.2 Temperature variation in BLT Figure 6 shows the variation of temperature in the sublayers. The temperature of outer sublayer adjacent to the free stream is close to the free stream temperature. The sublayer nearer to the wall has a temperature close to that of wall. It can be observed from the Figure 6 that the sublayers nearer to the wall are less affected than the sublayers nearer the free stream. In fact, the temperature of sublayers nearer to the wall is higher than the free stream temperature during the initial period of compression stroke at TDC becomes equal to it. Beyond this, the free stream temperature starts increasing and the sublayer temperature also increases. Figure 6 Variation of temperature in the sublayers with crank Figure 8. Variation of charge (air/ fuel mixture) velocity with crank Figure 9. Variation of velocity with normalized boundary layer thickness The variation of mixture velocities over the boundary sublayers are shown in Figures 8 and 9. It can be observed from the Figure 8, that the velocity increases as the piston moves towards TDC and reaches maximum value at TDC. After TDC the velocity starts decreasing. The velocity in sublayers follows the free stream velocity changes. As in the case of temperature, the velocity variation in the boundary sublayer close to the wall does not change much. This is seen from the Figure 9, where the velocity variations are plotted against the normalized BLT. The variation in the velocity of sublayer close to free stream is wide and follows the free stream velocity. 3.4 Effect of Reynolds number on boundary layers Figure 7 Variation of sublayer temperature with normalized BLT The temperature of the sublayer nearer to the wall varies between 350 K and 390 K. The corresponding variation in the free stream temperature is from 390 K to 1087 K. This is more clearly seen from the Figure 7, where the variation of sublayer. Each vertical plot represents the variation of temperature inside the sublayer. The variation in the temperature increases as the normalized BLT approaches unity. The lower limit of temperature variation in a BLT closer to free stream is lower than Figure 10 Variation of Reynolds Number with crank The variation of Reynolds number with crank is shown in Figure 10. As the piston moves towards TDC, the Reynolds number increases exponentially and reaches a maximum value of at TDC. The sudden increase in Reynolds number is due to the squish motion of air-fuel mixture, near TDC. 3.5 Catalytic activation The catalyst coated on the combustion wall surfaces activates the charge prior to combustion. The

4 intermediate components formed in the initial period accelerate the combustion and hence the overall reaction rate improves Variation of fuel concentration in boundary layers The fuel consumed at the catalytic wall surface due to catalytic activation creates a concentration gradient in the fuel. The fuel molecules start diffusing from the free stream into BLT. The level of fuel concentration in different sublayers at various crank positions is shown in Figure 11. The corresponding variation of fuel concentration against the normalized BLT is shown in Figure 10. It can be observed from the Figure 11, that the fuel level in the sublayer near the free stream shows a sudden increase near TDC. It may be recalled from the earlier discussions and from the Figure 2 that the BLT suddenly decreases near TDC due to the effect of high Reynolds number. Due to this, the effective distance that the fuel molecule diffuses becomes less near TDC and hence the fuel levels in the sublayers are higher. Figure 13 Variations of surface reaction and fuel diffusion rate in a boundary sublayer near to the wall Peclet Number In calculating the thermal flame quenching distance, the Peclet number (Pe) plays an important role. The variation of Pe with crank is shown in Figure 14. It can be observed from the figure that Pe maximum at TDC. Figure 11. Variation of boundary layer fuel concentration with crank The variation in the fuel concentration in the sublayers nearer to the wall is less compared to the sublayers nearer to the free stream as seen from the Figure 12. This shows that the diffusion rate of fuel is the main controlling factor for reaction rate. Figure 14 Variation of Peclet number with crank Effect of catalytic coating on flame quenching The Figure 15 shows that variation of BLT and flame quenching distance with crank. From the figure we absorbed that the flame penetrates inside the BLT upto a distance of m. Hence further