高功率密度液力变矩器空化特性研究

高功率密度液力变矩器由于其内部流速高、局部压力低而易出现空化现象,导致其液力性能恶化。针对液力变矩器内空化现象进行试验及数值研究,通过对不同转速、不同速比及不同补偿油压力下液力变矩器性能测试,获得空化随工况及供油条件变化规律。构建基于Rayleigh-Plesset的全流道瞬态空化仿真模型对不同工况下液力变矩器内部两相空化流动进行预测,利用应力混合涡模拟湍流模型精确捕捉涡流状态,实现对有/无空化下液力变矩器内部流场及液力特性的计算。结果表明,液力变矩器在高泵轮转速、低速比及低补偿压力下容易发生空化,空化程度随着速比的下降而升高,在起动工况时达到最大。在空化工况下,液力变矩器导轮流道内产生大量空泡,空泡阻碍油液流动,导致循环流量降低,进而使液力变矩器传递功率的能力下降,起动工况下能容系数降低高达31%。全流道瞬态空化模型能够实现液力变矩器空化特性的精确预测,对变矩比、能容系数及效率的最大预测误差由无空化的30%降低至5%。

Investigation on the Cavitation Characteristics of High Power-density Torque Converter

High power-density torque converter is prone to cavitation due to high circular velocity and low local pressure, consequently leading to performance degradation. Both experimental and numerical studies are carried out on the cavitation phenomena in a torque converter. A series of hydraulic performance tests are performed under different wheel speeds and charging pressures to reveal the relation between cavitation and operating/charging conditions. A full wheel transient cavitation model based on Rayleigh-Plesset equation is built to capture the two-phase cavitating flows inside torque converter under variant operating conditions. The stress-blended eddy simulation (SBES) model is adopted to calculate the turbulent behavior inside the torque converter. Both experimental and numerical results revealed that cavitation might occur under high pump speed, low speed ratio and low charging pressure conditions, and the cavitation degree increased with a decreasing speed ratio. The worst-case-scenario for cavitation is the stall operating condition. Large amount of cavitation bubbles are generated in the stator domain, blocked the main circular flow and reduced the mass flow rate, resulting in capacity degradation as high as 31%. The full wheel transient cavitation model is able to predict the cavitating hydraulic performance precisely. By considering cavitation, the maximum prediction error for torque ratio, capacity factor and efficiency is reduced from 30% to 5%.