Influence of products of combustion and addition of water vapor on the turbulent combustion of methane mixtures

Abstract

An experimental and theoretical study of the effects of the combustion products and water vapor on the turbulent combustion of methane mixtures in engines with spark ignition is presented. In order to compare experimental data for a spherical mock-up chamber, with known turbulent field, with the conditions at the end of the compression stroke of an actual engine running with an initial pressure of ~i.0 MPa and a temperature of 550-650~ the air was replaced by a mixture of 02 + 1.6N 2. The initial temperature in all cases was 385~ Because of the selected gas composition, the velocity of the experimental laminar flame was the same as that in the combustion chamber of the engine, under the specified conditions at the end of the compression stroke. The correctness of this procedure has been confirmed by [i] from measurements of the laminar velocity on rapid compression rigs. Systems involving partial engine loading and recirculation of the residual gases require a knowledge of the combustion properties of mixtures with specific proportions of the combustion products. Data on the effect of water vapor concentration on the normal combustion velocity and the turbulent burnout velocity can also be obtained by the constant volume method. For the study of the combustion of mixtures diluted with the combustion products, a predetermined portion of the fuel mixture was burned and the combustion products mixed with the remaining portion of the unburnt fuel mixture. Water vapor was produced in the combustion chamber by burning a stoichiometric mixture of hydrogen and oxygen at the selected pressure and subsequently mixing the combustion products with the unburnt fuel mixture. The experimental results for three groups of fuel mixtures with different coefficients of excess oxygen (~) are shown in Fig. i, where the velocity of turbulent burnout (w) is plotted against the root mean square pulsating velocity u' E. The normal velocities of the laminar flames (u n) are given by the points of intersection on the ordinate axis. As will be seen from Fig. i, the dilution of any initial mixture of CH 4 + 2=(O= + 1.6N=) by the combustion products or by water vapor decreases the velocities of both laminar and turbulent combustion. This can be explained by the lower combustion temperature after dilution. The decrease in the turbulent velocity caused by dilution of the fuel mixture with the combustion products is constant throughout the entire range of the values of =, whereas dilution with water vapor has the greatest effect on nonstoichiometric the effect being smallest when ~ = I.i. An important trend, illustrating the effect of the diffusion-thermal structure of the flame front on turbulent combustion, is apparent from a comparison of the w(u'z) curves for mixtures of the same dilution with values of = equal to 1.4 and 0.8. When ~ = 1.4, the