Validation Criteria For Cavitation Inception In A Diesel Injector Nozzle Under HCCI Engine Conditions

Combustion of injected liquid fuel in diesel engines is dependent on the effective atomization to increase the surface area of fuel and hence achieve higher rates of mixing and evaporation. The reduction in fuel droplet size leads to higher volumetric heat release rates, prompt ignition, increased flammability limits, and lower engine emissions. Also, understanding the details of the internal injector flow and the connection to the spray would help engineers to reduce engine emissions. Furthermore, under the injection conditions in modern diesel engines cavitation phenomena often occur in fuel injector nozzles. Cavitation in diesel injector nozzle introduces vapor bubbles into the flow, and increasing the maximum velocity in the nozzle whole as a liquid core. The flow velocity is increased because of two reasons when the fuel is cavitating. Firstly, if there is vapor along the wall, the liquid will have a slip condition boundary, thus allowing the velocity of the liquid to increase. Moreover, cavitation inside diesel injector nozzle leads to an increase of the spray cone angle as well as flow outlet speed so it is expected to improve the air fuel mixing process.In order to analyze the flow and cavitation conditions inside the diesel injector nozzle we established a three dimensional numerical grid for the real size diesel injector nozzle by two different codes which commercially available for grid generation. Although, the numerical calculations for a single hole nozzle have been performed in order to reduce the computational time. However, the same diesel mass flow rate is implemented in the calculations with assessment of the grid sensitivity. The real diesel injection nozzle physical domain was broken up into hexahedral cells, Tetrahedral; with pyramid grid layers near the nozzle walls using a hybrid mesh generation technique. The use of hybrid grid improves the stability and convergence rate and improves the capabilities of different codes to capture the large gradients that occur at the rounded orifice inlet, since it follows the flow direction and creates less numerical diffusion. Due to the first cavitation bubbles appearing at or just after the nozzle rounding, it was deemed necessary to refine particularly there. As regards the cell size in the simple contraction nozzle, the cells in the hole, which is the most critical zone of the domain, and when cavitation is expected to occur, range from 0.06μm in the orifice core to a minimum of 0.02μm in the wall. The complexity of the numerical problem for the diesel injector nozzle is due to the convergence of the solutions and the accurate results can be solved with skewness values between 0 to 0.3, and high aspect ratio between 0.85 and 0.98.Clearly an optimum amount of cavitation is desirable and it is important to understand the sources and amount of cavitation for more efficient analysis of the cavitation phenomena inside the injector nozzles. According to the traditional approach towards cavitation modeling, cavitation inception begins when the local pressure drops below the vapor pressure of the fuel at a given temperature, thus was traced before by Reynolds and cavitation numbers. As expected, both of cavitation and Reynolds numbers cannot provide a detailed description of the in nozzle flow pattern, as it depend on the flow boundary conditions no matter what happens inside the nozzle internal flow and cavitation. As the first two new criteria, the vapor voidage and quality has been applied to check the predictive capability of two different cavitation numerical models, Zwart-Gerber-Belamri model and Singhal et al. model, accounting for the onset and development of cavitation inside real size Diesel nozzle holes against referenced experimental data. The results indicated that the Singhal et al. model is more reliable for cavitation prediction than that achieved by Zwart-Gerber-Belamri model. Since, the ZGB model fails to capture the transition from incipient to fully develop cavitation as the value of vapor present in the nozzle is almost the same along the nozzle lengthThe quantitative comparison between the results obtained from three different turbulent models performed by other new criteria of data analysis by integral dimensionless theory based on infinitesimal calculations for Reynolds and cavitation number along the diesel injector nozzle domain. The results indicated that, the RNG k-ε, and realized k-εmodels clearly performed better than standard k-εModel where an almost constant property gradient was present. The constant behavior of the infinitesimal Reynolds and cavitation number calculated indicted that, the standard k-εturbulent model delivered very poor results for both the vapor present inside the diesel injector nozzle in cavitating conditions, as well as the pressure distributions in the nozzle. However, the RNG k-ε, and Realized k-εturbulent models show an acceptable agreement with the referenced experimental data of turbulent kinetic energy profiles as well as the basic nozzle flow structures and cavitation distribution.To validate the suggested criteria for cavitation inception the experimental activity have been done on a diesel HCCI engine to research the mixture preparation using the indirect injection system, referred to as external mixture formation (PFI). However, by introducing the fuel externally to the combustion chamber we can use the turbulence intake process to create a homogeneous charge regardless of engine conditions. This eliminates the need for combustion system changes which were necessary for the internal mixture formation method. With this method, the combustion system remains fully optimized for direct injection and also capable of running in HCCI combustion mode with nearly ideal mixture preparation. The key to the external mixture formation with diesel fuel is proper mixture preparation. The investigation has shown that a diesel engine can run on a homogeneous fuel/air mixture that is generated externally in a special fuel vaporization box. In our study, diesel fuel aerosol was injected into the intake port. However, we used the fuel vaporization box in which the diesel fuel entered in a liquid state. The engine combustion with a homogenized mixture via fuel vaporizer is demonstrated. It was found that, the effect of fuel premixed ratio increment is more pronounced between injection pressures is between 75 and 200 bar. Furthermore, for the same engine load by increasing the pressure between the mentioned values the fuel premixed ratio is increased rapidly. Never less, increasing the injection pressure more than 200 bar will add more loads on the tested engine pump as well as increase the pumping loss in the system without remarkable improvement in the overall fuel premixed ratio for the whole system. Also, the optimal operating conditions for our case will locate in the nozzle injection pressure range between 75 and 200 bar, and the temperature effect on the fuel premixed ratio is more pronounced as the temperature increased until 225℃. Regarding the engine emissions data, with a fixed engine load and speed of 1.8 BMEP and 1200 rpm, respectively, the increasing of CO2 emissions is slightly different in high injection pressure conditions of 150 bar, and 200 bar. Although 75 bar was the lowest tested pressure, but the rate of CO2 formation at that pressure with increasing temperature is very highly increased when the temperature located between 25℃until 150℃while it stay constant from 150℃to 225℃. Furthermore, raising the fuel temperature and the injection pressure especially in high load case will enhance the combustion in the engine as it reduces the HC presence in the engine emissions. It can be observed that the engine operated with diesel vapor-air mixture exhibits a significant reduction in HC at all loads. Also, the lowest engine load examined was the lowest values of CO emissions for the same value of injection pressure and engine speed. However, increasing the fuel temperature at the same injection pressure with fixed engine load will decrease the amount of CO emissions formed as a result of combustion process. For the same examined system temperature the highest pressure applied recorded the lowest CO emissions in the test cases. Also, the result indicated that the nitric oxide and smoke levels were very low and HC levels were almost about 50-150 ppm in the engine tested case.