investigation is carried out to the concentration of combustible fuel-air mixture inside the BLT. This is shown in the figure16. It can be observed fuel-air concentration in the BLT is well above the lean limit. But the flame extinguish in spite of availability of combustible mixture. The possible explanation for these phenomena may be that, the flame front losses its heat to the wall and extinguish. Figure 12 Variation of fuel concentration over normalised BLT Surface reaction rates The catalytic coatings on the combustion chamber walls oxidize the fuel molecules in the sublayer adjacent to it. The reaction rate and the diffusion rate are calculated from the flame quenching model. The results are presented in Figure 13, for both the catalytic surface reaction rate and the diffusion rate in the boundary sublayer adjacent to the wall. The results presented in the figure are for the copper catalyst. From these figures, it can be seen that the reaction rate which is less than the diffusion rate in the initial periods, suddenly increases and reaches a peak value and then drops. This shows that the overall surface reaction is controlled by the reaction rate during the initial periods and by diffusion rates during the later periods. Figure 15 Variation flame quenching distance and BLT with crank Figure 16 Variation flame quenching distance, BLT and fuel concentration with crank

5 This is in contradiction to the explanation offered by some investigators [Jones 1996] regarding the increased hydrocarbon emission from the catalyst coated engines. Even though the catalysts deplete the fuel near the wall, the diffusion of fuel from the free stream supplies enough fuel for the flame to propagate. In effect, the flame quenches not by the depletion of fuel but by the thermal quenching. Hence, the catalytic coating does not affect the quenching distance and actually reduce the hydrocarbon emissions by consuming the fuel in the quench layer. Even after the flame quenches, the surface reaction may continue and consume the fuel remaining in the sublayers. 4. CONCLUSIONS Based on the present investigations on flame quenching the following conclusions were arrived. The thermal boundary layer thickness is less than the laminar boundary layer thickness for most of the crank period, except near TDC. The squish motion near TDC suddenly increases the Reynolds number which results in decreased boundary layer thickness. The results of the present flame quench model indicate that the flame quenches due to the heat loss to walls. The depletion of fuel due to the catalyst coated on the combustion chamber walls does not affect flame quenching. The catalysts coated on the combustion chamber surface do not contribute increased hydrocarbon emissions, but actually reduce it. Each catalyst has a specific surface reaction, based on their reaction rate the catalyst can be rated as: copper > chromium > nickel > base (standard). REFERENCE 1. Daniel, W. A. Flame Quenching at the Walls of an Internal Combustion Engine, 6 th Int. Symp. on Combustion, The Combustion Institute, pp , Dhandapani, S., Nagalingam, B. and Gopalakrishnan, K.V. Some Experimental Investigations on Catalytic Combustion of Lean Burn SI Engine, paper , 25 th ISATA Jubilee Intl. Symp. on Automotive Technology and Automation, Florence, Italy, 1-5 June, Frideman, R. and Johnsten, W.C. Wall Quenching of Laminar Propane Flame as a Function of Pressure, Temperature and Air-Fuel Ratio, J.Appl. Physics, Vol. 21, pp , Heywood, J. B. Internal combustion engine fundamentals, McGraw-Hill, New York, Jones, R. L. Surface and coating effects in catalytic combustion in internal combustion engine, surface and coating technology, Vol , pp , Leyford-Pike, E. J. and Heywood, J. B. Thermal Boundary Layer Thickness in the Cylinder of a Spark- Ignition Engine, Int. J. Heat and Mass Transfer, Vol.27, No.10, pp , Nedunchezhian, N. and Dhandapani, S. Study of flame quenching and near-wall combustion of lean burn fuel air mixture in a catalytically activated spark-ignited lean burn engine, Combustion and Flame, Vol. 144, pp , Pfefferle, W. C. The Catalytic Combustor: An Approach to Cleaner Combustion, J.Eneregy, Vol. 2. No.1, Rychter, T. J., Saragih, R., Lezanski, T. and Wojcicki, S. Catalytic Activation of Charge in a Prechamber of a SI Lean-Burn Engine, 18 th Symp.(International) on Combustion, The Combustion Institute, Thring, R. H. The catalytic engine - platinum improves economy and reduces pollutants from a range of fuels, Platinum Metals Review, Vol. 27, No. 4, 1980.