燃油温度对柴油液相喷雾特性的影响

    Effects of fuel temperature on the characteristics of diesel liquid-phase sprays

    • 摘要: 在低温环境下柴油黏度增大流动性变差,抑制了柴油机柴油喷雾的雾化和蒸发,直接影响柴油机的冷启动可靠性。为解决上述问题,采用背光法试验对比研究高温、低温柴油在不同工况条件下喷雾宏观结构的变化规律,并利用MATLAB对拍摄的喷雾图像进行数字图像处理获取喷雾宏观特性参数。试验结果表明,喷孔直径0.12 mm时,随着喷油压力的变化柴油温度对喷油量的影响比较明显,黏性力的持续作用减慢了靠近壁面的流体层的流动速度,抑制柴油喷出,柴油温度低于0℃时随着柴油温度降低喷油量减少,高压时柴油温度降低导致液滴间增强的黏性力在动能和惯性力的作用下反而促进柴油持续喷出,喷油量随着柴油温度降低而增多。喷油压力为75 MPa时,柴油温度降至−20 ℃相较38 ℃柴油喷油量增加了近23.87%。并且小孔径下柴油温度降低液滴间黏性力增大,导致喷雾液滴尺寸增大,液滴所具有的动量越大,削弱柴油喷雾径向发展趋势,轴向运动能力增强,随着柴油温度的降低,喷雾贯穿距增大而喷雾锥角减小,柴油温度从38 ℃降至−20 ℃,柴油喷雾在0.70 ms时刻喷雾贯穿距增大了39.89 mm。喷孔直径为0.28 mm时,喷射压力小于75 MPa,喷油量随着柴油温度的降低而减少,相同喷油压力下柴油更多的动量消耗于克服更大的内部摩擦力,使其到达最远距离的能力减弱,降低柴油温度会导致喷雾贯穿距和喷雾锥角均减小,当喷油压力为75 MPa时,0.70 ms时刻−20 ℃柴油喷雾贯穿距相对38 ℃柴油喷雾贯穿距缩短了30.86 mm。相同喷射压力下增大喷孔直径,−20 ℃柴油喷雾贯穿距减小,喷雾锥角增大。随着喷油压力的提升柴油温度对喷雾宏观特性的影响更加突出,喷油压力越高降低柴油温度造成的喷雾贯穿距缩短趋势越明显。研究结果可为柴油机冷起动过程低温柴油喷雾特性提供支撑。

       

      Abstract: A diesel engine is the most energy-efficient powerplant in agricultural machinery. However, the performance of cold start has been limited to the diesel fuel viscosity in the low-temperature environment. Fluidity deterioration has also inhibited diesel spray atomization and evaporation, leading to the low reliability of diesel engines. In this study, the backlight test was conducted to compare the variation in the spray macrostructure of high and low-temperature diesel under different working conditions. The digital image processing of the spray images was then carried out using MATLAB to obtain the macroscopic characteristic parameters of the spray. The test results showed that the much more outstanding impact on the injection volume was obtained in the diameter of the spray hole 0.12 mm with the various injection pressures at diesel temperature. The continuous viscous force slowed down the flow velocity of the fluid layer near the wall and then suppressed the diesel emission. Once the diesel temperature was lower than 0 ℃, the diesel temperature was reduced by the amount of injection. The diesel temperature decreased to enhance the viscous force among droplets at high injection pressure. The continuous ejection of diesel was then promoted under the action of kinetic energy and inertial force. The fuel injection amount increased with the decrease in diesel temperature. When the injection pressure was 75 MPa, the diesel temperature dropped to -20 ℃, compared with 38 ℃ diesel injection quantity increased by nearly 23.87%. The diesel temperature decreased under small pore size, leading to the increase of viscous force among droplets. There was an increase in the droplet size and the greater momentum of droplets. Thus, the radial development trend of diesel spray was weakened to enhance the axial movement. Furthermore, the spray penetration increased with the decrease of diesel temperature, whereas, the spray cone angle decreased. The diesel spray penetration increased by 39.89 mm at 0.70 ms when the diesel temperature decreased from 38 ℃ to -20 ℃. When the nozzle diameter was 0.28 mm, the injection pressure was less than 75 MPa, and the injection quantity decreased with the decrease of diesel temperature. More momentum of diesel was consumed to overcome the greater internal friction under the same injection pressure. The farthest distance to reach was weakened. Diesel temperature was reduced to decrease the spray penetration and spray cone angle. When the injection pressure was 75 MPa, the diesel spray penetration at -20 ℃ at 0.70 ms was 30.86 mm shorter than that at 38 ℃. The spray penetration of -20 ℃ diesel decreased with the increasing nozzle diameter at the same injection pressure, whereas, the spray cone angle increased. Secondly, there was a more prominent influence of diesel temperature on the macroscopic characteristics of the spray with the increase in injection pressure. The higher the injection pressure was, the more outstanding the shortening trend of spray penetration caused by the decrease in diesel temperature was. Therefore, the injection pressure and nozzle diameter can be expected to improve for better evaporation and gasification of diesel liquid phase spray at low temperatures.

       

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