Research On Spray Characteristics Of Large Size Marine Diesel Engines

Spray characteristics of diesel engine have appreciable impact on the air use rate and gas-mixture uniformity, and is one of the most important influence factors on combustion. Now, many researchers have done many studies on vehicle diesel and infrequent about large size marine diesel. Because it has wide difference between large size marine diesel and vehicle diesel, empirical formulas and models based on much assumption and simplification both have their sphere application, and it would bring error using these formulas and models to predict spray or combustion of large size marine diesel without proof. Therefore, this study had done these:(Ⅰ) Analyzed interrelated literatures, the dominating influence factors had been confirmed: injection pressure, chamber gas density and temperature and nozzle hole structure(oil characteristics’ influence is not contained in this study). Through a constant volume bomb(CVB) test bench, designed 11 testing program, got images taken in different situations, gained spray characteristics information by binary image, then analyzed different influence factors on spray characteristics. The results showed these:1) When injection pressure changed from 70 MPa to 160 MPa, spray droplets’ velocity increased, at the same time, droplets’ diameter decreased and drag force increased, so penetration increased and growth rate decreased; While spray cone decreased a little with injection pressure increase.2) When gas density changed from 11.5kg/m3 to 80.5kg/m3 with filling N2 or SF6, drag force increased, so penetration decreased; When gas was same, spray cone increased with gas density increase;When gas was different, because SF6’ molecule number was less than N2 even if its gas density was higher, its effect of break-up gone down, so spray cone may decreased with gas density increase in different chamber gas;3) When gas temperature changed from 298 K to 393 K, the temperature was too low and the vaporize effect was not obvious to the whole spray, so penetration changed little; While more droplets surrounding of spray vaporized to transparent vapor with gas temperature increased, but shadowgraph methods couldn’t record vapor, so it lead to the recorded spray cone decreased.(Ⅱ) Based on the CVB spray experiment, using CFD(Computation Fluid Dynamics) software AVL-FIRE to study the parameters of WAVE and KH-RT break-up model’s influence on penetration, to calibrate the break-up model, took a precision compare between WAVE and KH-RT break-up model to confirm the applicable spray model. The result showed: Towards the penetration calibration in the CVB condition, there was little difference between these two model, while WAVE model just needed alter only one parameter. So WAVE model was confirmed to be the applicable model to large size marine diesel engine, and the key parameter C2 of WAVE was confirmed too.(Ⅲ) Using the calibrated model, changed the boundary conditions to high temperature like diesel engine combustion chamber, to study the influence of injection pressure, gas density and temperature, nozzle hole structure on spray penetration in vaporizable condition. The results showed:1) Spray characteristics at hot conditions were different from cold conditions. At cold conditions, oil vaporized little and liquid would interact the wall; At hot conditions, oil vaporized much along the spray development, liquid penetration would be max while gas penetration would go on.2) When gas temperature was 800 K, injection pressure changed from 70 MPa to 160 MPa, spray energy of motion increased, so penetration of gaseous phase(POGH) increased; at the other hand, droplets diameter decreased and vaporized more quickly with injection pressure increasing, and droplets lifetime decreased, so max penetration of fluid phase(MPOFH) tended to decreased.3) When gas temperature was 800, gas density changed from 11.5kg/m3 to 54.5kg/m3, drag force increased, so both POGH and MPOFH decreased.4) When gas temperature changed from 600 K to 800 K, spray energy of motion changed little, so POGH changed little; While droplets vaporized more quickly with gas temperature increasing, droplets lifetime decreased, so MPOFH decreased.( Ⅳ) In this study, it influent spray cone much that nozzle hole diameter and Length/Diameter rate changed from 0.94 D to D and 6.5 to 8.5 respectively, but influent penetration little. Maybe, the size of large bore marine diesel engine’s nozzle hole is too big, nozzle hole diameter and Length/Diameter changed within the specification limits of this study influent droplets’ velocity little. The virtual reason could be researched by nozzle flow simulation in the future.This research grasped the spray characteristics of independent R&D oil supply system of large size marine diesel by CVB spray test combining CFD simulation, and completed spray model calibration, which could provide the basis of combustion system development and meeting the needs of High-efficiency and Low-emission of large size marine diesel engine.