喷孔锥度和燃油温度对柴油喷嘴内空化流动特性的影响

Effects of Hole Taper Coefficients and Diesel Temperatures on In-Nozzle Cavitation

为弄清喷孔锥度和燃油温度对柴油喷嘴内空化流动特性的影响,阐明喷嘴内气液两相空化流动初生机制,采用背景光高速显微成像技术获得不同锥度系数及燃油温度下简化竖直单孔柴油喷嘴内空化流动及近场喷雾光学图像。结果表明:随着喷射压力增大(0.2~1.0 MPa),柴油喷嘴内燃油逐步从单相流动发展为片空化、线空化、空气倒吸和超空化等多种气液两相空化流态;云空化、线空化和空气倒吸现象的初生强烈依赖于燃油喷射压力,且具有显著的瞬态不稳定性。与圆柱形喷孔相比,渐缩锥度喷孔对各类空化流态具有明显的抑制作用,且锥度系数越大,抑制作用越显著。同时,提高燃油温度(288 K、303 K和323 K)有利于各类空化流态的初生及发展。此外,喷孔内部空化两相流动加剧了燃油射流的不稳定性,促进了射流破碎雾化,有利于增大喷雾锥角。研究为揭示各空化流态特性及其对喷雾特性的影响规律奠定了理论基础。

This study focuses on the effects of hole taper coefficients and diesel temperatures on the in-nozzle flow regimes,and reveals the cavitation inception mechanism.A self-circulating visualized system for the cavitation and near-field spray patterns was set up with a high-speed camera and a macro lens for the simplified vertical plain nozzles.The results show that the in-nozzle diesel flow will change from the single-phase flow to gas-liquid two-phase cavitating flow including the sheet cavitation,cloud cavitation,string cavitation,air suction and super cavitation with the increase of injection pressure(from 0.2 to 1.0 MPa).As the inception of cloud cavitation,string cavitation and air suction heavily depends on the diesel injection pressure and shows significant transient instability.Compared with cylindrical nozzles,tapered nozzles strongly suppress the inception and development of various cavitation regimes,and a greater taper coefficient means stronger suppression.Additionally,improving the diesel temperature(288,303 and 323 K)is beneficial to the inception and development of the cavitation regimes.Moreover,the inner two-phase cavitation flow worsens the fuel jet turbulence and increases the spray cone angle,eventually promoting the jet atomization.The study provides a theoretical basis for revealing the characteristics of cavitation flow patterns and their influence on spray characteristics.