The Research On The Combustion Characteristics Of The Engine Fueled With LHV Gas

ABSTRACT:In recent years, due to the shortage of the global petroleum resource and the serious pollution to living environment, the research on clean alternative fuels is becoming an important direction of engine techniques. Lower heating value (LHV) gas has been considered as an alternative gas fuel nowadays. In this study, the characteristics of ignition, combustion and emissions about the engine fuelled with LHV gas was investigated using fully coupled multi-dimensional CFD and detailed chemical kinetics model combined with experiments. It is useful to promote the effective and clean combustion in engine fuelled with LHV gas and the study has important theoretic and engineering practical value.Based on discrete particles method, the ignition model for engine fuelled with LHV gas is developed. The model is composed of four parts, Electric energy deposition sub-model, Discrete particle velocjty sub-model, Burn rate sub-model, spark plug protrusion and electrode heat transfer sub-model. In the discrete particle model, a convection velocity has been considered. Using fractal theory, a coefficient used to describe the wrinkling effect of turbulent combustion has been defined. In flame propagation period, a self-adapting turbulent combustion model based on Damkohler number was built. The model has been used to calculate the burn rate in proper way by comparison of turbulence and chemical kinetics time scale. At the same time, detailed chemical kinetics model has been combined with CFD.Based on the analysis of coupled simulation software consisting of KIVA and CHEMKIN, the parallel calculation system by MPI and OpenMP is built. The system combines the advantage of Message-passing model and Shared-memory model, so the time consuming is reduced markedly.Based on the parallel simulation system for engine fuelled with LHV gas, the characteristics of ignition and combustion was studied. The simulation results indicate that with the increase of turbulent intensity and swirl ratio, the velocity of flame kernel development increases. Meanwhile, fractal dimension inceases and the combustion process are enhanced. Ignition energy mainly influence the initial stage of fame kernel radius. With the increase of the volume fraction of inert gas in fuel, the velocity of fame kernel radius development and fractal dimension decrease. It shows the same behavior if the excess air ratio increases. The normal exhaust emissions like NOX, CO have clear distribution regularity in cylinder. The amount of CH2O is two order of magnitudes lower than that of normal emissions. UHC emissions mostly reside in the vicinity of cylinder wall, and the compositions are mainly composed of unburned CH4, CH2O, CH3OH, C2H4 and C2H6. Meanwhile, with the increase of the volume fraction of inert gas in fuel, the higher temperature area decreases at the same crank angle. At the same time, the flame development duration increase remarkably, but the amount of NO emission reduce. Advancing spark timing can increase the maximum pressure value, but the amount of NO emission also increase.The experimental bench for engine fuelled with LHV gas was built and the combustion characteristics of the engine has been tested. The results show that the inert gas in the LHV gas fuel has great effect on the flame development duration, but less effect on the rapid combustion duration. Meanwhile, the inert gas causes the center of the heat release curve to move apart from TDC and with the increase of inert gas volume fraction, the degree of departure is enhanced. The level of inert gas fraction has strong influence on the cyclic variations at low load operations. With the increase of inert gas fraction, cyclic variation of the engine fuelled with LHV gas is strengthened. When the level of inert gas volume fraction is lower than 20%, the combustion process has good stability under all tested load conditions and the indicated mean pressure has strong linear correlation with the maximum pressure. But if the level of inert gas fraction in blend is higher than 30%, the engine fuelled with LHV gas shows poor stability performance, especially in the case of the low load conditions. Hydrogen addition can decrease the flame development duration and rapid combustion duration, but hydrogen gives the larger influence on the flame development duration than on the rapid combustion duration. At the same time, hydrogen addition into the LHV gas decreases the cycle-by-cycle variation. For a specified inert gas fraction, the indicated mean pressure shows higher and concentrated value when hydrogen addition is introduced. This effectiveness becomes more remarkably at high hydrogen addition fraction. Strong independency between indicated mean pressure and peak pressure is presented with the increase of hydrogen fraction. With the increase of inert gas fraction, CO and HC emissions increase, but NOX emission decreases markedly especially with CO2 dilution gas. Hydrogen addition can decrease CO and HC emissions effectively, but the amount NOx emission increases obviously meanwhile.